Minisplit Make and Model Recommendation
I am building a 1.5 story house in the midcoast region of Maine (CZ – 6), inland from the coast about 20 miles. It will be a double stud build insulated with DP cellulose (I have estimated a total wall R value of about 32), triple pane windows, and an R-50 ceiling/roof.
I estimated heat loss to be about 18,000 BTU/hr, including losses due to exfiltration. About a third of the non-exfiltration losses occur via the walls of the full basement.
I intend to heat primarily with wood so, in a sense, the heat pump is a backup system. That said, the unit needs to be capable of keeping things warm when I am away or can’t burn wood for some reason. In the past I have kept bedroom doors closed and have not actively heated the upstairs (2 bedrooms and a bathroom), I expect those habits to continue. The basement will be unfinished and unheated. I do not care about the cooling side of things wrt the heat pump.
Given the above, I am tending towards a a ductless unit on the first floor, and resistance baseboard heaters on the second floor. Others have suggested different approaches in prior Q&A but this seems like the right balance for me at this time.
With that all said, three makes/models of heat pumps have been recommended to me. They include the Mitsubishi MSZ-FS15NA and MSZ-15NAH and the Samsung Max Heat 18K (RNS18ABT and RXS18ACT) and 24K (RNS24ABT and RXS24ACT) models. The max heating capacities of the Mitshubishi model are 16K and 14.4K at -5 and -13 deg F. The corresponding data for the 18K unit are 17.7K and 12.5K, for the 24K unit they are 21.2K and 14.6K. The max heating capacities of one or more of the units are light relative to my heat loss calculations but I think I need to balance this with the fact that I am not trying to actively heat the basement or second floor.
Do any of you have any opinions about the appropriateness of these units (in terms of capacities, brand and model reputations, etc.) given my situation? Are there other makes and models worth considering in addition to, or instead of, these? Thanks for any help you can provide.
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Replies
Rob,
I’ll give you a bump. Are you installing rigid foam on the basement walls? Just curious. With the basement, it would be pretty easy to install a ducted unit and provide good coverage for the basement and first floor. FWIW. If you have never lived in a tight house, you might be surprised by how easy it is to create fairly consistent conditions throughout the living space.
Hi Steve,
I will be applying R10 worth of foam board to the interior of the basement walls, that insulation is reflected in my heat loss estimate. I suspect I will also frame and insulate a 2x4 wall inside the foamboard which would more than double the R-value. My heat loss estimate drops 2,000-2,500 BTU/h with that change.
The basement will be unfinished and will be used for utilities, storage, and a shop. I am not really concerned about actively heating it, figuring it will be comfortable enough for the intended uses. I am more interested in optimizing the heat pump for its primary purpose, heating the first floor (actively) and the second floor (with waste heat from below) when I am not burning wood. In all likelihood, I expect the heat pump might be used to keep the house warm less than 10% of the time.
I would argue for some type of ducted HVAC as a better way to achieve comfort (with the wood burner as a backup source) in your tight house. You could create two zones (one for the main floor and one for the upper floor) and still dump a bit of conditioned air into the basement without treating it as a "zone." One of advantage of this approach is that you could have AC for the few days or weeks each year when it is needed. A system with a dry-mode would also give you a way to reduce humidity during the shoulder season.
I know that burning wood is supposed to be carbon neutral, but I'm not a fan of using this sort of combustion as a primary heat source. Wood burning introduces quite a bit of pollution into the atmosphere and can be particular hard on people who have respiratory issues.
Also... What is your ventilation strategy? Tight house absolutely need air exchange.
Hi Steve,
I'm still devising a ventilation strategy. I absolutely understand I need one.
I’m not familiar with Samsung. At this size I like the 15k Fujitsu. I would be comfortable with the 18k Mitsubishi also. I wouldn’t try to get ultra conservative. I would focus on finding an installer you trust at a competitive price.