ERV/HRV – Ratio of supply and exhaust ducts
I recently moved into a home that is 25 years old. All of my previous homes were newish and had HRVs. Proper ventilation and indoor air quality are important to me, so I am going over retrofitting options. Based on my research, I’m learning towards an ERV for its ability to run all year round (I live in zone 7 with cold dry winters and hot humid summers).
Anyway, my question pertains to ducting. The easier solution would be to tie into my AHU, but I’ve read (mostly on GBA) that this is not an efficient method. I’m thinking of installing dedicated ducting to the ERV. Based on accessibility, I could add exhausts from all 3 bedrooms, but I can only access one common area to supply the fresh air. My question is: does my indoor exhaust and supply ducting need to be made equal? Do they need to have a 1:1 ratio, or can I have 3 exhausts and 1 supply? I could potentially increase the diameter of the supply.
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This is from an Venmar installation manual....it's all you need :-)
3 exhausts and a larger supply is just fine, but you'll want to balance your 3 exhaust ducts (either at the grille, or by installing duct dampers) so that airflow there matches your occupant loads.
I found this document on designing balanced ventilation from PHIUS very helpful, even if it’s not a passive house.
That is pretty much the setup I run at home. A length of flex at the end of the bedroom pickups is a good idea to limit sound. I've also done a DIY intake filter using a standard register and section of washable filter to keep most of the dust out of the ducting.
If you are in cold climate, keep in mind the fresh air feed will be bellow room temperature. The best would be to put the fresh air feed right into the return of your furnace this way if the furnace is running, it can be used to tamper the supply air. If you have an accurate pressure gauge (which might be need for balancing anyways), you do have to check that the return is not restrictive and causes the ERV to unbalance when the furnace runs, usually a connection near the return register is a pretty safe bet.
Interlocking the furnace fan to the ERV is optional. A better setup is if you have a smarter thermostat that can drive both ERV and furnace fan, you want it setup to run the ERV 24/7 at low speed and the furnace fan only as needed to mix the house air.
Akos makes a good point on supply temps. Even at lower CFM settings and a decently efficient ERV/HRV, you will likely need to install an inline heater to condition incoming air. You’ll see air in the high 50 F range on very cold days. If you dump fresh air near your house thermostat(s) it will cause a definite temp imbalance in your home. In zone 7A and about 80% efficiency, even at 60 CFM my setup needs to add 150-250 watts of heat on -10 to 0 F days…and more at higher CFM profiles on the system. If you dump into the furnace return, those BTU requirements will be met by that system, at the expense of power use on the furnace fan. I would not do this unless my furnace fan was an ECM model.