So little heat energy is required at 60F-70F outdoor temperatures, it wouldn't make sense to install the extra heat pump complexity for that small range of heating.
Got a source?
I'm not Al in Ohio, and I don't have a definitive source, nor did I sleep in a Holiday Inn Express last night
, but I'll offer up two observations and some back-of-the-envelope calculations:
Observation: Just because Trane says that a heat pump can be >effective< down to 25 or 30 (Deg F), doesn't mean it is more >cost efficient< than other available options might be. There's no doubt that air-to-air heat pumps use more energy to supply heat as the outside temps go down. Clearly at
some point, depending on the cost of alternatives (which >don't< have efficiencies that change with outdoor temps), the heat pumps aren't the most cost effective option
Observation: Builders do all kinds of things to reduce their costs that ultimately increase long term costs to home owners. If a central AC unit (or mini-splits) are needed anyway for cooling, the incremental costs of adding a heat pump are small, compared to plumbing a gas line, installing a gas flue (or vents in and out), giving up floor space for a separate furnace, etc. The same goes for other auxiliary sources of heat (oil, propane, etc).
Promised back of the envelope example: I couldn't find any ready-made, impartial "with numbers" examples of heat pump costs vs alternatives at low temps, so I did the hard way. A
Fujiitsu mini-split model ASU/AUO12RLFW1 uses 5.9 amps at 220VAC = 1300 watts (outdoor unit only). It has a nominal heating output of 16K BTU/hr (outside temp not provided in specs), but only produces 12.7K BTU/hr at 17 deg F outside temp (and 10.2K BTU/hr at 5 deg F outside temp.) Since Golden Sunsets (post 45) is in New England, let's use their electric rates: 19.93 cents per KWH in Jul 2018. So, for this heat pump to make 10.2KBTU at 17 deg F, it takes 1.3 KWH of electricity, which costs
25.9 cents.
Natural gas (2017 avg, New England residential) costs $13.32 per 1000CF, which provides 1,037,000 BTU. So, to make this same 10.2K BTU of heat (with a 95% efficient furnace) requires us to burn
16 cents worth of gas.
Notes:
1) I didn't include the cost of electricity needed to run the fans on the internal mini-split units, but I also didn't include the fan for the furnace. It would probably be close to even.
2) This is at 17 deg F. It's a
lot colder than that for much of the winter in New England, the efficiency of the heat pump would be even lower and the advantage of gas would be even higher.
3)
Even at its "nominal" heat output of 16K BTU (outside temp not given, but you can be sure it's pretty warm), the heat pump
still is not as cost efficient as gas, given these prices. The heat pump cost is 1.6 cents per KBTU, while the gas furnace requires 1.56 cents per KBTU.
There are lots of variables not included here. Some heat pumps do better/worse at low temps. The cost of fuel oil, wood, wood pellets, propane, or other alternate fuels will almost surely be higher than natural gas.
Even with a well designed (and expensive) heat pump, it takes a lot of energy to extract 100 deg+ temps from air that is 30 degrees. I don't find it hard to believe that it costs more to make heat this way than to simply burn natural gas.