Exhaust-ducted HRV vs. simplified
Hello! I’m looking to put in an ERV, after I realized I could probably DIY for not much more than CFIS. I am stuck planning the ducts and wondered if you could lend me some insight.
I have an open floorplan 2500sqft place. 2 stories plus basement. Fairly tight envelope, and we continue to seal here and there. I expect to be around 700cfm50 soon.
I think running supply ducts is basically out for me, but I think that’s OK because my return air grills are in all the right places already, so as long as I can seal up my HVAC ductwork, I hope that will work well.
For the exhaust ducts I’m stumped. The moisture-producing baths are on the second floor. I think the only reasonable way to get ductwork up there would be to run it up the furnace flue chase. Fortunately it runs between them, but it is completely walled off and well-sealed. (Also not sure if it’s big enough to fit a person inside to do the work)
If not that, I could run a simplified system, but I read all kinds of negative things about how well they function, and my furnace is a single speed PSC, 375W.
Or, perhaps I could run exhaust ducts to the first floor half-bath, laundry room, and kitchen/living-room. This would allow for someday possibly adding those upstairs exhaust ducts and would not require operation of the furnace fan- but without the furnace fan, stale air from upstairs is a long way from the HRV’s exhaust vents. (But, I do have bath fans)
Last thought, I read that up high on the wall is the best mounting point for exhaust vents. Given I’m accessing from the basement, floor vents for the main floor would be a million times easier- are they a reasonable choice, even if not ideal, or a terrible mistake?
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Replies
Pshyvers,
There are lots of issues here.
First of all, if you install an ERV, you need to install fresh air ducts (supply ducts) as well as exhaust air ducts. I'm not sure what you mean when you write that "running supply ducts is basically out for me." Perhaps you mean something like, "I'm just going to install 2 feet of supply ductwork that terminates in the basement." Or maybe you mean something else -- I'm not sure. But, like it or not, your ERV has supply ducts (fresh air ducts).
Second: only you can figure out how to run your ductwork. Opening up a chase to see what's inside isn't that difficult. Get a drywall saw or a sharp utility knife and open up the chase. Drywall is easy to patch. If there's room, there's room. If there isn't, patch the inspection hole and come up with another plan.
The usual way to retrofit ducts in a completed house is to choose a location -- in the corner of a room or a closet -- and install the ducts. Then box the ducts in with framing and drywall.
When you talk about a CFIS system, I'm guessing that you are referring to a central-fan-intergrated supply ventilation system. You can certainly design that type of ventilation system if you want. It's a good option, as long as the installation includes all of the essential parts, including a motorized damper on the fresh air duct and an Air Cycler control (also called a Fan Cycler control), and as long as the system is properly commissioned to make sure that the proper air flow is being delivered, and as long as your furnace or air handler has an ECM blower to keep your electricity bill in a reasonable range.
For more information on central-fan-integrated supply ventilation systems, see Designing a Good Ventilation System.
Yes, I wasn't completely clear about the supply ducts; my plan there is to connect the ERV's supply to the furnace return. I have seen it repeated many times when the furnace fan is off, the fresh air will exit the return grills, which is fine because the return grills are well-positioned for supplying fresh air to bedrooms and living spaces.
The CFIS system I have read a lot about, and it does seem like a nice KISS option! But lacking an ECM blower, among other issues, I decided I'd go for the ERV.
Thanks for the words of encouragement!