Improved air sealing brings other challenges
In Kansas 9 Jurisdictions allow an Energy Rating Index analysis as an alternative compliance path for meeting the 2012 IRC chapter 11. Other builders use the HERS index for marketing purposes. Use of air sealing recommendations and increased quality of installation have seen air change rates move down from 5-6 ACH pre-implementation to 3 ACH or less in a growing number of audited homes. Early antidotal results are showing problems with supply only strategies. The 6″ pipe to the return is causing summer humidity problems in some cases. Exhaust strategies pose the risk of back drafting gas appliances. Cost to remediate these issues is becoming a great concern to the builder community. I’m also seeing a greater number of high cfm kitchen hoods over gas stoves. While code is calling for fans over 400 cfm to require make up air, with ACH under 3 I can see a hood fan under 400 causing a larger negative pressure in the CAZ zone. What is the best solution (other than an ERV) to battle these potential IAQ issues.
Is anyone further along on the learning curve? I could use some suggestions.
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Bruce,
The first article that I recommend that you read is this one: Designing a Good Ventilation System.
The problems you have described are unfortunately common, and home performance contractors have been battling them for years. Builders who are just beginning to think about improved airtightness should probably hang out on the GBA website for a while and study up.
1. "The 6-inch pipe to the return is causing summer humidity problems in some cases." My first guess is that the builder with this problem forgot to install a motorized damper on the air intake and forgot to install an AirCycler control. The result is predictable: overventilation, which carries a huge energy penalty and risks overloading the AC system's ability to dehumidify the indoor air.
2. "Exhaust strategies pose the risk of backdrafting gas appliances. ... I can see a hood fan under 400 cfm causing a larger negative pressure in the CAZ zone." For more information on addressing this issue, see Makeup Air for Range Hoods. In most cases, builders who care about airtightness no longer install atmospherically vented appliances; instead, they specify sealed-combustion appliances.
Where about are you in KS? I'm in the SE we have adopted no energy code, looking at 2012.
Not a good idea to mix ventilation with a forced air system, if I understand you right. The cfm requirements for heating & cooling are an order of magnitude higher than that for HRVs, making heating/cooling ducts ridiculously and sub-optimally oversized for ventilation-only. When using heaing/cooling ducts for HRV it generally requires running the heating/cooling system air-handler at least on a duty cycled basis to guarantee sufficient mixing & ventilation air in each room, which doubles or triples the energy use of the ventilation system. the recovery unit blower has a lower CFM (approx. 100 PSF) compared to FA (what 1-2xs more) so one could expect back pressure at the recovery unit. If you are feeding the FA blower (option 1) then the FA blower fan has to be on to pass recovery air, the FA blower is also a resistor to flow when it is not on. If you bypass the FA blower send(option 2) it does not have to be on, but, when it cycles from zone demand there would be more pressure at the FA blower than recovery unit in the mix, hence back-pressure and in the shared return, lower COP. The only ducts in a energy efficient homes these days should be HRV/ERV.