The air vented out is always replaced with make-up air. Some of this air could come down your chimney. Backdrafting a wood-burning fireplace is generally easy; hence the often necessity to open a window to keep smoke out of the house, particularly when starting the fire.

600 rated cfm (call it 400 actual cfm realistically) can probably leak into your house without special MUA components and without too much pressure loss, but I would expect that pressure loss to backdraft your fireplace unless you open a window when actively using the fireplace.

1200 rated cfm (call it 800 actual cfm realistically) is another kettle of fish. For that I would want at least a hole in some wall, floor, or ceiling that was ducted to the outside and which had a damper, preferably a damper that operated when the hood fan operated.

In this case you probably wouldn't need a blower in the MUA duct. However, if you want to filter the air, or heat the air with a finned radiator type heat exchanger, then a blower may be needed because the pressure loss could be 0.1 to 0.2 inches w.c., and this will backdraft anything except perhaps a blown oil burner. Gas combustion appliances can backdraft at 0.03 inches.

So, I guess I would believe that the need for an MUA duct blower for the 1200 cfm case somewhat depends on when you use your hood vs. when you use your fireplace, and whether combustion appliances are in an airspace connected to the kitchen or have an isolated source of their own MUA.

Ultimately, specifics depend on how leaky the house really is, excluding the fireplace.

kas

We have a hundred year old house with a fireplace and a gas boiler for our radiators, and it is also leaky. So, we had the same question as you when we installed a range hood recently.

I think part of the answer depends on where you live and what the code is there. Where we live, code is strict and they really don't differentiate between much between the newer tight houses and the old drafty ones.

Thanks for your input kaseki. I wish I were an engineer and could give an actual figure on how leaky the house is, best I can say is VERY :).

Along with the hole out the roof for the swamp cooler there are also (6) 8"X5" openings to the crawl space. The other gas combustion appliances (boiler and H2O heater) are in the utility room which accesses the crawl space. The utility room is also below the kitchen. As for the source of their make up air I'm unsure... kinda makes me wish I'd asked the tech. who installed the new boiler last year a lot more questions.

So, thinking this through if we were to go with the 1200 cfm hood and connected a damper to a fresh air inlet (hole) to the outside and to nothing on the inside (as we have no forced air and therefore no ducting), the damper would open when I used the range vent and let in MUA for the time the range vent was in use, and close when it wasn't. Realistically, how much more air could it be pulling in than the 6 other holes in the crawl space and the one in the roof? hmmm. I'm probably missing something in my logic but it seems silly to have only 1 of 8 openings controlled unless it's absolutely necessary to have that 8th hole and it's in a location that I want to be able to shut is my thinking.

Currently I have a non-functioning electric range with OTR MW recirculation hood (the catalyst for the whole remodel). Needless-to-say I'm excited to have proper range ventilation and want to do it right, but I'm starting to think that a damper for additional MAU might not be needed.

I do know this, the 3 times a year my DH takes the time to get me a really good fire going, I'm sitting in front of it and am no where near the range. :)

needinfo1 did you end up putting in a damper for MAU? If so, where is it located and is it just a damper connected to a hole in the wall as I described above?

As for me, I'm looking into MUA for peace-of-mind. I know its part of the IRC which I believe my municipality has adopted.

reinette:

If the utility room (which accesses the semi-open crawl space) is also connected, air-flow-wise, to the kitchen, you already have your MUA source, it just isn't limited by a damper. If the utility room is not connected to the kitchen, then the combustion appliances are ok, but the kitchen still needs an air path to outside. If every room is connected, then you have as much MUA as you might need, albeit from ground level across a potentially musty crawl space floor rather than from a source higher up.

I can't determine from your response how the attic roof hole for the cooler might be connected to the kitchen.

So, in my view, if the sum of the areas of all holes to the outside that freely connect to the kitchen (accounting for screening) is as large as the hood duct, you probably need not worry about any MUA safety issue. I hope the fireplace has a damper so that when not in use you don't draw odor in through the chimney.

kas

kaseki-

Again thanks for your expertise and guidance in this area. The swamp cooler is through the ceiling on the second floor landing. This is freely open to the stair case which is also freely open to the dining room and adjacent kitchen.

I'm fairly confident that we will not need additional MUA especially in the warmer months. These old homes were defiantly built with air-flow in mind. However, I have decided that if I go with the 1200cfm model, I will have a damper installed in the mudroom off of the kitchen.

Much depends on your local regulations. Here in MA dedicated MUA is not required by code and when I crank the speed up on our 1200 cfm blower I just open a window. Our house is tight enough that the hood won't move any significant amount of air without that, at the lowest speed the ERV and available leakage seems to be an adequate source. Our furnaces and water heater all have sealed combustion with their own air supplies, so backdrafting is not a concern. For the few minutes a week that the hood runs at high speed and needs to be vented I'd really rather not cut another big permanent hole in the house envelope. If the hood were in use a lot at high speed I'd look into bringing an outside air supply in at the floor in front of the range in an effort to create a laminar flow situation like they use in restaurants to minimize drawing conditioned air out of the house.