Minisplit heat pump newbie
Hi. I feel like an impostor on this site because I know very little about green building. Forgive my ignorance. I read blogs elsewhere that recommended this site for expert info on energy questions.
I have a 1,300 sq ft, 2 story home in zone 5A and have an oil boiler. I’m looking for alternative (read: cheaper) heat sources and have investigated pellet stoves and wood stoves. Someone then recommended mini split heat pumps. After doing some reading I had some HVAC companies visit to give me price quotes. Now I’m confused:
One company stated I need 4 Mitsubishi mini splits (2 on each floor) at ~$13,000, but he needed to go back to the office to calculate the heat load for my house before giving me a firm price or indicating which models I needed.
The other company priced out 2 Mitsubishi 12,000 BTU interior units and a 20,000 BTU exterior unit at ~$7,000 with installation.
Here are my questions:
1. Why the great divergence between the two?
2. Can you calculate heat load from the floor plans to the house?
3. How critical is it to calculate heat load when the house is so small? Is it possible to eyeball my needs?
4. Do the prices sound reasonable given the # of units? They seem much cheaper online, and even with labor, the quotes seem high.
5. Do you recommend non-Mitsubishi units that might be less expensive?
Thank you.
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Replies
J,
Great questions. The best way to proceed is to do a heat-load and cooling-load calculation (called a Manual J calculation), but it's often possible to use a rule of thumb to size ductless minisplits, especially if the installer is smart and has lots of experience.
The trouble is, many heating and cooling contractors oversize the equipment, because they really aren't that good at estimating heating and cooling loads.
Many homeowners are using one ductless minisplit per floor to heat and cool their homes. This works best if the house is compact, and if it has a well-insulated thermal envelope that has a low rate of air leakage. Even then, you are likely to see room-to-room temperature variations. These variations bother some families, and don't bother others. If you leave your bedroom doors open during the day, the variations in temperature will be smaller.
A unit in the hallway may work for heating, but it might not work for cooling if one of your bedrooms has large west-facing windows. Individual features of your house affect the heating and cooling loads.
For more information on Manual J calculations, see:
Saving Energy With Manual J and Manual D
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Calculating Cooling Loads
For more information on ductless minisplits, see:
Just Two Minisplits Heat and Cool the Whole House
Thank you for these links. It's reassuring to see that small homes can get by with 1 mini split per floor.
I doubt I'll be able to calculate my heat load on my own. My expectation is that on the coldest nights of the winter I may have to use my oil boiler, which is ok. Also, i don't have A/C in the house, so any cooling provided by the mini splits is going to be fine.
I guess I still want to get some more estimates to find out if $7,000 is a reasonable cost.
Thanks again.