Minisplit heating system for high altitude
I would like to relate my recent experience and outline the present configuration and performance of my minisplit heating system to solicit comments on whether it is reasonably sized and balanced, and how I might improve it. We bought this house 5 years ago and are trying to improve it as we can. It was terribly leaky and we have air sealed everywhere we can. It is fairly tight now, certainly not great, but enough to get the interior humidity up into the 40+ percent range during the winter.
I have a 3 year old retrofit Mitsubishi H2I (MXZ-8C48NA outdoor unit, MSZ indoor heads) system in my 1250 sq ft ranch style house, built in 2000, located at 8100 ft in the Colorado front range. The climate zone seems to be generally identified as 5 by county, but the location is nearby high mountains identified as zone 7. The house is 2X6 construction with FG batts, and the windows are new Pella Impervia. The bedroom half of the house has a conventional attic with 16 in of cellulose, and the front great room has a vented cathedral ceiling with R-24 FG bats. The house sits on a full basement with about 1/2 of the perimeter fully buried, and the other half gradually exposed to a walkout at one end. The walkout end of the basement is finished with 2X6 walls and FG bats. The climate over the last 5 years has been similar to the front range plains, but generally 10-15 degrees C cooler. We have a lot of wind from the late fall through spring, often 30-40 mph, and gusting to 50+ for days.
The outdoor unit is 48K feeding 3 indoor units upstairs: 9K and 12K in the bedrooms, and 18K in the front great room. The basement has a 12K unit in the main room at the walkout end. The system was sized by eyeball by the HVAC installer company before I learned much about such things from this site. I used to think it was significantly oversize based on what I have read, but I recently learned several things that suggest it may actually be about right or even a bit small. Experience over the last three years has been very good, and we stay at 70 degrees F overnight when it goes down below 0 degrees F, and even managed 68 overnight on a -17 degree F night. When it is predicted to go below 0 degrees F at night, I turn off the basement unit to maximize heat for the main floor. I recently wanted to put in an additional 9K unit in the basement so I could warm the back end to 70 during the winter (my workshop). Typical temperature in the basement without heat is low 60’s, down to upper 50’s in very cold weather. The Mitsubishi Diamond dealer I contacted thought this would work since the Mitsubishi manual for the outdoor unit said the outdoor unit would accept up to 62K total indoor unit capacity (130% of the outdoor unit rating). But when the HVAC contractor checked with Mitsubishi to determine the appropriate total refrigerant charge, the answer was very different. They said that the system was already maxed out because the outdoor and indoor unit ratings were derated 24% due to altitude, and further due to the total lineset length (about 140 ft total for all the units). I did not find any of this in the unit specifications in the install manuals. Considering this, it seems I actually have a 36K system or even less (the derating for the lineset length is unknown). If I add up the heating capacities for the upstairs indoor units (derated for altitude), they are close to the derated outdoor unit capacity as that capacity drops below 10 degrees F (down to about 80% at -10 degrees F), which happens 5-10 times per winter. Does this add up to a reasonable system balance or design point? It seems to work well in practice. I realize we are designing by doing rather than computing the heating requirement, but this is where we’ve ended up and it seems to be working.
Apparently, to add additional heating capacity for the basement I will need to add an additional outdoor unit, probably a 24K unit derated for altitude to 18K, which will also handle the current basement indoor unit since I have found that it would be better to split the heating up between the upstairs and the basement. In the shoulder seasons we often wish we could have the basement on heat while we have cooling upstairs to counter high solar heat gain in the late afternoons. There is only one location around the house where I can shield the outdoor unit(s) from severe winds. Can I locate the additional outdoor unit close to the present one?
We have not found a HVAC or heating in the Colorado front range that actually seems to know the engineering of minisplit heating systems. I have low faith in whether the system I have is properly charged, and am having trouble actually getting any contractor to measure what is in the system now and determine what it really should be. I can never get any actual numbers, just arm-waving declarations, and Mitsubishi technical won’t talk to me directly. Can anyone recommend a good HVAC contractor/engineer in the front range that actually understands these systems? One further question I have no idea about is how all the derating affects efficiency, and how to optimize? All I presently know is that it works better and is cheaper to run than the previous propane heat. Any comments or guidance would be appreciated.
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