Mitsubishi minisplit not as efficient as expected – Is something wrong?
In July we had a Mitsubishi Hyper Heat system installed, MXZ-5C42NAHZ in our raised ranch house in connecticut. We have 5 wall mounted units in the house, 2 6k units (smaller bedrooms), 1 9k unit (larger bedroom) , 1 12k unit (finished basement) , 1 15k unit (living room/dining room/kitchen area)
We were amazed at the efficiency in the summer months, July Meter read 1176kwh, August Meter read 1261 kwh, September Meter read 1223 kwh.
The heating portion is where I am not sure if something is wrong. December meter read was 2119 kwh – This was having all 5 units set to 66-68 at all times. I also had a wood stove going in finished basement for about half the month, which keeps that room above 70, and radiates through the house – I was assuming this prevent the mini split from working as hard. When I got this bill I was blown away at how much energy it was using.
For the next month, I turned off the basement unit, had woodstove going for nearly entire month, and the other 4 units I reduced temperature to 62-64. When i received this bill it was almost the same, 2007 kwh.
Does this seem high? Am i doing something wrong? Am i not helping anything by having the units on but the wood stove cranking? It almost seems like on is on, and will be a similar usage regardless of what temperature I have units set at and I am wasting time with using the wood stove concurrently?
The house had electric baseboard heat before this which I would only use when the woodstove was not cutting it, however I was told by installer I would have significant energy savings with this mini split vs the electric baseboard heat…. so far I am not convinced and am not very excited about the 17k bill to have this unit installed…
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Seems reasonable. 1000kwh input should give around 3000kwh of heat (COP 3 for December).
That around 10 million btu's, which works out to about 14,000btu/hr.
Heating your house is usually the most energy intensive thing that happens on your property. So yes your bill doubled from the summer(added 1000kwh), but that's probably between $100-$200 dollars depending on your rates.
You should expect heating portion to be cut to between 30-50% of your previous electric only consumption. But if you're heating previously unheated spaces (basement) you can expect to erode your savings.
There are also often significant design problems with overly zones multi splits with oversized heads, and outdoor units that may push your effective COP as low as 2 or lower.
I guess I just don't understand how using the wood stove in combination with the mini split is not doing more. It seems if mini split is on its on and cranking away at electric meter. The months were similar temperature outside and first month only used wood stove half the time, and had basement unit on ( i would assume not doing anything since wood stove was keeping room warmer than unit was set at) - all units were around 66-68 - 2nd month had basement unit off and wood stove cranking entire time - all upstairs units in low 60's.... however was only about 100kwh hour difference.... for some reason it really seems that on is on and temperature setting inside does not matter, and either does supplemental heat... when that woodstove is cranking it is radiating through floors and i wouldnt even expect the large living room unit (15k) to be necessary, i always assumed it would only be drawing power if it was required for heat.... there is a constantly running fan on all units but most of time only blowing cool air when its not heating
what was your previous heating system?
what was your previous a.c. system? You mention being amazed at efficiency in warm months...
what is your domestic hot water? electric? oil? gas?
Is your home well insulated/weatherized?
Heat pumps are not magic, just because many use them with success in cold climates. The trick is a well insulated home that allow for smaller systems and smaller heat loads/losses.
Multiple heads, long refrigerant lines snaking around the house and inadequate levels of insulation make for a long winter.. Not to mention cleaning multiple heads annually for mold accumulation.
Try controlling your fans manually set to high not on auto.
He stated his shoulder season consumption was around 1200kwh, and Dec around 2200kwh.
Previously was on baseboard electric (and wood stove).
I'm in MA and have two Mitsubishi 3C30NAHZ2 each connected to 3 indoor heads. Your story is similar to a few others with Mitsubishi multi-zones, including mine. I would suggest shutting off any heads you don't need and putting the fans as high as you're comfortable with on the others to save electric consumption. I put power monitors on my units to help me troubleshoot the high consumption.
Also, I wonder if your outdoor unit shuts off once your rooms reach their set temps. Mine seems to do that and at least one other user here noted that as well for their Mitsubishi multi-zone system.
Do you have a sense of what your base load is of that ~2000 kWh so we can get a sense of what the heat pump alone is consuming? Maybe crunching some numbers with the consumption, the average temperature during your billing cycle, and the heating degree days (HDD) may also be worth it to understand the heating demand of your house during the period and if it makes historical sense with what you've seen in past winters and estimated COP.
The outdoor unit shuts off as in the fans stop spinning ? Yes that does happen I have been out there when they are not going. Reading in June of last year was 1026 kWh , so that would basically be no heating/ac . I have electric everything it is basically my only utility bill. The units work better with fan set to high? I have had the fans on auto figured this would be better at adjusting based on when needed? High you def hear them could I do medium? Still better than auto?
Yes, if the fan stops running on the outdoor unit then it has shut off (or is in the middle of a defrost cycle). These heat pumps work best when they modulate up and down, not power cycle from on to off and back on again. My experience at least is that the modulation isn't great with the multizone Mitsubishi systems. Mine will run at a power significantly above minimum then shut off upon reaching room set temp, then when the temp drops a degree below that, the unit will turn on again at that same high power consumption level. I'm just wondering if you're experiencing a similar scenario. I'm just looking to accumulate data on how these things operate and why a lot of multizone users are posting frustrations with efficiencies.
That said, 1000kWh for heating sounds great to me for the winter, even this mild one. I'm using a bit more than double that. My house was built in 1968 and is 3400 sqft, including a partially finished basement that I am heating. My insulation is stock for that time period, except for my roof that I had spray foamed w/ closed cell to R38 and gable ends to R21. How big is your house and how well insulated / airsealed is it? If you're only using 1000kWh, then I suspect a MXZ-5C42NAHZ is quite a bit oversized for your house... or perhaps it is spec'd right and you're offsetting a lot of the load with the wood stove. I probably could have gotten away with that unit for my house if I ducted the second floor and really tightened and insulated up my basement. Also, a 12k head for the basement sounds really excessive. Did you have a Manual J done on your house before sizing this system?