Heat pump options
We are building a 20×32 foot cabin in inland Maine. The downstairs is open except for the bathroom and there are two bedrooms and a bath upstairs. The cabin sits on piers with a crawl space. It is pretty well insulated and with a design temperature of -2, the Better Builder Manual Z tool computes the heating load as 18K. I estimate that the downstairs load is about 12K and the upstairs about 6 — lots of big windows downstairs to look across the lake.
While one option would be to put a single unducted unit downstairs and let the heat find its way through open doors to the bedrooms, some family members are skeptical of that plan and want to shut their bedroom doors.
A second option would be to put an unducted unit downstairs and the smallest ducted unit I can find upstairs.
A third option might be to put a unit upstairs with three ducts, one of which would send heat downstairs. Could that work?
I appreciate your thoughts.
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Replies
I know it is scurrilous but consider the option of adding a few resistances heater to option 1 for use on the coldest nights of the year when the door happens to get closed. Yes, the BTUs they produce will cost 4X what you could get from a heat pump but they are cheep and easy to install and use zero energy if you leave then turned off.
Option 3 sounds like 80% of the heat load is downstairs but only getting 25% of the heat. One unit could carry the building but you will need more than 4 ducts.
Walta
You may be right that a little resistance heat would be sufficient for the bedrooms.
Is it possible to have vents of different sizes with the compact vent heat pumps? So that one big one going downstairs could take 2/3 of the heat and two small ones for the bedrooms each take 1/6? Or are do they all have the same capacity?
davidsmartin,
Being familiar with past NH snow levels I would urge careful consideration of where and how you place the outside unit as much or more than the inside. The weather has altered a lot in the last 50 years, but I do recall helping rescue a cabin roof from 5+ feet of snow way back when. Even something like Martin's snow shed roof might not keep enough off your unit after a deep snow.
I would support the use of cove heaters in the bedrooms and bathroom even if you choose a second delivery unit for upstairs. Balancing total capacity and variable output range of ASHP s
is tricky if you wish to avoid short cycling. Even with a the newest units you will not get a COP of 4 when the temps are nearing zero, so the energy cost penalty is not near so great. It is also nice to know that you are not dead in the water if the ASHP fails or freezes up. Service calls may take much longer than you like.
In regard to your heat load calcs, the floor may be as big an energy loser as the large windows looking over the lake. It will pay to make the effort to more accurately define where the losses are. In my own current design I was shocked at the losses through a floor over a garage which I consider to be as open to ambient as a crawl space would be. Even analyzing your past usage of whatever energy source you currently use will be helpful in keeping it real.