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Will an ERV/HRV do anything meaningful in the Phoenix desert heat?

kurtgranroth | Posted in Green Products and Materials on

I’m building an 800 sq ft house in Phoenix AZ and since my goal is to be extremely air tight (certainly better than 3 ACH50), my thoughts move towards mechanical ventilation.

HRVs and ERVs dominate the mechanical ventilation discussion but I find myself struggling to see if they do anything at all meaningful in a Phoenix summer. That is, each refer exclusively about how efficient they are in heating the incoming air. Heck, “heat” is even in the name for HRV.

Well, heat is the enemy in Phoenix and saving it is the last thing I’d want to do for almost all of the year. Instead, I’m looking at the outside air coming in at 115 degrees to mix with inside air at 80 degrees. Yes, heat exchange is heat exchange and so I’d expect both HRVs and ERVs to temper the incoming hot blast of air with the outgoing chilled air. But… how much? Are there any stats or graphs or docs of any sort showing this aspect of the heat exchange. That is, if a product is 90% efficient at 10C, then how efficient might it be at 40C?

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  1. GBA Editor
    Martin Holladay | | #1

    The purpose of an HRV or ERV is to provide fresh air to the interior of the house, while simultaneously exhausting stale air. Its purpose has nothing to do with cooling the house or providing comfort. If that's your goal, use an air conditioner or swamp cooler.

    All ventilation systems have an energy penalty. The energy penalty of an HRV or ERV is lower than that of other types of ventilation (like exhaust-only ventilation or supply-only ventilation), even in your climate, because the unit will transfer some of the heat in the incoming airstream to the exhaust air, thereby lowering the temperature of the incoming air.

    1. this_page_left_blank | | #2

      If you start from the position that fresh air is a requirement rather than an option, then an HRV/ERV saves energy. Ventilation is 100% non-optional, whether it's mechanical or passive. The only case where an HRV/ERV will not save energy is in a house that has no active heating or cooling.

    2. kurtgranroth | | #6

      Yeah, this house will have a mini-split to handle the cooling loads (and minimal winter heating).

      What I'm specifically interested in with regards to HRV/ERVs is the energy recovery efficiency in an extreme cooling capacity. There will definitely be SOME recovery, but how much? The HRV/ERVs that I've looked at typically show how efficient their units are at recovering energy at various temperature points using graphs... but all of said graphs are in extreme heating capacities and don't go near the temperatures I'm interested in. There's no reason for me to assume that the efficiencies are even close to linear, so those graphs are useless to me.

      The reason I care about this is because an HRV/ERV solution will almost certainly cost more than a simple supply-only (or maybe exhaust only) solution. If my house was in Maine, then I could probably get a ROI timeframe easily enough with the existing data. But is there any reasonable ROI timeframe here in Phoenix? How do I know?

  2. BirchwoodBill | | #3

    The Minotair HRV also has cooling capacity, that may be enough for Ventilation and conditioning.

    1. this_page_left_blank | | #4

      The Minotair has a cooling capacity of around 5kBTU/h. I can't imagine that being enough to cool even a tiny house is Arizona.

    2. charlie_sullivan | | #5

      It can make the incoming air a little cooler than and ERV or HRV can. But its energy consumption is much great. I'm pretty sure it's going to use less energy to use an ERV or HRV and provide cooling separately.

    3. kurtgranroth | | #7

      The Minotair is a very interesting looking unit, but it's extreme overkill for my tiny house. Plus, while it is difficult to find any prices for it, the prices I have seen suggest that one such unit will be many times more expensive than ANY ventilation + mini-split solution I end up with.

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