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Community and Q&A

Passive Preheating of Air for an HRV or ERV

rockies63 | Posted in General Questions on

I was watching a video about rooftop solar and how the homeowner/builder had designed the roof so that air passing underneath the solar array would be channeled into the home at the ridge to help heat it in the winter (the air was 20 degrees warmer  after passing under the array).
I was wondering if the same concept could be used to heat incoming air for a cold climate HRV or Erv? I know that passive air heaters mounted on the exterior of a house have been around for a while (the air from inside the house enters through a slot at the bottom of the unit, rises between a black metal back-plate and the glass surface and re-enters the house at a higher temperature through a slot at the top) but could such a design be built below the fresh air intake vent for an HRV/ERV to preheat the incoming air? 
Basically, it would be a box mounted on a south facing exterior wall with a black metal back plate, a glass front and have an open vent at the bottom to bring in fresh outside air (not indoor) and then the air would be funneled through a slot at the top into the ERV/HRV’s fresh air intake vent.
Think it might work, or is there a device already on the market that passively does this?

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  1. DennisWood | | #1

    You won't find a product like this (aside from the large commercial installations of Solar Wall), for most home owners as they don't make sense. You'd be far better off investing in a more efficient air exchanger which has benefits 24/7 vs the limited heating window you'd get with a solar air air pre heater. In a cold climate, you may only be preheating effectively for 4-6 hours (and only on clear days), and then you'd need an alternative strategy for the rest of the time. I'd rather spend my money on a unit like the Tempeff or Zehnder which are in the 80-90% efficiency no (or very little) post heat required.

  2. Tim_O | | #2

    Sometimes the numbers help guide decisions. Let's say your ERV runs at 100CFM and has 80% efficiency. This equates to 20CFM of raw outside temperature air coming into your home. If you are somewhere in a cold climate, maybe your design temp is 0*F. This comes to about 1400BTU/Hr heat loss, or 410watts. You may find it more cost effective to just add another panel to your array to get that power.

    There are solar air heating panels commercially available and lots of DIY ones. Maybe something like this would be viable. However, you would have to disconnect in the warm seasons or have a diverter in line in some way or another.

    If going through the effort of DIY'ing something creative, my first choice would be something like the GeoCooling/heating devices that Zehnder and CERV sell. Using a short ground loop, single speed pump, and radiator to provide preheating AND precooling.

  3. Robert Opaluch | | #3

    Your idea certainly could help, but probably not make a large difference in your ERV's performance. Air has little "thermal mass" or heat capacity. So unless the air in your device gets fairly warm or hot vs. cold outdoor air temp, it won't make a large difference in your system's performance. I'm assuming you would build this experimental device yourself at little cost. So if you are interested, no harm in trying it out, and it would help to some extent. Please post the results here showing the increase in incoming air temp and the average volume of air you bring through the device. And as already noted, if you need to cool your interior in summer, you want a bypass to skip summertime heating of your incoming air.

    You didn't mention your location, climate zone or how sunny/overcast your winters, but assume from your chosen name "rockies..." you have sunny cold winters. In climates like just east of the Rockies from New Mexico to Montana and Calgary, you'd get substantial wintertime sun on a south-facing facade. Likely your device would make some difference in your system performance. In the Pacific Northwest/BC, Buffalo NY or other areas affected by the Great Lakes, there are overcast winters and minimal opportunity for solar gain, so your invention wouldn't work very well.

    Of course, just having more south-facing window glass would heat your interior directly, without using air to absorb and move that solar heat gain to the interior.

    This web site is a fantastic resource for providing solar heat gain data for design:

  4. rockies63 | | #4

    Thanks for the info!
    Now, some people may have thought this idea was silly but just by asking it I learned about two products I hadn't known anything about.

    One is the Tempeff RGSP-K dual core heat recovery device.

    The other is the Zehnder ComfoFond-L geothermal heat exchanger.

    I'll have a look at them and see if they might be a good option. BTW, I'm in climate zone 6B, west of the Rockies.

    1. Tim_O | | #5

      I believe the Tempeff is a commercial sized product. But basically it's a high efficiency HRV at 91%.

      The ComfoFond is one, CERV/BuildEquinox has a similar one. Both are in the neighborhood of $1000 I believe. But the concept lends itself to DIY as well I think, if you're into that. Bonus is that in the summer you get some free cooling and dehumidification.

      Another option, but not passive, would be something like the CERV or Minotair with built in Heat pump. Now your 415watts of heat loss are cut in half or less depending on the COP at that operating temp.

      1. Trevor_Lambert | | #7

        The ComfoFond was $2000 back in 2018, and I can only assume it's more now. The Geo-boost for the CERV2 looks to be $1000, but presumably only works with the CERV2, and the total cost of the CERV2 is pretty hard to justify.

        1. Tim_O | | #8

          I don't see any reason you couldn't use the Geo-boost with another ERV. It's just a water-air heat exchanger with 8" ducts. I thought about pairing one with the ERV supply combined with an A2W heat pump too.

          1. Trevor_Lambert | | #11

            I had assumed it had controls built into. If it's just as you describe, then you could use it with any system. But it also means it's very expensive for what it is. There used to be a company that sold a similar product for about $400, and I balked even at that price and built my own.

            Don't forget that this Geo-Boost is only one of many components you'll need to make this work. Circulating pump, pump controller, temp and pressure gauges, blow-off valves, fittings, it starts to add up.

    2. Trevor_Lambert | | #10

      The ComfoFond is difficult, probably impossible, to justify economically. If you care to DIY the whole thing, it MAY be possible to justify. I have a DIY ground source ground source pre heater, and while I don't exactly regret it, if I had to do it all over again with all the hindsight, I don't know if I'd bother. I think the all-in cost was around $1000, but the time commitment was substantial.

      In your climate, the Tempeff dual core isn't going to necessarily preclude the need for pre or post heating. You can read my response below for some explanation. You're almost certainly going to have air delivery temps below comfortable levels without pre/post heating during the coldest days. You might be able to manage this with careful supply vent positioning.

  5. DennisWood | | #6

    Tempeff is the product I brought up here a few times. They definitely have a residential line starting at 110 CFM. There is no preheat or post heat typically required when you are at 91% sensible efficiency. These units work like a pair of Lunos, but with much larger aluminum heat sinks. They work to much lower temps, and offer up to 75% moisture recovery which is a plus for very cold (read dry) conditions. See attached.

  6. Trevor_Lambert | | #9

    "There is no preheat or post heat typically required when you are at 91% sensible efficiency."

    This is the first I've heard anything like this. I guess you'd first have to decide what factors are making it required in the first place. If you're talking about delivering air at a comfortable temperature, I don't see how that statement makes sense. A typical, half decent ERV/HRV is going to be in the 75-85% range. Is getting that up to 91% going to make a whole lot of difference to the delivered air temperature? Let's look at a couple of scenarios. At a room temp of 22C, outdoor temp of 0C, delivered air is going to be about:
    17.5C at 80% eff
    20.0C at 91% eff
    That might be significant, but not earth shattering. As long as the air vent is more than two feet away from you, I don't think you're going to be bothered by either of those. I don't think pre or post heating is strictly required in either case.

    What about at -20C?
    13.6C at 80%
    18.2C at 91%
    That's more significant, but it ignores the fact that you can't actually operate a (conventional) HRV/ERV continuously at -20C without risking frosting the core. So you're either looking at pre-heating, or some sort of recirculation/cycling, regardless of whether your efficiency is 80% or 91%. The requirement criterion is frost protection strategy, not efficiency.

    The Tempeff units do boast some impressive numbers. If they do not require pre or post heating, I suspect that has more to do with the dual core than the efficiency. In fact, I just read some of their material, and this is pretty much what they say.

    "Because the cores switch cycles every 60 seconds, frost
    does not have a chance to build up"

    The Zehnder CAQ350 HRV has a 92% efficiency but would still require pre-heating at temps much below 0C.

    Edited to add:
    I've taken a few moments to review the efficiency charts of Tempeff and noted that the 91% figure you're citing is only valid at 1.7C. It's almost certainly going to be lower at temps like -20C. So if you live in a mild climate that rarely sees below freezing temps, yes you could get away with no pre or post heat. If it regularly gets to below -10C, then you're still going to need pre or post heat. Also of note is that you only get those high efficiencies with an oversized unit. For up to 110CFM, that's their smallest unit the RGSP 300. If you want 200CFM, which I would recommend is the minimum for boosting purposes for virtually any sized house, that means getting the RGSP 600 if you want to see that 91% efficiency.

  7. DennisWood | | #12

    NRC tested the unit in Nunavit (as in the Arctic). I believe it performed better in testing than published data.

    It works completely different from a typical ERV/HRV. There are two aluminum cores, around 10"x12"x48" which alternatively store and release heat. Moisture recovery is via the condensed water that is deposited in a core during the heating cycle (exhaust air flowing over a cool core), then recovered when flow is reversed. A core at their reverse cycle likely never drops below zero.

    Supply temps at -25C were 18 C. If that is being integrated to existing HVAC, there would be no need that I could see to post heat. If standalone, multiple ducts at 18 C delivered to the ceiling area..again, I don't see a need for conditioning.

    In speaking with their sales manager, they sold quite a few units during Covid to schools specifically as they did not need to bring in 220V etc for post heating.

    I've never had my hands on one, but I figure the NRC Arctic testing is about as scientific and objective as it gets. Cost for a residential setup in the 110 CFM range is somewhere in the $5000 USD range.

    1. Trevor_Lambert | | #13

      A quick cursory glance at the document seems to indicate that there's a modification of duty cycle to maintain the high supply air temperature. However, this had only a minimal effect on true ventilation rate, so that does put it head and shoulders above the standard, single core HRV/ERV. It does appear to be a superior product, accompanied by a premium price.

  8. DennisWood | | #14

    Agreed. But if compared with a Zehnder it may work out overall to be less expensive as you don't need a 220V circuit for preheat, or the expensive of installing and running the ground loop system. No drain either if the cores are kept horizontal.

    The K version has the fan and core components separate so you have some options for installation as well.

  9. rockies63 | | #15

    Here are a couple of great videos explaining how the the Tempeff Dual Core works.

  10. DennisWood | | #16

    I had seen the 2nd video (not worth recommending) but the first one I had not seen...and it's excellent. A lot of insights there on how the system works.

    Thanks for posting these :-)

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