Makeup Air for the Range Hood
A range hood won't work well in a tight house without make-up air.
My own house is pretty tight — about 1.5 ACH50 — and when I run the range exhaust fan, I can hear air whistle in through the door gasket. The FHB House is even tighter — 0.39 ACH50 — so I knew going in that we had to make provisions for make-up air when the range exhaust is operating. Make-up air needs to be introduced into the kitchen but not too close to the range hood itself or the air will be drawn directly into the hood and the air rising off steaming pots will spill into the room instead of being sucked into the hood. But on the other hand, I am reluctant to dump make-up air into the room where it may feel uncomfortable.
I opted to run a 6-in. duct through the first floor system, alongside the exhaust duct, and terminate it in the kitchen cabinet toekick space just to the left of the range. There’s a space between the toekick and the bottom shelf of the cabinet, so air can freely flow out from the toekick area.
A 6-in. intake duct is a big hole in a tight house and one that I want to control to prevent heat loss.
So we installed an automatic mechanical make-up air damper (Broan). The 6-in. damper fits inline on the intake duct and has a small low-voltage motor that opens a gasketed damper valve. The motor opens the valve when air flow is sensed by a venturi switch mounted to the side of the exhaust duct.
The microwave / exhaust is ducted through a wall shared with a closet, so I installed a small snap-in access panel where the venturi switch in mounted. That way if there is ever a problem with the switch it can be easily accessed.
To get a sense of whether there was any pressure differential inside to outside the house when the system was operating I used a manometer to measure the baseline pressure and compare it to the pressure differential when the exhaust was operating. The baseline pressure differential was about -0.4 Pa. When the exhaust fan was turned on and before the mechanical damper opened the pressure dropped to -28 Pa. This shows that the exhaust fan was depressurizing the house quite a bit for a few seconds while the damper opened. Once the damper was fully open the pressure difference settled at -7.6 Pa. So the exhaust fan put the house under a slight negative pressure.
And how does the exhaust fan perform? To test it I put a flow hood over the exhaust vent to see how many CFM were exhausted. For a baseline I opened a door so the exhaust fan had all the makeup air it needed. The hood measured a flow of 101 CFM. Next I measured the flow with all the doors and windows closed and got a 87 CFM reading. This shows that there is some restricted makeup air flow that reduces fan performance. Still, even at 87 CFM, the range exhaust fan is evacuating plenty of air to keep most of the cooking fumes and moisture vapor from lingering in the house.