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Community and Q&A

Questions on heating upper floor and controlling indoor humidity

Stephen Lim | Posted in Energy Efficiency and Durability on

We just bought a house in Seattle (2000 sq ft. 2 floors + basement) that has an integrated water/forced air heat ducted system on the main floors and basement and convectair wall heaters on the upper floors. We are looking at improving our energy efficiency and also controlling humidity issues we are having (in the 55-60% range on the 2nd floor and basement)

We just had an energy audit done and our contractor recommended the following:

– attic insulation and seal
– a single unit Daikin Quartenity DHP that can be used to set the humidity level, positioned at the top of the stairs (pointing down the hall)
– Panasonic whispergreen fans in the upstairs bathroom and basement bathrooms

I have a few questions that would be great to get advice on:

1. Would it be worthwhile installing the DHP on the upper floor? We don’t spend a lot of time up there (bedrooms only) and I’m wondering if the heat distribution from the ducted system would be sufficient. The installed cost was 6K although we do qualify for a 1.5K rebate

2. Is it worth installing the DHP that can control humidity levels? The cost difference is about 1-1.5K compared to Daikin’s standard model which has a similar SEER rating (the model we choose doesn’t affect our 1.5K rebate)

3. The 2nd floor bathroom fan is located on and vented through the wall and the contractor is suggesting reorienting the 2nd floor one to vent through the ceiling. Is this worth it (installation cost is significant) or should we just replace it with the Panasonic WhisperWall version instead?

4. Could we install the Panasonic spot ERV in the bathrooms instead of the fans? I noticed that the the price difference online or in another posting between the whispergreen fan and the ERV was not very much.

Thanks in advance!

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  1. Expert Member
    Dana Dorsett | | #1

    Seattle's outdoor dew points are quite temperate year-round. If you have reasonably moisture/vapor sealed foundation & slab and a mechanical ventilation system-HRV/ERV or even an exhaust-only system, the outdoor dew points in Seattle are ALWAYS low enough to dehumidify to 45% RH @ 70F in winter, and under 60% RH @75F in summer simply by adjusting the ventilation rate. (Or running it under dehumidistat control.)

    The rare exception will be the 1% of summertime hours when the outdoor dew points break 60F, but those days coincide with higher air conditioning loads, which is when any minisplit will be doing considerable mechanical dehumidification. Applying the difference in cost between a Quaternity vs. comparable heating/cooling mini-split on a whole-house HRV system would pay for about half the HRV. Running the ventilation system under dehumidistat control during the cooler month usually works out great from a ventilation rate point of view, but from late-May thorugh early September it's probably better to just duty-cycle it at some nominal rate, and let the mini-split do the dehumidifying whenever there is a cooling load. It'll be enough.

    If the WhisperGreens can be setup for dehumidistat control that alone might get you there. If the basement is running higher RH, a 70-pint dehumidifier set up to drain into a sink or sump converts the air humidity into sensible-heat (which is what you WANT in the basement at least 3/4 of the year.) But if you have a significant moisture issue that shows up as efflorescence on the slab or wall it's better to treat the source rather than band-aid it with a dehumidifier.

    At Seattle's electricity prices it'll be cheaper to heat with the mini-split than with oil or propane, and maybe even with natural gas. If you pass on the Quaternity for another solution, it's still a good idea to use a mini-split that can both cool AND heat, since the cost delta is in the very low 100s USD. During the shoulder seasons when the heat loads are low a decent mini-split will average well over 4 for a COP in your climate, and in the low 3s even in January. The Quaternity is unique in that it can dehumidify to a particular RH set point in either heating or cooling mode whereas other mini-splits would always be cooling slightly when in their dumbed-down "dehumidify" mode, dumping that heat-of-vaporization outdoors (which is great when there's a cooling load, not so great during the heating season.) But the $1-1.5K cost delta is a pretty expensive dehumidifier compared to a $200 room dehumidifier that always dumps the heat-of-vaporization into the room.

  2. David Meiland | | #2

    Seems strange that the humidity issues are limited to the basement and second floor in a house with forced air. I would dig a bit deeper on the source(s) of humidity before deciding on a solution. A lot of houses with forced air are going to suffer from excessive dryness in the winter (I am in your general vicinity). Was a blower door test and/or duct blaster test done? What was the nature of the energy audit?

    Also, hard to know if you should replace the fans without knowing what's already in place.

    Much as I love DHPs, I would not install one if the main goal is to dehumidify and you don't feel the need for extra heat. I will say that, depending on your ductwork, I might consider abandoning it and switching to 1 or more DHPs as primary heat. I would need to know a LOT more about the house and energy use to determine that, and since you just bought the house I'm not sure if you have the data yet.

  3. Stephen Lim | | #3

    Thanks for the advice and happy new year

    Sorry i didn't mean to indicate it was just an issue on the 2nd floor and basement - it is pretty consistently high on all floors but i t5% higher on the 2nd floor and the basement. Pretty sure that is tied to us showering on the 2nd floor and the laundry where we air dry some clothes is in the basement. Levels are actually 5% lower now that visiting family have left!

    The energy audit was a blower door test. Air infiltration was .35 ACHn. With air sealing which we will do I'm this will improve.

    Not sure exactly what fans we have but I did just burn the 2nd floor one out by leaving it running too long...

    Sounds like the Quartenity DHP might not be worth the incremental cost. With the rebate we are still thinking it is a pretty good deal to install a cheaper version. On the ventilation side I think i am leaning towards installing the wall mounted fan on a programmable timer upstairs and a continuous run in the basement and leaving the HRV for down the road.

  4. David Meiland | | #4

    Sounds like you have a way to monitor RH, so I would do some experiments. Stop air-drying laundry for a couple of weeks, and make sure that the bath fan runs for 15-30 minutes after every shower. If you don't have an effective fan (measured at 50 CFM or better actual performance) you need that. I like WhisperGreen ceiling fans but I like inline fans in the attic more, since you can get more CFM with less noise. Install a 10-20-30-60 Leviton pushbutton timer and make sure everyone uses it. See if you can get RH down to around 45%.

    If you really want to get nerdy about it, order a few of these for your RH monitoring, I find them incredibly handy:

    Further air sealing of the house is going to change things and require more mechanical ventilation, so be sure you stay on top of it. Any gas appliances inside the house or basement?

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