My return water temps are 115F (46C) on a P98 design heating day, and obviously cooler on warmer than design days. Cooler is always better, but "baseboards require 180F [82C] water because that's what's on the spec sheet" is a commonly-held but mistaken belief.
Ah this is school knowledge of thermodynamics: the smaller the delta the more efficient the heat pump. For human comfort look at the iso7730. Also the system is self regulatory with such low temps.
Heat pumps and furnaces behave very differently here.
For a furnace you’re talking fractions of a percent difference in efficiency across a wide temperature range so by far the most critical issue is total heat losses to the outside. A heat pump’s efficiency is far more variable making total losses to the outside less important.
First, sooner or later you have to replace your furnace with a heat pump. Second, modern furnaceses are condensing. Return temp should be as low as possible maximise condensing.
We agree return temps should be lower, but to determine how critical this is you need to out numbers on that. In steady state operation at maximum load a difference in return temperatures of 15f is ~1% efficiency. But steady state at maximum load is an extreme situation the average return temperature is well below that theoretical maximum thus an even wider difference is needed for a seasonal difference of 1%.
Redundancy is critical in areas that get really cold. That may eventually mean turning to hydrogen, but a backup gas furnace for a well insulated home really isn’t a major CO2 contributor. More relevant to the discussion it further reduces the impact of minor changes in efficiency or comfort.
As said, at some point you will likely replace your furnace by a heat pump. Then all investment to low temperatures will pay off. A modern gas furnace is no backup for a heat pump, as it also needs electricity. I would recommend a low tech solution like a wood stove for backup. I have one in my cellar. In case of a long power outage I would likely just install this through some open window and do some redneck stuff to make it airtight.
Backup doesn't necessarily mean "in case of grid failure". A gas boiler or a simple electric boiler is a valid backup to a heat pump being inoperative, in the middle of defrost cycle, or otherwise just unable to provide enough BTU/hr into the building due to an extreme cold temperature.
Particularly in temperatures where the heat pump is at/under 2.0 COP for a few dozen hours per year, an electric boiler makes a great deal of sense. It's almost as clean as the heat pump in the extremely rare times when it's needed for supplementation due to extreme cold temps, can allow for a more suitably sized heat pump to cover the 99% of heating hours [meaning greater efficiency there], is extremely reliable due to its simplicity, is fully automatable with existing controls, doesn't require local gas supply network, and is a pretty good match for a heat pump.
Trusting in the power grid for both is obviously not redundant, but it’s cheaper to get independent power for a furnace than heat pumps. Actual redundancy means you want completely independent systems so you shouldn’t then use the same heat delivery loops/pumps/fans either.
Wood stoves require you to physically be home and are kind of a pain to use thus make a poor backup on their own, though a solid 2nd backup.
PS: Most areas are better off with solar thermal + heat pumps. Solar thermal however requires sunlight which not everyone gets in the depth of winter.
I dont see the point in making an emergency option convienient. Forget solar thermal, this is to much hussle for anything. PV plus battery more likly the solution you want.
Less about convenient than being to go on vacation in the winter.
Work out the amount of solar + batteries you need in the winter when the power goes out and your COP is less than 2 because it’s very cold and you need more BTU’s than normal because it’s very cold. Solar thermal is just significantly cheaper on a large home in a cold climate.
No, this does not work. Solar thermal will not work without power. Any battery solution monitoring your home temperatures are enough. Then you can travel home or ask your neighbor.
