Minisplit woes
Hi all, hoping someone who in an expert in Mitsubishi can weigh in!
I’ve had difficulties with this installation from day 1, mostly incompetence with the electricians subcontracted by the hvac company that I’m currently working on resolving (could this be at all to blame!?)
My main concern is my electric bills, we have a MXZ-3c30nahz2 and a MXZ-3c24nahz2. The whole house is heated with 4 heads, two on each unit. Two 6k on the 24kbtu and a 12 and 15k on the 30kbtu. I had originally wanted them split with a 6k on each but the installer messed this up and it was too late to have it repaired when I realized the error. Anyway! I’m using 3000kwh per month in the winter in southern VT- this is almost $600 at our utility rates. Most of this must be the heating, we keep the house set around 67 but the 15k zone rarely reaches past 65 most of the day. Old house but pretty good insulation levels. Still finding it hard to believe electric usage is so high. I’m pretty comfortable most of the time but that one zone that has a hard time keeping up is often a bit colder than I want.
Can someone advise on whether this sounds like an installation error or design error? My contractor originally had everything specced much larger and I talked him down. (Original quote was two 30k units, 9ks upstairs and same downstairs)
happy to provide more info, having a hard time figuring out where to start here! Thanks in advance
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What was the old heating system? Do you have any idea of what that level of usage was?
Old heating system was an oil furnace but for various reasons it was inadequate. Most importantly there were no ducts upstairs in that setup. We used it for a year but had the temp set pretty low. Something like 800gal of oil for the heat season and kept the temperature around 65. Hard to base much off that though because there was no insulation at that point and heat was inadequate.
I would install an energy monitor like an Emporia Vue 2 to get a more detailed look at your electricity consumption. Perhaps something else like a bad well pump with a leak is drawing excessive power.
You can run through the math here using the oil consumption. Having the house at 65F is not a problem, use base 60F for the heating degree days.
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Without doing the full math, I would guess your 800 gallons of oil put the house in under 2.5 tons of heat loss, so installing equipment capable of about 5300oBTU output at 5F is silly oversized. There is no way that will ever run efficiently.
Your best bet is to use only one of the units, either the 3C30 or the 3C24, for heating the whole house. Running both for cooling is fine as the cooling season is much shorter.
Generally I agree, but the thing I wonder about is why the house is unable to reach the 67F setpoint.
Some zones are just fine and capable of reaching their set point but the 15k zone (also physically the largest area) runs constantly at full fan
Runs constantly is what you want, properly sized and operating mini split runs 24/7, the fan never shuts off, always blowing a bit of hot air.
Not reaching setpoint, or at least maintaining an even temperature, is a different story. As DC said, most likely undercharge. This effects all indoor heads but since the rest are way oversized, they would still reach proper setpoint.
As for the output:
https://ashp.neep.org/#!/product/26170/7/25000///0
https://ashp.neep.org/#!/product/26173/7/25000///0
Most hyper heat units produce above their rated nameplate capacity at 5F.
Is this system actually capable of 53k output? If that’s possible why would the one zone never be satisfied? That’s what I’m having a hard time with. My understand was that oversizing leads to short cycles
City water, all electrical appliances are brand new and seem to be functioning normally. I can’t believe it would be anything besides the heat system. It does seem like the outdoor units are a tad oversized but would that really account for this?
If the compressors were under-charged with refrigerant you'd see this kind of behavior.
Hm, is it possible for only one of the indoor units to be undercharged or would that change how all the units on that compressor function?
The charge is for the outdoor unit and all connected indoor units.
Ok so it looks like this is a combination of a low charge and oversized equipment. Is there any world that I can get this fixed? I have no idea where to start. I watched a video on an installation of one of these units and the guy was talking about holding a vacuum for 24 hours, this is certainly something that never happened with this installation. I can’t count how many problems there have been with this and I’m already $20k deep on installation so far but if I’m going to be spending $4k each year to run it I might as well get this fixed.
Please help I have no idea what to say to my “diamond” contractor to get this fixed. Is any of it under warranty? It feels like negligence from the installer but I’m worried that because I requested changes to their recommend equipment (smaller units) that they’ll blame me for not going with their recommendation. I’m completely overwhelmed by this problem….
Thank you everyone for helping with this
Maybe I am missing something here, but the question of whether a 3000 kw electricity bill is too much depends on what month you are talking about...does 3000 kw for an entire electric bill sound so far off?
If you were using 800 gallons of oil in a heating season, at 43.9 kw per gallon of oil that is 35,120 kw of heat. If you assume your oil furnace or boiler was only 80% efficient then it took approximately 28,100 kw of heat (35120x.8) to heat your home (or 28,100 kw in heat loss) for an entire season. I assume that the coldest months of January and February were the worst so it is not unreasonable to assume it takes 6000kw per month to heat your home (or 6,000 kw in heat loss) in these months. So if your electric bill was 3000 kw total in January or February then that would not be surprising if you assume these Mitz multi units operate at a COP of 2 (2 kw of heat per kw of electricity input) during the coldest months in Vermont. I don't think these units are much more efficient than that in the coldest months in Vermont (Zone 6?)
Many folks on this site are far more knowledgeable than I am so if I am off base in my calculations, please say so. Thanks
Thanks for this, I’m not knowledgeable enough to check your work but just wanted to add that the other element at play is I significantly improved air sealing and insulation since the oil heating year:
Attic started at ~1 inch of cotton, now has addl. r-26
Floor over unfinished basement started at 0 now has r-30
Basement walls started at 0 now have r-15
Sill plates are now sealed and some upstairs walls now have r-15
Most of this was done this past year so I’ve had two heating seasons with mini splits, this year i assumed they would really shine with the improved insulation but numbers are about the same for electric usage in the two years I’ve had them- also this year has been a much warmer winter in VT! (zone 6 btw)
Oh, I see. That is strange; especially since this has been a very warm winter , although recall that late December had a very cold spell where much of Vermont was below 0F...maybe that skewed the numbers temporarily....see if your next electric bill goes down....(BTW: my family is going to ski in Stratton in a few weeks, I hope there is snow !!!!!)
I have had many mini splits installed and sometimes it is hard to figure out what is going on. Leaks can be an issue. I noticed you said they did not do a nitrogen pressure test. Most of the HVAC guys I have used didn't either. Nor did they use a torque wrench to tighten the fittings. :(.... Also, note that with many Mitsubishi units, refrigerant should be added if the line sets exceed 25ft in length.....
I would recommend starting with running a simulation in BEopt which is free software from the National Renewable Energy Laboratory (NREL). It isn't overly complicated to learn and there are some good videos on how to use it as well as info here on GBA. This will give a good idea about how much energy use should be expected. From my experience in a cold climate like Vermont you will see higher electricity use in December and January than you might expect but the other months will be less with much lower use in the summer than a standard AC unit. For reference an efficient new 2800 sqft house I designed that is all electric with Mitsubishi units in northern Utah (where it has been a relatively cold winter) used 2500 kw in December but other months have been been much lower.
Wasn’t able to get beopt up and running because I have a Mac (and only a Mac, no virtualization option right now either). I did run a coolcalc for my own sanity and coming back with a combined load of about 42k btu for the house. I have a heated floor in a large bathroom upstairs which accounts for some of the upstairs load. Looking like maybe 12k upstairs and 27k downstairs is realistic. Right now I have two 6k upstairs on a 24k outdoor unit and a 15k and 12k downstairs on a 30k outdoor unit.
Correct me if I’m wrong but that seems properly sized to me? Of course the outdoor units are capable of much more. Is that a problem? I don’t fully understand the oversizing concern tbh- does it lie in indoor or outdoor unit sizes?
The current configuration was at my insistence as the installing contractor wanted to put in much larger units. This is what we ended up with. How wrong is it?