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We bought a mountain house that has only electric-resistance heat (mostly baseboards, plus a couple of in-wall units with blowers). To reduce electricity consumption, I want install a ductless minisplit heat pump for the main common living space (living/dining/kitchen) on the upper floor of the house. The lower floor contains the bedrooms, and I will continue to rely on electric baseboard there (at least for the time being) since bedrooms are usually kept considerably cooler, plus it'll be rare for all three of them to be occupied. Because the house is at 4000+ft elevation, I would use the minisplit little if any for cooling (nobody in the area has A/C).

I am unsure what size unit to install, as well as the number and placement of the indoor unit(s).

See the floor plan below. The area is about 875 sq-ft. The house was built in 2001, so presumably pretty good insulation, and double-pane windows (but lots of them on the west wall). Ceilings are vaulted, sloping from 8ft up to 11-1/2ft. The DoE website specs a winter design temp of 15 degrees for the county, but the house is about 1000ft higher than the reference, so I assume 10 degrees.

I've talked to two knowledgable sources. One says 18K btu/hr (or maybe even 15k) is the right size, that to go larger would cause the unit to not be able to throttle down low enough, during shoulder seasons, to avoid short-cycling; the other guy says the first guy is an idiot, and I should go at least 24K. First guy pointed me to this calculator, and I get about 19K heating capacity required, for the entire upper floor.

https://hvac.betterbuiltnw.com/Site/Building.aspx?BuildingId=9740

I'm leaning towards the low choice, because that separate NE bedroom/bathroom/closet has its own electric baseboard, plus the big room has a propane gas-log fireplace.

The other question is about the indoor units. I'd like to have only one unit - to avoid a second line-set, and because there's one central location where I can easily route the line-set from the basement, up through a closet on the lower floor, and to the spot marked by a red 'X' in the floor plan. This is a very central location, and the room has a ceiling fan, so I hope it will provide enough circulation into the entire room. I'd also much prefer a wall unit, although it can be as low as 6ft above floor level; a floor-mounted unit would require an undesirable furniture re-arrangement, and there are many fewer choices for such units.

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3 Answers 3

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10 degrees above zero? Easy peasy. Much more choice in units at that temperature rather than mine - but then, mine will be more efficient at that temperature than the ones that only just manage it. But the much less expensive "self-install" ones (if you are so inclined) actually work at that temperature so it might pay off for you.

Window area and level of insulation are the real factors here.

I heat two floors of just over 1000 square feet each in a -15F climate with one wall-mounted 12K BTU head (each with its own compressor, as it's more efficient that way) per floor. I turn off the upper floor (I leave the fan on to circulate, and sometimes turn on an additional circulating fan) when load gets very light.

If you have a lot of window area (even good windows are terrible insulation .vs. even typical walls) and more typical insulation in your 875 sq feet you MIGHT need to go as big as 18K BTU/hr.

But enough with the hand-waving. you might have real data and precision available

If you have your power bills archived or can get that data, you can know to a much better level of approximation what you'll need, since you have been heating with resistance heat. If your power company uses smart meters and gives you 15 minute or 1-hour data to download, you can get very precise about your (whole house, not just this floor) heating load, at 3412 BTU/KWh - just look at your winter overnight power usage in the coldest months. That will give you real data on your as-built heat load in your precise location.

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  • We just bought the place. I think POCO will give me monthly bill amounts (along with kwh usage) but that's only so much help, not knowing how much it was occupied. Commented Jun 11, 2022 at 14:06
  • Actually, the baseboard is all in the bedrooms. The big room where I want to put the minisplit is heated by two wall-mounted electric resistance blower heaters. Each is on a 30amp breaker, so perhaps as much as 30*240*80%*3.412 = 19653 btu/hr, or a total of almost 40K btu/hr. Maybe I can get the faceplates off them and see how much they really are, or use a clamp-on ammeter at the breaker connection. Commented Jun 11, 2022 at 14:09
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    Doesn't matter baseboard .vs. wall mount blower as far as KWH usage - it's all resistance heat, it all comes out the same. It's highly unlikely that your resistance heat is operating full bore at max capacity, so get as much actual KWH used data as they will give you and work from there. Since it costs pretty much the same and there's no penalty for oversizing, (it just cycles off more) I expect resistance heat to be over-capacity as installed. How much power was actually used is the important data.
    – Ecnerwal
    Commented Jun 11, 2022 at 14:37
  • It might honestly be worth it (galling, but worth it) to put metering on those heaters and live through one January (Northern Hemisphere - presumably July for Southern) in order to accurately size your replacement heat based on real load. HVAC estimates are notoriously bad and generally fudge too big.
    – Ecnerwal
    Commented Jun 11, 2022 at 14:53
  • I suppose it doesn't need to be all that cold outside. The btu/hr per degree of temperature differential should be pretty linear, i.e. pretty similar regardless of outside temperature. So I wait till it goes down to 40 degrees (pretty common here ) and how hard the electric-resistance heaters work to get it to 100 degrees in the house. Commented Jun 11, 2022 at 14:58
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Be careful with "BTU" numbers on heat pumps - those typically refer to the air conditioner mode, which you have little interest in.

Think about the resistive electric heat you now have. Does it comfortably keep your house warm all days of the year? Figure out the "wattage of heat" that is right for you. Now here's the trick. Your resistive electric heat has a COP (Coefficient of Performance) of 1.0000, meaning you get 1.00 watts of heat (3.41 BTU/hr) for each watt of electricity you use.

The COP of a heat pump is variable based on outdoor temperature. If you don't want that problem, look at a ground-sourced heat pump, but the latest heat pumps respectable COP down to surprisingly low temperatures.

Modern heat pumps can run at or below COP 1.0, making them their own emergency heat... but since you already have baseboard heaters that are dirt cheap to maintain, why add the wear & tear to the heat pump engine?

But you need to get data from the manufacturer about the heat pump's performance at all the temperatures you might get. (You don't want to be like Texans who were blindsided by their arctic freeze and cracked a lot of water pipes).

The deciding factor on heat pump size will be the machine's performance at your reasonable worst-case temperature. If they give you BTU by temperature, that's great, but otherwise if they give you COP by temperature, you can multiply that by power consumption to get effective wattage of heating.

  • COP x power used in watts = useful heating in watts
  • COP x power used in watts x 3.41 = useful heating in BTU
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  • I suppose it depends on who makes your heat pump as to what the ratings mean. For my mini-splits, the nominal number is the cooling, and the heating is actually a bit more than that. As with the OP's plan, I very rarely put them into cooling mode (other than trying to remember to exercise them every 2-4 weeks in the "off" season)
    – Ecnerwal
    Commented Jun 11, 2022 at 23:25
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    Single wall mount should be fine, based on my install working fine with one per floor that's bigger than your floor. They (should, and if not, you need to find out when shopping and buy a different one - mine do) blow air downwards when in heat mode, very effectively yet very quietly.
    – Ecnerwal
    Commented Jun 12, 2022 at 13:40
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    @RustyShackleford The freon work HAS to be done by a pro and warranty requires they do the whole job. But you can do a lot. #1 is the right-of-way for the lineset: a hairy, open-ended and expensive wildcard for the installer, and their bid reflects that. So I say if you the homeowner can pre-install those lineset routes e.g. with conduit, so they bid it knowing that pulling the lineset will be a 2-minute task, should help a lot. Plus pre-wire the electrical. Commented Jun 14, 2022 at 19:00
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    @Rusty Watch some Youtube videos on mini-split installs, and pay very close attention to the fitment issues, glitches and annoyances that suck up most of the installer's time. Your job is to make sure none of those exist, the installer puts the paper template on the wall and gosh, there's an empty line-set conduit exactly to the mm where the template wants it, an electrical conduit exactly in the right place, the mounting screws even line up on the studs (or there's a plywood backer waiting). So the work the pro must do, goes like butter. Show the installer that prep work is done. Commented Jun 14, 2022 at 19:25
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    @RustyShackleford yes as long as they are sweeps. Commented Jun 25, 2022 at 1:20
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Alternate/additional methods to increase the comfort during cold times, especially in seldom used weekend houses or if the heating system is too weak - and/or to save much energy and money:

  1. Bed heat mattress pads.

  2. Hair dryer next to the bed for instant warming in very few seconds. Can also serve as instant sauna and replaces a 30 minute hot shower to get the blood circulation going in the morning woth way less energy. Does also partly desinfect the bed: bacterias, viruses, ticks, fleas, lice etc. have little chance to survive the 60 to 150 °C hot air stream. Needs only a tiny amount of energy, since the air in the bed will be heated up in a few seconds.

  3. Another hair dryer close to the outdoor shoes can remove any moisture and any olfactory nuisance in a few seconds. Can also be used to warm up a the inner side of jacket or cloak in a few seconds before leaving the home.

  4. For chairs a car heated seat cushion plus variable switched mode power supply can produce a cosy atmosphere just as quickly as in a car.

  5. Sitting at a desk (home office) can be more comfortable with no. 4. plus an adaptation of the far east Komatsu concept. Long sleeves/towels/bed linen etc. hanging from the table to the ground at 3 or 4 sides ( more layers=better isolation) build a quite isolated area where a small thermostatic electric heater (300W) warms feet and legs.

4 and 5 should be controlled by a motion sensor adaptor (with 5 min. delayed turn- off time).

All 5 points need nearly no energy compared to traditional room heating.

Moisture can be easily monitored by a unit with 3 wireless sensors plus built-in sensor.

To reduce/avoid moisture, "dark-airing" should be used, i.e. airing the rooms at night resp. preferably before sun rise in the morning, since the air conveys statistically the least amount of water at that time.

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    I'm not among the downvoters (can't be bothered, rarely do unless outright wrong/dangerous), but this in no way even approximates answering the question.
    – Ecnerwal
    Commented Jun 11, 2022 at 14:47
  • @Ecnerwal 1. "To reduce electricity consumption," is the main issue, since this is the begin of the second sentence after the short introduction sentence. 2. The question which size of heat pump is needed turns out to be another problem. The answer clearly 1. shows how to save energy and money in this case, which could be also interesting for other readers. 2. helps to answer the 2nd question, since a heat pump which normally would be too small could be "healed"/supported by these methods to get the needed comfort.
    – xeeka
    Commented Jun 11, 2022 at 15:39

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