4 ms·

Which is not exactly efficient. 40C or less is desirable.

sokoloff 1y ago
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.
- looofooo0 1y ago
  This is bad. Return water should be less 35 degree max. Actually 30 degree after the heat pump would be ideal: https://www.flow30.de/ https://www.flow30.de/
  - jillyboel 1y ago
    [citation needed]
    - looofooo0 1y ago
      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.
      - Retric 1y ago
        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.
        looofooo0 1y ago
        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.
        Retric 1y ago
        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.
        looofooo0 1y ago
        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.
        Retric 1y ago
        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.

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.

looofooo0 1y ago
This is bad. Return water should be less 35 degree max. Actually 30 degree after the heat pump would be ideal: https://www.flow30.de/ https://www.flow30.de/
- jillyboel 1y ago
  [citation needed]
  - looofooo0 1y ago
    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.
    - Retric 1y ago
      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.
      - looofooo0 1y ago
        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.
        Retric 1y ago
        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.
        looofooo0 1y ago
        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.
        Retric 1y ago
        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.