Something 3: Fireplaces can also backdraft when in use, so either don't run it when your hood is on full power, or find a way to have a really good draft.

Something 4: If the climate requires heating of the MUA, there are two basic configurations generally suitable for residential use.

In the first, air is heated in the duct with some form of thermostatically-controlled heater, usually electric (although I use hydronic). Garden Web member cooksnsews provided this image at one time. Typically, this would be in a basement or attic.

The second approach involves dumping the MUA into a room (basement, mud room, etc.) that has a higher powered heater than the room itself would require. Modine, for example, makes such heaters. This may simplify duct complexity and control complexity.

The commercial approach may be seen in the Greenheck literature, and on the roofs of myriad restaurants, where among the up-blast blowers may be found a rectangular structure with a triangular overhanging section.

Heating required may be pulled from this chart based on the thermal capacity of air.

Something 5: Introduction of MUA to kitchen.

The MUA must not disturb the flow of air into the hood or the plumes will be displaced or broken up. Put it into a separate room, or via the kitchen ceiling using a diffuser aimed away from the hood, or via the toe kick areas away from the hood. More area of insertion means lower velocity and less disturbance.

Something 6: Even if there were no car in the garage, heavy duct-mounted components have to be suspended adequately for the conditions, such as an earthquake-prone area. Strut material bolted to the joists may be useful to contain any suspension failure. I would put the blower close to the exit and install a silencer between it and the hood.

Thank you SO much for all your thoughtful suggestions, kaseki! I sent the Fantech link to the contractor, where you can put in all your numbers and it will draw a fan curve diagram for your specific situation, asking him to please run the numbers (which I don't know) for both the FKD 10 and the FKD 10XL, since they both have the 10" ductwork that the hood uses. He wrote back, "The FKD 10 might be enough, but get the XL. Bigger is better." I don't think he input any numbers. :(

OK, consider this plot for the FKD 10. https://shop.fantech.net/en-US/fkd--10--mixed--flow--fan/p103505

If we conservatively assume that the passive MUA pressure loss (outside to kitchen) is 0.4 inches at the flow rate we will get from this recursive calculation, and the hood and duct another 0.4 inches, ditto assumptions, (kitchen to outside), then we can enter the graph ordinate at 0.8 in.wg and follow a horizontal line to the right to the maximum power line to find that the predicted volumetric airflow rate is about 620 CFM. What you wanted.

You only have a 30 x 22 inch hood, so a ripping big fan is not really needed, in my view, unless it is needed to overcome pressure losses that I don't know about.

Perfect! Does this need a 10" passive MUA system (since there is 10" ductwork on the exhaust side), or is an 8" a better option? Broan makes a universal MUA that has passive damper opening for 8" and motorized damper opening for 10".

How long does your MUA duct have to be? What filtering will it have? I don't have time to read up on Broan MUA details, so if you know of any pressure loss vs. flow rate information, or can extract any from Broan, please spill it here. Please reread "Something 4:" above and explain what your MUA path and heating scheme (if any) is going to be.

In other words, useful answers to your questions do not exist outside of the overall context.

Since it is unplanned, as yet, I do not know how long the ductwork is. The contractor discussed putting it into the return of our HVAC system, in order to utilize its existing filtering & heating system -- which would be about an 8' duct run.

I would NOT do passive MUA for this (a 620 CFM exhaust system) even if OK by code. You should have powered MUA matched to your exhaust blower. Besides any risk of backdrafting (like accidentally forgetting and having a fire going while exhaust running or other), not having proper MUA forces outside air to be pulled through wall cavities. If the outside air is more humid this can lead to mold in the wall cavities and regardless of outside dew point it can be pulling chemicals and such built up in the wall cavities in to your home.

If you choose to do passive anyway the general rule of thumb, IIRC, is 2.5-3.0 times the size of the powered exhaust duct. Not diameter but cross sectional area so I believe a 15" passive for a 10" powered? This is assuming 0.0" static pressure for the passive MUA and should be enlarged to account for static pressure.

On the problem of finding a good HVAC contractor.

The U.S. HVAC industry is about 15-40 years behind the EU.

Their (U.S.) lack of understanding of Indoor Air Quality (IAQ), energy efficiency and other areas of their business are surprisingly deplorable. Most do not understand any of the theory behind why they do anything, only fill in the blanks if-then stuff they learned in 1 year of tech school 15 years ago (a lot of which info was then 10-25 years behind what was known in Europe.

So good to know! Fan tech makes a powered MUA 750 cfm (8" duct) and 1200 cfm (10" duct). Which one best "matches" the FKD 10 (905 cfm, 10" duct)? I was thinking the 750 cfm version might best match the FKD 10 fan curve you drew kaseki, but don't know if matching the 10" duct size is more important?

pittsburrito wrote:

"Since it is unplanned, as yet, I do not know how long the ductwork is. The contractor discussed putting it into the return of our HVAC system, in order to utilize its existing filtering & heating system -- which would be about an 8' duct run."

In such a case, with the furnace blower off, the duct run length is the MUA feed to the furnace plus the effective length of all the ducts from furnace to rooms evaluated in parallel and weighted by their size. I wouldn't even want to analyze that. Also, feeding the return means hood pulled air runs the return ducts in reverse, and the feed ducts in forward. I would wonder about the initial dust disturbance that might result.

A powered MUA means that some control system is needed. At the easy end, the MUA blower is matched to the hood blower and they run together. If the MUA has lower pressure loss, pressure balance can be achieved by adding MUA path restriction. (I think I saw an iris device somewhere recently -- maybe at Fantech.) Or a portion of the MUA may be ducted back to the MUA input. This is done on a large scale for large building MUA when the MUA blower is run at constant speed. The feedback control damper is pressure controlled. Usually such buildings operate at positive pressure.

At the hard end, MUA blower control can be based on measured interior to exterior house pressure differential. This scheme addresses any combination of exhaust blowers, their speeds, and fireplace use. Where such schemes are based on MUA blower power control, they may require a proportional-integral-differential (PID) computer controller to ensure stability given the lags in the system loop.

Whether passive or active, I would suggest that the MUA be independent of the furnace system. In any case the house heating cannot effectively warm the MUA as it is designed to handle closed house heating loss, not open house breeze heating loss. (Maybe a gale in the case of opaone's system.) :)

So, we've purchased Fantech's 750cfm makeup air system, and the contractor wants to dump it into the HVAC's air supply ductwork "so it will be pulled in through the supply vents as needed." I am concerned about this plan, because we have a toekick supply vent under our kitchen sink, which is 7 feet away from our range hood's intake (diagonally). Am I a homeowner who knows enough to be dangerous, or is this a bad idea?

Here is a photo of our situation:

http://www.simpleimageresizer.com/_uploads/photos/1310b3e8/1570455161852670183278_50.jpg

You are (IMHO) a homeowner who, given unlimited time like all the rest of us (ahem) could benefit by reading the last 10 years of hood and MUA threads here and then any references that they may point to. Failing that, let's address the two questions implied above.

Toe kick. You want any MUA supplied by the toe kick areas to not disturb the flow of air into the hood. So it is good if they are located away from the hood, and also if they point away from the hood. The toe kick area applied to MUA (if that were the only source of MUA) should be roughly that of the hood entry aperture or the MUA air velocity will be undesirably high. Also pressure loss may be an issue.

MUA feed via the overall air supply ductwork. Since this keeps coming up, get your contractor to estimate the total pressure loss between the Fantech system and the house interior (should equal duct loss plus register loss). Add this to the pressure loss of the Fantech system itself (Fantech can supply). Then use the Fantech blower fan curve to see just what MUA flow rate will result. Alternatively, you might inform Fantech of the purported pressure loss of the ducting system and let them tell you what the resulting MUA flow rate will be. You want it to be commensurate with the 620 CFM or whatever your hood blower will actually be able to move against the hood baffles and ducting.

The Fantech MUA system arrived last night, and fortunately, it's installation instructions are very clear about it not being tied into the HVAC system in any way (written in bold print). So I have official backup on my position!

Among other places, Fantech recommends the MUA outlet could be positioned behind the refrigerator. Of all the options they offer, this appears to be the most viable option in our situation.

Is there a difference between venting through the wall behind the fridge and venting through the floor underneath the fridge, in terms of MUA effectiveness? I'm thinking the MUA would be more diffused coming from underneath the fridge, as there is a staighter path between the wall behind the fridge and the hood (up over the countertop into the left side of the hood. See photo: https://photos.app.goo.gl/qLcWTNo7T3DjawBZ7

I'd start with tech support at Fantech.

Strange, I received a new post email "pittsburrito said:The Fantech MUA system arrived last night, and fortunately, it's installation instructions are very clear about it not being tied into the HVAC system..." but that post doesn't show up. The most recent post I see is @kaseki from 2 days ago.

Anyway, ideally the make up air should be dumped directly in to the kitchen. Unless you are in an extremely moderate climate, that will require conditioning (cooling, heating, dehumidifying) of the incoming air which can be expensive. The less expensive alternative is to dump it in to the return duct (NOT supply duct) prior to the furnace/AC. This will allow your already existing system to condition the air as needed. The key to this working is that the MUA system and HVAC system must be interlocked so that the HVAC blower turns on anytime the MUA system is on.

My guess is that Fantech's warning is to not dump it in to the return air duct unless there is a proper interlock.

Equally strange, this thread showed up for me this morning at the top of the forum listings, but there was nothing new in it.

I would call Fantech tech support.

I don't know if they're on the other end of the phone line but many high end appliance manufacturers are there and they will clarify things in a few minutes.

Here are the 3 specific reasons Fantech says in the installation manual to not connect their MUA system to an HVAC system:

"Fantech deems it unacceptable to use central HVAC system

equipment and duct work for treatment and conveyance of makeup air.

Reasons for this include:

• Furnace manufacturers often limit the temperature of air entering

the heat exchanger to no less than 50-55ºF. The equipment warranty

could be voided if the limit is broken. [My edit: i.e. could crack the coil]

• Residential HVAC heating and cooling systems typically cycle on/

off with one or two stages of capacity on a signal from the space

thermostat. To put it another way, the central HVAC system is

intended to maintain a condition in the living space, and it only

operates when the living space condition becomes uncomfortable.

Since makeup air comes directly from outdoors, it is often quite

uncomfortable. The uncomfortable makeup air would be conditioned

only when the space thermostat is calling for action. [My edit: just as opaone has pointed out above.]

• The temperature difference between the indoor condition and the

outdoor condition can at times be significant. Cold makeup air

could cause the formation of condensation on the exterior of poorly

insulated duct work and equipment. Very humid makeup air could

result in condensation on the interior of poorly insulated duct work and equipment. [My edit: i.e. foster an environment for mold to grow, which in my area of the Southeast is likely.]

So we are back to my question from yesterday:

Install a dedicated MUA duct to:

* a floor vent under the fridge, or

* a wall vent behind the fridge?

I'm leaning towards the wall vent, in order to protect the fridge coils from humidity (mold growth). The back of our fridge is a solid sheet of metal (so it's more easily cleaned than the coils underneath). But the GC wants to go through the floor because it is easier to put a vent there. Thoughts?

I agree w/ the wall vent. I'm not sure there is enough clear airway below/around a frig. In addition to humidity/mold on the coils, that much airflow could clog them with dust much faster than normal.

Where are you located? How often will you use your hood? Electric inline heaters can get quite expensive to run if you have much cold air. Will hot/humid air being dumped directly in to your kitchen be an issue? I'm 100% in favor of the air going directly to the kitchen but you do have to think about conditioning it if necessary.

Fantech's verbiage above strangely ignores simply fixing the issues.

First, I have never heard of a heater core cracking because of cold air and in our case I checked w/ Carrier and they said that there is no minimum temp on the furnace we currently have. Even so, we do use an inline electric heater to heat the air to 40°f before dumping it in to the return duct and will do the same in our new house.

Most HVAC systems can be interlocked w/ the MUA to at least run the blower and many can heat/cool/dehumidify as well.

Properly insulating ducts bringing cold air in from outside is a common and standard practice.

So those are really non-issues that they're raising.

The real issue is that hoods do work better with proper airflow to the hood. From this standpoint two of the better locations for introducing MUA is under the range so that the air flows up in front of it or in the ceiling in front of the hood to create a bit of a flow under air curtain (many commercial hoods actually have an MUA vent built in to the front of the hood to accomplish the same thing). These can all be uncomfortable for someone cooking though and so why they are rarely done in residential situations.

So for me the cost of properly conditioning the air to introduce it directly in to the kitchen is not worth what I can determine is the minimal benefit over dumping it in to the return duct and using our existing HVAC to condition the air. It may not be as good of air flow as a direct vent on the opposite side of the room but I don't believe it's too much worse. I'd rank the options as:

1 - Under Range or hood curtain - Best / score = 10.

2 - Wall vent elsewhere in kitchen - Better / score = 5.

3 - In return duct and using HVAC - Good / score = 4.

Using HVAC to condition MUA depends on whether the HVAC system can meet the requirements of my Sep 28 colorful graph. The same BTUh apply for differential temperatures when cooling the air.

I'm located in central North Carolina.
We have more uncomfortably hot weather than uncomfortably cold weather here. We've opted to not get a MUA heater until we try it out. If it is uncomfortably cold, we can add the heater to the MUA later.

The hood will be used at least twice daily (breakfast & dinner).

In commercial settings, one successful MUA configuration is to bring the MUA down behind the cooking unit to spread out on the floor underneath from where it can rise to the hood. Some residential gas ranges are rumored to prohibit that. I think a cooktop would be OK, but generally we don't have ranges and cabinets with legs such that a mop can clean underneath. Instead we have cabinets flush to the floor. In this case, air introduced behind a cooktop or range can only get out via the toe-kick areas, and those are generally blocked cabinet to cabinet making the zone for MUA release relatively small. This causes pressure loss and high velocities, which can cause turbulence at the zone where the MUA interacts with the hood flow.

Better in my view is to either ensure that a wide section of toe kick is open for MUA insertion, or use a wall or ceiling vent. For a ceiling, a diffuser should be used to direct the MUA away from the hood giving it some opportunity to spread through the kitchen and approach the hood at a lower velocity than it has leaving the diffuser. A wall vent may have a similar requirement, depending where it is. I would be concerned that a refrigerator wall space would be too restrictive unless the air path to the open kitchen area was large enough to slow the air down as it passed the refrigerator. This space might not be available with built-in units.

Hmmm, I can't imagine dumping a bunch of hot humid NC air directly in to the kitchen unconditioned. My experience there is Bennetsville SC and Hilton Head which may both be massively more hot and humid than where you are though.

Floor vents will collect their own salad, so should be evaluated from a hygienic point of view.

Both passive and active MUA should be filtered. Whether filtering passive MUA will lead to an unsafe negative pressure condition in the house will depend on what combustion appliances are present and what their MUA configurations are.

I completely agree, opaone, however fantech only sells heaters for MUA, not air conditioners -- which brings us full circle back to the idea of connecting it to the HVAC, which Fantech "deemd unacceptable." It's a Catch 22 for sure! This is why I am so confused about what to do. The contractor just wants a passive MUA connected to the HVAC and call her done to pass code. It's only me pushing to have the MUA be implemented effectively.

There is a direct vent fireplace in the same great room as the kitchen. There is an on-demand hot water heater in the attached garage that shares a wall with the kitchen. There is a gas furnace in the crawl space.

Manufacturer's spec is recognized by code as a requirement by the building inspector and should be by the contractor and likely by any manufacturers impacted. Installing systems incorrectly could void warranties for the hood fan, the MUA fan as well as the HVAC system. In the event you plan to run your AC while running the MUA and you want that type of system, other manufacturers make them.

Combustion appliances not using air connected to the kitchen (garages are, I hope, not connected) don't count. Fireplaces probably won't operate correctly unless the MUA is above ambient pressure or very accurately balanced for near zero pressure differential.

The issues described above are why I sometimes suggest dumping the MUA into a room or basement that can be over heated/over air-conditioned and let that air easily propagate to the kitchen. By "over" I mean more than the room would need without a significant breeze moving through it.

By the way, it is unclear in this discussion whether the existing HVAC is up to dealing with significant heat loss. Usually, heating is designed to deal with the worst case house heat loss with windows closed for the coldest day. Cooling is designed to deal with the worst case house heat gain with windows closed on the hottest day. Neither of these conditions apply to "Pro" cooking equipment MUA air flow levels. 620 CFM actual may be marginal for your HVAC, or overpowering.

We are having to abandon the active MUA option and use passive instead. There is not enough local support to help us with the design & install, not even from tech support at Fantech. Even the kitchen designer recommends the passive approach routed through the HVAC, which is the only the GC has ever done it, and the HVAC company has never even heard of an active fan MUA system in a residence. No one can figure out how to condition the MUA for humidity & temp cooling without connecting to the HVAC, and since Fantech deems that unacceptablle, we'll have to change to a MUA system that does allow it.

Thank you all so much for your valiant efforts to assist from afar! If we had your level of expertise here, i'm sure it could be figured out. Meanwhile, we'll have to go with what the locals here do for MUA (and pay a 25% restocking fee to Fantech!).

That is a shame.

I know. I'm disappointed, too. No one here knows how to design an active make up air system, much less one that doesn't connect to the HVAC. Fantech tech support says they can't help either, because they are not local to see/design for our situation. They only sell parts.

Fantech does do heated, right? Did you need cooled also?

Yes

This is why I suggested a entry room (basement, say) approach. Any HVAC company should be able to set up a heat pump system and air condition the room the MUA is being dumped into using a unit designed for the flow rate. For example, 620 CFM at a cooling delta_T of 30 degrees needs to remove (per the chart I published further up) less than 20k BTU/hour. This is 1.7 tons of air conditioning.

The Fantech system should heat when it needs to using its own thermostat. If not, then a suitable 6 kW heater would do with its own room thermostat Something from Modine, perhaps.

The control of the Fantech system might just be as simple as a flow vane in the hood duct that energizes the MUA. I think that is there already, although I haven't investigated their MUA system. You just need to show the inspector that the unit blows air whenever the hood flow is over 400 CFM, or maybe less measurement complex, whenever the hood motor control is set for more than x% power.

Your twin goals are satisfying the code, and ensuring combustion appliance safety. Your methods may vary. Making the fireplace work under all cooking conditions is probably a bridge too far in your area.

That would be great, if we had such a space, but we don't. We are living above a crawl space, and have no unused rooms/spaces. We have a small half-bathroom (the only 1st floor room with a door), a dining room which has a large opening to the foyer, and the Great room (living room + breakfast room + kitchen) which is open to both the foyer and the dining room. We have a 2 ton HVAC system for that 1,000-ish sq. ft.

Upstairs has it's own HVAC system, and there is not much air flow between the upstairs & downstairs, due to the way the stairs are built (walled-in and going around the 3 sides of a small coat closet). There is just not a good place to dump unconditioned air here.

The only gas appliance inside the home's living space is the direct vent fireplace (with on-demand lighting, rather than a standing pilot light). So backdraft danger is not acute. We rarely use the fireplace, and can remember to not run it and the hood at the same time.

However, we DO want sufficient MUA to not be pulling in air from the crawlspace or creating negative pressure inside. I hope that a passive MUA system routed through the HVAC will be sufficiently effective.

That's quite disappointing.

In the parade of homes this year we went through about 22 homes and all but 2 or 3 had active MUA. Most dumped MUA in to the return duct, I think 3 conditioned the MUA and sent it directly to the kitchen. From memory about 1/3 of these were Fantech, 1/3 Braun and 1/3 various others (electro-controls, etc.). All of the ones connected to the return duct were interlocked w/ the HVAC system to turn it on to move and condition the air as needed.

All of these houses had HRV's or ERV's. Most connected to the return duct but 3 properly pulled stale air from baths & kitchen and sent fresh air directly to bedrooms.

FWIW, all of these HVAC folks are about 20 years behind those in Europe.

That North Carolina is so far behind us who are behind Europe & Asia is really disappointing. One more in a long list of areas where the U.S. is no longer a leader and not even a close follower.

Where is this 2-ton HVAC system in the layout?

Do you have any idea what the nominal CFM rating of the HVAC system is?

I'm thinking that a 2-ton HVAC cooling capacity for 1000 sq. ft. is pretty good, and probably can suffice with 620 actual CFM on most warm days. You might find it not keeping up with whatever the interior temperature setting is, but the result shouldn't be stiffeling in the summer. For your HVAC to work, it seems to me you would need the following in your configuration. (To avoid ambiguity, let me call the heat exchanger part the heater/ac.)

• The return to the heater/ac needs a damper that allows air to return only and not flow in the wrong direction.
• The MUA path to the outside needs a damper that only allows air to be pulled in.
• The MUA is connected to the return on the heater/ac side of the return damper.

If this configuration leads to cold registers in winter, add a heating component to the MUA.

If this configuration leads to warm air at the registers in summer, embrace Nietzsche-ism.

Hilarious! We do enjoy a warm house: Our AC is set at 78 in the summer. It's the humidity that could be difficult to control.

I'm glad you think connecting to HVAC might work for us. (Not ideal, but could still work.)

I'm not sure what "nominal" meams in this context, but our HVAC company mentioned that our hood is moving half as much air as our entire 2 ton HVAC unit. So, if our hood can move 600-700 cfm, would 1200-1400 cfm be the nominal cfm of the HVAC?

Nominal should mean the zero static pressure blower flow rate.

"So, if our hood can move 600-700 cfm, would 1200-1400 cfm be the nominal cfm of the HVAC?" That would be the implication, although he may be referencing the nominal flow rate, and not the lower actual flow rate of either or both. What we don't know is what the house pressure would fall to when the heater/ac blower is not on. It seems, though, that you don't have a combustion appliance safety problem. Your hood blower, thus, may have to work against not only the pressure loss of the hood baffles, but also against the heater/ac filter, heat exchanger and duct/register loss. All this should be considered in choosing the hood blower nominal flow rate and the MUA feed.

It might still be a good idea, or a planned for idea, that the heater/ac blower be turned on whenever the hood duct is flowing significant air.

Humidity gain will be partially mitigated by the ac function of the HVAC, and the rarity of running the hood for long periods of time. At worst, adding a supplemental dehumidifier to the household could be a successful countermeasure.

Just a note that for passive MUA if you do it, the duct for 600 CFM would need to be about 18" internal diameter. This is based on the 42sq"/100CFM plus 2". But done in my head so may be off.

I would certainly try to do active in to your return duct if you can though. Have you looked at Broan or Electro-Controls as alternatives?

opaone: what pressure loss is that rule of thumb based on -- the not-exceed value of 0.03 inches w.c. that gas combustion appliances need for safety? If so, doesn't the diameter depend on length? Or, is this perhaps a commercial rule for special cases; most restaurants will want active MUA so that they can keep the dining areas positive w.r.t. kitchen areas?

The OP appears to be in a municipal zone where the requirement is undefined MUA when the hood blower is above 400 CFM rated, and not based on pressure achieved during operation vs. types of combustion appliances present. With the "furnaces" in the crawl space and attic, there really is no safety issue, so the real functional purpose of the MUA, besides claiming that one is present, is to allow the hood to develop a reasonable portion of its zero static pressure flow rate.

Cont'd. I personally would put a blower in the MUA path, but also be sure that the return path to the heater/ac have a one way damper so that none of the MUA flow reversed the return beyond the vicinity of the heater/ac mechanism.

At this distance, I don't know what Fantech's basis for objection was. Perhaps they are assuming that their control system is in play, whereas in this configuration, the MUA is simply replacing the air expelled from the hood via the heater/ac. The heater/ac blower seems adequate for this (we don't have a fan curve for it, but the positive pressure side will be looking into a negative pressure household). When the heater/ac blower is not running, as when the household is neither too cold or too hot, then a blower in the MUA path seems desirable.

Control could be managed by a configuration where the MUA blower is powered ON via on a relay pair performing {hood blower ON} AND NOT {heater/ac blower ON}. This would be relatively easy to configure with some DIN rail relays, DIN rail fuses, and DIN rail connection blocks (whatever they are called -- too lazy now to look them up) built into a NEMA enclosure.

If the inspector's or OP's reticence requires, a PE somewhere in the vicinity could build it.

What's a PE?

Yesterday, I called Broan to ask which Universal Make Up Air system of theirs I should use in my circumstance. Without waiting to hear my circumstances, the Broan Tech support said to use the 8" duct MUA version for a hood that pulls 500-1,000 cfm, the 10" duct MUA for a hood pulling more than 1,000 cfm, and the 6" duct MUA for a hood pulling less than 500 cfm.

So I am surprised by opaone's recommendation to use a 14" duct! (Broan doesn't make passive MUA units larger than for 10" ductwork)

Opaone has a commercial hood and really impressive ducting.

A PE is a professional engineer, a member of a state society that still operates according to medieval guild practice of forcing all people in a certain trade to be members. The concern is that people who are not engineers may call themselves engineers and build unsafe devices. In many states, it is illegal not only to call oneself an engineer without being a PE, it may also be illegal to construct and sell certain classes of electronics devices without being a member of that guild. In some municipalities, it may be illegal to construct certain classes of devices for one's own use. I imagine NYC is one such place. After all, if one burns down his own apartment, he is also likely to burn down other apartments.

So, an organization is constructed that is intended to enhance safety, but which, not uniquely, also engages in rent-seeking (in the financial sense). Public safety is always a difficult area in which to achieve balance. Thus my suggestion of hiring a PE if you need a blower controller and don't feel competent to build it yourself.