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Community and Q&A

Mini ductless HVAC and air quality

TMK2 | Posted in Green Products and Materials on

Live in western WA. Climate conditions:  experiencing +100 degree summers for a week or so and 10 degree cold weeks in winter. MOST importantly, we have been experiencing poor Air Quality (AQ) due to fire season smoke. For the last 4 years, my wife has had to leave the area due to poor AQ from smoke.

We have a 12,00 sf remodel house with upgrades to the present insulation code. The house still leaks air and should not be considered airtight.

The present heating system is propane fireplace( about 12 years old and experiencing issues. We have a lifetime warranty but getting it serviced usually involves long waits( stores only make money selling new units) and questionable knowledgeable technicians.

Last year received an estimate to install 3 ductless units for $16,000 via Costco contractor program. The units would not pay for themless nor improve AQ.

Second quote this year is $9,555 for a Fujitsu AOU36RLXFZ1 seer 18.0 HSPF 9.4 12 year warranty. Two Fujitsu ASU12RLF1 indoor units , all installed after $1,000 PUD rebate.

No one can tell us about the AQ treatment of this system. Is this a reasonable price? Construction market in Puget Sound is RED HOT. Prices going thru the roof. I live in a rural area 40 miles from anywhere ;very difficult to find any help. I’m not into a DIY because 1) 74 yo after a stroke, 2) wife throws a fit when I do anything making life unbearable.


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  1. Expert Member
    Dana Dorsett | | #1

    These things don't have much of a filter, but just recirculate the air within the room. If the air in the room is lousy, it stays lousy, if it's great, it stays great. If you need an air filter to deal with wood smoke haze, that's going to have to come from somewhere else- no furnace or heat pump is going to fix that.

    But regarding the proposed ductless solution...

    For a 1200' house a 3 ton cold climate heat pump is LUDICROUSLY oversized for the heat load at +10F, and the individual 12RLS heads are ridiculously oversized for their room/zone loads too. When married to a multi-zone compressors these things don't modulate, and being crazy-oversized it's likely to short-cycle into much lower than advertised as-used efficiency, and lower comfort too. If it's possible to heat the place comfortably with a propane fireplace, a single head MODULATING ductless would be a better choice, and it would be a lot cheaper too.

    The 36RLX can deliver over 36,000BTU/hr @ +5F, which is probably way more than your house needs. (It's about what my sub-code antique 2x4 framed 2400' house plus 1600' of insulated basement needs at +5F.)

    With glass in the windows and doors that shut a 1200' house would have a total heat load of less than 20,000 BTU/hr @ 10F, and even a pretty leaky house would have a heat load less than 25K. A tight 1200' house could be under 15K for a load if it has decent (and not too many) windows. I have relatives on Whidbey Island heating a house more than 1.5x that size with a single 1.5 ton Mitsubishi (that delivers ~22,000 BTU/hr @ +10F.) I have another relative in Port Orchard heating a ~900' house with the same unit (and it's a bit oversized for her actual loads.)

    I suspect you'd do just fine with a Fujitsu 15RLS3 (or maybe a 12RLS3) with the head mounted in the same room as the fireplace. The 15RLS3 is good for 20,500 BTU/hr @ +5F, the 12RLS3 is good for 16,500 BTU/hr @ +5F, and have MUCH higher HSPF efficiency than the 36RLXFZ, and they MODULATE, delivering very stable room temperatures, with far less on/off cycling.

    Either one of those should be less than $5K, all-in installed price, maybe even $4K.

    Alternatively a Mitsubishi FH15NA or FH18NA would have comparable capacity at temps you care about.

    With a ZIP code (for best weather data) , exact fill up dates & amounts and the AFUE rating of the fireplace it's possible to put a firm upper bound on the design heat load of this house using these methods:

  2. GBA Editor
    Martin Holladay | | #2

    A ductless minisplit is a heating and cooling appliance. It has nothing to do with filtering the air. In that way, it's just like other types of heating and cooling appliances (a wood stove, a furnace, a boiler, or an air conditioner).

    If you want an air filter, get an air filter. Normally an air filter would be associated with a ventilation appliance (perhaps an HRV or ERV) rather than a heating or cooling appliance.

  3. Expert Member
    Dana Dorsett | | #3

    Some HVAC equipment can be retrofitted with HEPA filters but so far as I know that's not the case with mini-splits. But the filter can only work when the blower is operating, and the cooling loads can high, low or negative even when there's a forest fire up wind.

    Forest fires put out a lot of not-very healthy stuff- a really great filter on an air cleaner might be able to reduce the indoor PM2.5 to something reasonable, but something else would be needed to manage the VOCs in wood smoke. This is a serious public health issue.

    But unless you're right in the fire zone it won't hit the levels New Delhi is seeing this week. Between the burning of crop residues, trash bunrning, motor vehicle emissions and the Diwali celebration fireworks the PM2.5 levels this week are completely over the top, way beyond merely "dangerous" levels. (The only effective filter for that would be a train ticket out of town.)

    Fortunately most of the time western WA is downwind of the Pacific and the forests aren't burning. The annual average air quality of the region (except maybe King & Pierce Counties) is better than the east coast average, except during forest fires. The PM2.5 averages are consistently some of the lowest in the nation:

    But the turpenes released by the coniferous forests and the human activities take the overall air quality down a notch compared to remote arid less populated parts of the US, say the Big Bend region of Texas.

  4. TMK2 | | #4

    OK three questions
    1) if I need an air filter plus heater / AC what type of system do I need and where do I find it?
    2) If I plan on adding onto the house (say 750 sf) am I better off upsizing now or adding a second heat pump/ mini?
    3) Who do I work with to get an honest designed system & install?

  5. Expert Member
    Dana Dorsett | | #5

    1>Air seal the place to as tight as you can make it. Active ventilation using a heat recover ventilation HRV with a HEPA filter can take a good chunk of the smoke out. Use an activated charcoal room air purifier/filter & HEPA filter to manage peak smoke pollution, which will further reduce particulates, but (with the activated charcoal) also reduce volatile organic components of the smoke.

    2> Upsizing the mini-split may not be necessary- size granularity comes in pretty big chunks Run a Manual-J on both the heating & cooling loads for the house as it is now, and another that includes the addition. A tight 750' addition built to code minimum or better would add maybe a half ton, maybe less to the sizing, but unless there is easy air communication/convection to that shared space you'd be better off with a second half ton or 3/4 ton minisplit that modulates to a low enough level.

    3>Get somebody who makes a living on the accuracy of their numbers (rather than making it installing & maintaining equipment), such as certified professional engineer or RESNET rater to run an aggressive (per the instructions) Manual-J on the place and specify the equipment capacities, or dig up the specs on the equipment yourself and put it out to competitive bid with the relevant contractors. The contractors may want to propose something else, and it's OK to listen to their suggestions- some are actually familiar with the characteristics of what they are selling (sadly, many or even most are pretty clueless when it comes to ductless.) But caution them about your desire to avoid gross oversizing, and to have a fairly low minimum-modulated output so that it actually modulates.

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