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Community and Q&A

Can I make mini splits more efficient at extreme temperatures?

Phillip Roshak | Posted in Mechanicals on

I renovated a home in northern New Hampshire with meaningful air sealing and additional insulation. I installed Mitsubishi hyper heat mini splits upstairs and down, but have baseboard and wood stove backups for when the temperatures drop below 20f and the mini splits need help. Overall, I am happy.

My question is about the outdoor units. Below a certain outdoor temperature the mini splits lose some energy efficiency.

Would a passive solar box, insulated on sides and with a black concrete slab on top of rigid foam, south facing, containing the outdoor units preheat the outside air when temps get really cold? Would such an arrangement keep the mini splits at more efficient operating levels such that I would need less frequent use of my backup heat during December-march?

I suspect such a passive solar box would require some attention and I would have to open the glass in warmer months. Humidity inside the box and its impact on the unit would have to be considered. Cost would have to be considered, but building such a box does not seem technically difficult. But is this idea viable? If the outside air inside the passive solar box could be kept above 20f for a meaningful amount of time it seems like it could be helpful. A metaphorical turbo for the mini split?

Appreciate any thoughts!

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Replies

  1. Nate G | | #1

    Intuitively, harvesting solar gain to heat the air around the outdoor unit makes sense. That said, the sun is least likely to be shining when you need to harness its heat; the thermal storage capability of your system would have to be very high. And as you alluded to, you wouldn't want to let the unit overheat in summer. That would actually decrease its efficiency since it would be trying to move heat outside if you use AC. At that time of year, it would probably prefer to be shaded.

  2. Ven Sonata | | #2

    Heat pumps bring out the mad scientist in all of us. The solar heated heat pump is usually the first idea, but it needs to be discarded because of interference of any enclosure with free heat gain. ( it is different with domestic hot water heat pumps at the tank since they heat during the non heating season. But if you are heating your house AJ BUILDER has a good idea. It also occurred to me. Daytime temperatures are significantly higher outside therefore use your heatpump and store the energy inside for nightime use. Water storage is easiest. People who use wood boilers do this, they can store at least a days heat in a water tank ( best kept inside with only modest insulation...but if space is an issue outdoor well insulated ground buried is a more expensive option. Explore web sites for wood boilers with storage and just use the storage dimension.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Phillip, There is a way to improve the efficiency I think. My idea is to have you run your mini split during the warmest part of the day and store heat in your home for the coldest part of the day. To do this you have to overheat your home by several degrees by night fall. Then set back the temperature and not reset it till the warmest part of the day. Last night here was -15. Today the temperature by noon will be 30 degrees or more warmer. As you know the COP improves as the temperature rises. Of course if you like an even temperature in your home, you are back to the woodstove

    The one benefit for having the unit warmed by the sun might be when it is off. Off it might sun warm and help with frost. Off it moves too much air to realize a lot of sun gain as it is moving so much air and the air is the same temperature close anyway around a home.

    Anyway, use the mini 11-4 and the woodstove overnight and I think you could gain 1/2 a COP. Dana would have to figure if that has any value. It might be worth $100/winter?

    Reading another thread, if you are not using the heat pump for long periods of time (hours) then keeping the unit warm via the sun etc would stop the preheat from kicking in. Saving less than 100 watts per hour. 1.5 cents an hour saved for the time period or less. I'll send anyone a quarter to cover this cost, first request fulfilled only.

    Read this other thread, very informative.

    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/27925/disappointing-phantom-load-272-seer-fujitsu-9

  4. Charlie Sullivan | | #4

    I actually just a few days ago downloaded climate data and set up a spreadsheet to estimate the benefit of AJ's plan. My estimate for my climate (central NH, zone 6) was an 11% savings in annual electricity use, from a very rough calculation. I was a little disappointed that it wasn't higher, but that still might be worthwhile.

  5. Expert Member
    Dana Dorsett | | #5

    At the VERY volumes air moved by a mini-split even at half-speed there's no way to get a measurable benefit out of any sort of passive solar scheme. The most important thing is to give it good access to it's source (the great outdoors), with plenty of clearance on all sides so that it's always blowing air through the coils that's near the outdoor ambient temp, undiluted by the super-chilled air coming out of the front of the coils.

    Sizing them so that they modulate at part load most of the time increases efficiency dramatically. At +15F and an indoor temp of 70F a pretty-good 1-ton will have a COP greater than 3.0 these days, whereas at full speed it can be below 2.5.

    Take the FH09, for instance: http://usa.mylinkdrive.com/uploads/documents/4560/document/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

    At +17 the "rated" capacity it can deliver 6700 BTU/hr with input of 600 watts.

    3.412 BTU/watt-hr x 600 watts= 2047 BTU/hr input, so at that modulation level at +17F the COP is 6700 / 2047 = 3.27- not bad!

    But it's full-speed capacity at +17F is 12,200 BTU/hr at an input power of 1440 watts.

    3.412 BTU/watt-hr x 1440= 4913 BTU/hr input, for output of 12,200 BTU/hr, a COP of 12,200 / 4913= 2.48, not nearly as good- about 30% LESS heat per kwh(!) even though it's running at the same outdoor & indoor temperatures.

    At +5F the max capacity 10,900 BTU/hr, max input 1500 watts. 3.412 BTU/watt-hr x 1500= 5118 BTU/hr input, for 10,900 BTU/hr out, a COP of 10,900 / 5118= 2.13.

    That's still not bad, considering, but it's not 3+. Unfortunately the submittal sheets don't give part-load figures at +5F, but you can bet that it's substantially better at half-speed than flat out, even at +5F or lower.

    As temps sink even lower, capacity & efficiency slowly drop unless you let the inside temp drop at the same rate as the outdoor temp, keeping the same delta-T. Most people aren't crazy about letting the place drop to 50F on a -15F night just to have the same ~COP 2.1 efficiency & capacity that it has at +5F out/ 70F indoors though.

    Supplementing the overnight heat with a wood stove as AJ suggests works, since it would then let the thing cruise along at part-load where it's operational efficiency is higher. Even at 0F if it's running at only half-speed it'll have COP of something like 2.5, and at it's lowest speed it might even hit 3 (but might not.)

  6. Ven Sonata | | #6

    It may help to reflect on this term " cop" it is very misleading. Cop 2 means you just cut your heating bill in half! Wow. That is good and fairly easy to make such a machine. You put in one kwh and you get 2 back. But what if we could get cop 9!!! Hmmm, actually it will only increase your savings by another 40%! It means you put in one kw you get 9 for free but that is 90 % drop in heating bill. But cop 2 gives you 50% drop in heating bill.
    It is impossible to get more than about 5 cop even on the best geo thermal heat pump. What is cop 5? 80% reduction. Air source at cop 3? 66%! Only 14% savings for $20,000 more on geothermal.

    So: lets say $1000 heat bill.
    Cop 2 = $500 heat bill,
    cop 3 $333 heat bill,
    cop 4 $250 heat bill,
    cop 5 $200 heat bill,
    cop 6, $166 heat bill. You see the law of diminishing returns.

  7. Jin Kazama | | #7

    Dana you are bringing up and interesting point.

    Supplementing with a wood stove when temperatures drop at lower than say -15c will not only be more economical because of the wood energy cost, but will also help the minis go back to their efficient operating range, pushing the savings even further.

    I was investigating fire ( wood and pellets) heat source in the last 2 weeks, to learn and see if i could offset some of my pretty large highly glazed house heating bill when my mini's go near COP of 1 at full load under -20c ( obviously coincides with higher heat loss ).

    Current pellet pricing scheme for Quebec drives their cost to par with resistance heating, completely ridiculous ( @ 8KBTU/lbs @ ~5$+taxes per 40lbs bag VS 0.088$Kwh cost ). They are going to kill a market that was just born a few years ago with the current prices..anyhow

    So back to wood burning, we pay around 100$/maple chord around here. So it is still economical to use an efficient wood stove compared to base resistance heating. ( ~ 3X less expensive ) But it does not take much to push wood out of question when COP of minis enters the equation. Even at 2.0COP , just the labor involved with wood burning ( unless wood burning is sentimental or you have the wood for free ) scars the involved economy.

    That said, you raised a point i had not thought about. Do we have data on efficiency of recent minis at lower than -20c temperatures with different loads? If i recall correctly, the agency tests were done down to -15c? A combo of wood and minis used through winter nights might just be a deadly combo if your idea works. Would be a double-sided saving scheme.

    Ah life .. i am still debating whether i use my wood resources for home heating, or if my new diy maple evaporator will chew through it too quickly !!! :p

  8. Jin Kazama | | #8

    There is also the temptation to use the money one would invest on a wood stove, chimney and wood cords to buy more PV panels !!! :)

    Ven : i really enjoy your recent participations on this site and i am surely not alone, keep it coming fellow west Canadian :)

  9. Marc Labrie | | #9

    Don't neglect the significant sol-air temperature rise on a south facing wall in the winter that can be use to preheat the air adjacent to the outdoor unit.
    http://levelton.com/beepmodule1/u4-impacts/arct501_4-1a.pdf

  10. Aj Builder, Upstate NY Zone 6a | | #10

    Jin, I have to point out that the ideas that are being discussed are mine. Dana and others have backed the idea. My ideas go way back here. I have always figured it made sense to gather heat when it makes sense from the sun and to use stored heat such as the stored heat in warmed mass or the stored heat in fuels such as wood when the sun is not shining brightly upon this planet.

    aj- the one who understands whens't the sun shine-ith and whens't it does not.

  11. Jin Kazama | | #11

    We discussed this idea a few months ago ( or was it years..damn time ) when i asked around if it could be possible to use dark steel siding on commercial/industrial building to help gather heat that would be channeled through to the mini split intake to help raise efficiency during daytime. If i recall Dana was again pointing that the quantity of air that the exterior unit requires makes this a hard task.

    On a residential building, i do not believe one would have sufficient siding space to achieve that,
    without destroying the building aesthetic value. Maybe a simple geo loop with glycol , 1 pump and a fan+radiator could be used? probably not worth the additional complexity.

    On an industrial building, with 20ft high sides and a lot of width for each mini splits...i'd try it for sure.
    Same design as is used on many buildings now to preheat make up air. Again, we are using a scheme that boosts efficiency during sun hours, where it is less needed.

    AJ i believe we all agree that storing heat in mass within the envelope is beneficial, but there is a hard limit on it..and it is usually in terms of $$$$ investment.

  12. Expert Member
    Dana Dorsett | | #12

    Marc Labrie: The volume of air going through the outdoor unit on a mini-split at high speed is measured in thousands of cubic feet per minute. Any local heating of the air from placing it on a south facing side of the house is completely dissipated in the first minute.

    While those local air temp micro-climate next to the sunny side wall may mitigate heat loss from the house in low-winds, it does nothing for the efficiency of a mini-split. The mini-split creates it's own high-wind condition, breaking up that cozy micro-climate.

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