Kaseki, one additional question:

There's a couple different flavors of MUA units. Some of them are wired to the range hood where it activates when the range is turned on. There's another one I'm looking at (a "universal type") that detects air and then opens.

I may be wrong on the above, but that's what I think I'm seeing.

I think somehow in the end, it would be beneficial for the damper to stay off if the lowest fan speed is being used on the hood (which I think is like, 280 CFM or so). That would probably be a better design so that it isn't on all the time just because the hood is on at its lowest setting. Not sure if this is common, but I'm sure I'm not the first that asks.

Not sure if the automatic one detects air speed and is smart enough to know when to open, or if it operates by any time it detects air speed, it will open the damper.

I'm sure with the wired variety, and if you're good with electronics of the range, that you can possibly wire it in to only be used if the user presses the middle or high switch. But that's kind a bet.

Just curious on your thoughts.

I think I identified a good place for the air intake, which is about 10 feet away from the stove (and behind a sectional wall) at the end of the hallway where the garage door entry and laundry room entry doors are. A little draft there is not likely to be noticeable by anyone as it's not a place you hang out at.

While I don't know what specific suppliers are doing, normally an electronic sensing system will have a threshold below which there is no action, and above which there is. In an entirely passive MUA except for a powered damper, opening the damper need not be proportional. All one needs is a signal. One such is current sensing of the hood power. (There is an entire thread here somewhere where that was discussed in some detail. One complication is that the hood lights could actuate such a scheme.) Another (I think Broan uses it) senses airflow in the hood duct. Clearly either of these will want a threshold setting. I can't answer who has what, however.

If I had the components and found premature [defined by me] damper opening, I would build in a threshold circuit if the electronics weren't too opaque for revision. However, I can't expect others to do so. So inquiry directed at the manufacturers is advised.

Note that my unfinished MUA system is presently running passive, and the damper action is merely the inability of warm household air to rise past the heat exchanger and temporary filter due to low pressure difference across them when no exhaust blowers are running. Eventually a damper will be added along with a blower and filter pack. Blower control will be based on the differential house to outside pressure. This is mainly due to having two kitchen vent systems along with other exhaust fans. My only combustion appliance (oil burning boiler) has its own MUA. At present measurement indicates that this passive system is adequate for my hood, defined as sufficient air velocity at full blower power. However, this hasn't been measured with all blowers in operation. I expect some degradation justifying the active system plan. In any case, letting the house pressure fall any significant amount, even without a back-draft issue, is still undesirable. So I suggest erring a bit on the side of early damper opening.

Got around to talking to my HVAC friend.

He wants to mount it to the return side of the HVAC system. That way we can run a short duct to the gable end of the house-side.

He likes the design of the make-up air coming from vents, rather than having a dedicated make up air vent. He says the make-up air has more benefits being given to the whole house rather than just localized to the kitchen. There needs to be a balance of air pressure within the building envelope.

I tried to ask him more details about operation, but he said the damper would be interlocked with the furnace blower, but he is considering to add a pressure switch and relay so that it could work with the hood too. I think this means that the damper would open when the blower comes on, but the damper could also open if the hood is on too. This has benefits of providing filtered fresh air for more than just hood operation.

He prefers the Honeywell unit that has a thermostat interlocked so that the damper doesn't open if it's too cold outside. So you do have some conditioning that is occuring with how he is thinking of it.

Sounds like he knows what he's doing and is going to come help. He's agrees that an 8" damper is what I need.

He knows how to wire the Honeywell Fresh Air System to the hood (and obviously to the air handler). But would anyone dare to fathom how we could wire up to the hood in a way to where the lowest setting (270CFM) doesn't kick on the air handler?

I think that would be the best solution, but may require some ingenuity. The Z-Line 625-30 doesn't mention any electrical things in the installation guide. I did find the electrical diagram however.

I see a multiwinding motor; this will be harder to sense current draw except on the common return, if there is one. Maybe a white lead -- I can't quite read the drawing. Alternatively, given a DIN rail and a bunch of current sensors and relays, one could sense all motor windings except the lowest speed and do an OR circuit for the damper. Or sense common minus the low speed sense. Or sense common and sense low speed, and use a NOT AND circuit. These are guesses because just which windings are used for various speeds is not known (so far). And current sensing of a normal waveform vs. a possible chopped or synthetic waveform may require a different approach than simple magnetic coil measurement. My first guess, though, would be that for economy, the windings' power and/or number of poles would determine the speed and the waveform would be normal ac.

Note that inhibiting the MUA damper when it is cold outside means that there is no MUA when it is cold and the house pressure will fall and the hood will be partially gagged at higher flow rates.

When the damper is open and the hood running, the amount of heat needed is given approximately by this chart. If you overpower the furnace internal pressure, you may find MUA is flowing both to the heat registers and the return registers.

Cool. I'm sure we'll figure that out.

Am I correct to assume that because we are hooking the MUA to the return line, that any air that it pulls will be somewhat conditioned? ie, does the air that it pulls from the damper really stay cold while it runs through the filter and the furnace before it makes its way to the registers? Assuming it will warm up the air some. For the summer, it should cool down the air somewhat to because the air will move over the AC coil. Am I correct?

Let me know which items you disagree with below:

Benefits of hooking up MUA to return line:

1) Potential for less ducting. You're only connecting the return to the outside--in many cases these are pretty close to each other.

2) No new air filter. The air handler air filter, which is what you're used to, is the damper's air filter as well.

3) Conditioned air. Because the air runs through the air handler (and filter), then the air will be somewhat conditioned?

4) No new vent or cutting holes in the interior.

5) Equal re-pressurization throughout the entire house for a direct to return line installation, instead of just adding pressure to one area.

6) It's also wired (or should be) to add fresh air to the house based on the control panel settings. This has benefits of improving indoor air quality--totally independent of how it's connected to the range hood.

Things that have me second guessing the return line:

1) Because MUA is going to be wired in to the system blower, that means every time the hood is on, the blower turns on. For someone that runs the hood 30-60 minutes a day, that's going to lead to more cycles and more energy usage.

So you can see why I'm curious on trying to make sure that this is doing its job, adding fresh air in the house (regardless of the hood wiring--it's going to be wired to work full time if the outside environment permits), and adding make-up air when needed. I don't think 270CFM is needed if we envision that this "low" setting will be used the most--think about soups, steaming things, etc. For stir frying and other things, you can bet that we'll crank it up, and for that everything should activate.

School me if I'm off base or overthinking anything.

Here's a better pic of the diagram.

I can probably bring the Z-line inside and take a look to see how it exactly looks inside. Maybe with my electrical tools and some help then I can actually play around to see how it actually works.

You certainly don't want to wire it up in a way where if someone turns the light on that the damper would activate...that would be a bad move!

OK, the motor has a white lead. If you can determine that this connects to the neutral house wiring lead, then current in the motor lead should be proportional to whatever the motor torque is, and this will be related to (but not linearly) the CFM.

1) Presumably true, as viewed from here in S. NH.

2) Also true, but only so long as the furnace motor is driving air through the filter and the air flow is not less than the hood air flow. Otherwise, if the furnace is moving X CFM and the hood is pulling Y CFM (Y > X), the difference Y - X moves partly via the return ducting, and, in proportion to the differential pressures here and there, some additional will be pulled through the furnace filter. A one-way damper in the return would correct this.

3) True if you allow a range of meaning to "somewhat."

4) That would be the point.

5) Equal pressurization is true, but the idea of your last phrase is not true steady state because the house pressure between rooms with open doors or clear passageways where air is added to one area will be sub fractionally inch of water column different in pressure. The only cause of pressure difference is air moving through meaningful restrictions.

6) Yes, but the hood MUA will be higher in CFM than house replacement air requirements, as far as I know. @opaone can probably comment on replacement air requirements. I've never studied it.

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1) Yes, but in a restrictive filtered/registered ducting system, you would ideally want an MUA blower anyway. What you need to do is have the thermostat (hot / cold) control the heating and cooling including the furnace blower, and yet also be able to separately run the furnace blower based on MUA needs (e.g. hood blower speed) even when no heating or cooling is required. This is a controller issue and amenable to solution by relays if not otherwise easily configured.

Great response. Sounds like I'm in for a nifty solution.

I have a NEST, and it allows me to schedule and run the "fan". Currently, as we know, the fan just recycles the indoor air since my system is closed. After a fresh air system, the fan will pump in some outdoor air, as long as the Honeywell's controller allows it to be such (i.e., making sure air outside isn't too cold/too hot).

Thanks for the information regarding the wiring. Sounds like this is all doable.

I think the solution that I'll be installing this weekend should be a great solution and not only solve the code requirement, but to also get an upgrade in interior air quality. I would tend to think most new hoses probably have one or two of these installed by default.

I'll post pictures of our progress if you are interested. The goal this weekend is to install the fresh-air system. Then a month or so down the road when I'm installing the range hood is to get that wired in appropriately and for the "fresh-air system" to be wired up as a "make-up air" system as well.

"for the "fresh-air system" to be wired up as a "make-up air" system as well."

I may be missing something here but this is a rather concerning statement. These are very different appliances that serve very different purposes. Each are individually critical to occupant health.

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If this info is above but I missed it tell me and I'll search through when I get a moment... Where are you located (e.g., what is weather like)?, how old is your house and how well sealed? Rangehood is 600 or 900 CFM?

What MUA are you installing? I saw mention of the Honeywell damper - is that it?

Boise, ID.

600 CFM Range Hood

House built in 1993. Still has original 1993 windows. We have a flap cat door, but that shouldn't play into much since that isn't a permanent fixture. Front door needs to be re-gasketed. Otherwise, it's pretty average for the age when it was built.

My HVAC guy likes the Honeywell y8150 and the associated controller for it. I think he's going to add it as a "fresh air" system, but also wire it up as a make-up air system as well.

A few thoughts...

For any range hood over 400-600 CFM you should have a powered MUA that matches your hood CFM. The damper you've planned will be insufficient (both for lack of power and severely undersized for passive anyway) and result in risk of backdrafting any convection gas appliances such as your gas fireplace and furnace as well as sucking air in through various leaks in your envelope. It will also significantly reduce the performance of your hood due to increased supply side static pressure.

The CO from backdrafting is a serious health problem and risk of death. Outside air being pulled in through leaks can not only impact indoor air quality (whatever is in the walls gets sucked in to the house) but can result in moisture, mold and framing deterioration. You don't want any of these.

For fresh air the number one element is removing CO2 and VOC's from inside and replacing that with fresh outside air. How does your HVAC guy plan to remove CO2 & VOC's?

The hood will be 600 CFM, with a 7" duct.

We are planning on using a the Honeywell 8" damper. You're saying this is going to be too small when hooked into the return side of the duct?

The ornamental gas fireplace I believe has a damper on the outside wall for it.

Yes. If you have a powered MUA then 7-8" will be fine. For passive MUA a duct needs to be about 4-7x the size of the powered duct IIRC. Larger if there's a filter (and there s/b). IIRC for 600 CFM passive you can't have more than about 0.03" pressure drop across your filter though this depends on the fan curve of the range hood.

Your range hood will lower the pressure in your house resulting in air infiltration which will take the path of least resistance so it will pull a little through your little passive duct and then try to pull the rest through leaks or other openings (flues for convection gas appliances). These overall will have considerable static pressure resulting in your range hood performing at perhaps 150 CFM instead of 600 CFM.

I assume your range is on the ground level? Is there a level above this that will create additional stack effect pressure?

For a powered MUA, those are those external units that typically sit on buildings and what-not, correct?

This is the honey well system. It's "powered", but I assume it's still a "passive" system.

Or is it "powered" because it works in conjunction with the air handler?

I find it hard to believe that people with 600-1000 CFM hoods are adding active systems on their roofs for residential, or are putting in 32-56" passive vent ducts for outside air. I seem to be missing something here...

That is passive, not active or powered. A powered MUA has a blower that matches the CFM of your range hood.

The most common residential MUA is: http://www.electromn.com/gen/makeup_air_ii.htm though Fantech and others are also not uncommon.

More: https://www.fantech.net/fileadmin/user_upload/fantech/Support/Media_Center_USA_Files/e1574-makeup-air-system.pdf

Most states require powered/active MUA for hoods over 400-600 CFM (varies by state). This is not code-gone-wild but a critical element for your families health.

In addition you should likely have an ERV or HRV for fresh air (U.S. code was about 20 years behind in the early 90's). Get an IQAir and stick it on a bedside table for a few nights and then in your kitchen for a week or two to see what your CO2 levels are. If above 700 ppm then you're not properly exhausting CO2 & VOC's.

Sounds like you're saying regardless to monitor CO2 levels. I have Co2 detectors through the house, have never once gone off. But if that goes off, that's probably at a WAY different (very bad) level than what you're saying. :)

I would rephrase the question to you that I have a 400CFM microwave range hood right now. The house was built that way, passed inspection/code that way. There's no make-up air system installed. It's hard to believe that going to 600 CFM would all of a sudden require a a $1500 active unit which I know most people are not installing with their new range hoods. And I feel like I'm being pretty conservative...many people are putting in 1000-1200 CFM monsters in their house!

So it "passes code" because it's <400 CFM, but that doesn't necessarily mean that it's "correct" because I doubt the house has a 400 CFM air leakage somewhere. Maybe?

Our requirements, pretty standard:

Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system.

I'm sure that since he's an HVAC professional, he's probably going to measure the pressures and such, not just come out, stick it in the wall and hook it up and then leave.

He's probably going off rule of thumb for now, which according to Broan is to size your MUA duct at least the same size as your exhaust duct. Broan also believes that the codes imply that the buildings envelopment is also used as part of the equation, NOT just the MUA unit. So you technically do not need to intake 600 CFM of air with the MUA if the house has "X-amount" of leakage.

As seen, my state does not require an active/powered MUA unit. Are you sure about "most states", because that's a pretty serious implication. I can see that maybe being the case for new houses, but for a 90s home?

For my state, It just needs to be automatically controlled to the exhaust system, and the air (including the house's building envelope as part of the equation) needs to intake approx the same amount of air during operation.

Nash, my 1200 cfm hood has a 10" duct going out, and the passive MUA has to have that same size coming in according to our city inspector. I never use my hood on full power, but because I needed a 48", it came with plenty cfm. I can crack a window open f needed to. Still alive and kicking ;)

Adding a bit of perspective, what suffices for safe MUA pressure loss depends on what the combustion appliances are. In the case of this thread, as I recall that the only potential issue would be trying to operate the fireplace with the hood blower operating. (What does the furnace run on?)

So potentially, the house could operate at a level of negative pressure that was safe from CO back-drafting and not too burdensome on the hood attainable flow rate, so long as the fireplace is off. Unless the fireplace is sealed from the house, it won't take much negative pressure to overcome its draft, unless the fire is roaring.

The meaning of passive MUA, as used here, is no MUA blower. Pressure loss may be slight if a large diameter short duct and no filter are present, and significant, at least for natural draft gas combustion appliances, if a filter is present.

Active MUA, as used here, means with MUA blower and with the implied capability of keeping the house pressure close to neutral. If you are using a furnace blower to aid passive MUA, then the MUA may be treated as marginally active, depending on parameters I don't know, such as how much CFM the furnace will flow when on.

Some of opaone's suggestions above may not directly relate to a narrow view of MUA (which I generally take to avoid having to write endlessly), but which are pertinent to health. Note that CO2 and CO are different and it is CO that is a back-drafting issue and human and gas combustion sourced CO2 that is an insufficient air exchange issue.

I think at this point we need to know what the furnace blower can do, CFM-wise, when pulling air from outside and distributing it through the house ducting, given that the kitchen hood is pulling 2/3 of it free air rated CFM (thus achieving 400 CFM). The microwave oven probably couldn't do 200 CFM, depending on how gagged its design is.

My combustible appliances are:

• 1990's carrier Furnace (in garage)
• Water Heater (in garage)
• Ornamental sealed fireplace (pretty sure it has a MUA right behind it)
• Gas stove (no make up air)

Yea, I don't expect the current microwave vent to do anywhere close to 400 CFM. It really sucks (or doesn't) when it comes to cooking. The vent right now runs into the wall, and up the wall to the attic, so it definitely has a 90 degree turn once it goes in the wall. Semi-rigid ducting (4") leads to the roof.

When I re-duct for the new hood, I'm going to delete what's there, and simply go straight up the ceiling with rigid ducting. The crown molding and cabinetry will hide the ductwork.

Thanks for all the time to help/inquire.

OK, so you have a gas furnace, but it is in the garage and presumably its burner is not affected by house pressure. BTUh seems adequate. We don't know from the data whether the furnace blower can correct the house pressure against the MUA filter and the hood blower. I'm not sure how to interpret the 0.5 inch w.c. pressure listing on the label, but I would guess it is the maximum loop pressure loss from furnace through the house and back into the furnace. Without a furnace blower fan curve, and one for the hood, we are only guessing. However, one can build to plan, test, and if the furnace seems to be struggling at full hood blower power, then a modest blower in the fresh air path (also controlled by the hood blower being on) should be sufficient, I would guess.

First, keep in mind that code is the worst that you are legally allowed to build. U.S. code is also, still, well behind international standards, other countries as well as behind science.

The vast majority of houses built in the U.S. in the 80's, 90's and 00's had inadequate ventilation, kitchen exhaust and kitchen MUA. So yeah, it 'met code' in 1993.

There are reasons that codes are updated - we learn stuff. If you want to continue living in a house that will cause health and learning problems for you and your family, and then potentially make those problems worse with your current plans that is your choice.

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You are confusing CO and CO2. Very different elements. Do some research, get an IQAir (or Netatmo or URAD A3), see if you have good, poor or awful ventilation in your house currently.

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Your HVAC guy and his lack of understanding of what he's doing is putting your and your families health at risk.

My guess is that you already have poor ventilation and poor indoor air quality. Your and your family's cognitive abilities are likely impaired when in your home with doors and windows closed and you may be experiencing other problems as poor sleep or headaches if it's really bad which was not uncommon in 1993.

Now you want to add a 600 CFM hood but with poor MUA. First, that hood is struggling against static pressure in your home so will not perform well. It will pull some air in through your little damper but not much.

Assuming you're not using your fireplace and your garage is well sealed from your house then you shouldn't have backdraft problem (CO, NO2, etc.).

It will pull air in through whatever leaks in your envelope it can. There's a building science issue here of creating moisture problems within your wall structures. The air coming through the leaks is also picking up whatever is in the wall cavities and bringing it inside. That may be nothing but could be fiberglass, mold spores, creosote, or other VOC's. Also consider that the biggest air leaks are often also the places that rodents and insects use - find a bunch of mouse poop and you'll nearly always find air being sucked in to your house over it (this latter is probably not a huge health concern but is gross to think about).

There needs to be a little perspective applied here. I don't know if we even know what climate you live in. What's the sqft of the house? All of these play a part. The code you are up against is IMC 505.2 and its been around for over a decade. In my area the local code authority had adopted a revision to be over 600cfm before it needs MAU and it can be passive but they will not allow it into the furnace without it being conditioned prior.

Opaone is correct that our homes do not have the same IAQ that commercial space's have most times and that's pretty sad. A ERV however it may help with your IAQ will not provide make up air for your hood unless you plan to run the exhaust from the hood through the ERV.

Keep these items in mind.

1. if you live in a climate with below freezing temps ducting into your furnace with too much outside air can lead to safety issues. A furnace can safely heat about 20% outside air when the temps are low. Its a heat exchanger issue with possibility of cracking. If you have the largest furnace typically used in residential construction that's 400cfm maybe pushed to 500cfm.

2. If your house is say 2,000sqft with 9ft ceilings your hood/MAU would change the air in the house every 30minuts at 600cfm. If the house is 4,000 sqft its every hour. You can start to see when is MAU more important and how it will effect the house.

3. In a passive system the hood will draw from the path of least resistance. I would not worry about trying to wire a damper to open at a certain speed. If its low speed it will be pulling less air than when its on high speed.

4, When I was going to do mine what I had was a fan that energized when the hood was engaged. The fan was downstream of an electric duct heater that would engage when the air temp was below 55*. I ducted it to a toe kick in the cabinet across from the range. Its simple but effective for what i need. There is a filter prior to the EDH. In the end I opted for a 500cfm hood so I would not have to deal with it. My house has an ERV that feeds into my main furnace for fresh air, Radon mitigation system and two fans that we turn on in the summer at night to utilize free cooling (t-stat controlled) . I feel I have enough leakage and fresh air to have a safe environment.

My house is about 1770 Square Foot with 8ft ceilings. Single story. I do have bout 300 square feet that has cathedral ceilings (10ft or so).

Regarding wiring the damper, I'm more worried about wear and tear on the air handler that the act of turning on the range hood will fire up the blower each and every time. I've mentioned that it's about 30-60 minutes a day. I'm anticipating that we would find the low speed setting (270cfm) to be adequate for cooking most of the time.

We have freezing during the nighttime for 2-3 months, and hot temps for 2-3 months. I'm curious if I would be able to turn the Honeywell "OFF" on the controller for those months, but the damper/air handler blower still being allowed to open when the range hood is activated past a certain CFM. I'm sure my HVAC guy will know that. The answer is probably "yes".

That is, if my assumption is that if a passive MUA is connected to the return side, that the blower has to be running for it to be effective at all. Definitely correct this assumption if it's not the case! If that's not the case, then I'm making this overly complicated.

How I see it, is that I'm getting both MUA and FAS benefits.

FAS: Wired up to air handler. You can set a schedule ,yadda-yadda for it to run the system fan to draw in fresh air. If temp conditions allow, it will draw in fresh air during heating/cooling cycles as well. But as mentioned, I may want to turn this "OFF" for certain months. Lots of calculations go into configuring this. My HVAC knows his stuff and will do all that.

MUA: In addition, wired up to the range hood for the sole purpose of it to open "when required". What I don't know is how much configuration can I have for "when required" (every single time the fan is turned on, even on the lowest setting?), AND if connected to the return line (which it will be), does the air handler blower has to be running for the MUA to be effective? If the blower HAS to be running, then you can see why I'm trying to ask some of these configuration questions.

For the purpose of MUA, if the air handler blower doesn't need to be running when the damper opens, then I could care less about wiring it up in a certain way. IF the blower has to be running, then all of a sudden I will start to care about energy usage as well and outside air temps.

Sorry, just getting educated! I'll be getting a ventilation and ducting upgrade either way!

I may have left out something in my last message. Assume that the MUA is connected to the return at the furnace.

If the furnace is off, and if the return has a one-way damper before the intersection with the MUA, then when the hood is on it will pull air through the furnace, past the blower and past the furnace filter. There will be some pressure losses by the time the air gets into the house. These may not matter assuming what we seem to know, other than to reduce the achieved CFM of the hood.

If the furnace blower turns on, then the pressure loss in the house will be less (higher pressure) and the hood flow rate will be higher than with the furnace blower off. House pressure will not go positive, however, due to the return working whenever the hood flow rate is less than the furnace flow rate.

What is awkward about this scheme is that the hood flow rate will depend on the furnace state. But it would be consistent and repeatable if the furnace blower is always turned on with the hood (at least for higher hood blower settings).

I am unclear what the issue is with operating the furnace blower more often. In the winter surely it operates a good fraction of the daytime. And if the air is also cooled in the summer, likely it operates for a good fraction of the day then too. Obviously lifetime, whatever it is, will be reached sooner, but ultimately, replacement cost should not exceed what would be needed anyway if the MUA had its own blower with its own lifetime.

The value of an MUA blower in this configuration is that it can be linked to the hood, and no messing in the otherwise functioning furnace circuitry is needed.

I am unclear why a gas-heated air furnace would have a risk from sub freezing air. Is there an intermediate water stage?

You can buy a sail switch system -- I have used them twice and am using at my new house. Yes it's designed for a hood and any grease. The sail switch is nice because it does not come on when my units are running on low ..... it's not needed. You order it based on the duct size on the unit ..... it comes with a damper and transformer. It's independent of the hood. The sail switch calls the intake damper to open and it dumps the air into the HVAC system

Kaseki every furnace has a temp rise its designed around which basically is this case would be the mixed air temp. For the purpose of this hood if its 0* outside at 600cfm on a 1600cfm furnace (likely given the size of house) mixing 68* return air, the temp of the mixed air entering the furnace is at 43degrees. Obviously depending on other variables, dewpoint and humidity you could create condensation on the heat exchange which leads to rust, leads to cracking, leads to CO. Also the HX is not designed to have those swings in expansion and contraction. That's why in high outside air systems a stainless steel heat exchanger is required. All dependent on climate. The discharge air temp on that scenario could be in the low to mid 60's depending on the furnace. Here is an article explaining some of it. Typically and engineer in my area will not design a furnace over 20-25% outside air because of the heat exchanger and the discharge air temp.

Furnace Air Temperature Rise - HVAC School (hvacrschool.com)

Thanks Gary. My experience is blown oil boilers and hydronic heating, so the limitations of other schemes has not been experienced.

With VS equipment -- You can duct it through a dehumidifier or a simple air exchange (aprilair) -- this will inject it into the supply. I have had 1200 cfm blowers for 30 year .... you don't leave them on high for very long. On the return you duct them back from the furnace -- not right into the unit

This may be one of the best threads online about MUA. Let's keep it going. Lots to learn here, and I'm sure more information/advice will be given.

I'm meeting today to plan with my HVAC guy. I'm sure I'll have some of his opinions before the end of the day. All of the issues that were mentioned here will be asked.

I'm sure we will get into a discussion of passive + engineered opening vs passive + return vs active. Since he serves many houses in my area, I'm sure he will know about air quality and will have an opinion on that.

For newcomers to this thread, I HIGHLY reocmmend reading this. It is the best article I have seen about MUA, and it's written in a way that non-HVAC guys (like me) can understand:

Part 1:

https://www.phrc.psu.edu/assets/docs/Publications/BB0312.pdf

Part 2:

https://www.phrc.psu.edu/assets/docs/Publications/BB0412.pdf

This article reiterates what has been said here. It goes over some pros/cons of each method. It reiterates that passive MUA systems, in general, cannot intake large amounts of air to compete with a high powered range hood. It goes into the topic that was discussed here already about how much your house may/may not leak depending on when it was built, etc.

For a new tight house, passive won't work unless you get stupid on your sizing. For an older house, passive + the house leakage, depending on the CFM of your exhaust, can be sufficient. But there should be some measurements and math involved, not just sticking in a MUA to "meet code".

MOST people probably don't install MUA units when they buy new range hoods--these people typically go 1000+ CFMs. Kuddos on me for actually being somewhat conservative with 600CFM and asking critical questions.

"For an older house, passive + the house leakage, depending on the CFM of your exhaust, can be sufficient."

That was the belief 20-30 years ago. Today we know a lot more about the problems that this causes.

I didn't learn much today other than what I've already saying.

The big thing I learned is that he disagrees that an 8" duct/damper that's tied to the return will be limited to 150ish CFM. He said it will pull in a lot more than that when the blower is on. He likes the idea, even though it's more effort, to not kick on the blower at the lowest range speed although not really required. The house will be constant supply of fresh air cycles from the outside.

He also disagrees calling a MUA that's hooked to the return line as a "passive" system.

Other than that, we are going to proceed with the plan of adding a fresh-air system (which he said is code for any new house) and to also hook up the damper directly to the hood.

I don't really have any other gas appliances in the house other than a sealed gas fireplace (this does not equate to anything as it draws and exhausts air from the outside) and my gas range. I think this makes my house pretty simple to do.

I can imagine another house having more complexity that has a gas-drier, gas-this, gas-that, furnace/water heater that's indoor, etc.

Pretty excited that I will have upgraded ventilation, but also a MUA system. My total cost should be around $500, and that's enough to also spot him some Benjamin's for helping. For others that are reading this, it's likely to be near 2x this price if you don't have any help and hired it out completely, and if you have a house/situation that's as simple as mine and are using the common passive "make up air" from Broan or Zephyer. So that $850 hood range that you bought would need +$1000, minimal, for it to function safely correctly, in the best case.

The basic concept is that given a perfectly sealed house except for a passive MUA duct/path, the MUA flow rate will exactly equal the actual hood flow rate, and these two identical rates will be such that the pressure loss of the hood and its ducting, plus the pressure loss of the MUA ducting, all of which are a function of air flow rate, intersects the hood blower's fan curve (see figure).

With an MUA blower, the two flow rates are still exactly equal, but the actual flow achieved by the hood system is more complicated to determine as it depends on what the control, if any, of the MUA blower is trying to control to. Both blower's fan curves need to be monotonic as in the figure or RPM hunting is possible.

(The brown line is the resistance curve.)

One goal of the duct configurations would be to make their contributions to friction pressure loss small relative to the pressure loss at the hood baffles. As the MUA accretes devices such as a heat exchanger and filter, MUA pressure loss rises and the pressure in the house falls. (In a real house, this also causes some leakage.) For fan curves I have seen, the pressure drop will penalize a VaH faster than it will penalize typical conventional in-line and external blowers, and likely conventional in hood blowers, depending on blade type and throat it operates in. If an MUA blower and control loop succeeds in keeping the house pressure equal to the outside pressure, the VaH is less relatively penalized, and its actual flow rate, given minimal ducting to the outside, should be close to its rated (non-magic) value.

What I see as in issue here, at least conversationally, is how good is good enough if one wants to be better than code minimum but not over-the-top in approaching perfection. And I would suggest that MUA ducting have room for a blower and determine in situ whether it is needed after the house is sealed up.

@kaseki, Quoted you for truth!

"What I see as in issue here, at least conversationally, is how good is good enough if one wants to be better than code minimum but not over-the-top in approaching perfection. And I would suggest that MUA ducting have room for a blower and determine in situ whether it is needed after the house is sealed up."

Today is make-up air installation day (and hood range exhaust day).

My HVAC tech believes that just installing a fresh air system and controller is enough to satisfy code. But I disagree because of this blurb that is in code:

""shall be automatically controlled to start and operate simultaneously with the exhaust system."

But he does like the idea of tying it into the the range hood. Today I should be able to post some updates. His statement is that the fresh air system is going to be on often, so it doesn't matter.

But in the summer time (dead of heat) I may turn the fresh air system off. Curious if the honeywell set to "off" but it still being wired to the range hood (I see this an an "override") will still activate the air handler?

Here's some possible outcomes today:

Sloppy case: Fresh Air system installed and programmed which "should" satisfy code (though I doubt this).

"fine" case: Fresh air system activates any time range hood is on. This meets the letter of the code, but it's kinda sloppy. Wouldn't it be nice if it stayed off for the first 2 fan settings and didn't draw any outside air in the house unless it "had" to?

Best case: Fresh air system overrides only when last two fan settings are used (>400CFM speeds).

What is a 'fresh air system'? Are you referring to an ERV or HRV ventilator?

https://www.honeywellhome.com/us/en/products/air/ventilation/fresh-air-ventilation-system-with-truezone-damper-y8150a1017-u/

Not too sure, but I live in a "moderate" zone so either would work. That is the system that we are installing today up to the return line.

Last week I "stubbed" the new exhaust pipe into the attic. The old system was 6". The new system is 7". A lot of work for just 1" of an upgrade!

I'm curious if the new 7" roof cap will fit without any deck cutting (current is a 6" roof cap). Probably not, but that curiosity will be in the back of my mind until I find out later today!

"He also disagrees calling a MUA that's hooked to the return line as a "passive" system."

He can disagree all he wants but he's wrong. It is very much passive.

Your air handler blower pulls air from the return duct. That air in turn comes from the source of least resistance which will primarily be the returns inside your home.

ONLY when pressure inside your home is below the outside pressure + the static pressure of your passive duct will it pull any air through the passive duct. HOWEVER, before it does that, your house will pull air in through any other places with lower static pressure which includes many leaks through wall cavities or windows or whatever - but frequently places that you don't want make up air coming from and especially if the outside air is higher humidity and higher temp than inside which will result in moisture condensation build up in your wall cavities.

""shall be automatically controlled to start and operate simultaneously with the exhaust system."

For a passive system, this may refer to having a controlled damper that opens when the hood is on and closes when it isn't on. I'm pretty sure Honeywell makes these, inter alia. Using instead just a spring balanced passive MUA damper may have deficiencies in windy weather. Of course, the spring balance also adds some pressure loss.

Yep. And wouldn't you agree that the range hood and a passive damper (even if it's hooked into the return line of the air handler and is on its on program/schedule) needs to be wired together? Basically, the range hood needs to be wired into the fresh air controller as an override in order to satisfy the requirement.

I talked to an inspector and he said that statement only applies if your kitchen does not communicate with the rest of the house (had a door, can be closed off, etc). That does not apply in my case since the return air grill is not impeded and is always available.

He says "yes" that make-up air is required, but "no", it does not need to have a dedicated duct/motor/wiring in order to meet code.

But, in order to keep the house pressure near the external pressure, it is necessary that the damper open when the hood is on. There might be some minimal air flow that can be made up with leakage, e.g., bathroom fan, but as I have seen with my more powerful system, the hood blower has a minimum flow rate that is still significant. This is either due to lack of good starting control of the blower when running a very low speed, or due to the ineffectiveness of baffles to extract the larger grease particulates when the air speed through the baffles is too low for the centrifugal effect to be sufficient.

TL;DR: Add a circuit that opens the damper when the hood blower is on.

Here is the ACTUAL wiring diagram of what I have (printed on the unit). Ignore the previous stuff.

Keep in mind that 'code' is "the worst quality that a vendor is allowed to provide".

'Code' is not good but only not horrible. And not horrible based on what a group of people on a committee believe.

At one time exposed asbestos was 'code'.

Lobotomies were once considered an appropriate medical remedy.

U.S. 'code' is generally 7 to 42 years behind other countries.

Of 211 chemicals not allowed in consumer products in EU countries, 207 of those are by code allowed in consumer products in the U.S.

The U.S. has the lowest life expectancy of all developed countries and the highest rates of preventible chronic disease. And we spend over twice as much on medical care as other developed countries.

A child in the U.S. is over twice as likely to be killed by someone driving a car as a child in Europe and a U.S. child riding a bicycle is 11 times as likely to be killed by someone driving a car as a child in Europe and if walking they are 17 times as likely to be killed - and this is because of what U.S. code allows traffic engineers to design.

When someone says 'it meets code' this is what they are signing up for as acceptable.

The "actual wiring diagram" shows two blowers with two speeds each (unless there are some additional phase controls in the electronics). The white wire from the "plug" should be neutral, and can be used to detect current flow. However, the lamps will be on this circuit, so they may or may not have an influence. (LED lamps, for example, would have low current draw.)

For the mental experiment that follows, assume that there are no combustion appliances being affected by house pressure and we are just trying to keep down flow through the walls. One could run the hood at full power with whatever the MUA was set to do, with windows and doors closed, and check around light switches and windows for drafts. Or one could measure differential pressure. While we have pressure limit values for different combustion appliances, I haven't seen one for keeping down air infiltration through walls. Maybe @opaone has a recommended value (other than zero).

In my view even if you are feeding exterior air into the furnace system the MUA duct needs a heater. It may only need to supplement the furnace to the degree that the air that gets into the furnace is within spec for cold air. Electrically heated air would be the easiest to deal with, in particular if your furnace system isn't hydronic. For the CFM being bandied about here, I think Fantech has a heater in that range. See my graph way up this thread.

In summer you have the choice of tolerating hot air flowing into the house or cooking outside.

hood range is connected. It definitely exhausts air. Goodbye greasy fumes

we still have some work to do to connect the damper and such

We've got all the intake and exhaust ducting done. Waiting on him to come back out so we can do all the wiring/testing/adjusting.

The intake is shown above. This connects into the 14" return duct.

I took out the Honeywell controller. On the "Off" position, it says "remote switch only". This is awesome and actually answers my question without having to look in the instructions.

"On" can be used from Mar-June and from October to November-ish and will act as a Fresh Air system, given the balancing adjustments are made and such from the HVAC guy.

"Off" can be used for the extreme months. Even in "Off" position, it will STILL open after we hook up a switch from the range hood to the damper. This is great news.

Basically, "Off" is make-up air only, and "On" will be fresh air AND make-up air.

The only thing that will deserve some extra credit is if we can wire it in a way for the Honeywell switch to ignore if it's on low speed. Not a big deal though.

I thought of one awesome benefit that I didn't think about for make-up air. I have a small theater room that gets warmer than most rooms. We installed a jumper duct to the hallway where the main return is. That works great, as long as air handler is on. And it works "OK" at best.

But with the fresh air system, this should help regulate the theater room temps in this situation which happens a lot:

6:00PM. Outside is 66-69F after the end of a pretty hot day. House T-stat is 71F. Theater room is 74-75F.

Before the fresh air system, the only way to cool this room down would be to turn on the A/C. This increases the chance of freezing and is not really solving the problem.

But I'm thinking now with the fresh air systems kicking on at usual intervals, blowing in 66-69F air from the outside should help reduce theater room temperatures and at least provide more air flow.

Exciting progress update on the HVAC project! We've completed all the intake and exhaust ducting, eagerly awaiting the next steps for wiring, testing, and adjustments. Removing the Honeywell controller was a smart move; I'm impressed with its intuitive functionality. The option for "Off" being dedicated to make-up air is fantastic, and the addition of a switch from the range hood to the damper is a game-changer. Integrating the fresh air system not only enhances air quality but also offers unexpected benefits like regulating temperatures in our theater room. I'm particularly excited about the potential for Modern Copper to optimize our HVAC system's performance. Looking forward to seeing everything come together!