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I wonder how heat pumps would work for apartment building central heating systems. The circulated water has to be a lot hotter than heat pumps are capable of, a

I wonder how heat pumps would work for apartment building central heating systems. The circulated water has to be a lot hotter than heat pumps are capable of, as far as I've understood. Is there already a system that could viably replace gas in the existing building stock? Another problem is that most people in Berlin/Germany are renting, so the incentives do not align. The building owner doesn't pay for the heating, so they are unlikely to do any investments where the upside is for the tenants only.

tialaramex 6mo ago
Heat pump hot water is very common in Europe. Just as it's possible for the pump to make a box so cold that food stays frozen despite the hot summer's day in Florida, it's also possible to heat up a tank of water this way, it just costs more energy. Unlike modern natural gas boilers which have only a very small tank of water kept hot and use a fire to make more on demand with only a few seconds of delay (you might not even realise this is happening, but of course it isn't actually instant and so there is a small tank of very hot water) the heat pump will need a large tank because you're going to put about a day's supply of hot water in the tank and gradually, over a whole day, re-heat it as needed.
- distances 6mo ago
  The problem is the old building stock (say, large apartment buildings from early 1900s) which isn't energy efficient at all, and on cold winter days require very hot water to keep warm. The radiators were designed with assumption that the water can be boiling hot (coal/gas). It doesn't seem feasible to renovate everything, so I'm wondering what are the practical options. A hybrid system where a gas boiler is kept for the peak demand?
  - tialaramex 6mo ago
    There's no chance at all that you're using boiling hot water (373K) because that stuff is both dangerous and unreasonably expensive to make at volume. My guess is that what you're thinking of is the water being say 320 to 340K, which is obviously far warmer than you'd heat a home, but isn't boiling water. Heat pumps will be more efficient at lower temperatures, so maybe it's keeping that tank of water at 320K while your gas boiler was 330 or even 340K -- but the gas boiler is more efficient for lower temperatures too, ask any manufacturer or installer, the way to get lower costs is to turn down the local thermostat, it's just that people are used to "instant" heat from a gas boiler and won't tolerate answers like "Run it for a whole day" whereas that is what your heat pump needs. It is true that because of this "Run it all day" approach an uninsulated home is so expensive to heat that it's impractical, when I grew up there was ice inside my bedroom window when I woke on a winter's morning, it had been under 270K at night, and a gas boiler would heat it in an hour or so -- but that's not because it's literally impossible with heat pumps it's just far too expensive.
    - distances 6mo ago
      No, 47°C to 67°C is way too low. The buildings are not actually circulating boiling water, but not too far either: > Typically, heating systems in Europe use water flowing through pipes and emitters (radiators) heated to high temperatures (70-90°C). [0] The radiators are often designed for these temperatures. Supplying with lower heat will not work without replacing the radiators and/or renovating for better energy efficiency. And heat pumps, AFAIK, can't supply 70-90°C. Thus my original question of what is the current plan to solve this problem. [0] https://www.ifeu.de/fileadmin/uploads/Publikationen/Energie/ifeu_rap_2023_Towards_low_flow_temperatures.pdf https://www.ifeu.de/fileadmin/uploads/Publikationen/Energie/...
      - tialaramex 6mo ago
        So, firstly yes, exactly, even in the 19th century they aren't using boiling water, it's just needlessly expensive to make. 90°C seems unnecessarily hot to me (as you can see, by the time I was born it wasn't being used to heat homes) but it isn't boiling. And yes, some older buildings will need refurbishment, these building often pre-date electric light so that's happened before. The 1970s house example starts with a 70°C flow, but with a combination of insulation and replacing radiators they get to 41°C flow which is a much cheaper 318K in real numbers. The thing is making 90°C water was expensive and unnecessary even when gas boilers start being commonplace, the manufacturer will tell you that you should turn it down until it's heating the home as slow as you can tolerate. That got more true as the boilers got more efficient, because the efficiency is from recovering more heat energy from burning methane and you do that at lower flow temperatures, the heat pumps are just more of the same. Notice how those diagrams show gradually reducing flow temperatures over the years, in the 1930s recovered heat from an industrial district at 90°C is plausible, but the gas boiler manufacturer fifty years ago will go white when you say you need 90°C -- he's going to suggest 70°C, and his successors will keep bargaining you down because the efficiency numbers are better as flow temperature reduces and while "instant heat" feels good in the moment, the bills for the gas you're burning will not. We can pump these temperatures but they don't make economic sense. Unlike low outside air temperatures this is just economics. The low outside air temperatures mean you need to defrost the pump, which if it got cold enough might literally become impossible, but if you want to pay far, far more money to heat water to 90°C that would be technically possible, it's just silly, like burning $50 bills to keep warm - use singles they're 50x cheaper.

tialaramex 6mo ago

Heat pump hot water is very common in Europe. Just as it's possible for the pump to make a box so cold that food stays frozen despite the hot summer's day in Florida, it's also possible to heat up a tank of water this way, it just costs more energy. Unlike modern natural gas boilers which have only a very small tank of water kept hot and use a fire to make more on demand with only a few seconds of delay (you might not even realise this is happening, but of course it isn't actually instant and so there is a small tank of very hot water) the heat pump will need a large tank because you're going to put about a day's supply of hot water in the tank and gradually, over a whole day, re-heat it as needed.

distances 6mo ago
The problem is the old building stock (say, large apartment buildings from early 1900s) which isn't energy efficient at all, and on cold winter days require very hot water to keep warm. The radiators were designed with assumption that the water can be boiling hot (coal/gas). It doesn't seem feasible to renovate everything, so I'm wondering what are the practical options. A hybrid system where a gas boiler is kept for the peak demand?
- tialaramex 6mo ago
  There's no chance at all that you're using boiling hot water (373K) because that stuff is both dangerous and unreasonably expensive to make at volume. My guess is that what you're thinking of is the water being say 320 to 340K, which is obviously far warmer than you'd heat a home, but isn't boiling water. Heat pumps will be more efficient at lower temperatures, so maybe it's keeping that tank of water at 320K while your gas boiler was 330 or even 340K -- but the gas boiler is more efficient for lower temperatures too, ask any manufacturer or installer, the way to get lower costs is to turn down the local thermostat, it's just that people are used to "instant" heat from a gas boiler and won't tolerate answers like "Run it for a whole day" whereas that is what your heat pump needs. It is true that because of this "Run it all day" approach an uninsulated home is so expensive to heat that it's impractical, when I grew up there was ice inside my bedroom window when I woke on a winter's morning, it had been under 270K at night, and a gas boiler would heat it in an hour or so -- but that's not because it's literally impossible with heat pumps it's just far too expensive.
  - distances 6mo ago
    No, 47°C to 67°C is way too low. The buildings are not actually circulating boiling water, but not too far either: > Typically, heating systems in Europe use water flowing through pipes and emitters (radiators) heated to high temperatures (70-90°C). [0] The radiators are often designed for these temperatures. Supplying with lower heat will not work without replacing the radiators and/or renovating for better energy efficiency. And heat pumps, AFAIK, can't supply 70-90°C. Thus my original question of what is the current plan to solve this problem. [0] https://www.ifeu.de/fileadmin/uploads/Publikationen/Energie/ifeu_rap_2023_Towards_low_flow_temperatures.pdf https://www.ifeu.de/fileadmin/uploads/Publikationen/Energie/...
    - tialaramex 6mo ago
      So, firstly yes, exactly, even in the 19th century they aren't using boiling water, it's just needlessly expensive to make. 90°C seems unnecessarily hot to me (as you can see, by the time I was born it wasn't being used to heat homes) but it isn't boiling. And yes, some older buildings will need refurbishment, these building often pre-date electric light so that's happened before. The 1970s house example starts with a 70°C flow, but with a combination of insulation and replacing radiators they get to 41°C flow which is a much cheaper 318K in real numbers. The thing is making 90°C water was expensive and unnecessary even when gas boilers start being commonplace, the manufacturer will tell you that you should turn it down until it's heating the home as slow as you can tolerate. That got more true as the boilers got more efficient, because the efficiency is from recovering more heat energy from burning methane and you do that at lower flow temperatures, the heat pumps are just more of the same. Notice how those diagrams show gradually reducing flow temperatures over the years, in the 1930s recovered heat from an industrial district at 90°C is plausible, but the gas boiler manufacturer fifty years ago will go white when you say you need 90°C -- he's going to suggest 70°C, and his successors will keep bargaining you down because the efficiency numbers are better as flow temperature reduces and while "instant heat" feels good in the moment, the bills for the gas you're burning will not. We can pump these temperatures but they don't make economic sense. Unlike low outside air temperatures this is just economics. The low outside air temperatures mean you need to defrost the pump, which if it got cold enough might literally become impossible, but if you want to pay far, far more money to heat water to 90°C that would be technically possible, it's just silly, like burning $50 bills to keep warm - use singles they're 50x cheaper.