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Community and Q&A

Heat Exchanger in Lieu of HRV

scottie_miller | Posted in General Questions on

I’m planning a 300 SF off grid cabin in Leavenworth, WA, (climate zone 5, ~6,000 HDD).  I’ll be using the cabin in the winter when the temps are below freezing and often dip into the single digits.  We will have a small solar system and 12V/120V power, but limited battery storage.  Reducing loads will be important.  I’m hopeful that we can facilitate heat exchange from our exhaust air to our incoming fresh air, but may not be able to accommodate an HRV’s continuous power draw.

Does anyone have experience with sourcing air to air heat exchangers without built in fans and electronic controllers?  Something that would work with 4″, 5″ or 6″ ducting?  Thanks in advance and appreciate this wonderful GBA resource!

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  1. Trevor_Lambert | | #1

    I think this is going to be difficult to find. Your only option might be to fabricate it yourself. I've seen a video on youtube on how to do it. Of course, it's unlikely to be as efficient as a commercial model.

    What will be moving the air through it? If you're counting on passive air exchange, I doubt you're going to get much fresh air through it.

    1. scottie_miller | | #3

      @Trevor #1 - I suspect fabricating an air to air heat exchanger is beyond my DIY skills, so I may be out of luck here! The intention is to hook up some inline fans in the exhaust/intake lines, use the fans when needed and rely on a syphon/venturi type cap and thermal chimney whenever the wind is blowing or sun is shining. Unfortunately without a passive heat exchanger (the guts of an HRV without the controllers/electronics/fans, etc.) the whole plan falls apart!

  2. walta100 | | #2

    I though the off grid solution is to crack open a window if the air in the cabin feels stuffy. It is not automatic or elegant but is the time tested solution.


    1. scottie_miller | | #4

      @Walter #2 - I suppose the simple open a window option may be my fallback here, but in a cold climate this sure does come along with some heating penalties!

  3. Expert Member
    Akos | | #5

    At one time your could buy fanless HRVs (ie Vanee VFE125). These were pretty much an HRV core in a box with fittings. Don't know if they are still made, quick google search is not showing up with much. You could probably DIY something pretty close to one with a replacement HRV core, most manufactures sell a core for a reasonable cost.

    You would have to figure out how to move air through and balance the airflow.

    If you have enough height, you can use stack pressure with a stale air pickup near the ceiling and fresh air supply at floor level. Won't create much pressure, but might be enough to move enough air through an oversized core.

    1. charlie_sullivan | | #6

      Stack effect driven ventilation gets reduced by the heat exchange. If you have 80% heat exchange efficiency, you only have 20% of the stack effect. It seems like there would be some way to configure the ducts to still get full stack effect, but there really isn't. High exhaust and low exhaust in the room doesn't do it, once you bring the ducts back to the same elevation for the heat exchanger. You could put a really high stack on the exhaust duct after the heat exchanger, and just use the 20% of the heat that is still in that air, but it would be hard to get that flow started.

      1. Expert Member
        Akos | | #12

        I was thinking about this a bit this morning. I'm having a hard time to fit heat exchange into the my thinking. Stack effect is about buoyancy of hot and cold air and height creating a pressure difference between inside and outside. Even with heat exchange along the way, this pressure difference exists and you'll get flow.

        Maybe a better way to think about it. Putting a heater in front of a leaky basement window will not reduce the amount of flow through the leak.

        1. charlie_sullivan | | #15

          Putting a heater in front of a basement window will not reduce the flow. However, if you route a duct up from that basement window to a heat exchanger where it gets heated, and then back down the basement, the air in those two duct runs is different temperatures. So it's different from just heating the air as it comes in. The duct going up is at the same temperature as outside air, so you might as well skip that and have the inlet at the higher elevation. The duct going down is at nearly room temperature, so that doesn't do much either, and you nearly the same lack of stack effect that you have with have inlet and outlet at the same elevation.

          You can come up with different plans, but once you work out all the temperatures in all the ducts, you aren't left with much stack effect. If you have a specific proposal that you think works, describe the specifics or make a sketch, and I'll show you why it doesn't work.

      2. Jon_R | | #13

        I'm with Charlie. There is no free lunch - you can't use heat to power a system and not lose any heat in the process. Some rough calculations suggest that 20% (leaving < 1 pascal) isn't enough to move the air around (with any reasonably sized ducts).

        1. Expert Member
          Akos | | #14

          It seems to work (uses both stack effect and wind), maybe not well enough, at least the company no longer makes it this version. They still have a passive vent though.

          1. scottie_miller | | #18

            @Akos #14 - I love the Ventive concept! I was looking at this a few months ago. Alas, it appears to be UK only, and the current crop of products are all full of electronics and connected through the cloud to a central data facility, etc. The original product could have been a great fit here. Thanks for bringing it up!

    2. scottie_miller | | #7

      @Akos #5 - Aw man, the Vanee VFE125 looks like exactly what I need! It does appear to be out of production so unless I can source a used product I'm probably out of luck here. DIY'ing an enclosure for a replacement HRV core could be the right concept. I'm "handy", but not "fabricating sheet metal handy", so I may be biting off more than I can chew with this.

      Re: airflow balancing, I am imagining using an inline fan in both the intake and outlet air ducts and then using a syphon/venturi type cap and thermal chimney to drive exhaust air when the wind blows and/or the sun shines. This will leave the airflow unbalanced with an exhaust only ventilation system when fans aren't powered. With a very airtight structure I'm hopeful this will drive air movement through the intake heat exchanger duct. The stack effect will probably help marginally but the intake/exhaust will at least have to pass each other through the heat exchanger which may limit the effectiveness.

      Thanks for your thoughts!

  4. scottie_miller | | #8

    @Charlie #6 - Excellent point re: efficient heat exchange negating the stack effect, (something I hadn't thought about previously). My intention here is to use a syphon/venturi type cap to drive exhaust airflow when the wind is blowing, (my only data point is a Q&A comment from Dana Dorsett in 2014 noting a 4" venturi cap can drive 65cfm in a 4MPH wind) and a thermal chimney surrounding the exhaust duct to create heat driven air movement. Obviously the wind doesn't always blow and the sun doesn't always shine, but I'm hopeful that this setup will create sufficient air flow in the exhaust duct to drive down electricity usage for ventilation without a huge heat loss penalty. In a very air tight structure I'm hopeful the exhaust only passive system will be able to drive intake airflow through a heat exchanger. This is all a moot point if a passive heat exchanger isn't available. Hmmmm....

    Thanks for taking the time to point out the heat exchanger/stack effect trade off!

    1. charlie_sullivan | | #16

      The venturi cap plus solar heated stack could work. You can also get very efficient, low-power 12 V fans. It might be easier to start with a unit that comes with fans and rip them out than the fabricate an enclosure from scratch.

      But really, I think the Lunos recommendations are the right solution. Either the Ego or the E2.

      1. scottie_miller | | #19

        @ Charlie Sullivan #16 - The Lunos options are certainly the easiest powered option that would work. @47 deg latitude, overcast winters and a small solar system the point of this exercise is to reduce power load wherever possible. Maybe dropping the idea and simply going with the most efficient powered option is a good fallback. Thanks!

  5. Expert Member
    PETER G ENGLE PE | | #9

    I'm not sure I would stress out too much about this. A 300 sf cabin is tiny. Do a decent job of insulation and it'll be heated by body heat and other interior loads. If you're planning on any sort of wood burner for heat, it's going to overwhelm the space so cracking a window really doesn't carry much of a penalty. Then again, if you're trying to heat with electric resistance in an off-grid cabin, power usage could be a real issue. Have you taken a thorough look at your overall energy balances? FWIW, I do sort of like the idea of wind-driven passive ventilation for background ventilation when it's unoccupied. Probably would keep the place feeling fresher. Alternately, what about a commercial HRV on a switch? Use it when you need it. The engineering and fabrication are already done. Maybe something simple like a Lumos system. Could be run on a timer when not occupied.

    1. Jon_R | | #10

      +1 on a Lunos or similar (TwinFresh?). At a paltry ~100 Wh/day, just size the PV solar system to handle it. Or if you aren't there much, just crack open a window.

    2. scottie_miller | | #20

      @ Peter Engle #9 - I've given thought to an HRV system on a CO2 switch, (or a Lunos system modified with a CO2 switch) which is another way of reducing power consumption where ever possible. When I started designing this project ~5 months ago I assumed we'd land on wood space heating, but at every turn I found issues that I'm unwilling to compromise on, (too many BTU's too quickly with an efficient burn, too much mass [and cost] in a masonry heater for the limited foundation to support given code limitations, significant thermal bridging in the flue pipe, etc.). Where we've ultimately come down on this is to use solar thermal DHW and use a large unpressurized H2O thermal heat store with a thermosyphon radiator system for space heating. This will be backed up by resistive electric heat if it's ever needed. We've modeled the build in WUFI and the heating load is pretty minor, so the small battery system is the biggest limiting factor on pretty much everything. The robust building envelope makes the heat exchange for air intake all the more important. At some point we may throw in the towel, dramatically expand the PV/battery system, install a heat pump and take the easy way out. Until then I'm full speed ahead on trying to cut consumption at every turn! Thanks for your thoughts, they're very helpful!

  6. Expert Member
    Akos | | #11

    One problem you'll have with an HRV, or at least when occupied in the winter, is frost buildup inside the core. I can't see a simple way of managing defrost with a passive system.

    A better option is to use an ERV core, since these transfer some of the moisture to the incoming air, need much less defrost. My ERV at home doesn't need any defrost until bellow -10C.

    1. scottie_miller | | #21

      @Akos #11 - Great point on the frost buildup. Average minimum temps are below 30 deg F roughly 5 1/2 months/year with the coldest temps in the single digits F. My intent here is to have the intake air flow through an earth tube to temper the cold, (which I'm well aware will be met on GBA with quite a few eye rolls) to remove the defrost need for the HRV, decrease the delta T between incoming/outgoing air and allow for air conditioning in the few summer months it will be required without any power requirement. This certainly doesn't force me to use an HRV core in lieu of an ERV core, but my (limited) understanding is that the HRV will be slightly more efficient for our climate, (Zone 5, Leavenworth, WA). I'm certainly open to using an ERV core as the moisture transfer in the winter will probably be a value add and the summer humidity is relatively low. Thanks for the idea!

  7. dlfdk | | #17

    I had the same issue sourcing an HRV core for a DIY ventilation system in my tiny house. In my case the DIY solution came to be not because of power draw concerns, but more as a reaction to the exorbitant prices store bought products come with. Like the Lunos. And I wanted a centralized system, which wasn't available in a size fit for my 120 square feet.

    So I went searching, and finally found a company willing to supply one. Took a lot of convincing, though. A cross flow heat exchanger capable of supposedly 95% heat recovery at my flow rates. It was combined with noise reducers, regular ventilation fittings and two high quality 12 volt computer fans. At max they're specified to use 6.6 watts each. At that level they keep the CO2-level in my house around 600ppm when it's just me and a bit more when I have company.

    If price isn't a huge concern, I'd probably suggest you opt for a Lunos or Lunos-like system. Since they don't need ductwork or fittings, their power efficiency is unbeatable. All houses - especially the small ones - built with airtightness in mind ought to have a ventilation plan that goes further than opening a window when the air gets stale. Even if electricity is at a premium. In a small house, the time between closing the window and the CO2-levels spiking is surprisingly short.

    If energy recovery isn't vital, you could probably create a solution based on stack effect, motorized dampers and a decent quality CO2 or particle sensor.

    1. scottie_miller | | #22

      @dlfdk #17 - Ummmmm, can I buy you a drink and ask you a million questions about your tiny house build??!?!?!!? It sure sounds like you found the exact product I'm looking for. If you know a supplier/manufacturer that will provide a high efficiency heat exchanger with 35 cfm to 65 cfm max flow rate, I'd be willing to trade my first child for their phone number! (My first child isn't worth much, so I may have to trade something more valuable). Anyway, the Lunos or stack effect/venturi cap exhaust solution connected to a CO2 sensor may be the easiest option here, but I really want to incorporate a ducted ventilation heat exchanger if the right product exists on the market. The point that air quality degrades in small spaces so quickly is exactly why I'm putting so much thought and effort into finding the best possible solution here. Thanks so much for taking the time to reply!

      1. dlfdk | | #25

        I got mine from At the time I was looking into the viability of selling standalone HRV cores, which certainly played a part in the company's willingness to supply me with a sample for prototyping.

        I'm still surprised such a product doesn't exist. It would be rather straightforward for an existing manufacturer to make. An insulated box, stick a core in it, then add the necessary fittings for the ducts to interface with later. Let the rest be up to the installer/customer to figure out.

        With the way things are today though, I can't really recommend my DIY solution. Forgetting the hassle of sourcing the HRV core, actually building the box for it to sit in was way more work than I expected. First of all, the airstreams must be kept separate all the way through. That requires gaskets and what not, carefully maintaining airtightness where it is needed. After that you have to insulate the thing, at least the cold half, or you'll risk making the whole project completely meaningless. Then comes the challenge of getting this Frankenstein contraption to interface with standard ventilation ducting and fittings. And on and on it goes.

        All of this can be done of course. My own system functions well. It just isn't a particularly fun or productive way of spending one's time.

        If I had to do it over, I'd probably go with the cheapest possible centralized HRV system I could find. And then use that as a base for modifications. It would almost certainly be easier to tinker a bit with the fans - or even swap them out - to get the needed power consumption you're after than building something completely from scratch.

        1. scottie_miller | | #27

          @ dlfdk #25 - Thanks for the clarification on your process and the link to the HRV core you sourced for your project. I misinterpreted your previous comment and thought you sourced an entire heat exchange unit, not just an HRV core that you had to fabricate a box for. I’ll take your advice here to source an HRV unit and pull the fans/controller, etc. in lieu of fabricating a box for a stand alone core. Now I’m on a new journey to find a unit that will allow easy disassembly of the appropriate components! I’ll keep you posted if a find the right product in case you’re interested in revisiting this type of project down the road on your next build. Cheers!

  8. walta100 | | #23

    What am I missing? You have enough battery/generator or some other power source to heat the cabin with resistance thru 2 weeks of cloudy days and nights or whatever the local worst case is but you are concerned about the 90% smaller load from the HRV fan?


    1. scottie_miller | | #24

      @Walter #23 - The DHW and space heating will be solar thermal, not PV, and the PV/battery storage will be extremely limited, so every watt matters. We will have a backup electric resistance wall heater but it will have very limited functionality and will only be installed because it’s very inexpensive and adds a small bit of redundancy. Cheers!

      1. creativedestruction | | #26

        Still need pump energy for the solar thermal. I'm with Walter, right-sized ventilation for your 300 SF is a very small load compared to just about anything else. If you had more space I'd say hook it up to one of those bicycle power generators if you really need more watts.

        1. scottie_miller | | #28

          @Jason S #26 - Actually the solar DHW will work on a thermosiphon and won’t utilize pumps/electricity to operate. I have confidence the DHW and primary space heating will be possible without electricity, (backup resistive heating is planned). All of that said, I understand your point that an HRV draw is minor in comparison to many common electrical loads in a standard home. If I strike out on my passive heat exchanger idea I may just take your advice and put in a standard ducted HRV or Lunos pair. Cheers!

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