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Maximizing Efficiency of Minisplit System

Mike Thompson | Posted in General Questions on

Hi all – I have a 2600 sq ft house in Massachusetts and am heating the house with mini splits for the first time.  We installed 5 Mitsubishi Hyper Heat units.  The backup system is a 30+ year old oil burner with baseboard heat.  I’m trying to maximize efficiency with the two systems but am stumped about to do it.

Right now, I have the mini splits set to 68 F and I just leave them there 24/7, like the installer recommended.  I have the oil burner thermostat set to 65/66.   Does that sound right?  Am I just wasting electricity if they are set to 68 while the house is empty for 7-8 hours per day?

When outside temps drop into the teens or single digits, should I turn the mini splits off entirely or should I just turn up the other thermostat and let the oil burner do the heavy lifting?

Lastly, does anyone have any advice for integrated controls?  The Kumo Cloud app is terrible and I have Nest thermostats.  So, I can’t use IFTTT to turn units on/off based on outside temps.

If anyone has any tips for juggling mini splits and oil heat in New England, I’d really appreciate it!

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Replies

  1. Trevor Lambert | | #1

    Hyper heat models should easily handle the entire load down to sub zero temps. Running the oil burner as little as possible is the goal.

    Common wisdom on minisplits is that daily temperature setbacks are pointless at best, counterproductive at worst. Letting the temperature drop and then having them try to heat the house up again quickly forces them to run at a less efficient portion of their range.

  2. Rliebrecht | | #2

    Hey Mike, we also just made the switch. The answer revolves around heat load and zoning.

    Firstly, how does the heating capacity of your heat pump system(s) compare to your heat load? We have two indoor/outdoor pairs, each rated at 18,000 BTUs. We just went through a cold snap (-13F) and found our heat pumps kept up just fine without any backup, despite the fact we're in an early 1990s home with crummy skylights and poor windows we're working to remove/replace over the next year. Technically our heat load (probably around 40,000 BTU/H after some upgrades earlier this year) outstrips the two heat pumps, but as I said, the pumps were not maxed out for any lengthy stretches.

    Secondly, I'd be thinking about zoning. I wouldn't want to turn on an oil burner that will distribute heat to my bedrooms during the daytime and my living room during the night. So what if it's cool in your bedrooms during the middle of the day on a Saturday?

    We have two smart thermostats for our heat pumps. We rent out our basement, so I put a Mysa thermostat in there because it's got a simple wall-mounted interface for tenants. Mysa's thermostats aren't particularly "smart" (they display the room temperature but rely on the user to set the heat pump temp), but they are apparently working on it. Upstairs in our part of the house, we use an Ambi Climate 2. It has various levels of "smart" control: from basic setpoint control and monitoring to thermostatic control (the thermostat tells the heat pump what temp to run at to achieve a particular room temp) to comfort-based AI (the thermostat learns what temp range you find comfortable and predicts heat needs based on weather, time of day, etc.) Ambi is really fussy, and I had some terrible customer service the one time the thermostat stopped working. The reporting is valuable - I check it daily.

    Both Ambi and Mysa integrate with smart home platforms, so there may be a way to sync up HVAC systems - we decommissioned our propane furnace and have room-by-room electric resistance as backup (yet to be needed), so I'm not sure what's possible.

    1. BFW577 | | #3

      If your looking from a cost perspective it wouldn't be hard to look up the cop's on your mini splits and compare the running costs to your oil burner at say 17 degrees. Efficiency Maine has a great fuel comparison tool at the below link.

      Prices for all fuels are way up in New England. I'm in CT and I believe electricity rates went up 26% and thought I read Massachusetts was seeing a 33% increase. These are all due to natural gas cost increases.

      Check out the savings in Maine with a heat pump vs propane and oil right now. Maine is using recent state surveyed fuel prices in the default data. It may still be significantly less expensive to run the hyper heats in Massachusetts with current high heating oil prices. I ran some numbers here in CT recently and a good heat pump was still much cheaper over oil even at some very cold temps.

      https://www.efficiencymaine.com/at-home/heating-cost-comparison/

  3. Josh Durston | | #4

    With the high price of oil, I would keep the oil off until it becomes a comfort issue, and forget about negligible cost savings. When it's really cold it would be nice to have some heat off the baseboards. I don't think there is a temperature where oil would pull ahead cost wise by a significant margin, but it might help to counteract your mini splits declining output as it gets seriously cold sometimes.

    I find my mini split keep up to setpoint just fine, but at some point as it gets colder my house becomes less comfortable in the corners, due to a dropping mean radiant temperature, even if the air is technically at setpoint. Then I switch to my wood stove and high efficiency NG boiler/panel rads and it's cozy, but not really any warmer, mostly due to heating at the perimeter of the rooms. A new high performance house probably wouldn't have my problem to the same extent.

    Depending on your pumping arrangement some hydronic systems can use pretty substantially electricity that often isn't factored in to operating costs (especially if you're zoned with a ton of pumps). Or pumping can be pretty cheap with a properly setup system with a ECM pump (my Grundfos pump usually says it's drawing only 10watts.)

  4. Wooba Goobaa | | #5

    Hey Mike. You have 5 indoor heads and how many outdoor units?

    1. Mike Thompson | | #6

      We have 2 outdoor units. The first is a 20K BTU/hr unit connected to a 6k and 12k head. The second is a 30K BTU/hr connected to a 6k, a 9k, and a 15k head.

  5. Nick Defabrizio | | #7

    Air to Air mini splits are interesting. For about 6 years I have used two single head one ton Fujitsu hyper heat mini splits to heat my main floor: 1500 sq ft (medium tight construction) in zone 5. The issue has always been heat distribution, not so much capacity or efficiency. My units run all day, without set backs and I believe that is the most efficient use. However, no matter how long they run, the heat does not travel well down a short 10 ft. hallway to the bedrooms (the units are in my great room/living room, dining, kitchen and hall)...For example, today, with the temps around 20 F I have to run the temperature to 73 F to get the two bedrooms to 66F-even though the bedrooms are at the end of a hallway that is only 10 feet long. Shutting one of the bedroom doors ups the temp in the other bedroom 2F.

    1. Rliebrecht | | #8

      Similar layout to our house. We found pointing a tower fan out of the hallway lifted bedroom temps by a few degrees simply by pulling cooler floor air out, which then draws the higher warm air in.

      1. Nick Defabrizio | | #10

        Yes, that makes sense....I don't have the room to do that but I may put in a room to room fan eventually..

    2. Mike Thompson | | #17

      My first floor is pretty wide open, so the rooms are heated pretty evenly with the mini splits. Upstairs is more of a problem because only 2 of the 4 bedrooms have units installed (and we also have a narrow hallway). I ended up putting Nest temperature sensors in the rooms without splits so the oil burner will kick on if/when they get too cold. But a fan seems like a great idea. I may try that out.

  6. Jennifer M | | #9

    Your situation sounds like mine, except that I rely solely on the mini split, with a wood-burning fireplace insert for back-up (which I haven't needed). I am in CT, with a 27k btu MRCOOL DIY condenser and two indoor units - a 9k btu below the window at the end of the bedroom hallway, and a 12k btu at the ceiling in the central living area (this weekend I am adding a 9k btu to the kitchen). My 1953 ranch is very leaky, but the minisplit runs all day most winter days at around 75-77. On the coldest days (today!) it is set at 85. My house is warm except at the very edges of the rooms that border the great outdoors. Is there a reason yours is set at such low temps? Am I negating any savings by setting it to my comfort level?

    1. Nick Defabrizio | | #11

      I was thinking about the MR Cool DIY unit for the bedrooms and basement. How do you like it?

      I just don't want to spend another $10,000 for a multi-head Fujitsu unit (and their multi-head units are not rated as super efficient anyway). I had read somewhere that the heat output on the MR Cool DIY unit is a bit on the low side, but you seem to like it ?

      1. Jennifer M | | #12

        My quotes for your system in my home were $30k, so I was predisposed to loving the MRCOOL! However, yes, I really do like it, and the install was truly DIY.

      2. BFW577 | | #13

        I don't think any of the Mr Cool units are using vapor injection compressors that most good cold climate units like hyper heats have. The performance really drops off fast on them usually after 32 degrees and they usually are only rated to 5 degrees. For their cost they still aren't bad units but they are nowhere as efficient as other stuff out there.

        1. Nick Defabrizio | | #14

          Yes but..... I am on the Southern edge of Zone 5 in Northern NJ. There are only a relatively limited number of days where it hits 5F or lower and then only for two or three hours per day. If you trust the NEEP numbers, the COP numbers at 5F for these Mr Cool units are not that far off, particularly compared to the Fujitsu and Mitsubishi multi head units. BTU's drop off fast on the Mr. Cool units but the remaining rooms I need to heat are small or a basement with low BTU requirements.

          In the end, in places like NJ or Conn, the install prices for the high end units have become insane. Look at what Jennifer is saying: $30k to put in hyper heat units! Who but wealthy enthusiasts can afford that in a retrofit situation? And that is on top of the need to move toward efficient electric cars, cooking, heating and cooling. I believe that cheap, efficient plug and play DIY mini splits are a key to broad and deep adoption of heat pumps in cold climate retrofit situations. I am rooting for MR Cool to do another generation that is more efficient

    2. Mike Thompson | | #15

      Our house was built in 1988 and is moderately well insulated. I'm pretty comfortable at 68 and I wanted to be more conservative - at least until I see what the electricity bills look like. I may invest in a energy monitor (like an Emporia Vue or Sense) and play around with temperatures to see what kind of difference it makes in electricity use. I'd be curious to see if there is any significant difference of a setpoint of 68 versus 70 or 72.

  7. Mike Thompson | | #16

    Nick - I never looked into a DIY option. I'd be curious how much I would have saved. Our HyperHeat units cost about $25K (5 heads/2 outdoor units). But we got $5,000 back in rebates and a zero interest loan for the rest. (Massachusetts has some really good incentives right now).

    1. Nick Defabrizio | | #22

      That $5k rebate is nice! NJ just transferred the rebate program to utilities. Guess what happened? Right: the rebates shrunk dramatically and are zero for those like me who get power from a municipal co-op.

      In any event, to give you a price comparison, a 27K Mr Cool DIY unit with two or three heads is around $ 2,800 complete (except for outdoor 240V electrical hookup) and a 36 k unit with two or three heads is around $3,300.

      The issue with MrCool DIY multi units is that they are not as efficient or strong at very low temps. They have only been rated to 5F and they lose more steam than Fujitsu/Mitsubishi hyper heat units as outside temps drop. Also, their turndown ratios do not seem as high. Thus, ultimately, even though the HSPF is about the same, your back up boiler will run a bit more and your electric bill will be a bit higher.

      Fom a purely economic perspective, if you assume a cost of funds at current mortgage rates (3%), it will take quite a long time to make up the $12-14k difference between the MrCool units and the Fujitsu units you had installed. In states where there are lower rebates the differential gets larger. This is the conundrum I am wrestling with in deciding whether to complete the full conversion to electric heat by adding another hyper heat unit or two. I have a very efficient Energy Kinetics boiler/baseboard and no rebates so the economic payback differential if I add a few more Fujitsu hyper heat units is much longer. I am converting purely for "civic duty" reasons...but I do not have unlimited deep pockets to do so.

      1. BFW577 | | #24

        I think your underestimating how poor the Mr Cool units performance is in cold. They essentially are rated at 47 degrees and immediately start losing capacity below that. From 47-17 degrees they lose 60% of their maintenance capacity. They don't have basepan heaters either so they are more of convenient cooling solution in my opinion.

        Here is the neep rating on the popular 12k diy unit. I would imagine a Mr Cool Multi split would perform worse than the more efficient 1 to 1 unit.

        https://ashp.neep.org/#!/product/30946

        1. Deleted | | #27

          Deleted

        2. Nick Defabrizio | | #28

          Actually the numbers: 6kBTU at 5F, are close to the load calculation for my bedroom at design temp for my area. Meanwhile the larger models (18k or 24 k) seem well matched to my mostly underground basement. Then, once temps approach zero, even my Fujitsu hyper heat models in the main room start to strain and I turn on my Energy Kinetics boiler.

          I think the issue with the Mr Cool units is their turndown ratio of only 1.97 and how much of an inefficiency penalty will I pay for the rest of the winter if I go with these versus two more Fujitsu models? I am struggling. When the delta gets close to $6k, I start to get cold feet (no pun intended). I can't imagine that pure economics will favor the Fujitsu units, especially when some of the additional electricity cost will be absorbed by my PV units.

          By the way, I am assuming the numbers for the MrCool 27k multi unit published by NEEP (https://ashp.neep.org/#!/product/30943) are similar to the performance numbers for the same size DIY unit. If so, the output losses are not as bad-going to 17kbtu's at 5F

  8. Richard Levinson | | #18

    Re: setbacks

    Personally, I've found that contrary to conventional minisplit wisdom, lowering the heat at night for my Fujitsu ducted unit does save a noticeable amount of electricity. However, there are a lot of factors that would go into whether or not it would save energy in a given situation:

    -difference in COP between higher output and lower output; some units have drastically different COPs (like 3-4x) at minimum vs. maximum output. The Fujistu has a smaller disparity in COP between outputs, so there's less of a penalty when running at higher output to bring it back up to temperature. But some are much more efficient at lower outputs, so turning down the temperature might be counterproductive.

    -normal nighttime vs. daytime temperatures; if you have generally warmer days and cold nights, then the increase in COP in warmer temperatures can sometimes be beneficial if you're ramping up the temperature during day time hours. On the other hand, if you have more consistent temperatures between night and day, then there might not be an advantage to shifting to higher output during the day.

    -how long the setback is for. During the week, I'll normally have the temperature be 2º colder for 16 hours of the day (11 pm - 3 pm). If it's only for 8 hours though, the math would probably be different.

    I think in most situations (particularly in colder climates) setting back the temperature probably wouldn't have a huge benefit, but there are situations where it definitely does.

    1. Rliebrecht | | #19

      I'm up in British Columbia and I noticed and improvement in our bill with a setback overnight. But we use an Ambi Climate to automate the setpoint, so our results vary depending on outside temp. We also only set back a couple degrees C.

    2. Mike Thompson | | #21

      Thanks for this. We're a family of 5 with school-age kids, so If I were to use setbacks, it would be probably be about 7 hours at night (10 pm-5 am) and/or about 7 hours during the day (8 am - 3 pm). I'm just not sure if it would make a difference or not. I'm thinking about attaching an energy monitor to my circuit breaker so I can actually keep track of the data (rather than waiting for my next electric bill). But the cost of that system will probably offset any savings - at least for the first year.

    3. Brad | | #29

      I've read the conventional wisdom to keep the set point constant for a heat pump, but I haven't seen the data & I'm not convinced that the generalization is valid either. In my case I program the thermostat to warm up slowly in 1 deg increments so it mitigates the full on hit to efficiency. Also in my case we have large outside temperature swings (avg about 15-45 in the winter), so running at night is the least efficient. My house also get warmed up by the sun so allowing the walls and contents to cool at night allows for more thermal storage and lower peak temperature in the warm room.

  9. Mike Thompson | | #20

    Here are the specs for our system. I'm guessing that the average low temp in Western Mass during the winter is around 20F. We'll get into the single digits from time to time, but not very often.
    Mitsubishi MXZ-3C30NAHZ2:
    SEER = 18.0
    EER = 12.5
    HSPF = 11.0
    COP @47F = 4.0
    [email protected] 17F = 2.65

    1. PBP1 | | #23

      I see you mentioned: "We have 2 outdoor units. The first is a 20K BTU/hr unit connected to a 6k and 12k head. The second is a 30K BTU/hr connected to a 6k, a 9k, and a 15k head." But not sure where in MA you are located? I lived in Western and Central MA and remember some -20F days around 1984.

      I'm in Montana and have a single Mitusbishi MXZ-3C30NAHZ2 (rated at 28k heating @47F) with a 15k, 12k and 9k ducted (the sum of the heads is to be no more than 1.3 times the rating). Also, COP is less with ducted. Mitsubishi provides the specs for different ducted combinations. I think COP @47F is around 3.85 for my combo.

      In a month with average temperature of 24F, the ASHP consumed about 1500kWh. From my data, baseline consumption for the ASHP and three heads is around 160 W (e.g., powered on with very low to no demand).

      My single ASHP can keep 2,100+ sq ft two-story at 68F at -5F (overnight low). I also have a lot of glass (six sliders) and exposed first floor (on piers), second floor is mostly over unheated two car garage. Heat load is 28k, accurately predicted by HERS rater and HVAC guys. There are many threads as to oversizing and from what I've seen few threads about people that have undersized. I think the "green" advice has been to undersize an ASHP wrt heat load. I believe Marc Rosenbaum (MA) has written extensively about sizing. Also, I think Dr. Bailes (EnergyVanguard) has the 24k Mitsubishi ASHP and has blogged about its operation and oversizing (https://www.energyvanguard.com/blog/can-you-oversize-a-mini-split-heat-pump/)

      I have used set backs at night, but no more than a couple of degrees (from 70F to 68F). I believe it makes some small difference. One head gets turned regularly to 74F+ (upstairs zone where someone likes it warm during the day).

      I use an Efergy True Power Meter on my breaker box, which has everything but the ASHP and then use the outdoor meter to get the difference as to the ASHP. A plot of daily usage in kilowatt-hours (kWh/day) versus average monthly temperature gives a "smile" where you can discern a baseline along with a heating slope and a cooling slope. It's not the most scientific, but provides some insights (subtract about 13kWh/day on left axis for estimate of ASHP usage to meet load).

      1. BFW577 | | #25

        You should look into the Efergy online engage hub. You can connect up to 5 sensors to it and get live 15 second power data right on your phone. Since you already have the transmitter on your panel you would just need the $79 hub. You can have up to 5 sensors on the hub. I have sensors on my solar, whole house, and both my mini splits.

        Here are some snapshots from the ap showing my live mini split power modulation and daily usage to give you an idea of what you can see. Its been invaluable seeing the data from my mini splits on how to most efficiently run them. Its also been truly eye opening how little power they consume.

        1. PBP1 | | #26

          Thanks, great idea to expand the utility of the transmitter. The readout appears quite clear and useful ;-) Appreciate the info!

  10. Mike Thompson | | #30

    I just installed an Emporia Vue on the subpanel that powers my minisplits. I'm going to collect some data and will report back!

    1. BFW577 | | #31

      I actually just installed the vue a few days ago. Very impressed with it so far. The 1 second resolution is awesome. You can see the inverter ramp up in real time. Anyone running a heat pump should install one of these. Makes it so easy to see consumption and make changes and compare energy usage.

      Also a sensor on your furnace or boiler circuit makes it really simple to calculate your real time heat loss. You can see exactly how long your furnace ran each hour to easily calculate your btu/h heat loss.

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