Make-up air?

[zuren]

I am researching what I feel is the issue and was hoping to get some guidance from the community here...

Over the past couple years, I've been making strides with making our 1979 ranch home (with walkout basement) more comfortable and efficient. Weatherstripping around doors was poor and I have been slowly installing rigid foam (with expanding foam seal around the perimeter) along the rim joist of the foundation (there are areas where the basement ceiling is finished, so I won't be sealing those areas; the previous owner just had fiberglass insulation along the rim joist). The windows are newer replacement-style that seem to be pretty tight. Unfortunately, the house may be becoming too tight...

Several things happened this year:

- Installed new bathroom exhaust fans (old ones were original with failing bearings)
- Installed new vent hood over the kitchen stove (old one was original with failing bearings)
- Installed new power-vent gas hot water heater (original was electric and failed)

My issue is that I'm noticing cold air back-drafting down the vent hood over our kitchen stove. The cold draft on the main floor of the house is noticeable and if our neighbors are running their wood stoves, we end up smelling it. I do not have data to back me up, but I feel like our furnace is running more because of this situation (thermostat is in the dining room, which is just off of the kitchen). The hood is new as of this year; we upgraded the stove to gas and needed a better vent hood since the old unit was dated and failing. I feared the damper was stuck open but before I launched into dropping the hood, I noticed that the hot water heater was running at the same time we were noticing the draft. I had my wife feel the cold draft as I went downstairs and opened the walkout door; the draft immediately stopped. Now every time I notice the draft, it is usually in association with one of these devices running:

- bathroom exhaust fans (usually the fan in the bathroom closest to the kitchen)
- clothes dryer
- gas furnace
- gas, power-vent hot water heater

I have a hard time believing that an older house like mine is so tight that I will require a make-up air solution, but based on that quick test, it seems that I may be tight enough. It is most noticeable when the water heater is recovering, and even worse if we are drying clothes or someone is using the restroom at the same time.

So does my diagnosis seem plausible? I've researched some solutions and install options but I will share those thoughts if everyone agrees that I need to help the house breath.

Thanks!

 

[Dana]

Yes, it's possible to make an older house tight enough that it'll cause backdrafting when moderate to high cfm powered vents are in use. (My 1920s vintage bungalow has reached that point.)

If you have any open flues depressurizing the house that could cause backdrafting during cold weather too. Finding and fixing those would be worthwhile too. Have you ever done a blower door test in conjunction with infra-red imaging to sort out the leak points?

It's time to declare victory, but don't stop air sealing. Random air leaks aren't always along the cleanest paths, or putting the ventilation air in the most appropriate locations & proportions. Instead, it's time to install appropriate ventilation.

Installing a heat recovery ventilation unit would increase the makeup air volumes and the air would be drawn in via a known clean path, and provide ventilation air in the right proportions where it's needed most. With an open basement it may be possible to do that reasonably with a ducted HRV, but there are ductless ceramic core oscillating types that change direction every 50-100 seconds, some even operating in pairs (eg Lunos e2). HRV ducts are tiny compared to heating ducts, and it's generally better to run a separate duct system for the ventilation. Heat and ventilation aren't needed in the same proportions everywhere, and ventilation is needed even when there is no heating or cooling load.

ASHRAE 62.2 (used by building codes in some areas) spells out 7.5 cfm per person (defined as the number of bedrooms +1), and 0.03 cfm per square foot of conditioned space. eg: For a 2000 square foot 2- bedroom you'd have 7.5 cfm x 3 (=22.5 cfm) plus 2000 x 0.03 (=60cfm) for a total ventilation requirement of 82.5 cfm. Building Science Corporation rails against this as excessive overkill, and an energy waste, and rcommends reducing the area multiplier to 0.01cfm per square foot, along with the same 7.5 cfm per person. That would make it 42.5 cfm.

An HRV may or may not provide sufficient make up air path for all of your big cfm exhausts, but it's a start, and you know where that air is coming from, not from under the slab, not down a dirty flue. When it's time to replace furnaces or water heaters, direct vented sealed combustion units should be considered to guarantee you're not dragging in backdrafted combustion exhaust.

Furnaces as a rule tend to be grossly oversized for the actual loads, and right-sizing it helps reduce those parasitic effects. Heating duct systems that aren't well balanced can pressurize some rooms, depressurize others whenever the air handler is running, contributing to backdrafting issues. An Energy Star ducted HVAC system would have no more than 3 pascals (0.012" water column) room-to-room pressure differences at all air handler speeds, at all operating conditions, doors open/closed. This is tough to measure with inexpensive tools, but any doored off rooms with a supply register needs a dedicated return path. If there isn't a return register, a jump duct of one sort or another needs to be implemented- door cuts alone aren't usually big enough to really cut it at heating/cooling cfm.