Ah, the difficulty here is first, the current sensor would seem to have to detect power in one of three wires to the motor. However, it might work fine using the neutral return from the motor if there is a common neutral. Else three sensors might be needed, with the damper control sides wired in parallel.

The second issue is the cable. One can buy cable made for all sorts of combinations of wire size and number, but the minimum distances may be unaffordable, or at least impractical, and the cable might not be rated by UL for this purpose.

Conceptually, multiple 14/2 w/g NM (Romex) cables could be used, although I am unclear whether this motor has a common neutral or three feeds and three returns.

That aside, there are flexible metallic and non-metallic conduits that should not be more annoying to run than NM cables that one could use, and in such a case, if you can find 16 AWG or 18 AWG THHN wire in different colors you would be OK. Or just run widely available 14 AWG. In lieu of multiple colors, colored electrical tape can be used at each end. Perhaps KitchenAid sells a suitable cable, so long as it is in conduit. If you were thinking of rigid conduit (required in some totalitarian cities), that should not be needed in most cases.

You can either run the neutral return through the current sensing switch's sensor, or all 3 of the hot wires. As long as the gadget can sense current in any of the wires, it'll do its job.

Don't put both hots and neutral through, though. They'll cancel out. Either the neutral return or all 3 motor winding wires.

For your blower connection, I seem to recall a code requirement that all conductors related to the circuit are run in the same raceway. So I don't think you could just zip-tie two 14-3 NM cables together, though I've seen it done. Our code expert Ron Natalie may have a final answer on that one. Color coding would be a problem.

Flexible electrical nonmetallic tubing (ENT or "smurf tube") is easy to use, but I'll bet the attic will get too hot for it.

Flexible metallic conduit (Greenfield) would work. It's more awkward to work with than smurf.

I've heard of, but never used, a high temperature nonmetallic flex similar to liquid tight. I expect that it's expensive. Wouldn't hurt to ask, I guess.

As long as the ground is green and neutral is gray or white, the other wires can be any colors you want. Use stranded wire, not solid.

You'll find a better range of wire and electrical supplies at a real electrical jobber, the kind that closes at 5:00 pm and has trucks with electricians' names in the parking lot.

Thank you everybody for the input.

I cannot find a terminal for triggering MUA on this hood. There is an accessible terminal strip but that is lighting wire only. The switchbox is more complicated and less accessible than I expected. In my simple mind I figured the on/off switch for the blower would be accessible and provide 110v power and I could connect MUA to terminals there and not have to worry about the multispeed aspect of the blower. But the switches are all contained in a plastic housing so the only easily accessible connections are the lighting terminals and the 6 wires accessible from the junction box. If connecting to the hood controls was not possible I figured I could always add the Broan flow sensor to the venting and trigger the MUA that way. I could still do that but that system seems more complicated and therefore prone to failure than a direct wiring connection. I hadn't considered a current sensing switch. I haven't figured out 100% what all 5 wires to the blower are for. The wiring diagram shows resistance values as: Blue-Red: 18, Blue-Gray: 14.3, Blue-White: 21.6 (min.), Blue-Black: 9.8 (max) so I assume blue is being used as a common neutral and the standard of black (high), red(low), and green/yellow is being followed. I figured I could check it with a meter once I got the hood and blower installed.

To those concerned about MUA coming from the garage it is not. The MUA vents to fresh air from an external wall. I considered installing an active MUA system and I know I am borderline on pressure differential with a passive 10" duct on a 1200 CFM blower but the house is 2800sf built in 1998, H20 heater and furnace is in garage, and the fireplace is a sealed unit.

If there isn't an obvious better solution for the wiring I will just follow the MFG instructions and purchase 1/2" metal conduit and feed it with individual wires. I figure it will be about a 16-18ft run.

I looked at the manuals for these units online. They don't make this easy! Shame the engineer who designed the hood and blower didn't think about adding an accessory terminal for a damper.

However, I think I've worked out a code-compliant and safe way to make your damper work. Let's see if Ron Natalie approves. :)

To use a current sensing switch, you have to run ONE wire of the power to the load through the CSS's current transformer (sensor). I'd do that without voiding the warranty on the hood (wow, is that system expensive!) by running the supply for the hood to a junction box in a convenient location. I'd connect a loop of NM cable about a foot long between the power feed and the cable going to the hood. The loop would exit the box through a knockout and re-enter the box through another knockout. Following me so far?

The trick is that I'd use only the hot wire in this cable loop. The neutral from the feed would go right to the hood neutral inside the box. So now I have a one-wire-only loop I can feed through the switch's current transformer. That would connect to the CSS, secured to another j-box which then feeds the damper.

Here's a somewhat crude doodle, drawn to rather random scale.

Now, here's the sneaky part. You use an adjustable sensitivity CSS. Set it so that just turning on the light in the hood doesn't trip the CSS, but turning on the blower motor at any speed does. I'm counting on the light having a lower current draw than the blower.

Nice idea! To minimize future effort, determine ahead of time whether the hood lighting is to be halogen or LED, and install whichever you want. Otherwise, a change later may require fiddling with the CSS sensitivity control.

The only other issue I can see is that the original post called for 18 AWG motor feed conductors. This suggests the motor's different speeds all have modest current draw. Hopefully, the current draw from a set of halogen lights is not comparable to the current draw of the lowest motor speed.

They don't want to make this easy - they want you to spend twice as much for their ducted switch+damper MUA solution. We found this hood "open box" for 1/3 the cost of new and bought it to match our "Kitchenaid" rangetop. Then I found out I either had to buy their 3 speed blower or replace their controls with a variable speed control. Anyway, I really appreciate the extra effort you put in to help make this work. The doodle really helped me understand your suggestion and realize that since the exectrical box and MUA are located in the garage I could avoid a lot of extra work by wiring it all up out there. The Halogen lights are 50W each and the blower is rated at 750W (7.5A). So it seems to me that there should be quite a range between 100W (lights) and 750W (blower) even if the higher impedence speeds to dial in an adjustable sensitivity CSS. I want to fully understand the theory behind this idea. I assume the original suggestion of a CSS on the blower neutral would work but this idea to sense the current in the supply line was suggested to make the installation per the MFG doc? (i.e. therefore avoiding warrantee disclaimers)? Next - The MUA system includes a transformer to reduce the voltage to 24v, does the CSS take 110V or 24V input, or does it have it's own transformer to drop the voltage? And can you recommend a specific model? I am not very clear on exactly what these look like.

I just looked an example up on Amazon (Current Sensing Switch,Normally Open Current Sensing Switch Adjustable AC 0.2A -30A (SZC23-NO-AL-CH Model)). It provides a switch terminal that opens and closes depending on current. So it can close the circuit to whatever power is supplied to the MUA damper. It requires no power of its own.

CAVEAT: I only glanced at this unit, and do not know all the parameters of your MUA, so this example may not be appropriate for your use. I think, however, it is typical of how these devices are configured. @DavidR may have a more optimal device in mind.

Yeah, I was looking for a way to avoid opening either unit up. I saw how much the hood and blower set cost and just about fainted. If I were spending that much on something I wouldn't want to void the warranty. And trust me, they'd be likely to do that, especially if you dodged paying them even more for their authorized solution.

After looking at the Broan datasheet, it seems to me the easy way is to just buy the MD10TU damper instead of the MD10T. The U version is supplied with a pressure switch to open the damper when it detects blower pressure. Just wire to instructions and pop open a cold beverage.

But if you're really determined to do it the hard way :) ---

There are lots of current sensing switches out there. You don't find them at the big box stores because they're usually used for industrial applications, typically to shut down a motor when it's either overloaded (such as a machinery jam) or underloaded (such as a broken belt or gear). But you can get them from industrial supply houses, even from Amazon.

Many are available, including cheap and cheerful Chinese models of unknown reliability. Prices run the gamut from around $20 to maybe $80. Your favorite search engine is your friend.

As an example, I picked one that appeared to be of mainstream quality, the Veris H738. You would connect it as shown in the wiring diagram on page 2 of the datasheet in the section titled "relay controlled directly by status contacts." Your 24 volt transformer goes where it says "Relay Coil Power Source," and your damper goes where it says "Relay Load." The NM cable loop in my doodle above goes through the current sensing hole on the H738.

WARNING! Make sure your damper's 24 volt control circuit doesn't draw more current than the H738 can handle, which is about 5 amps. I'd guess that it almost certainly doesn't, because the damper installation instructions say to use AWG 20 bell wire. If it does, though, post back, and I'll explain what you can do about the situation.

One it's installed, turn the blower on low, and calibrate the H738 as explained on page 2 of the installation guide for monitoring undercurrent.

With a little imagination, you should be able to adapt the above instructions to almost any CSS on the market.

The MD10T is already installed but I can pick up the pressure switch kit for another $32. My gut is telling me that is the way to go at this point simply because my confidence in avoiding "What the heck was this guy thinking?" is higher with that solution.

But that brings up another question. I am replacing a downdraft cooktop with a rangetop + hood. The cooktop was wired with 12/3 romex connected from a 20A breaker to an in-cabinet junction box which feeds cooktop and blower. One load conductor of the 12/3 romex is unused and capped at both ends. The new rangetop and hood/blower each spec 15A circuits with the rangtop recommending a dedicated circuit. From my perspective it would make sense to terminate the existing 12/3 cable at a junction box above the range hood, Then wire the range hood and rangetop to the junction box. Does that make sense? (and if so what gauge wire should I use)? Do I really need separate circuits for the rangtop and hood/blower? (low load and common purpose seem like a reasonable load combination for a single circuit).

This is a gas range, right? At that circuit rating, I sure hope so!

If the range instructions call for a dedicated circuit, it's a code violation to install it otherwise. If it's only recommended, I guess you're OK.

If the hood instructions don't require a dedicated circuit, you can connect the hood to a general purpose circuit. But not to the counter receptacle (small appliance) circuits.

I would probably dedicate a circuit each to the hood and range if I had the panel space.

You can use AWG 14 cable for a 15 amp circuit. Our resident code expert Ron Natalie will correct me if I'm wrong, but I think that you now have to use combination AFCI/GFI breakers for these circuits.

I believe since you are hardwiring both the cooktop and the vent, they do not require AFCI.

" The U version is supplied with a pressure switch to open the damper when it detects blower pressure. " I don't think that is what this is for, ie sensing lower pressure in the vent pipe. Rather, it is for sensing lower pressure in the whole house. If the damper is out in the garage, the sensing pipe (hose) will need to stretch to the inside of the house, not just the garage.

This is a gas rangetop - sorry I didn't specify that earlier, and the installation instructions recommend a dedicated circuit - not require it. The hood instructions just specify a 15A circuit. I would add another circuit but I don't have much panel space left and pulling 55ft of wire to the hood is a challenge. My thinking was that the power draw on the rangetop should be minimal and intermittant (spark), the hood/blower startup should not be coincident, and I wouldn't be using one if the other was down.

The U version of the MUA damper states "a sensor is attached to the exhaust duct to detect if the range hood is operating" and in looking over the installation the whole sensor is installed on the air duct (part inside and part outside) so I assume it is comparing pressure inside the duct with that outside to determine if air is flowing or not. The sensor is installed in the vent duct (not with the damper) and the only connection between the two is a 24V conductor. I would run power from the panel to the transformer and then a 24V line would connect the transformer output, damper, and sensor in series.

After looking around a bit finding suitable conductors and conduit for the hood to blower connection at a reasonable cost may be tougher than I thought.

I don't recall if it was the Broan part or not, but our vent hood was installed with one of the pressure sensor-type devices for MUA operation. It was unreliable and very sensitive to orientation, apparently. DH changed the setup to current sense. I would recommend going that direction from the start if possible lest you experience the same frustration and do-over.

"finding suitable conductors and conduit for the hood to blower connection at a reasonable cost may be tougher than I thought."

I guess it depends on how you define "reasonable." Using prices found at a big box store online:

100' of 1/2" Greenfield flex $58

100' AWG 14 stranded THHN $19 * 6 = $114

Total $172 not including fittings

You can add a couple of bucks and use 3/4" Greenfield for easier pulling.

After the cost of the hood, blower, and damper, that smells like peanuts to me.

Thanks Michelle for the input on your pressure sensor MUA. My initial reaction was to put a sensitive device like that in a somewhat "dirty" environment was asking for trouble. In my case I can install it to be very accessible so unless I find lots of cases like yours I will probably stick with the pressure sensor. I like the CSS option and understand the science but my confidence in executing a professional looking installation with a CSS is low (and would mean pulling another dedicated circuit for the hood). Kitchenaid pairs their MUA damper with a short duct section with what I would call a "flapper switch" which is expensive and I'm sure over time gets coated with grease and impacts duct performance (and I don't want another reason to take the duct apart).

DavidR you are right in that the cost of the electrical is peanuts relative to the overall project. If we had it to do over again we never would have purchased the open box "deal" on the Kitchenaid hood which put us on this long and expensive path. A hood with variable speed blower controls and MUA terminals would have simplified everything and enabled inexpensive blower options.

It looks like Platt sells AWG 14 stranded in the required colors and 3/4" Greenfield steel or aluminum conduit by the foot so if that will meet the requirements the cost should not be too high. Outside of Romex cable I don't have much experience with wiring so my confidence in getting the right thing is low there (and this pandemic situation doesn't make getting help from the suppliers very easy). So I really appreciate the input. I have plenty 14/2 romex and it is tempting to use that and color code the ends but I want to make sure this is done right and I just don't know enough to be confident with that approach.

Using NM in conduit would be iffy as far as code compliance. You could try to convince your inspector that the conduit is providing physical protection, but I don't think that will fly when it's flex conduit. And I think you'd get dinged for not running all the circuit conductors together.

The right way to do it is to bite the bullet, buy the color coded individual conductors, install the flex, and pull the 6 conductors through. It shouldn't be too tough to find online resources with the info you need for working with metal flex.

If the somewhat hacky nature of using a CSS bothers you, it seems to me that your safest and easiest course is to pull out what you have and replace it with a more orthodox (and more expensive) integrated system from your choice of a single manufacturer. You can then recoup some of your losses by selling what you have on the big auction site.

" The right way to do it is to bite the bullet, buy the color coded individual conductors, install the flex, and pull the 6 conductors through. It shouldn't be too tough to find online resources with the info you need for working with metal flex."

Thank you. That is what I wanted to hear. I am trying to save time and money but I want to do it the right way and not worry about it. I haven't worked with metal flex before but it doesn't look terribly complicated. Terms like "NM in conduit" and THHN don't mean much to me until I look them up. If I understand correctly I should be safe to use 14 AWG THHN/THWN stranded copper in the required colors pulled through 3/4" Greenfield flexible metal conduit. I assume even with stranded 14 AWG six wires could be tough to pull through 1/2" flex. Am I better off with steel or aluminum conduit?

Regarding the MUA I read the Amazon reviews for the MD10TU and they were all positive so I am going to add the pressure sensor to make my MD10T into a MD10TU and install the pressure sensor where I can get to it easy to clean or replace it. If I have problems with it I will swap it out for the CSS solution. It's always good to have a backup plan.

In my opinion ...

If one is going to do electrical wiring in a USA household that someone else might live in one day, then the parts of the National Electrical Code (NFPA 70) relevant to one's project should be understood. The NFPA [YEAR] Handbook contains the NEC and a wealth of knowledge. Having a Handbook that one has delved into may make the code inspector more confident that you did the right thing where he can't easily inspect. (And you'll sleep better.)

In this case, a flex cutter tool will be worth the time saved trying to safely make cuts with a hacksaw.

In addition to the flex, you will need flex connectors for each end, and red caps to protect the wires. Conductor lengths in the boxes should be long enough to stick out six inches.

The Handbook will provide requirements on distances between fasteners and boxes.

Box sizes have to be large enough (cu. in.) to meet rules for numbers of conductors of given sizes. (Motor interface boxes that are part of motor assemblies seem to be immune to these rules, but the other end of the flex won't be.)

The Handbook has tables of numbers of conductors of given sizes allowed in conduits of various sizes, but I would go for 3/4 even if 1/2 was allowed if there were no other impediment, such as a motor control box with a 1/2 knockout that I couldn't punch out larger if the path is tortuous. When pulling through flex, be careful to not scratch the wires. This is why connector assembly before pulling is required (except for 90-degree connectors.)

Note that "real" 1/2-inch flex is not the skinny stuff used for pendant lighting whips that also uses 1/2 knockout connectors.

The Handbook, while expensive and updated every three years, can, even when out of date, provide help for future projects, or at least help scale the scope of a job that you might outsource to a real electrician.

kas

Thank you for the tips and I understand where you are coming from. You make a good point about the handbook and I will see what I can do to get my hands on one before tackling the wiring. It has been a long time since I remodeled our previous home and I have discovered quite a few code changes already - it makes sense to have a good reference. My original intent was to outsource this work to a licensed electrician (along with the HVAC and plumbing and flooring and ...) but the current situation has made that impractical and doing the work myself in this environment just makes sense. I really appreciate there are folks out there willing to provide advice to help us make good choices.

Kaseki makes some good points, but I'm not sure I agree on the rotary cutter. They're nice, but is it worth the cost for just a few cuts? I've used MC cable many times, and I've never bothered to buy a cutter. (Too cheap, I guess.)

It takes more time to cut the armor by hand, but it's not tough.

For MC, bend the armor sharply where you want to cut it, so the coils pop apart. Cut the coil with a pair of diagonal cutters that you don't care about. You can also use a hacksaw. Trim the metal smooth and even with the dikes.

With Greenfield, you can just use a fine-tooth hacksaw.

As Kaseki said, don't forget your red plastic anti-short bushings. Make sure they're visible from inside the box. Your inspector will probably want to see them.

I wouldn't recommend stranded wire. I did once in the past, and it's difficult to make the right connections, and I later found out that it's not code-complaint. Using Greenfield (empty MC cable), you can pull your own wires and the fittings for the boxes are readily available. Make sure the boxes have the right sized knockouts.

"later found out that it's not code-complaint."

Citation needed, please.

Stranded is much easier to pull.

The trick for connecting stranded wire to screw terminals without having the strands untwist is to twist the strands counter-clockwise.

PS: Some pros use crimp-on fork lugs with stranded wire.

I have also tinned stranded wire a few times. The last 1/8" or so is all you need to tin - a quick application of the soldering gun and touch the end with solder. A solder pot would be perfect for this, but now we're about to fall overboard. :)

I use crimp-on fork lugs for screw terminals with stranded wire (only type of single conductor wire I have in quantity) and use crimp on ferrules for Allen screw type terminals. Note that a proper crimper is recommended to assure crimp pressure (defined by not releasing until the crimp condition is satisfied).

I don't believe that there is any restriction on stranded wire; however, one can run stranded in conduits and then use a wire nut to connect it to a solid conductor flying lead attached to a screw terminal if one has wire nuts and no lugs at hand.

Tan wire nuts provide the widest range of diverse wire size connections.

Left-hand twist, eh? Thanks, just in case I'm ever pressed for direct attachment.

"Note that a proper crimper is recommended to assure crimp pressure (defined by not releasing until the crimp condition is satisfied)."

Well said, Kaseki. The cheap 1/8" thick dull black steel big-box store crimpers packaged with a bag of 100 assorted terminals (the whole deal made in China) don't do the job. A good ratchet crimper effectively cold-welds the terminal to the wire. For large gauge wire, a hydraulic hex crimper is the way to go.

As a tool hound, I always wanted a hydraulic crimper, even though I hardly ever do anything requiring a crimp that large. Worth noting, perhaps, the Klein J213 pliers have a feature on the handle side near the pivot that can, if one is strong enough to fully close the pliers, deal with the next size lug larger than yellow (10-12). I think I only needed to use it when crimping lugs to #8 copper grounding conductors for the purpose of bonding together parts of my aluminum deck and its roof structure.

The blower will be supported by attic trusses (flow direction = horizontal). I am in the process of designing the support structure but the suppliers' approaches to noise reduction appear to be diametrically opposed. Some isolate their blowers with springs or rubber gaskets. The Kitchenaid installation fastens 4 lag screws through the blower housing directly into wooden structure on either the inlet or outlet plane. I see no reason to cantilever a 50 lb blower so I plan to provide attachments on both inlet and outlet planes but I am unclear whether it would be better to hang the blower from upper truss members (to isolate the energy) or securely attach directly to the ceiling truss joists (to increase mass to dissipate energy). I know vibration management can be complex and suspect the best answer may be dependent upon blower weight and balance but I would be interested to hear what others' experience is. I can secure the blower safely either way - the goal is to reduce noise.

I would hang from the rafters and keep the vibration away from the ceiling joists where it will be more noticeable. In either case, motor vibration will propagate via the ducting unless isolators are used. I think Fantech has some parts that may help.

Rather than springs, damping type rubber isolation is called for. Many mounts exist for machine tools that may be applicable.

If you have a long enough run (or design a long enough run), you can use the length to allow insertion of a silencer on the hood side of the blower.

A serious fanatic would send the motor armature and blade assembly to a balancing shop.

The instructions call for 18ga wire, not 14ga. The fan doesn't pull more than 7 amps. 18ga in colors may be harder to find ad HD or Lowes, but maybe an auto or marine parts place.

Thanks for bringing the specs back up, Weedmeister. I took another look at the hood, blower, and installation instructions for both last night and seeing the six thin 18 AWG stranded wires in integrated 2x3x6 junction boxes each with 7/8" knockouts really made me think connecting them with 14 AWG solid in MC would be overkill. There must be a way to source to the specs and code without going broke in the process. The Blower is branded KitchenAid, Jenn-Air, Whirlpool, and Maytag so maybe if I make a few phone calls to support lines it would help.

If the connecting wire is operating at 120 Vac, then it has to meet certain requirements established by UL and enforced by NFPA 70. Among these is the rated insulation breakdown voltage. This might not be met by automotive wire. Also, for power transmission, 18 AWG wire is only rated at 2.3A. [https://www.powerstream.com/Wire_Size.htm] I would only use this size outside a chassis for bell wiring, thermostat wiring, or intercom wiring.

For short wiring distances within a hood, it may be OK, but for remote feed to a motor, I would use conventional and widely available 14 AWG. Beyond low electrical resistance, it has more damage resistance. Check with your Code Enforcement Officer. Check large electrical distributors to find out if they will pull less than 100 ft of each color.

Don't forget to include a green grounding [bonding] wire in the conduit.

Edit: As noted in the link referenced above, the current rating is very conservative. The NEC allows higher values (14A/18AWG; Table310.15(B)(16)), but these depend on multiple factors, including ambient temperature, number of current carrying wires within the bundle (maybe only two in this case), and are further limited by the conditions specified in 240.4(D)(1), which asserts 7A. Unless the hood control includes a fuse or breaker meeting the conditions of this paragraph, the circuit is actually only limited by the 15A breaker of the branch circuit, in which case I would go with 240.4(D)(3) and use 14 AWG. [NFPA 70, 2014] Don't forget NFPA stands for National Fire Protection Association.

The "tech support" lines I tried today must be short-handed. They were not very available nor helpful (at least the ones who actually answered). I struggle with a reference that shows max current for 14AWG as 5.9A but the principle makes sense to me. McMaster-Carr sells 25ft lengths of each color THHN 14AWG stranded, as well as FMC, grommets, and adapters. I checked the charts which show 1/2" FMC should hold 6 conductors of 14AWG just fine but I defer to experience here whether it would be more trouble to pull the wire through 1/2" FMC or increase the knockout size to acommodate 3/4" conduit.

The 16 ga marine wire I have is rated for 600v and 105*C. (It's also oil/gas resistant.)

I spotted 18/7 wire at Lowes the other day. That's 7 conductor 18 ga.

18 ga is rated for 7 amps continuous. 14ga is rated for 15 amps, not 6.

This is a bit off topic but I removed my 25 year old GE Profile oven the other day and was surprised to find the 2 pole 50A breaker, AWG 6 circuit to be connected to the self-cleaning electric oven with an AWG 16 pigtail. I am sure they roughed in 50A for every house in the development in case the customer wanted something special but it just seemed wrong for the conductors to be smaller than the individual strands on the supply.

The breaker current rating should have been reduced to the level of protection required for the pigtail (taking into account that ovens often use connection wire rated for higher temperatures (e.g., 105C) and thus higher currents than the same gauge THHN).

That's what I would have thought. I would think the breaker should be sized to match the limiting factor be it the wire, pigtail, or device. This was new construction when we bought it but I have since learned that there were so many houses being built at the time many of the inspections were "drive by" and the contractors sometimes took shortcuts.

Just a note to convey my appreciation for all the help and to provide feedback on the installation. I installed the blower in our attic connected to the hood with THHN 14AWG in 1/2" Greenfield flex. That worked fine. I "hung" the blower on a truss I attached to the rafters. I had to design the truss to be collapsible so I could build it on the ground and get it up into the attic but that worked great too. I attached it directly to the roof truss. I also bought a 10" duct silencer that installed vertically in our closet and the results are REALLY quiet. To be honest I am a bit worried we will forget the blower is on because we can't hear it (and this is a 1200CFM blower). In the end I decided to purchase the Broan airflow sensor switch for the MUA damper but I am not 100% happy with this arrangement. The MUA damper opens completely when the blower is on high, 1/2 way on medium, and not at all on low. The problem is that even on low the blower generates a lot or pressure differential. I may end up going back to the current sensor switch.

Photos of your folding truss!

What blower is this? Keep in mind that a blower rated for 1200 CFM will do 1200 CFM in perfect free air conditions with 0" static pressure. Add in all of the static pressure of your hood and ducting as well as whatever there may be from your house (MUA) and reality is likely more like 500 CFM.

Thanks for the update.

It would be interesting to know whether at the lowest speed for which the damper doesn't open there is significant house pressure drop (significant relative to back-drafting pressure limit rules). It may be that this flow rate can be supported by house leakage without there being detectable air moving past wall switch housings.

Another thought is whether, assuming a horizontal duct with damper, the damper can't be balanced to open on its own in proportion to hood flow rate. Boiler type furnaces taking air from inside the house (basement) used such devices to keep air from moving through the boiler and cooling it when it wasn't firing. The minus of this approach is potential banging when the exterior is windy and pressure balance is impossible without an elaborate physical means of excluding wind effects at the duct intake.

Nice kitchen.

The house is 25 years old and has significant leakage but since the kitchen/dinette/family room space can be closed off with doors I don't like the idea of that much air being pulled through the leaks (much of it comes through old style ceiling can lights), especially since it is easy to leave the fan on since you can't hear it. The wall switch housings are better sealed so I don't detect much air coming in there. There are five furnace vents in the space and when the MUA vent is open (blower on MED or HIGH) there is less air that comes through the can lights. The Blower will actually pull open the doors until the MUA vent opens to relieve the pressure drop (it takes a few seconds for the vent to open).

The new 10" MUA duct is horizontal (as is the 6" MUA duct controlled by the furnace timer).

Thanks for the last comment. We are very happy with the results.

OK, I'll check 'significant.' What size is the present MUA duct and how does it get air to the kitchen? I'm thinking that a "bypass" duct with a balanced damper that opened by itself due to pressure drop would handle the low setting. Not sure what is easier to set up in your situation, duct or current sensor.

The new MUA duct for the rangetop exhaust is 10" DIA and includes a damper controlled by the flow sensor. It draws outside air into the furnace cold air return (about 3' long) so the draw into the kitchen is via the central ducting system and 5 standard sized vents in the enclosed kitchen/dinette/family rooms. There is also a 6" DIA MUA duct that is controlled by the furnace timer.

I'll have to leave it to your judgment which is easier to implement.

Thank you for the input. Considering that the MUA flows directly into the furnace return air I am reluctant to rely on negative pressure to open the damper. The MUA damper should open when the Rangetop blower is on but not due to the furnace or other fans inside the house coming on so I think I prefer the current sensor solution. That keeps the MUA damper closed unless current is flowing in the blower circuit and if I understand correctly there are current sensors that can be calibrated to switch even at low power.

Your point seems well taken; I don't have forced hot air, and hence haven't experience with its subtleties. The furnace return will certainly have negative pressure when the furnace is running, and house pressure when the furnace isn't running, so calibration of a gravity damper against house pressure somewhere else would be impossible.

Honeywell makes remote sensing barometric controlled dampers, but these tend to be largish.

MUA delivered through a furnace may supply MUA at varying pressure losses depending on conditions. However, since your furnace has its own MUA, this may not matter much from a safety point of view. The MUA provides for better hood blower performance in this case, and keeps the hood from pulling air through the ceiling as you note.

Two aspects remain unclear, and perhaps can't be determined without measurements.

• What are the pressure losses and are they noticeable observing the hood flow.
• Is the furnace capable of heating the MUA under your locale's outdoor temperature conditions while also keeping the house warm?

First, nice kitchen!

A few thoughts...

What MUA are you using? Most residential made in the past 10 years have current sensors that turn the MUA on to H/M/L based on the amount of current being pulled by the exhaust blower. You can then balance the system (pitot tubes & meter) so that the MUA is pulling in the right amount of fresh air.

If your MUA doesn't have its own damper it should have terminals to trigger an outboard damper to open/close. These are typically labeled NO/C/NC to work with normally open and normally closed dampers (but make sure you read the manual for it).

Make sure you are interlocked w/ the furnace blower. IT MUST RUN anytime MUA is needed. A non-running blower can be a massive amount of static pressure.

You are correct in not wanting to pull air through house leaks. What we are finding is that doing so is likely a key cause of a variety of respiratory problems and decline of cognitive function.

I don't have equipment to measure the pressure differences but I have done some qualitative tests to evaluate the velocity of air that comes through a known opening (small window) for H/M/L blower settings as well as the velocity of air exiting the blower exhaust. What I found is that the pressure difference between the kitchen/dinette/family room space is greatest on the LOW blower setting and less on HIGH and MED settings even though the volume of air exiting the house from the exhaust is H/M/L as you would expect. I attribute this to the lack of MUA on the LOW setting due to the closed MUA damper. The hood flow I am perfectly satisfied with although it bothers me that the low setting is lower than designed because of the lack of MUA. I believe replacing the airflow MUA switch with a simple current sensing switch will be a good solution.

We live in the PNW with a fairly mild climate with low temperatures seldom below freezing and never below 0 degree F and our furnace does not seem to have any trouble keeping up with the colder MUA inflow so far (although we have not had many cold days yet).

opaone: Our MUA damper is a Broan MD10TU with a motorized damper triggered by a remote flow sensor switch. Thanks for the input on interlocking the MUA with the blower - I had thought that would be a good idea for MUA performance but not considered the static buildup aspect. The 6" MUA damper originally installed with the furnace is interlocked with the blower so I guess I better research how that works. And thanks for the kitchen compliment.

Unless you open windows every time you use your hood then your MUA is very undersized and will pose a health risk to you and your family.