Wow! A rare case where the aesthetics part of the performance-aesthetics-economy: pick-any-two limitation is not a factor in hood selection. But first, given a sealed household, I would strongly recommend obtaining a furnace kit to use outside air. This will avoid the possibility of the MUA system not working perfectly and the hood causing negative house pressure and hence backdrafting of the combustion appliance. I know that such devices exist for Beckett burners, and would guess that there are adapters for other burners. As the OP correctly notes, a 200 cfm ERV is not going to cope with a 1200 cfm vent hood. (Actually, what would happen with a sealed house is that at full power, the hood would pull the cfm the ERV would let through into the now minus a couple of inches w.c. house pressure.) So MUA is essential, and in the mid-Atlantic region would be pretty uncomfortable in winter if unheated. Depending on one's tolerance for heat and humidity, cooling in summer might be optional or not. We are starting to get into the realm where performance-economy, pick any two is applicable. MUA can be roughly characterized as passive or active. By passive is meant no fan in the circuit. Active uses a booster fan to keep the house pressure stable. No-fan only works for simple ducting with a vent hood air flow controlled damper. (A recent post here provided some sources for these.) If a filter is used at 1200 cfm (or whatever flow actually could be pulled with all the doors and windows open, then the house pressure will drop. One can see on websites selling filter packs that the pressure drop at such flow rates is too large for combustion appliance safety, which require negative pressures not be greater than 0.03 to 0.06 inches depending on appliance type. Active is needed to overcome the filter restriction. It may be needed to overcome heating heat exchanger restriction and/or air conditioning heat exchanger (expander) restriction. The difficulty with active is balancing the MUA air flow rate against the variable hood flow rate such that the house pressure is near zero. This usually requires a control system of some sort, and can be a "project." What to do? What to do? Only one free-of-processing-controllers active MUA has occurred to me. In the simplest embodiment, the hood control is connected to two parallel wired identical fans, one in the hood exhaust path and one in the MUA path. Then the two will (try to) operate at nearly the same flow rate at every control setting, and one only needs to adjust the MUA duct restriction until it matches that of the hood path. More exactly, adjust the MUA path so that house pressure falls very little as the hood is turned on. Note that this doesn't account for bathroom fans, fireplaces, or other exhaust flows not due to the hood. In such cases, PID control of the MUA fan would be needed. In my case, due to a 1500 cfm hood, 1000 cfm over-oven vent, fireplace, and three bathroom fan household, I am building an active MUA that will use an axial blower in the 2000 cfm regime. It has to overcome the pressure loss of its intake vent, its filter, and its heat exchanger (which is hot water pumped from my oil burner). Control is via a Fuji PID controller operating from a BAPF differential pressure sensor. The motor power control is not yet selected, and a lot of sheet metal action is still needed, along with further attic revisions that I seem to have trouble getting to at the needed rate. I recommend the OP and/or his HVAC person read the "Kitchen Ventilation Systems Application & Design Guide" that may be found at Greenheck's web site. It periodically moves around, URL-wise, so a Google search by name may be fastest. Greenheck or one of its competitors may be able to provide what you need for considerably less agony than a do-it-yourself HVAC project would. kas

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.

Around here you can vent up to 350 cubic feet per minute (CFM) before you need make up air. If your vent will move more air than 350 cfm (even if you never intend to use the higher capacity), the mechanical code in your area will probably require make up air, ordinarily supplied through an energy recovery ventilator (ERV) connected to your range vent with a switch that automatically turns it on when the vent is in use. Even if your code does not require it, it is still a very good idea. This is not difficult to do, but it should have been planned for all along.

First, this is what the proper calculations would say about Risinger's installation. OR THIS Second, short circuiting a hood, air curtains, and generally any makeup air introduced too close to the range will produce counterproductive turbulence that will reduce the effectiveness of the hood to the point that he may as well have just put in a 400 CFM fan without makeup air. Third, the floor under the range is required to be sealed from drafts on most ranges. But, you can't usually shove ALL that air through the HVAC system either