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HRVs tied into Forced Air Systems

user-788447 | Posted in Mechanicals on

My understanding is that HRVs should have dedicated ductwork sized specifically for lower ventilation flows. The system should also be balanced and commissioned.

However I’ve been seeing HRVs tied into Force Air Systems. What are the consequences of doing this? and how to advise the homeowner if they have this situation? Is this setup just wasting electricity or might it be contributing some fresh air while taking advantage of the some heat exchange? Can these systems work together if designed correctly or are the different fan mechanisms fighting each other? Should the homeowner disconnect the HRV from the Force Air System if it has already been installed?

I figure that when the Forced Air System is off the HRV system doesn’t distribute well because of the oversized ductwork, but is still of some benefit to dump some pre-conditioned fresh air into the thermal envelope?

When the Forced Air System is running, whether cooling or heating, can an appropriately designed connection to an HRV work like a CFIS setup or is there something about the balanced nature of the HRV that changes the physics of this situation?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    J Chesnut,
    When an HRV is connected to forced air ductwork, of course there are benefits. The mechanical ventilation system introduces fresh air to the house, and the heat exchanger recovers some heat from the exhaust air stream. Most HRV manufacturers can provide recommendations on the best way to hook up an HRV to forced air ductwork if you want to do this.

    However, there are many variables in these systems. If the system is set up to energize the furnace fan whenever there is a call for ventilation, these systems can use a lot of electricity. If the furnace fan isn't energized, ventilation air distribution during the swing seasons may not be effective, especially if the ductwork hasn't been very well sealed.

    I think that the extra cost of dedicated ventilation ductwork is money well spent.

  2. user-723121 | | #2

    J.

    I see nothing wrong with dedicated HRV ductwork for exhaust ( from bathrooms, laundry, kitchen area) and then routing the HRV fresh air through the forced air system supply. Pushbutton timers in the bathrooms can be set for different run time intervals and this works nicely as a individual bath fan alternative. This is how I do it in a new build.

  3. user-1023211 | | #3

    Doug,

    Interesting idea, do you need to run the furnace/ac motor for this type of installation? Or is the internal fan on the HRV enough to power the air through the ducts?

    I'm also studying designs for an HRV in an existing home which already has ducting, I'd rather use just the ducts that are in place, but if necessary I could run a couple strategic exhaust ducts like you are suggesting to the bath, laundry, and kitchen.

    Thanks

  4. user-723121 | | #4

    John,

    No, the HRV will distribute passively through the furnace supply. We have added a number of HRV's to existing homes and typically would exhaust from the furnace return and supply through the furnace supply. In new construction it is nice to use the HRV in place of individual bath fans, if you can do this on your existing house, great.

    With a dedicated HRV exhaust and using the furnace for HRV supply you can duct into the furnace return before the furnace filter. I have heard some concern about cold HRV air going through the furnace and potentially causing heat exchanger problems. I can't say if this is valid, you could always tie in the HRV supply above the furnace.

  5. user-788447 | | #5

    Thanks Martin and Doug for the feedback.

    I need to learn more about the controls of these ventilation systems.
    I seem to have developed an overly skeptical opinion that the combination of HRV and forced air can't work well.

    Martin in a response to a reply to your "Designing a Good Ventilation System" blog entry you responded -

    Finally, you are 100% correct that any ventilation system -- including a system with an ERV or an HRV -- can be (and often is) poorly installed. (The worst systems I've seen are ERVs or HRVs hooked up to space-heating ductwork.) That's one reason I wrote the article -- to guide GBA readers and help them avoid problems.

    At this point it seems a CFIS system may utilize the forced air system better for distribution and might be easier to avoid over utilizing the furnace fan. But what HRV may lack in utilizing the force air system for distribution may be made up by the capacity to pre condition incoming outdoor air.
    Again I'll need to study the control systems more.

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