Make-up air run to furnace cold air return?
I got my rough HVAC inspection on my gut remodel. I passed other than the inspector insisting that I install a 6″ duct from outside through a barometric damper to the cold air return of the furnace. This seems strange to me. I know that code requires make up air for a kitchen range hood that exceeds 400 CFM (of which ours doesn’t), but I didn’t imagine it would be run through the furnace. Wouldn’t the furnace draw outdoor air freely whenever it ran, whether the range hood was on or not? The house should be fairly tight, but the 96% AFUE furnace and condensing tankless water heater are both closed combustion.
This is the same person who a month ago preformed my plumbing inspection. At that time he pointed to my stand alone ERV and asked if it was my heating system. I told him that it is an ERV and is designed to take air from the bathrooms and kitchen adjacent laundry area and deliver fresh air to the bedrooms. He said he has heard of ERVs but had never seen one in the wild. He also seemed skeptical that it would serve in leu of bathroom vents. He didn’t bring up the ERV in today’s HVAC inspection.
Does the inspector’s requirement make sense? Won’t the furnace’s outdoor connection mess with the ERV’s balance?
Douglas Higden
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Replies
I suspect that the inspector's idea is that the barometric damper will only function when the house is at negative pressure, opening up and allowing outside makeup air for the kitchen range. Unfortunately, it won't work that way. You are correct that the air handler will draw in outside air every time the furnace operates, because the air in the return duct will always be significantly below room pressure when the system is running. In effect, the air handler will pressurize the house until the point the return air pressure equalizes with outdoor air.
There a number of threads on this site that discuss kitchen makeup air, but there are, so far, no perfect solutions for kitchen makeup air. The most elegant solutions involve using a fan on the MUA system that matches the kitchen range exhaust at all speeds. Elegant, but not cheap and there is the issue of blasting cold outdoor air into the kitchen when cooking. A far lower tech solution is to have an unpowered duct with a damper interlocked with the exhaust hood. This will allow air when needed but does require the house to go negative before much air returns through the MUA duct.
And yes, any of the various MUA approaches that result in change in the interior air pressure have the potential to mess with the ERV balancing, at least when they are operating. For most houses, this is a small part of the day, so probably not a big worry.
I agree the smart move is to let him have his win and not start a war.
Seems to me after the inspections are done a can of Great Stuff foam might explode in that make up air duct and clog it.
Walta