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What’s a cost-effective way to ventilate while maintaining winter humidity in a cold and dry climate?

iLikeDirt | Posted in Energy Efficiency and Durability on

I live in the high desert, and it gets cold in winter (there’s some snow on the ground right now) but remains very very dry all the time. I’ve done a lot of air sealing work in this old house (last blower door test 5.2 ACH50) so that now, finally, when I keep all the windows closed and don’t run the bath fan, the humidity naturally climbs to a comfortable 35-40%. However, with no mechanical ventilation, the CO2 level quickly climbs to over 1200 PPM and continues rising, so what I do to keep it down is turn on the bath fan. I need to crack some windows to get the level down to below 800 PPM, but when I do that, the humidity drops to almost nothing again, which causes cracked and bleeding skin and seems to provoke illness more often, anecdotally. Basically I am undoing all my work when I run the bath fan. We’ve taken to using a humidifier, but after one month, it’s struggled to keep the humidity over 25% and used 120 KWh of power–which has nearly doubled our electric bill! Generating humidity and then sucking it right out of the house feels stupid, too. Not a great solution.

It seems like an ERV system is the logical answer here but I’ve had a difficult time finding information about the humidity recovery of the various ductless options I’ve looked at–ducted does not seem to be a practical option because with a slab, a vented unconditioned attic, and 8 foot ceilings, we have nowhere practical to run new ducts. I’m not particularly in need of heat recovery for the ventilation system–the heat load is extremely low–but I do want excellent humidity recovery.

I’ve liked the look of the TwinFresh Comfo32 CFM through-wall ERV, as the unit is cheap enough to put one in every bedroom and I can do all the work myself, but they don’t list the humidity recovery efficiency. The more-expensive Lunos eGO lists a humidity recovery of 20% which is not giving me a lot of warm fuzzies that the TwinFresh unit is any better, and all attempts to contact the manufacturer have been ignored. Are they any reasonably-priced ductless ERVs that have a humidity recovery efficiency that’s higher than this?

Alternatively, if anyone else has other ideas I’m all ears. In general I am a big fan of simple and elegant systems as opposed to complicated mechanicals but of course this is a retrofit where options are more constrained.

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  1. GBA Editor
    Martin Holladay | | #1

    The solution may be as simple as a different control strategy for your exhaust fan. Instead of turning the fan on and off manually, you might try to operate the fan with a timer -- say, for 10 minutes per hour -- or you might try one of the controls that responds to indoor relative humidity.

  2. iLikeDirt | | #2

    I'm not sure I see how that will make a difference. Without humidity recovery, there is an observed trade-off between CO2 exhaust and humidity loss. If I run the fan less, there's higher indoor humidity, but the CO2 level rises. Right now it's on about 16 hours a day--whenever we're not sleeping. I'm not manually turning it on and off every half hour or something.

  3. iLikeDirt | | #3

    Houseplants are a great idea but unfortunately despite the fact that I live in the sunny high desert, my house is terribly oriented and glazed for direct sunshine through the windows. I'll admit I haven't tried it, though, largely due to the wife factor (she's not really into houseplants).

    Are you talking about the Panasonic FV-04VE1 spot ERV? I didn't consider those because they're ceiling units, and my attic is insulated and vented, not within the conditioned space. Don't wanna locate mechanical equipment above the air barrier. It's also extremely challenging to work up there due to the low sloped roof. Also four of them (one per bedroom plus one for the main room) will pull a minimum of 68 watts 24/7 at the lowest settings, equaling at least another 49 KWh per month, which would increase my electric bill by about 33% and is a lot more electricity use than I was hoping for. I have also been unable to find the latent efficiency of this device listed anywhere.

  4. Andrew_C | | #4

    Old fashioned humidifers -
    As a test or temporary fix, you could try filling shallow pans (think pie plates, even the cheap aluminum foil type intended for one time use) with water and setting them in the sun (you've got a lot of that, right?) You may find they evaporate quickly even without a dog. You could even connect the pan to one of those pet watering bowls that utilize upside down 2L bottles. We have a friend with a wood burning stove, he just keeps a cast iron pan sitting on top of the stove and refills it.

    Good luck, cracking skin is no joke.

  5. charlie_sullivan | | #5

    The Lunos and the Twinfresh have the same technology, and both can be expected to have low humidity recovery. I don't know the number, or know how widely reported the numbers are, but a "real" ERV should do much better. Panasonic makes a small reasonably priced unit that seems to be high quality. I'd take a look at it and consider installation options.

    It would be interesting to experiment with plants to lower your CO2 levels, or get input from a biologist to figure out whether that could help significantly. As a side benefit, they'd add humidity. You might need artificial lighting to get some of them on a cycle of absorbing CO2 at night.

  6. charlie_sullivan | | #6

    I was thinking you might get creative with applying the Panasonic spot ERV units. Put in a soffet lowering the ceiling to 7' in a corner, put it in a closet, build it into a window seat? I don't know exactly.

    And according to it's 23 W on high. Where do you get the 68 W number?

    Note that low-tech humidifiers do consume energy, by providing evaporative cooling, which needs to be made up by your heating system.

  7. iLikeDirt | | #7

    17 watts on low (10 CFM) * 4 units (one in each bedroom + one in the main room) = 68 watts for 40 CFM for the whole house. I'd totally consider the panasonic units if I can find that their latent efficiency is any good.

  8. BillDietze | | #8


    Just remember that the Panasonic Spot ERV becomes, essentially, an exhaust only system below 20F. See

    As with so many cold-climate ERVs, beware (or at least note) the frost protection mode. Either they turn exhaust only, just recirculate the indoor air or require a heater when it's cold out.

  9. charlie_sullivan | | #9


    Sorry I misread your note on the power consumption. Bill's note on the frost protection is an important one, depending on how many hours you have of low a temperature each season. There are some that have better defrost modes, but they wouldn't be likely to be in very small units like this.

    On the electricity consumption, note that this unit has better CFM/W run on high than on low. You'd get the same airflow and less than half the electric consumption running one on high instead of 4 on low. It might get fussy to decide which to run when, but you might find that it doesn't matter that much as long as you get enough ventilation overall.

    There is probably a way to figure out the moisture recovery rate by comparing the sensible and total recovery efficiency numbers, but I haven't worked through that before and it would take more effort than I have time for right now. One thing you could do would be to find a unit with high rated moisture recovery, and compare its sensible and total numbers to the panasonic's sensible and total numbers to see how it compares.

  10. iLikeDirt | | #10

    I guess that is a good question. How important is distribution?

    You know, now that I think about it, distribution may be a solved problem: the air will get naturally mixed using the central forced air system. And the winter when the furnace will be on is actually the only time I even need to use the ERV, since I have plenty of humidity and fresh air in the summer from swamp cooler usage and the humidity is fine during the swing seasons with the windows open. Maybe one of those Panasonic units running on high will be just what the doctor ordered. I wonder if I can install it vertically in a wall instead of a ceiling.

    As for the specs, they say 66% heat recovery and 36% total recovery, which I guess includes latent. If this is simply a 50/50 split, that equals a 55% latent recovery efficiency. (0.66 * 0.55 = 0.36). That's not too bad. Is my math in the right ballpark here?

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