Resistance Heat vs. Wall-mounted Air Handler
I am looking at converting the upstairs of my house from an oil-fired hot water system to a mini-split heat pump system. The upstairs effectively has 5 areas, 4 of which are currently heated:
1. Master bedroom (~400 sq ft)
2. Office (~225 sq ft)
3. Kid’s Bedroom (~225 sq ft)
4. Bathroom (~45 sq ft)
5. Hallway/Landing/Stairs area (not currently heated, I don’t see any need to heat it).
The walls are all about 7.5′ tall from floor to ceiling. The floor has 10″ high joists underneath it, followed by the downstairs, which is heated by a pellet stove. As far as I know, there is no insulation in the floor.
Ceiling opens to the unconditioned attic, with fiberglass batts in between the 10″ high joists.
Looking at some rough calculations, I can add nominal 6 KBTU/hr cooling (~8.7 KBTU/hr heating) units to the Kid’s Bedroom and office, with a 12K BTU/hr cooling (~15 KBTU/hr heating) unit to the master bedroom and be just fine. However, I am worries about not getting enough heat into the bathroom then (see attached sketch). As I see it, my options are to either:
1. Add straight resistance heat to the bathroom (about .6-.7 kW of demand on a cold day). This is cheap and easy to do, but in-efficient. Something like the cadet in-wall electric heater. Couple of hundred bucks plus a little bit of wiring.
2. Instead, replace the planned Mitsubishi model FS 6K wall-mounted air handler in the office with a 9K model SEZ concealed duct unit. Then run duct through the attic so that that unit heats both the office and the bathroom. This would presumably be more efficient, but:
a. It’s a lot more costly
b. A lot more work to run the duct
c. While I don’t have a rating for the full system with the two different units plugged in in this location, if I look at each option as a single zone system, ecomfort.com shows the efficiency dropping from 24.6 SEER with the FS unit to 17.3 SEER with the SEZ unit. Plus, I will be running duct through the unheated attic. And I will lose attic space.
So, my questions would be:
1. Does anyone have a way to estimate the actual differential electricity usage between the two options? In other words, what is the equation used to go from SEER to actual energy usage.
2. Are there other options here that I haven’t thought of?
3. Unrelated, but I am most of the way through finishing my basement. Currently the basement is just heated by the waste heat from the oil boiler. Also, during the basement renovation, I disconnected the hot water heating loop in the boiler, and instead installed a heat pump water heater (A.O. Smith 55 gallon). The basement is noticeably colder already – once I remove the boiler (and thus the waste heat generation), any thoughts on if I will need to stick a 6K BTU/hr heat pump unit in the basement to keep it warm in the winter? Or at that point, would I actually save energy by just using the back-up resistance elements in the water heater during the winter (instead of first pumping the heat into the basement, and then pumping it again into the water). At the moment, I am thinking I won’t install anything down there, but will leave room in the system to add one if I have to.
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