Hello,

I have a 3500sq/ft house that I just had air sealed and insulation added. My current air penetration rates are:
CFM @ 50pa = 3680
ACH50          = 7.5
ACHn            = .37

Now my (Minimum Ventilation Guideline) is 2683 so if I were to air seal the house further, I will need to add a fresh air system to control indoor air quality. I am very confused in the type of fresh air system options. I live near Charlotte NC so we have fairly cold winters and hot humid summers. Which type of system would you suggest that would:
1. Control indoor air quality once the house is sealed beyond the MVG
2. Control humidity
3. Lowest possible energy draw since the whole reason behind this project is to cut energy usage

I dont know if this is possible or remotely cost effective but I was thinking it would be ideal if I had a system built onto my central HVAC that would clean/filter the air. Then have the ability to turn the HVAC fan on every so often to filter the air rather then adding fresh outside air.

Thank you all!

I'm curious - how do you know that the ACHn actually is?

ACH50 came from blower door test after air sealing work was completed. I did a lot of looking online and found that although ACHn is very difficult to determine, if not impossible  (ACH50 / 20) is considered to be an accurate way of calculating ACHn.

I would say that it is a very inaccurate way. The overly simplistic constant ratio is unfortunate, because it makes people not look at things like minimizing air pressure differentials. In your case, this is probably far easier than further reducing ECH50.

Once you seal up the house enough, you have no alternative but to actively supply fresh air, primarily due to the CO2 buildup. You can filter the heck out of the air and all it costs you is a filter and energy penalty. You can even add all sorts of dehumidification equipment, but you can't scrub the CO2 out. That's why there is a fresh air requirement. In order to bring fresh air in, you have to exhaust used air, so it is a good opportunity to also send out things like odors and other chemical nasties.

My guess is that in a home, CO2 is the lesser issue and filters could be used to reduce (not eliminate) fresh air requirements. Even better, control ventilation with a CO2 sensor.

Posted By jonr on 28 Aug 2012 09:50 AM
I would say that it is a very inaccurate way. The overly simplistic constant ratio is unfortunate, because it makes people not look at things like minimizing air pressure differentials. In your case, this is probably far easier than further reducing ECH50.

ok, to get technical;
ACH50 7.5
LBL 19.4 (zone 3, 1.5 story, normal shielded)
7.5 / 19.4 = ACHn .38

.37 was pretty darn close ;-)

But it's still an estimate that may be very different than the real ACHn (which is what matters) - like you would measure with a tracer gas. Let's call it ACHn-est. Note that one study found that ACH levels were 5x higher when the hvac was running. This 500% error gives you an idea of how coarse an estimate it is.

My guess is that in a home, CO2 is the lesser issue and filters could be used to reduce fresh air requirements.

Hmmmm....I'm just not sure as to what degree filterable particulates govern the air exchange guidelines. I think it's generally CO2, humidity and other indoor concerns like odors or chemical contaminants and radon, none of which are well addressed by residential quality filters.

It would be interesting to know if something like carbon filters could pay for themselves through reduced ventilation.

Activated carbon filters don't do well in high volume applications like whole-house ventilation.

Good points all. Install a Renewaire ERV ventilating bath & kitchen, dumping fresh air in the bed @ great room.

Go ahead with the collective eye roll as I mention ventilation is code governed; filters not among the approved solutions.

Ventilation setup is one thing, after you move in, it's up to you how much you want to run the HRV or anything else. Logically, a house with one person at home and few vocs requires a lot less than one with 6 people and lots of new carpet, paint, glue, etc.