So you have the Broan 6" MD6T and you have it unfiltered, unconditioned, and ouputting under the fridge for conditioning purposes? So you are relying on the exhaust heat of the fridge to cool the incoming air? How cold did it get this past winter for you? How cold was that make up air? Detectable at all? Here we had -20 real feel in NY with the wind chill at times. It was brutally cold and cold often. I think I might have to put in a heated MUA. So update on Broan. The lady I spoke to today was completely useless. To make a long story short she had absolutely no idea how to use the Make Up Air Specifier on her site. It was quite pathetic and my impressions of Broan's customer service = suck. I spoke to Dean at Electro Industries at length about all the factors I'm thinking about. We went over two options - one using the MA-10 (10kw all in one 10" heater with damper, blower, filter, and current detector to control blower speed) and the less expensive heater unit EM-WX1025R. The broan MD6T comes with a 24volt 20VA transformer so I could then use the one signal to turn on both the Broan damper and the heater unit. Both options unit can accomodate up to 637cfm in a 50 degree temperature rise. It's a 10kW unit though and would require a 50A or 60A two pole (240V) breaker and its own dedicated lines (of course). I can take care of the electrical so no problem there but man I'm guessing my heating bill (which was astronimical last year) will be more than I can tolerate this year. Temperature Rise Needed/CFM EM-WX1025R 10000 Watts 80/395 70/452 60/527 50/632 40/790 30/1054 MA-10 $1917 + freight EM-WX1025R $800 + freight. I think that should be sufficient - it's better to have 50 degree air coming in than 0 degree air. I'm thinking that maybe I can get away with non-heated air if I can get fresh air to come up that old supply register near the range. The opening is closer than the fireplace but it might work if I put a blower in-line as well.

We have not installed cabinets and MUA unit yet, but here is our plan. We have a 720 CFM hood going in over a Blue Star cooktop. MUA is not required in our area but having done research I decided I wanted it anyway. I had an HVAC engineer review our system re a return air duct issue. While he was at the house I asked him about the MUA. He noted that 720 CFM is pulling close to 2 tons of airflow out of the house/system. Definitely want MUA even if not required. Otherwise you are putting a lot of strain on your conditioned (heated or cooled) air. I purchased the Broan MD6-TU system (about $120). We will have it linked to the hood to turn on automatically. Our HVAC guy suggested it would not be wise to connect the MUA vent to the HVAC ducting due to the burden on our slightly undersized furnace. He suggested unconditioned exterior air. We will run the MUA duct out to the side of the house through the crawlspace. Will have a vent with critter screen to the outside. The actual MUA opening in the kitchen was initially going to be in the toe kick under the cooktop cabinetry. I decided to move it over a few feet to be under the wall oven cabinetry. With unconditioned air I did not want it blowing on my feet. :) [We live in Virginia--cold winters, but nothing like when we lived in New England!]. The HVAC guy did calculations and determined that the best efficiency for our set-up was to have the duct come up under the 30" cabinet and be open to that whole space. We will install (2) 12" x 2 1/4" registers in the toe kick to maximize flow in. He thinks that with it being only 3 ft to the left of the center of the hood area that it will draw to the hood very effectively thereby increasing the efficiency of the hood in removing odors. It also should not dump much unconditioned air into the living space because the hood will draw more effectively. Not sure if that is helpful (or even clear!). I really appreciated the HVAC guy's help--and he did it all at no charge as a favor to my awesome contractors. We are blessed... Good luck!

Adam with that much CFM on the blower, if you stand to close you might get sucked out hahaha, only joking.One question did they use the right size ducting material that the manufacture recommend if undersized this could be the source of your problem.

With all those combustion appliances, unless they have their own MUA or they are in a room sealed from the house and not from the outside, back-drafting of carbon monoxide is a risk unless the pressure inside vs. out is carefully balanced. Note that providing such MUA for the appliances may be easier/lower cost than installing an "active" MUA system. You will potentially be drawing 900 to 1000 CFM depending on the MUA system and duct length and other factors. So the MUA flow rate needs to be comparable. There are two approaches called simply (at least on this forum) passive and active. Passive comprises a large duct with a filter, perhaps, allowing outside air to be pulled in. It will need a damper, and this may need to be wired to the hood system in some way. And the air it brings will need to be heated in cold areas such as MA. The issue with passive is that the duct, filter, and register/diffuser have some pressure loss, and certain gas appliances, particularly those without helper fans, will back-draft at very low differential pressures. Active comprises is a large duct with a filter, damper and a blower that has to, in some way via some feedback system, balance house pressure with outside pressure. This is a more complicated system. There are a few manufacturers supporting such for residential uses. Let me put off for the moment searching for their names. Introducing MUA into the heating system involves a bunch of factors that your heating specialist would have to evaluate to determine practicality. I don't expect it to be a good idea at this flow rate level. Heating is performed one of three ways, in general. (a) an electric coil system, like that in an electric clothes dryer, is put into the duct path. (b) a hydronic heating water-to-air heat exchanger is put into the duct path. This would only make sense where hydronic heating is in use. I use the same Wolf blower and can supply up to 120,000 BTUh to the heat exchanger. (c) the MUA is introduced into a room in the house -- mud room, basement, whatever -- that has had its heating augmented so that it can handle heating the mass of incoming air. This can make heating simpler if there is a easy way to add a heater. A gas function house might easily accommodate a gas heater. See chart for how much heat is required. For electric heating convert BTU/hr to watts. You will find the number to be high in either case. The MUA should be introduced directly or indirectly to the kitchen far enough away from the cooktop to allow the air flow to be relatively non turbulent. If introduced at the hood, it must be directed across the room so it can use that distance to get more laminar. If introduced at toe kick areas (and the amount of toe kick area should be of the order of the intake area of the hood for a passive system, 10 sq. ft. in your case), it must also be directed such that it doesn't strike close objects and rise turbulently into the cooking zone.