Will bath fan exhaust adequately ventilate a tight house?
We built a pgh in Virginia (Zone 4) and got unexpectedly good results on the blower-door test: 0.5 ACH@50Pa. For ventilation, we have a Panasonic WhisperComfort ERV (40 CFM, balanced) and two Panasonic Whispergreen bath fans (30 – 80 CFM each, exhaust only). The house is 3160 ft2, 42,500 ft3.
We are de-bugging the ventilation system and I am seeing CO2 levels consistently above 2000, with only two occupants at present. (I also had septic odors drawn in from unused laundry traps, but that problem is solvable.)
I suspect that the house is too tight for my exhaust-based ventilation strategy to work. This is a problem of luxury! But it is a problem.
I would really like to achieve CO2 levels lower than 2000 ppm! I have a spare (sealed) 6″ make-up air duct that I could open, but that seems pretty crude. Or I could install some retrofitted Lunos fans, or perhaps I could retrofit a second ERV. But those ideas are kind of expensive.
Has anyone had any experience with a house that is too tight for their ventilation strategy?
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You should have 15-20 CFM/person of outside air delivered directly to the bedroom (or any other closed off occupied room) Twice that if you are using only exhaust directly from the bedroom (pulling in less fresh house air to replace it).
Non-direct ventilation is unpredictable - you don't know how air will circulate. For example, imagine an exhaust fan in a utility room - and the air leaks are in the utility room. The rest of the house receives little ventilation!
https://www.krueger-hvac.com/files/white%20papers/article_demand_control_ventilation.pdf
Where are your fans and where are you measuring? You could have a ventilation rate or a distribution problem.
A passive vent in the bedroom would be one option if that is where the issue is.
Were the fans commissioned with the flow rate measured? The Panasonic fans are pretty good with ecm motors, but sometimes have very convoluted duct work that overwhelm them.
Your house leaks 354cfm@50PA.
The panasonic fans actually do a decent job with pressure, so if your ducts are less than about 40' equivalent 4" then you still get full rated flow out of them. Combined with the ERV, that should be enough to ventilate the house.
The problem is that because you don't know where your air leaks are and how well they are distributed around the house, there might be little or no air exchange in certain rooms.
Opening the 6" duct won't change this problem.
You need positive air exchange in your bedrooms, either run a ducted ERV to them or spot ventilation.
I have personal experience with the inadequacy of a bathroom exhaust fan to achieve proper ventilation in a "tight" house. Trust the CO2 sensor. I tried to use a Panasonic bathroom fan with ventilation mode in a 2.25 ACH 5o, 1000 sf home and the CO2 levels went through the roof. Not as high as yours though. I would be worried if I had yours levels of CO2. I then installed a Panasonic Intellibalance ERV and I've been good to go ever since. For a 3000 sf house like yours a whisper comfort is wholly inadequate whatever the numbers tell you. Again, trust your CO2 sensor and how such a high CO2 level makes you feel. Probably sleepy and miserable. Even a single intellibalance ERV is too small for your house.
I hate to say it but there seem to be a usual cast of people who just like to comment here and do it on a wide range of topics. My feeling is that the wider the range of topics people contribute on and the greater the frequency of that contribution the less trust can be placed in them. YMMV. I always try to speak only on topics I have direct experience with. God knows there are vast areas in green building that I would never venture a written opinion on. That doesn't seem to be the case with many people.
Our home is not overly leaky, but neither is it vault tight, and I have to keep an eye on CO2 levels. Last fall, with outdoor temperatures on the decline, we had closed all windows and doors but had not at that point turned on our heat recovery unit. There's just the two of us, no pets, no combustion appliances, no scented candles, etc., and we're often both out of the house for much of the day, and yet CO2 levels quickly rose above 2,000 PPM (this in a fifty-one year old, 270 sq. metre/2,900 sq. ft. Cape Cod).
Once I discovered the problem, it took roughly twenty-four hours before the HRV was able to bring levels down to below 1,000 PPM.