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Does anyone have any experience with ductless splits for very low inside temperatures (3 degrees C)?

SanfordAndSon | Posted in Green Building Techniques on

We have a church in the Ottawa Ontario area (zone 6A). The church is only occupied for about 1 hour a week. We are using a very old oil fired FA furnace.(about 80% eff. 164K BTU input). I did some measurements last winter when the outside temperature was -20C. The furnace ran for about 5 minutes each hour to maintain 3 C inside. Would I be able to use a ductless split to maintain the inside temperature during the week and use the oil fired system to bring the temperature up for Sunday morning?

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  1. GBA Editor
    Martin Holladay | | #1

    As far as I know, most ductless minisplits have a relatively high minimum setting on their thermostats. I don't think they can be set any lower than 60°F or so (15.5°C). We've had this discussion on GBA before, so I'm sure that someone will chime in soon with more details.

  2. Expert Member
    Dana Dorsett | | #2

    Both Fujitsu and Mitsubishi units can be set to 10C/50F, but not lower using a different mode than the standard setpoint mode, Fujitsu calls it "minimum heat" mode. To do get that low with a Mistusbishi requires using a different mode than the standard setpoint mode but it can be programmed to something in-between 10C and the normal 15C lowest standard temperature setpoint adjustment.

  3. SanfordAndSon | | #3

    Can anyone direct me to the previous discussion on the topic of low set points for these units?

  4. GBA Editor
    Martin Holladay | | #4

    I seem unable to come up with the right search words to help me find the previous threads. Anyone else able to find them?

  5. bencarsan | | #5

    Interesting idea. Sounds like those two systems would play well together. For a Mitsubishi you will need the optional MHK1 wireless remote in order to be able to set the thermostat below 67 degrees F and 50 F is the lower limit there. With your furnace running as you say, it looks like you will need approx 11,000 Btus/hr (5/60*164,000*.8). The incremental cost on oversizing is probably favorable because a larger unit will run more efficiently and in a large space short cycling shouldn't be an issue.

  6. SanfordAndSon | | #6

    Can anyone tell me why these units cannot be set below 50 F? The typical degree days for our area is 4300 for 18,5C inside temp.Reducing the inside temp to 3 C it is 1050 a significant saving on energy. It does take about 5 hours to bring the building up to room temperature on a -20 C day. Our oil prices are on the rise right now (over $4 CDN a US gallon).

  7. charlie_sullivan | | #7

    I think the fact that they don't offer a lower temperature is a result of either thinking nobody would want that or being afraid that people would blame them if their pipes froze with it set to < 10 C.

    Speaking of which, if you set the temperature down to 3 C, I would think you'd be worried about pipes freezing, unless you can drain the plumbing during the week, or perhaps if you know for sure that the pipes are all in internal protected spaces.

    And even with a setpoint of 50, you'll have a dual benefit: the reduced HDD, and higher efficiency of the mini-split: it has better efficiency when it is delivering heat at a lower temperature. So even going down only to 50 you should see a major savings.

    I found one previous discussion here:

    In that I mentioned a proposed way to hack a thermostat to get a lower setpoint. But I'm not convinced that makes sense.

  8. Expert Member
    Dana Dorsett | | #8

    To set a Mitsubishi to temps as low as 50F can be done with the standard remote.

    There is a button on the standard remote labeled "SMART SET", which will allow the setpoint to be adjusted below the the standard 61F minimum, but not lower than 50F. See page En-8 (p 9 in PDF pagination) of the FH series head manual:

    --------------begin copy & paste

    1 Press during COOL, or HEAT mode to select
    SMART SET mode.
    2 Set the temperature, fan speed, and airfl ow direction.
    • The same setting is selected from the next time by simply pressing
    • Two settings can be saved. (One for COOL, one for HEAT)
    • Select the appropriate temperature, fan speed, and airflow direction
    according to your room.
    • Normally, the minimum temperature setting in HEAT mode is 61°F
    (16°C). However, during SMART SET operation only, the minimum
    temperature setting is 50°F (10°C).

    ------------------end copy & paste

    With Fujitsu units it's just a matter of pressing the button labeled "MIN. HEAT". ( Note, that function goes away if using their fancier Wi-Fi remotes.)

    Potential reasons for such a high minimum:

    A minimum temperature of 10C/50F is enough to provide plumbing freeze up control for even fairly leaky buildings, but taking it much lower doesn't have much margin.

    It's also true that a ductless heat pump that is RIGHT-sized for the load would have a much longer recovery ramp to comfortable room temperatures from 5C or lower.

    But you'll have to ask the product designers for their rationale to know what thinking is behind it all.

  9. SanfordAndSon | | #9

    Thanks for the references. I have a bit or reading to do. There is no plumbing in the church so pipes freezing is not an issue. When I fist found a thermostat that would go down to 1.5 C and tried it we came back the next week to find a number of potted plants and flowers did not make it. going up to 3 C seemed to work as long as the plants were of a hardy nature.

  10. GBA Editor
    Martin Holladay | | #10

    Thanks for tracking down the link to one of the previous threads on this topic.

  11. bencarsan | | #11

    Dana, thanks for that--I had forgotten about the smart set option.

  12. walta100 | | #12

    I think the 2 systems would work great together. Size and set the mini split to 50° 24/7 and set the oil burner to 40° except for 3 hours on Sunday morning.


  13. gusfhb | | #13

    Since the building is very large, is it possible that a small mini split would both be unable to reach the setpoint anyway, and for all its trying the energy cost would be so low as to be irrelevant?

    Doing some math with the observed fuel usage could give you an estimate of the heat loss number and thus the size minisplit required to not reach the setpoint.

    Also, while I have never done this:!2966!3!57005222157!!!g!104635272837!&ef_id=WinJjgAABS9orQZA:20171207230654:s&kwid=productads-adid^57005222157-device^c-plaid^104635272837-sku^5EFR9-adType^PLA

    Set to 40 degrees, wired in the power line

    I used to use an old refrigeration thermostat in my barn [cause I had it] to keep it at 40 ish when I wasn't around. The deadband was backwards so one had to figure out where to set it to make it be where you wanted it to be.

  14. SanfordAndSon | | #14

    We are trying to save money on the oil bill. The one measurement that I did indicated that at -20C outside, the building required about 11000 BTU/hr. I do not have any other measurements for different outside temperatures.
    How much energy would one of the 12K units use if it is run full out? Our electric cost between .18 to .20 / Kw h

  15. Expert Member
    Dana Dorsett | | #15

    At what INDOOR temperature did the building need 11,000 BTU/hr @ -20C outdoors? (+3C?)

    At maximum speed, and outdoor temp of -20C and indoor temp of +20C (a 40C temperature delta) a cold-climate mini-split delivers about 6000 BTU/kwh. At a delta-T of 23C (-20C outdoors, +3C indoors) it'll deliver about 8500 BTU/kwh running at the full-on maximum.

    A Fujitsu 12RLS3H should be capable of more than 16,000 BTU/hr at a delta-T of 23C, a Mitsubishi FH12NA more than 14,000 BTU/hr, assuming reasonably low outdoor humidity (which takes up some capacity for defrost cycles.) The nominal capacity tests are performed at a delta-T of about 29C, albeit at higher indoor and outdoor temperatures making it not exactly apples to apples, but it won't lose much (and might even gain) capacity operating at the lower temp. (To know for sure how much it changes takes and in which direction takes more analysis about the characteristics of R410A refrigerant than I have time for.)

  16. gusfhb | | #16

    Thought I should add, saving money by adding complexity is not a winning plan.

    What is your yearly oil budget?

    If you are self installing the mini split, it is going to cost 1500 to 3000 USD for the unit, paying for install, x2 to x3.

    And you will still need to maintain the furnace.

    So can you pay for the minisplit over, say 10 or 15 years out of the projected savings in oil, while still maintaining the furnace?

    It would kind of make more sense to replace the furnace with the mini split. It seems from the usage numbers that a larger single unit would do the job, as long as it could ramp up the temp on Sunday morning

    Doubling the Delta T using your numbers says a 2 ton heat pump would work, but would it pull up fast enough and is the building simple enough[IOW one big room] to use a single mini split to heat it?

    I'm not saying don't do it, but that with the costs with two systems, savings may be illusory

  17. Jon_R | | #17

    Without running the numbers, my guess is that lowest cost is to move the plants and flowers into a small room and let the remainder of the building go unheated during the week. Even if you have to use electric heat in the small room.

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