My house is heated by an old oil furnace, and the oil tank could spring a leak and flood our basement at any moment, so I'm hoping to replace it with a ground source heat pump this year. I'm trying to do my research now to figure out how big a heat pump I'll need, and how much energy it will consume.
First, there seem to be many different rules of thumb for estimating the size of heat pump required for a given space. I don't have a huge house, but the basement and garage are heated... EDIT: Here are some specific numbers:
One rule of thumb I read is (volume of heated space) x (indoor temp - outdoor temp) x .133. I figure our total volume is about 40,000 cubic feet (that's a 1,600 sqft footprint x about 25-ish feet of height, including the basement). We would want to set our thermostats to 68ºF in the winter, and the coldest days we expect to plan for here in the Boston suburbs are about -6ºF. So that works out to about 400,000 BTU.
Another rule of thumb: (total floor area) x (50 BTU/sqft for Massachusetts, in climate zone 5). Let's say our total floor area is 4,800 sqft (that is, 1,600 sqft x 3 stories, again including the basement). That works out to 240,000 BTU.
Using other calculators and various rules of thumb gives me a huge range between about 300,000 - 600,000 BTU (25 - 50 ton).
My gut instinct is that these estimates seem very high. Some of the comments already posted here before my edit suggest the same thing. I read this article, which suggests that I should not pay attention to rules of thumb at all, which seems like good advice. And Googling for typical residential geothermal heat pump sizes, it sounds like 3-6 tons is much more reasonable. So, my first question is: Why are the rules of thumb I initially used off by a factor of ~10?
Next, it's possible to estimate the power consumption of the system based on its published energy efficiency ratio (EER). Looking at the Energy Star data, it looks like it's reasonable to expect an EER between 16 - 19 BTU/watt-hour for a water-to-water, closed-loop heat pump. So, ignoring rules of thumb, supposing I have a 5 ton system producing heat at a constant rate of 60,000 BTU for every hour it's running (1 ton = 12,000 BTU, right?), it would be consuming about 3.2-3.8 kW of electricity. Let's meet in the middle and call that 3.5 kW. Note that my initial rule-of-thumb estimates had me consuming more like 30 kW, which would be an absolutely stupid amount of power.
Still, 3.5 kW seems a bit high. If I use that constantly all day, that comes to 84 kWh. For comparison, according to my electric bills, my house uses about 15 kWh per day total on average, maybe 30 kWh if I'm really blasting the air conditioning on a hot day. So I feel like I must be off by a factor of about 3-5. Or, to make this more tangible, my electric company charges about $0.28/kWh. So if I was blasting the heat constantly at 3.5 kW for a month, that would be about 2,500 kWh or which would come out to about $700 per month. That just has to be wrong; nobody would buy heat pumps if they were that expensive to operate.
I'm pretty sure the error here is that I won't be running the heat constantly at full blast; my thermostats will turn the system on and off as needed. But this adds an extra bit of complexity to the estimates that I don't know how to reason about; this is where I need help. My questions are:
- How can I estimate how much time the system will actually be running? Can I estimate this somehow using my existing oil heating system?
- Is it better (from the perspective of energy efficiency) to have a high-powered system that heats my space quickly and then shuts off, or to have a lower-powered system that heats my space more slowly and steadily?
- EDIT: I just learned that variable-speed heat pumps are a thing. I had been assuming that heat pumps can only turn on and off like a furnace, but perhaps that's not true. I have no idea whether this is a common feature, or how thermostats work with a variable-speed heat pump.