Best practices for HRV on a budget
I am building a simple but efficient home in climate zone C4. It will be a 3 bedroom, 2000 sq ft home. with us living mostly in the upper half. I was originally planning on putting one or two panasonic spot ERVs in but am know thinking it might be better for overall air quality, efficiency, and spreading the heat from my two headed mini-split, to put a fully ducted HRV unit in. I have been looking at the Fantec FLEX100H, and the Lifebreath RNC 95. Money is a huge issue but the price difference between two panasonics and one of these units is not much. But the Ductwork will either be a lot of money to pay someone to install or it will be a lot of labor on my part. So I have a few questions.
1) When running duct work in interior walls, do the ducts need to be insulated?
2) If I run duct work low in my attic space, well imbedded into the blown in insulation, why would I need to insulate? Wouldn’t duct insulation just displace space from other insulation?
3) If the duct doesn’t need to be insulated then could I run flexible aluminum duct (dryer vent style). not really crazy about the plastic insulted flex duct.
4) why do some HRV units have oval duct ports? Is this for oval duct in walls.
5) If a HRV has 5″ supply and exhaust ports, do the trunk lines need to be 5″ but the branch lines can me smaller? How much smaller?
6) A friend put in a panasonic ERV and after talking to the rep, he upsized the vent ducts to 6″ and used PVC instead of traditional duct. The rep said he would get more cfm. I could much easier run PVC, but there has to be a down side, right?
Thank you all for any advice you might have on my situation.
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Replies
I'm not going to try to answer all 6, but here are a few tidbits that might help:
1) The supply and exhaust ducts between the unit and the interior of the house do not need to be insulated when they are run in interior walls. The supply and exhaust ducts between the unit and the exterior of the house do need to be insulated whenever they are inside the envelope of the house.
2) There's not necessarily an advantage to insulating a duct before burying it in insulation. But in a warm, humid climate, if the duct is cool and the air in the attic is warm and humid, you might get condensation on the outside of the duct, and get the insulation wet. Insulated ducts generally have an airtight jacket around the insulation to help reduce that problem. That's never seemed like a great solution to me anyway. I don't know your climate very well, but I doubt you would have that problem anyway.
But you certainly want to make sure that the ducts are well sealed if they run through the attic.
6) Are you talking about PVC pipe? Smooth pipe has less friction for air flow than poorly installed flex duct, and probably even a little lower friction than well installed flex duct. And it's easy to seal to be air tight. I haven't checked, but I'm pretty sure it's more expensive. Also, some have raised concerns that the PVC might outgas and contaminate what would otherwise be pristine fresh air (if you are lucky enough to live in a location with pristine outdoor air).
Thanks for your help Charlie. I was curious of off gasing from the pipe. There has to be a reason this is not done more. When installing supply and exhaust lines inside of the building envelope that don't need to be insulated what are most people installing? Hard duct? aluminum flex duct?
Thanks again.
Kail,
Here is a link to an article that provides a lot of advice concerning ways to duct an HRV: Installing a Heat-Recovery Ventilator.
Green builders sometimes use PVC for exhaust ducts, but never for supply ducts. No one wants that "new car smell" to permeate their ventilation system.