Hmmm. Assuming that your MUA is matched to your hood in flow rate (more or less), to first order none of it will enter the register unless this foyer hallway is blocked from the kitchen. It should flow directly to the hood via whatever tortuous path is available. (This is true if the HVAC is off, or the pressure at the return register is not very low compared to the MUA.)

To second order, if the HVAC is running, some will go to the hood directly and some will be pulled into the register and provided to the hood via the ducting.

The best features of this scheme are: (a) the MUA impinges on the thermostat so the HVAC can try to condition it, and even if it over-conditions the part that enters the return, overall if the HVAC has the capability, the eventual mixed air at the kitchen should be approximately the desired temperature. And, (b), this location may be the least annoying among several of your options for unconditioned air to be dumped into. At least the cook might not notice. Also, (c) no one can whine that the MUA is unbalancing the HVAC system by direct coupling.

Be sure that the free area of the register or diffuser you use to interface between the hallway and the MUA system is low restriction. This requires that it have an area significantly larger than your MUA ducting.

Cooley and Hart have a lot of candidate registers and diffusers listed, and I think that there are also pressure loss data available. At the flow rate you are operating at, the register/diffuser pressure loss should be small relative to the other pressure losses in the overall outside-to-inside-to-outside loop.

by conditioning, do you mean cooling, heating, dehumidification...?

I believe that is what conditioning means, but perhaps I am wrong?

Kaseki, thank you for the Hart & Cooley recommendation. I checked into it and got lost in the engineering numbers! (Makes me want to go to school to understand the physics of all this!) There is a gapingly huge knowldege gap here, which opaone has validated.

My husband is recommending we just put in the Broan passive universal MUA system, knowing that it is not ideal, and try again in 5 years, with hopes the HVAC industry will have caught up by then, so that we can put in a correctly sized, matched, and effective MUA system. Not just one that passes code!

I asked since most only do heating, putting an inline thermostatically controlled heater in the MUA line. And that is a function of location, as to whether it gets cold enough to justify it.

Right. We live in central North Carolina where heat and humidity are a bigger issue than cold. MUA is only made for New Englanders? Midwesterners? The Southeasterners need proper MUA conditioning, too! Fantech prohibits connecting to HVAC for conditioning, we are SOL and passive MUA is our only option.

Code here says 6" duct MUA is all we need for our 600 cfm hood (making up only the amount of cfm over 400 (200 cfm in our case) -- which I find ridiculous. Someone on a previous thread said we'd need a 14" duct MUA -- while the largest MUA Broan even makes is 10". We have 10" ductwork for our hood, but Broan customer service says we only need 8" duct MUA -- and could not explain why it shouldn't match my 10" exhaust ductwork.

Our hood fan is variable speed, and we don't anticipate running it at 600 cfm often, so do we really want a MUA damper that is not variable? (No.) Fantech's is variable, but Broan's is either 100% open or 100% closed, regardless of the hood speed..

The underlying issue is how much complexity you are willing to deal with, given a lack of HVAC knowledge. For passive MUA, I would use either a passive damper or an electric damper, just to keep out air when it isn't needed.

The size of the MUA duct, assuming that it is long enough to matter, is determined by the pressure loss that can be tolerated. As I recall you don't have any interior combustion appliances, so the issue is how well do you want the hood to work. With an active MUA (blower driven), the duct can be any size that the blower can overcome at the flow rate desired.

Imagine this scenario: The hood blower and the MUA blower are identical, and the goal is to have the house pressure close to the exterior pressure. The two blowers are powered by the same circuit in parallel so they have the same RPM. To keep the house pressure relative to outside at zero (inches, pascals, PSI, whatever units you want), the pressure loss in the hood baffles and duct system has to exactly equal the pressure loss in the MUA system. The MUA doesn't have baffles, but the next best thing for gagging the flow, an exit register. Maybe it also has a filter. With a bit of pressure loss adjustment, the MUA could balance the hood flow.

Now consider that the blowers are not equal and the MUA is not driven by the same voltage as the hood blower. In this case, the house pressure could end up at any value depending on how the MUA was controlled. I'm sure you don't want to build a controller that works on differential pressure between the kitchen and the outside.

So, use a passive system with low pressure loss, or find a way to operate an active system based on pressure differential or maybe sensing the voltage to the hood blower.

Personally, I think you need to also consider how uncomfortable you will be for the normally brief time that your hood would be running a full power and heating or cooling was imperfect.

(cont'd)

The IRC requires MUA over 400 CFM, but requires it to fully compensate, not just make up the difference. One shouldn't assume that a relatively modern house is so leaky that the 400 CFM not made up is not going to cause a pressure loss that not only degrades the hood blower flow rate, but also could be a safety issue. For a tight house, I am certain that a combustion appliance without its own MUA would have a back-draft condition with 400 CFM trying to flow out of the kitchen, or even the 200 that would flow if the house were tight and no MUA were provided for the kitchen other than backwards through the dryer or hot water heater.

Without interior combustion appliances, you only need to do three things: (a) install an MUA system that the inspector will accept, (b) provide adequate performance for your hood to work well, and (c) condition the air to your tolerance level for the periods of hood usage. Unlike days of yore, one doesn't need to run the hood all day to make up for not capturing the effluent when it is generated. Lengthy hood use might be needed if broiling in an oven gets odor and grease everywhere, but this should be even rarer than high power hood operation for the cooktop.

Dumping your MUA near the furnace return register is a reasonable method of operation. If the duct length to that point will be long, and the MUA is passive, use as large a duct as is reasonable to fit. If you are going powered (active MUA), then you will need to control the flow somehow.

(cont'd)

There is a passive control method used for active MUA in large buildings that one could consider using. In a large building, the MUA is always on. In your case, the MUA blower would turn on whenever the hood was on, using a voltage sensor or a duct vane sensor. The MUA would blow into the house, but a pressure controlled damper (Honeywell makes, I think) would sit in a duct loop between the MUA blower output and the MUA blower input. This would "recycle" unneeded air around its loop keeping the house pressure relative to the outside in a reasonable range.

(cont'd)

Please list exactly what components you have in your already purchased MUA system. Our goal should be to make use of it.

The MUA comes as a complete set from Fantech, so if we installed a Broan passive damper instead, we would return the entire Fantech set intact. It includes:

Fantech 750 CFM Makeup Air System. Contains FMAC controller, FML8 intake hood, ADC8 shut-off damper, FGR8HV filter cabinet, PrioAir 8 EC fan, LD8 silencer, 2 pairs of FC clamps

Although it matches the variable speed exhaust system we had installed (i.e. the Fantech exhaust fan relays to the MUA fan exactly how much air to provide) it appears too complex a system to install here, given the limited state of our HVAC expertise in this area. (I kid you not: current N.C. code only requires the amount of cfm above 400 be replaced -- not the IRC recommended replacement of the full amount of air that is being exhausted.) So, our state is further behind than most!

For our apparently inevitable install of the Broan universal passive make up air system with motorized damper, do you recommend we match the 10" duct diameter of our exhaust system, or install the Broan salesman recommended 8" duct version, or the Broan website calculator recommended 6" duct version to provide MUA for our effectively 600 cfm exhaust fan. (Our hood's inline fan is actually rated 910 cfm, but will end up being around 600 cfm in our ductwork situation.)

As I noted previously, make the passive duct as large as will feasibly fit. How long will this duct be, by the way?

MUA ductwork will be around 10 to 12 feet long.

A 10" wide duct version is as large as Universal MUA comes from Broan. That duct size match the duct size of the hood exhaust system.

I apologize that I did not realize not just the active MUA should have a wide duct, but the passive MUA, too. I thought only the one with a blower needed to be large, but Fantach sized their matching MUA system at 8", so was thinking to follow their lead?

Arrrgg. This is all about pressure loss, and not matching duct size. A larger duct means lower pressure loss, but at some point (likely under 10 inches), the dominant pressure loss in a 600 CFM system will not be the duct loss. For the hood system, it is the baffles. For the MUA system, it is the register, and (where present) the filter. Fantech is using 8-inch because they have knowledge of their active system and can predict its flow rate with 8-inch duct, etc.

10-inch duct is quite large, and it is unclear to me how you will interface it with a register in the wall of your entry way. There can be more pressure loss in an awkward duct transition than in the duct itself. Please explain your intentions in interfacing 8 or 10 inch duct with a register in your entryway.

The passive Broan system would connect directly to our HVAC return (the way the contractors here say "it should be done.").

Broan does not provide any engineering info with their MUA, instead using "a rule if thumb":

If over 1,000 cfm exhaust, use 10" duct.

If between 600 - 1,000 cfm exhaust, use 8" MUA duct.

If under 600 cfm exhaust, use 6" MUA duct.

(MUA pressure losses not taken into consideration.)

We do have a MERV 10 or 11 filter in our HVAC, so we'll have more pressure loss than most.

The entryway register was only a last ditch attempt at a work around in order to install the active Fantech MUA system.

Broan's rule of thumb, while very generic and possibly rife with exceptions, is good enough if MUA flow rate is almost entirely determined by the pressure losses in your HVAC system, plus or minus its blower. You may want to have your HVAC blower go on when the hood blower goes on so the MUA is aided in getting past the MIRV filter. It would also keep your HVAC returns dust collection in place.

Were the directions (if any) provided by Fantech not sufficient? After all, your local HVAC (or local carpenter) need only install it and a local electrician hook it up. It seemed to me from what you reported that the MUA control took its direction from the hood blower motor circuit, so no creative designing would be required by the installers, other than duct fitting.

The Fantech system would still lack a cooling/heating scheme, and that seems to me to be your real issue.

You are exactly right. The Fantech lacking access to cooling/heating is the real issue.

pittsburrito, what did you end up doing and how is it working?

Smart idea, but it’s a bit risky. Blowing MUA near the return and thermostat could make the thermostat misread temps and cause short cycling. It’s not technically a direct HVAC connection, but still goes against Fantech’s intent. You might be better off adding a small inline heater or tempering coil to condition the air instead.

The original NC location issue was cooling, as I recall. An in-line coil is a good method of MUA heating if there are no impediments to the electrical work. The issue with injecting MUA into HVAC is how well the capability the HVAC can correct the temperature of what is essentially the original house with somewhat open windows. Usually heat pumps/furnaces/air conditioners are designed to deal with predictable leakage of heat into or out of the house, rather than barn-like conditions.

I can report that summer use of my MUA (presently passive, but in NH), injected into the hallway leading to the kitchen, does not cause any cycling of the Mitsubishi split system a/c that feeds the living room/kitchen area. Rather, the a/c causes a cross draft affecting the cooking plume that under some conditions I address by temporarily shutting down that unit. The other units in that part of the house deal with their rooms relatively unaffected. This is at hood flow rates up to an actual 1000 CFM.

@pittsburrito: Care to bring us up to date?