HRV duct questions
I’m about to pull the trigger on an HRV for my home and have a few general questions about duct going to and from the HRV unit. The unit I am considering is the Broan HRV160TE and they state that 6″ insulated flex duct should be used for the incoming fresh air and the exhausting stale air. Everything I have read here says to use hard duct wherever possible due to less resistance. I’ll have a 19′ straight run of fresh air duct going to the unit, and a 6′ mostly straight run of exhaust duct. Could rigid metal duct be used with a decent amount of insulation covering it or would condensation still be a problem? Is there any other material for these runs that could be used in place of the flex duct? I have read on a few posts about using pvc or polypropylene pipes instead – yea or nay? Or am I over thinking this and should just do what is stated in the install manual? Zone 7, Anchorage Alaska. 1972 split entry home with 4″ rigid insulation installed (remote wall) a few years ago, will be tearing out sheetrock soon.
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Oh boy... the 2 ducts going to the outside has to be insulated and air tight (for the insulation too).
Do you like noise? If not, go with flexible duct for the fresh air portion... Are you exhausting from bathroom? if so, go with flexible duct unless you like condensation in there...
For 160cfm, you really should be looking at 8 inch duct. 6 inch would be noisier by at least 10db. Maybe even 20db, you can find the equation online and do the calculation. You want to keep FPM below 500.
If you are going to run it 24/7 or at night, noise should be your primary concern (condensation first of course).
1500 sq. ft home, 3 bedroom, 2 occupants - would run it at 55 or 65 cfm 24/7. I have 2 Panasononic whisperlite fans already installed in both bathrooms and can't see taking them out, which I would use for humidity/smell evacuation. Cant see running HRV at high speed for any reason, really.
Jon,
It's a good idea to follow the manufacturer's installation instructions. Here's the advice given by David Hansen, an experienced HRV installer who works in Vermont:
"We avoid the use of flex duct as much as possible, because its interior corrugations impede airflow. However, because insulated flex duct prevents problems with condensation drips, we use it to connect the HRV unit to the outside vent hoods. The flex duct needs to be sealed to both the HRV unit and the house vapor retarder. We also use short lengths of noninsulated vinyl flex duct to connect the HRV unit to the house ducts.
"We keep our flex duct runs as short as possible, and we always seal any rips or tears in the outside cover of the flex duct. (If moist interior air comes in contact with the cold fresh air in the intake duct, condensation will saturate the duct insulation.) Where flex duct connects with the HRV unit, we seal the connection with silicone caulk and screw the duct to the collar on the HRV."
Except for the limited use of flex duct as described in the above paragraphs, it's best to use rigid duct. The usual advice: use galvanized ducts for the supply (fresh air) ducts; PVC is OK for the exhaust (stale air) ducts.
Oh please, unless you are running hundreds of feets, flex duct's slightly higher resistance makes 0 difference. Check it out yourself https://www.cdicurbs.com/ductcal
Compared to the resistance caused by the filter (up to 1 inch!!!), or the vent cover (some fancy one causes up to .2 inch!!!)
Also, you can buy flex duct with resistance similar to metal ducts, they just are more expensive. Really the only disadvantage to flex duct is that you may not have enough space to fit them.
I think Allison does a good job here of explaining why most of us who care about quality construction prefer rigid ductwork to flex duct: https://www.greenbuildingadvisor.com/blogs/dept/building-science/should-flex-duct-be-banned.
I have rarely seen a well-done flex-duct job. Just this week I've been in two basements with flex duct runs that looked like roller coaster runs, with some kinks for good measure. That said, they do reduce noise transfer, but that can be done with just a short run--or even better, a sound attenuator (similar to a car muffler). The fact that they come pre-insulated is nice, instead of having to insulate them yourself--or leaving them uninsulated. But good HVAC guys know how to insulate rigid ducts when it's in the specs.
Zehnder has nice, pre-insulated ductwork for their intake and extract runs to the outdoors.
Edit to add: To Jon, the OP: 19 ft is a long run for intake. You might find it just plain easier to install rigid metal duct with R-5 duct wrap than to try to support a floppy run of flex duct. If you have a flat surface to run the duct through--like the floor of an attic--then the flex duct would probably be fine. But you'll get slightly better performance out of the rigid duct.
I like the advice that David Hansen gives in Martins post. I did it the same way. Carefully seal around the hood and where it exits the house with some can foam. Make sure that the part closest to the outside is insulated well.