Help with mitsubishi heat pump

[tabner]

What is the model number on the outside unit? I can look up your performance data and give you some more information.

One thing you need to know is the minimum heat output on your 30k outdoor unit. It may have a range of 7k-30k btu for example. Let's say you turn on just the big 15k unit and it only needs 4k btu to maintain heat in the space. Since your outdoor unit can only drop to 7k btu the other 3k btu was probably being bleed off by the 6k units cycling the fan. Hot refrigerant is being pumped through those 6ks units even if you aren't using them. My guess is since you stopped the 6k units from bleeding the extra heat the 15k unit might overshoot the set point now. You would be surprised how little btus a space often needs. My downstairs 12k split will often run at minimum speed only delivering like 2k btu to maintain the space.

I'm in CT as well and use 2 splits for my main source of heat. Have you looked into solar? CT has one of the best incentives for panels. As you know CT has like the 3rd highest electricity rates in the US. Heat pumps and solar are a great combination here. I assume you got the state rebates on the mini splits? My electric bill is $9.62 every month and I will reach my ROI on my panels after 5 years this April.

sorry for the late reply Brian. Yes, i would like to do Solar at some point. For right now I am trying to heat my house as much as possible with my basement woodstove install, so the electric cost is hopefully going to be minimal. But solar is definitely on my to-do list, however i'd like to go all the way and do the whole house batteries as well (as opposed to the feed the grid and get credits back setup). So I figure no rush, technology gets better, and whole house batteries get cheaper, etc.
I am using the heat pumps a lot now though, i find they are much more convenient during this weather, as opposed to firing up the woodstove every day. And theoretically, with lows around 30 and daytime highs in the 50s, they should be pretty efficient/affordable?
I'm still not pleased with the performance of my 15k unit. It doesn't seem right to me. I have the fan and vane both set in auto mode, and yet it will never blow the fan on anything other than low. Even when it drops below temp overnight. I keep all 3 heads set at 68, heat mode, auto head, auto fan. in the morning the main floor (15k unit) will be 64 degrees. The unit will be calling for heat and trying to catch up, but it just won't get there until the sun comes up and the house gets some passive solar. However if i switch the setting to high fan speed, it catches up the temp within an hour. I'm not sure if having two units upstairs on the external thermostats, and having one downstairs with the original remote setup is creating problems. I'm going to have an external thermostat put in downstairs at my annual service, which is in two weeks, so we'll see if that helps.
I think at the end of the day i should have gone with the 4 head setup and a branch box, which was originally quoted to me. I have a feeling with the branch box i would not have seen these issues.

 

[Brian26]

sorry for the late reply Brian. Yes, i would like to do Solar at some point. For right now I am trying to heat my house as much as possible with my basement woodstove install, so the electric cost is hopefully going to be minimal. But solar is definitely on my to-do list, however i'd like to go all the way and do the whole house batteries as well (as opposed to the feed the grid and get credits back setup). So I figure no rush, technology gets better, and whole house batteries get cheaper, etc.
I am using the heat pumps a lot now though, i find they are much more convenient during this weather, as opposed to firing up the woodstove every day. And theoretically, with lows around 30 and daytime highs in the 50s, they should be pretty efficient/affordable?
I'm still not pleased with the performance of my 15k unit. It doesn't seem right to me. I have the fan and vane both set in auto mode, and yet it will never blow the fan on anything other than low. Even when it drops below temp overnight. I keep all 3 heads set at 68, heat mode, auto head, auto fan. in the morning the main floor (15k unit) will be 64 degrees. The unit will be calling for heat and trying to catch up, but it just won't get there until the sun comes up and the house gets some passive solar. However if i switch the setting to high fan speed, it catches up the temp within an hour. I'm not sure if having two units upstairs on the external thermostats, and having one downstairs with the original remote setup is creating problems. I'm going to have an external thermostat put in downstairs at my annual service, which is in two weeks, so we'll see if that helps.
I think at the end of the day i should have gone with the 4 head setup and a branch box, which was originally quoted to me. I have a feeling with the branch box i would not have seen these issues.

Hopefully the tstat for the downstairs unit will help. The issue with many of these wall units is the temperature sensor is in the return in the unit. It measuring the temperature way up high by the ceiling which can be way higher in the heating season than floor level.

Even with CT' s high electricity rates its still around half the cost to run a heat pump with a COP of 4 than oil/propane. Even at 17 degrees your unit is still cheaper to run with a COP of 2.6. At full output your unit is rated to use 1900 watts. That equates to a cost of .44 cents an hour to run at CT's .22kwh electricity costs. It probably spends most of its time running at half that consumption especially in mild weather. Remember those COP numbers are at full output. At lower speeds in mild weather your unit can achieve an higher COP closer to 6.

 

[tabner]

Hopefully the tstat for the downstairs unit will help. The issue with many of these wall units is the temperature sensor is in the return in the unit. It measuring the temperature way up high by the ceiling which can be way higher in the heating season than floor level.

Even with CT' s high electricity rates its still around half the cost to run a heat pump with a COP of 4 than oil/propane. Even at 17 degrees your unit is still cheaper to run with a COP of 2.6. At full output your unit is rated to use 1900 watts. That equates to a cost of .44 cents an hour to run at CT's .22kwh electricity costs. It probably spends most of its time running at half that consumption especially in mild weather. Remember those COP numbers are at full output. At lower speeds in mild weather your unit can achieve an higher COP closer to 6.

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Cool, thank you. I have the Tstat going in Nov 23rd, so i'll report back.
I really appreciate the info though, that will make me sleep better at night.
at .44 per hr even if it ran full bore for 4 months straight, that's about a $1,275 bill. I just did the math on what i used to pay for oil to heat my house and the 4 months of winter were probably around $1700. and that was to keep the house at 65, and somehow still feel cold all the time. And as you mention, my heat pump should not be at full bore for 4 months straight, so this is great news.

 

[tabner]

So with the new thermostat setup there is definitely an improvement. Rather than cycling on and off constantly, the 15k head runs more steady and in line with what i would expect. However it still seems to struggle to keep up temp, if i'm not using my woodstove to supplement.
Yesterday was a high of 35 at my place and the mini split ran all day long and was barely keeping it at temp. I am a little scared for single digits in January.
@Brian26 you seem pretty knowledgeable on these units. if I describe my insulation/floorplan, would you be able to guestimate whether or not my heat pump is sized correctly?

 

[Brian26]

So with the new thermostat setup there is definitely an improvement. Rather than cycling on and off constantly, the 15k head runs more steady and in line with what i would expect. However it still seems to struggle to keep up temp, if i'm not using my woodstove to supplement.
Yesterday was a high of 35 at my place and the mini split ran all day long and was barely keeping it at temp. I am a little scared for single digits in January.
@Brian26 you seem pretty knowledgeable on these units. if I describe my insulation/floorplan, would you be able to guestimate whether or not my heat pump is sized correctly?

Lets start with a simple check of the mini splits first. Its possible their is an issue with the system like a refrigerant leak. Multi splits can have like 12 flare connections and are prone to leaking because their are so many connections. The proper charge is critical on these systems.

I would measure the supply air temp coming out of the unit. Set the fan to high and set the setpoint to a really high number like 80 so the unit runs at max fan/compressor output. Wait 5 mins and see what the supply temp coming out of the unit is. You can do this with just the 15k unit running and then turn all the rest on max and measure those as well.