ERV/HRV draw from furnace return network?
I’ve got an ERV that has been operational for a while, with the fresh air plumbed in a dedicated run into the main living area, but the stale pickup is unfinished. The problem is my desired run through a utility chase will be very tight work (the chase is full of ducts, pipes, furnace intake & exhaust, etc) and will have to punch through two floors to boot.
Reading the manufacturer’s docs, their comments suddenly jumped out at me- they said, typically empty stud cavities will be used for RA (return air) to this unit. Suddenly it clicked, what if I plumbed the ERV stale pickup to the furnace return ductwork? This is not a scheme I see described often, but there would be none of the concerns present when plumbing both fresh & stale to the furnace return plenum.
I’ve got a backdraft damper on the ERV outdoor exhaust, so the furnace shouldn’t be able to backdraft the ERV when it is powered down. The only real concerns that cross my mind are:
1) Obviously the furnace return grilles are not located in the same places you’d ideally place ERV return grilles- but, it would be better than how it is today (a naked pipe end in the basement), and I could possibly cut an ideally situated new return grille into the existing furnace return network (with a damper to flow match)
2) The ERV would have to fight the furnace fan for air when both are running. This is what I’m really here to ask about- the ERV is pretty powerful, and the 4 large furnace return grilles have a pretty gentle flow today, so intuitively I’d hope the network can trivially supply an extra 100CFM without totally killing the performance of the ERV. However, while I have an anemometer & manometer, I don’t know the math to work this out to be sure. I know the balance of the ERV would change, but it’s already set to draw more fresh than it exhausts stale, and exhausting a little less stale some of the time would improve the thermal scavenging performance anyway.
Can anyone comment? Is the typical static pressure drop in a furnace plenum too ridiculous to consider this? Are there gotchas I’m not considering? Thank you for any input you can provide.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Measure the pressure in the return plenum where you want to connect with your furnace fan running on high speed. You can then look on the blower curve of your ERV and figure out how much this pressure would unbalance the system.
Most ventilation blowers are designed to have a steep pressure vs flow, for example on my ERV operating at 0.4" water, if I connected to a duct that would add in an extra 0.1" of pressure reduces the flow from 80 to 75 CFM, which is not much.
My feel is that if the pressure drop in your return is less then 0.1", with most ERV/HRV it would not effect the balance significantly. If it is more then that, I would look for other ways of connecting it. You can get a bit of extra help if you add in an elbow into the return duct facing the direction of flow.
It will unbalance it, but how much depends on the pressures involved and the type of fan in the ERV. Connect to the return duct and measure the change in ERV flow with and without the furnace running.