Oops, kaseki, not antss (memories of old GW threads from 10 years ago still pollute my brain). Sorry, kas.

The main problem with doing this is if you have a gas furnace, there is a minimum return temperature that it will be rated for. Any lower than that and you could have problems with the heat exchanger.

The other problem with passive makeup air dampers in general is that often with the most powerful hoods (1000 cfm+) it just isn't sufficient, even when interlocked with the HVAC fan. Especially if you have a tight house and/or gas appliances that can backdraft.

In reality, we run the hood at anything approaching full speed for maybe a few minutes per month. Normally, running at maybe 25% of capacity is more than adequate, especially as our range is located in an alcove which surrounds three sides. With the resulting containment, we don't need high flow rates unless we're really searing away. Given that, do you think this relatively minimal use could create problems with the gas furnace? Understand the concerns over passive damper but certainly better than nothing at all, I figure.

If it gets really cold where you live, there is such a thing as a thermostatically controlled in-line electric heater. Depending on the air flow rate, it can raise the air temp say 50*F (or perhaps less). So you set it to come on at 30*f, it will raise air temp to 80*f feeding your furnace, usually fine.

In terms of air flow cfm, this will determine the duct size, as well as any in-line filter you will have with it.

Hi. I would go with an electric or gas heater in this case, assuming that you have enough (or can wire for enough) power or add the needed gas lines. 100kBTU/hr is around 30 kW (125 A at 240 Vac). I'm too lazy right now to calculate the number of hood CFM that that would heat by what deltaT (see the specific heat of air), but it would be a substantial part of the likely air flow you would need. You could dump cold air into a basement and heat the basement with fast-acting space heaters, or run the air down a duct with heating coils. For the latter, Fantech might have something. Greenheck certainly does for the higher flow rate conditions.

There was an early topic here where Cooknsews provided photos of his heated blown MUA duct through his basement. Unfortunately, the photos that I have of it seem to be broken by use of some initial hex code that modern image readers refuse to accept as indicating a jpg, and the original thread picture has a block from Photobucket.

http://ths.gardenweb.com/discussions/2266424/does-this-sound-right-re-mua

Anyway, if you push 1000 CFM heated air into the furnace return system, you may successfully diffuse it into the entire house with whatever chilling results from insufficient heating, but the furnace air flow would be operating in reverse. If dampers force this air to flow in the normal direction, then see hvtech42's comment. The furnace itself will NOT be rated to provide this level of heat addition, and probably not this level of CFM. Best is to dump the air into the kitchen, a nearby room, or hall via its own ducted path.

Blown oil furnaces running hydronic heating systems sized for a large house can manage this level of heating, but won't likely be able to simultaneously heat the house and the hot water while heating this significant air flow. But this may not matter for short sear durations.

So, taking what you have and assessing possible paths is one step, assessing how the heat is to be added to the air flow (and how much) is the next step, then there is the hard part. If the system is heated but passive, it may not keep the house pressure up. It may need a blower and the blower must be controlled some way that is proportionate to the air flow. In a complex environment with various exhaust fans, chimneys, etc., with the MUA flow needed to keep the pressure close to ambient, the control may be equally complex. Passive into a heated space can avoid much of this because (as long as you give up filtering the intake air) there won't be a pressure loss due to heating.

Another scheme based on large building MUA is to blow into a cold space and vent the air pressure using an interior pressure controlled damper back around the blower to keep the interior pressure where it is supposed to be. Or just have an unblown return duct to the outside In the residential case the motor might only be turned on when the air flow from the hood is at one of the higher settings. (For large buildings such systems run continuously.) Past the cold space (which might be the interior of a duct) is the heater. It needs to be thermostatically controlled.

Yes, this is all a giant pain, and is the reason why restaurants (cost of energy sensitive -- customer complaint sensitive) have companies such as Greenheck calculate what is needed and provide appropriate systems to exhaust grease, keep down odor, meet legal requirements, and keep the customers warm or cool as needed.

Offerings from Fantech and Broan should be considered.

I really would avoid dumping freezing air into your furnace, even if not for very long. It just wasn't designed for that type of use. Even if you get lucky and it doesn't cause any problems directly, the manufacturer will use it as an excuse to void the warranty if you have unrelated problems down the road.

Dump it directly into the house, and either deal with the cold for the few minutes it will be on, or heat it separately.

You could dump it into the furnace return and preheat it, but I feel like if you're going to go to that trouble, you might as well just duct it directly into the house with its own heater.

As for passive vs. powered MUA: passive dampers can work OK, but the problem is that once you need a certain amount of MUA, so many ducts to the outside will be required that it becomes no longer practical. That's where powered comes in handy. Powered is also the way to go if you want to heat the MUA.

As expected, super helpful info from both of you. Greatly appreciated. One point I want to confirm: It's OK to dump MUA into a room distant from the kitchen, right? Some of the commercial literature focuses on returning MUA back into the kitchen, often close by the range. But in a residential setting, providing MUA to our basement storage room (which is, incidentally where the gas water heater and one of two gas furnaces are located) would be OK? As a storage room, it's probably the least annoying place into which to pump frigid winter air.

Ideally, the MUA should approach the cooktop/hood area as a smooth low-turbulence flow. It can do that from a perforated wall diffuser in a lab setting, or via a path from a remote location, so long as that path isn't blocked by doors. The long path approach has the advantage that the turbulence will be reduced better than being dumped from a ceiling diffuser in the kitchen. A disadvantage where perfect conditioning is not achieved is that the longer path may feel cold rather than just part of the kitchen feeling cold.

In commercial configurations, the cooking zone is usually hot, and feeding cooler air to the cook is an advantage. It also costs less. However, in restaurants, pressure balanced MUA that is fully warmed would also be supplied to the slightly higher pressure eating area to keep odors in the kitchen.

My MUA is supplied via a hallway in-line with the hood from a uni-directional ceiling diffuser at the far end.

Thought I'd share my experience though have reported in similar threads elsewhere. I faced a similar design dilemma in our new construction design with my HVAC guy however finalized the design. For a 1200CFM hood I'll be installing a 1200CFM MUA with 20KW heater unit in the mechanical room, venting conditioned air via a large air return vent into a rear hallway ceiling at the opposite end of the home living space, some 85' away. Advantage is when MUA is in use (during cold winter days) its output will provide heated/conditioned air through the length of the home without noise or significant turbulence, thats the theory anyway.

Eighty-five feet should just about straighten out any register turbulence, leaving only peripheral hall door frame edge ripples of no consequence. Due to the inertia of the total air path, delay in the air flow pattern organizing itself if the cook is late in remembering to turn on the hood may actually be observable by watching smoke movement.

Would you mind reporting the blower make and model to be used, and that of the heater?

Thanks.