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Community and Q&A

Multi-Split Efficiency Optimization

lp20th | Posted in Mechanicals on
I recently found this web sight and see there is a lot of good info here.  I am getting educated. I wish I found this site before I had a 2 zone mini-split system installed.

This past summer a Mitsubishi mini-split system was installed in my house. It consists of two zones supported by one outdoor unit. It serves about 1800 square feet of living space, a large open cathedral ceiling area of about 1200 square feet (zone 1) and a smaller area with 9 foot ceilings, of about 600 square feet (zone 2). The house is located in south western New Hampshire. The house was built in 2003 and is 2×6 construction. Its insulation and air tightness is probably typical for its vintage, nothing special.  I have been working on improving it where I can. The original/existing heat source in the house is a hydronic baseboard system served by a propane fired boiler. The mini split system was installed to provide a/c but also to reduce the annual heating costs and not rely on the propane boiler as much as possible. 

The outdoor unit is a MXZ-3C30NAHZ2-U1 3 ZONE H2i HYPER HEAT PUMP, 2 1/2 ton, 30K BTU. The larger living area has a MSZGL24NA wall unit at 24K BTU and in the smaller area there is a floor unit, MFZ-KJ12NA-U1, at 12K BTU. The system worked great this summer with the cooling and it so far its been great for the heat in both zoned areas.  The smaller area does heat up faster than the larger open area, but the large area will eventually reach its set point. So far when simply adding up monthly electrical and propane costs its about breaking even compared to the previous year. Not a real scientific analysis but a rough generalization. I was hoping for some savings.

My questions are related on how to operate this system to achieve the lowest electrical power consumption. I see a lot of “set it and forget it” recommendations but still question this. I do not have any power monitoring so I can only observe monthly electrical consumption.  I am looking for general advice and opinions on the best path to follow.
A case in point, in the morning on a cold day the system is running and providing heat. If its a bright sunny day and this house does have lots of south facing glass, the sun will warms up the living space up well beyond the units set point.  Is it less costly to just shut the units down during that time and turn them back on as the sun goes away? 
The smaller zone is for a master bedroom area, during the day no one is spending any time there, should it be shut down?  Do you loose system efficiency by running only the one larger zone?
At night is it better to leave the settings alone, turn them down or shut the heads off completely? A cooler sleeping temperature is what is desired.
Does the system consume more power to cold start after being off for several hours vs leaving it running all the time?
Thank you in advance for any insights, opinions and suggestions anyone can provide.

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  1. kyle_r | | #1

    The first thing I would try is shutting down the master bedroom unit and only running the one in the living area. See if you get the cooler bedroom temperatures you want without being too cold.

  2. PBP1 | | #2

    Do you have stratification in the cathedral ceiling area (easy to check with a Black&Decker handheld)? Ceiling fans? I have the same ASHP as you do with three zones, one being a 15k ducted in a 20'x30' room sloping from 9' to 13.5' and I had to make sure the air flow was sufficient to decrease stratification (low pressure drop web products washable filters). Have you tried moving your thermostat, ours is not hung on a wall so we can move it around. If you have it in the wrong place and stratification, it might not pick up the heating from the sun. In general, its best to keep a steady set point and a higher fan speed. My baseline energy usage is about 160 W for the ASHP and three air handlers when there's no demand for heating or cooling. For 2,100 sq ft in MT, a relatively cold month (ave temp around 20F), usage for the system is about 1500 kWh.

  3. lp20th | | #3

    The cathedral ceiling in the larger zoned area is sloped from 9 ft to about 12.5 ft in the center. A 2nd cathedral ceiling area, the kitchen, is adjacent and open to the main living area. The 24K BTU wall unit serves both areas. Both areas have ceiling fans and are used to keep the air mixed. The fans definitely help with heat distribution. The thermostat for the wall unit is built into the unit itself. Both the 24K and 12K units are currently run in the auto fan mode.
    Comfort has not been an issue. Operational cost is the concern along with what, if anything, can or should be done to minimize operational cost.

  4. Richard_L | | #4

    Looking at the specs for that unit, and it looks like the Coefficient of Performance is pretty similar between the minimum, rated, and maximum outputs, so there's likely not much penalty in setting the temperature lower at times you need less heat like there is with other minisplits. So you could try that and see if it both improves the comfort (i.e. not too warm overnight) and saves energy.

    It's also possible the units are short cycling at times, which will tend to hurt the efficiency. Again, it might make sense to just turn them down/off in the times when you're getting solar gain so they can run more or less continuously when you're needing the heat.

    It's hard to say more without knowing more details (construction details, climate zone, how many kW/h you're using for heat, etc.).

  5. lp20th | | #5

    Thank you all for the responses. Here in New Hampshire we are going through the 1st real cold snap of the season. Temps bottoming out to zero and below at night. The system is putting heat out fine but I am also cycling the the boiler a small bit as insurance the pipes don’t freeze. I do have one vulnerable zone where pipes are in the ceiling of an unheated garage. The ceiling is insulated but the air temp in the garage dropped to 30 degrees last night.
    Regarding running the multi-split to obtain maximum efficiency while minimizing power consumption, time will tell with some experimentation of lowering temperatures and/or shutting one or both heads off when heat is not required. Hopefully I can find the sweet spots.

  6. nickdefabrizio | | #6

    Your situation sounds so much like my situation in northernmost NJ....which is a similar climate to the lower elevations in southern NH. The only difference is I have two Fujitsu 12k single head units in the main room and a very efficient Energy Kinetics System 2000 oil boiler.

    I run the boiler to heat the finished basement and the bedrooms on cold nights and just let the mini splits run without set backs. They don't use a lot of electricity, but my oil usage hasn't gone down as much as I would like, even though the System 2000 boiler is efficient and low mass. It has a 100k btu (.85? ) nozzle and I guess it can't throttle down far enough to cut oil use beyond a certain point. I am planning to add additional split units in the basement and one bedroom to ween off oil on all but the very coldest days .

    My problem is that now, with mini split rebates eliminated and huge mark ups on pro installed systems in my area, it would cost too much to add two or three heads I need to save maybe 200 gallons of oil a year (I got a quote for $8k for the MXZ-3C30NAz unit-not even the hyper heat!)......So instead I am looking at alternatives that allow me to self install-like Mr Cool units . These may not be as efficient as better units but at 1/3 the cost, the difference in power usage is not that great over a season (I do not need much A/C) and it is much cheaper to add an additional PV panel or two to cover the differential. I am also looking at monobloc Air to Water alternatives running a few panel radiators (I like hydronic heat), but that needs more research

  7. lp20th | | #7

    Hey Nick, I believe we have some additional coincidental similarities. I have over 30 years of experience with an oil-fired System 2000 boiler. In 1987 I built a home in Northern Connecticut. I had a 1st generation System 2000 installed at the time. The house is around 3000 sq ft. and was built to be energy efficient with a sunroom, solar mass, good insulation, low-e glass , etc., all the right things done as was considered back in the day. I can't say enough good about that boiler. Just sold this house in October 2021 with the original boiler still operational. My 2020 oil consumption was under 400 gallons with 2 people occupying the house, of course it was more when the kids were living home but annual oil consumption was typically under 500 gallons in all those years. Over the years the typical parts were replaced on the boiler including the domestic hot water holding tank but nothing that any other boiler might see. The down-side for the System 2000 is any of the proprietary parts are expensive and its difficult to get them if you are not a dealer.

    In 2018 we purchased the New Hampshire house, a lakefront property, with the intention of eventually retiring there. This house has about 2800 Sq Ft of finished living space, which includes a finished basement. It has a Burnham Revolution propane-fired boiler. I was somewhat shocked over the 1st couple of winters at the cost of operating the boiler, especially not living there full time, vs the Connecticut home, around double for annual fuel costs. This will be the 1st winter season having full time occupation. I did find the boiler is oversized for the application and also have found several issues with this boiler and have remedied some. I also have tightened up the envelope of the house where I could as well. In any event I needed A/C and the mini-split seemed like the way to go as I get heat too. It was installed this past summer.

    I found a local Mitsubishi Diamond Level installer and with the New Hampshire rebate I was in for under $7K. I did the electrical myself. I could never get a price like that in Connecticut. I also believe I can get a few hundred in federal credits as well when I do my 2021 taxes.

    In hindsight I probably should have went with separate outside units vs the 2 zone system. It probably would be more efficient to operate. The finished basement still relies on the boiler for heat but most often is not occupied and the T-stat is turned down to 55. most of the time.

    I have done a rough monthly cost comparison of (electrical + propane), June through December, for 2021 vs 2020. Propane per gallon cost has been steady at $1.99 a gallon during this time, electricity costs may have fluctuated somewhat. Both will be going up in the near future. The 2021 monthly costs were slightly more, however its not a very scientific analysis. In 2020 there was no A/C and full time occupation did not begin until October of 2021.

    I have friend here in New Hampshire who self installed a unit for himself in a finished basement and also helped install a similar unit into another friends house. So far both are very happy with the systems. I believe they were Klimaire brand with pre-charged lines, purchased through Amazon and not too expensive. The install was easy too.

  8. BFW577 | | #8

    An inexpensive electricity monitor would answer all your questions and pay for itself pretty quickly. You can see live exactly how much load your unit is under and how much electricity its using each min, hour, day, and month. Seeing all this information will allow you to make adjustments to maximize the efficiency of your unit.

    With the electricity data its simple to compare data after you make adjustments. Perhaps setting your temperature 2 degrees higher stops your unit from short cycling and now your unit is running at twice the efficiency under steady load.

    Monitors have come way down in price. Emporia just came out with the vue that is only $150 and monitors your main panel and comes with 16 sensors for individual breakers.

    I use an Efergy Engane monitor and attached a few screen shots to show you the data you can see. I can't recommend one enough. After seeing the data myself I was able to make tons of changes to maximize efficiency of my units. The electricity monitor easily paid for itself.

    1. tim_st | | #11

      @BFW577 what kinds of things have you tried that you've found to work? I am seeing what feels like pretty high energy usage from my multisplits at night now that it's properly cold here (lows in the 20s or 30s), but it feels like there's too many possibilities to know what to test out in order to optimize performance..

  9. lp20th | | #9

    Thank you for the info. I will definitely look into getting a power monitor.

  10. jameshowison | | #10

    Can you set the bedroom unit to "thermal off"? Then when the set point is reached in the bedroom then fan will stop blowing. That will reduce over-heating in the bedrooms when the main unit is calling for heat, that might preserve heat in the lines for the main room.

    OTOH, overnight you might find the on-off of the fan in the bedroom annoying, especially overnight. I don't know whether thermal off in the wall units is a software setting (some are) or a hardware setting (some units require cutting a capacitor for this, which is bonkers and can't be undone).

  11. lp20th | | #12

    Both units have internal thermostats and handheld remote controls. When a set point is reached in a unit I do observe the fan speed slows down sometimes to almost not running at all. Would that be considered thermal off? I don’t feel much warm air flowing from the vents, sometimes it feels like the air is the same temp as the room air.

    The lowest set point when in the heat mode is 61 degrees. If I set both to 61 at bedtime I see the larger zones actual room temperature to be around 60 - 61 degrees. In the morning. The smaller zones room temperature however is around 66 degrees. Not a horrible thing but it definitely overshoots the set point. When both are set to say 67, the two areas tend to stay on track and the room temps match the set points. The smaller zone reaches set point quicker though which is understandable.

    1. jameshowison | | #13

      No, "thermal off" is when the fan turns off entirely, rather than just slowing right down. It's a setting somewhere but it's not well documented (it isn't in the service manual for this unit).

      But if you aren't seeing overheating in the small zone at the set points you want then likely they are reasonably balanced (ie the total BTU needs of both together to maintain that set point is larger than the outside unit's minimum capacity), so nothing to worry about.

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