Air Exchanges in a new house
We are building our retirement home in zone 3 at the coast in North Carolina. I’ve come to know more about some of the design considerations for HVAC thanks to this site and I’m looking for some reassurance given that all our savings are going into this project.
The house is 2,453 sq ft with a 433 sq ft bonus room above the garage. We have 9ft ceilings and 2×6 walls and the cavities have been well sealed. We have low E windows and doors and we’re planning dense-pack cellulose in the walls and R38 in the ceiling. The roof is vented. Top and bottom plates have been sealed. We have a tight envelope with OSB and house wrap on the outside. We’re using LP TechShield on the roof deck.
The Lennox HVAC system that has not yet been installed delivers 0.11 air exchanges and an average of 800 CFM (May peak higher). It is a 3-ton system with three zones and has an MER16 filter, UV light and a catalyst (Lennox Pure Air-PC014-23). We’re also using an ERV that delivers 200 CFM and is hooked into the master bathroom and outside air.
I am concerned about the low number of air exchanges and total sustained CFMs. With windows that an be opened but will likely stay shut like our current house do we need to be looking at more like 4 air exchanges an hour and more like 1800 CFMs? What do the experts think? Should I be concerned about humidity control and stratification at low air exchange rates with this system?
Why does the code call for 4.0 AEH in commercial buildings with sealed windows but not in tight houses where windows are not opened and the air is controlled?
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Combining ventilation into the heating/cooling system of a higher performance/lower load house is a bad idea, since in a tight house ventilation is necessary even when there are no heating or cooling loads. It's much better to use a heating/cooling system that doesn't introduce ventilation air, and let the ERV manage the ventilation separately.
4ACH is a much higher rate than is necessary for managing humidity, and would create a higher latent cooling load during the cooling season. Indeed, four air exchanges PER DAY would handle the humidity issues in most houses, but probably not the air pollutants of a household where everybody smokes stogies and uses spray cleaners, hair spray and other aerosols obsessively. Commercial buildings typically have potentially higher number of occupants per volume of space, and many potential types of indoor air pollution sources. If you use kitchen & bath exhaust venting consistently (put the bath fan on a vacancy sensor switch with a 15-30 minute time-out) the moisture loads are pretty low, unless your hobby is keeping rain-forest plant collections along with the tropical fish tanks.
Thank you for your response. I had a feeling that 4 ACH was probably overkill. We're looking at 1 ACH right now.We have a two stage system going in with larger vents giving us a heat throw of 11ft There will be continuous flow at low speed. The HVAC company says that it would be better to install an ERV connection to the AH so that the fresh air gets everywhere in the house. I'm hoping he's right.
I like your idea about putting a vacancy sensor switch in as well and plan to install one for each of the three bathrooms. No rain-forest plants or fish tanks here.
Thank you.
John,
The long-standing ASHRAE ventilation recommendation for mechanical ventilation was 0.35 ac/hour.
So it appears to me that your plan will result in overventilation, for which you will incur a substantial energy penalty. If you are installing a central-fan-integrated supply ventilation system -- and it sounds like you are -- I hope that the fresh air duct is equipped with a motorized damper, and that the operation of the damper and the air handler fan are controlled by a FanCycler or AirCycler. For more information, see Designing a Good Ventilation System.
Sorry, I meant to say that the HVAC air flow is around 1 ACH. The ERV is a 4" pipe into the AH I believe and delivers 150-200 CFM. Perhaps I still have a problem. The system is indeed a central-fan-integrated supply ventilation system and there is a motorized damper I believe. I'll check the article you reference - thank you.
More questions for my HVAC guy
Reading the "Good Ventilation" article it seems to be referring to a situation where there would not be an ERV but an outside air pipe that brings in fresh air (and moisture) and has a motorized damper and a controller (Air Cycler) for the damper and the AH fan.
My limited knowledge from reading about the ERV is that it has it's own small motor and mixes air from the bathroom and incoming air and exchanges air with the air handler and exhausts outside. Is it electronically coupled to the AH?
I saw that the most cost effective method would be to have a separate duct system for the ventilation but that sounds like an expensive option.
Please forgive my lack of expertise.
John,
This is a big topic, but you might want to read this article to get more information: Ducting HRVs and ERVs.
This helps a lot. I like the illustrations. The HVAC engineer is installing the hybrid approach with an interlock I believe. The ERV will install above the master bathroom and draw stale air from that area. Given the layout of the house (laundry and bath in the same area) it would seem easier to go with a fully ducted system.
The compressor and heat pump are two stage so the AH can run at low speed. I assume this helps the hybrid approach. I have asked the HVAC guy about this and how the ERV air flow is regulated.
Thank you very much for this information.
Update: The ERV will run when the HVAC fan runs. It is a hybrid setup for the ERV. We're using a Lennox XP25 (21 SEER) variable speed system with three zones. I can adjust the system CFM on the "iComfort" control.