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I was talking with someone recently and they recently bought a new electric water heater. I asked why they didn't go with a heat pump option, and they said the

I was talking with someone recently and they recently bought a new electric water heater. I asked why they didn't go with a heat pump option, and they said the dealer told them that heat pump water heaters are basically a "robbing Peter to pay Paul" situation where you end up taking heat you would have added to your house and putting it in your water instead. I guess in warm climates / seasons that is probably a net benefit, but I've noticed our electric co-op also promotes highly-insulated electric water heaters instead of heat pump water heaters. Not sure what I think of it all. We have an electric water heater and at first I was grumpy about the relative cost to run relative to natural gas, but I've come to terms with it. It's about the simplest device possible, and that has some value I think.

MayeulC 6y ago
A conventional heater can, at best, use 100% of the energy to heat both the water and the house. A heat pump would use some of that energy to capture some more from a heat source (outside air, or geothermal, ground temperature). It gets less efficient as the heat source becomes cooler, so it might not be such a good idea in very cold climates. Theoretically, though, it shouldn't get lower than 100%, and efficiencies can reach 300% pretty easily. I haven't done an extensive search on the topic, here is one of the first sources I found with the kind of graph I wanted: https://www.nordicghp.com/2017/01/heat-pump-effective-temperature-range/ https://www.nordicghp.com/2017/01/heat-pump-effective-temper... I read about 270% at -4°C (25°F). 1 watt of electricity in, 2.7 watts of heat out, not bad. The same graph reads about 480% at 18°C(65°F), and 150% at -20°C (-5°F). I know that gas heat pumps exist as well, and some systems are hybrid, so it sounds like a no brainer to me. On the flip side, they can be quite expensive, so do your calculations relative to the temperature history, and hourly electricity hours. It would be more efficient to heat when the air is warmer during the day (assuming air pump), and combine that with a well-insulated house. Solar (thermal or electric) is pretty interesting too, I wonder what kind of efficiency one can get with photovoltaic + heat pump (photovoltaic is more versatile, being more useful during summer).
- semi-extrinsic 6y ago
  I think GP is talking about heat pump water heaters that are working between the air in the room where heater is installed and the hot water. Very different from what you are suggesting. It could still make sense if it is installed in a room with lots of waste heat, e.g. next to a clothes dryer and other appliances.
  - war1025 6y ago
    > I think GP is talking about heat pump water heaters that are working between the air in the room where heater is installed and the hot water. Correct. Every heat pump water heater I've ever seen has the heat pump directly on top of the water tank. It's possible they make ones that tie into an external heat pump, but I've never seen one.
    - fomine3 6y ago
      In Japan, separated heat pump unit / tank is pretty ubiquitous and never seen union unit. https://echonet.jp/introduce_en/gz-000505/ https://echonet.jp/introduce_en/gz-000505/
    - MayeulC 6y ago
      Huh? That would indeed be pretty dumb. I would have at least expected a duct that pumps air from outside... Heat pumps make little sense if you don't have access to a thermostatic source, or indeed any "free" heat source.
      - war1025 6y ago
        The trouble with water heaters is you want hot water year round. Heat pumps are pretty ineffective once you get down close to freezing. So having an exterior air source heat pump would only make sense for maybe half the year in a lot of locations. Generally utility rooms in houses have a decent amount of waste heat anyway from being near the furnace, so it probably does work out decently a lot of the time. My in-laws have a heat pump water heater and it makes their basement noticeably colder in the summer, which is pretty nice.
extrapickles 6y ago
This is because a bunch of the units on the market source their heat from directly around the water heater (likely a conditioned area of your house). They have options for ducts to dump the cold air they generate outside during the winter, but this doesn't help as you then need to draw in outside air to make up the pressure differential (otherwise good luck opening doors). Units that have the radiator/compressor outside are 3-5x the price, and are hard to find.
- choeger 6y ago
  These "hard-to-find" units are pretty much the standard offering for heating new houses here in Germany. Where are you living?
  - war1025 6y ago
    Home heat pumps always have an exterior unit here. It's heat pump water heaters that don't [0]. [0] https://www.rheem.com/products/residential/water-heating/hybrid/ https://www.rheem.com/products/residential/water-heating/hyb...

MayeulC 6y ago

A conventional heater can, at best, use 100% of the energy to heat both the water and the house. A heat pump would use some of that energy to capture some more from a heat source (outside air, or geothermal, ground temperature). It gets less efficient as the heat source becomes cooler, so it might not be such a good idea in very cold climates. Theoretically, though, it shouldn't get lower than 100%, and efficiencies can reach 300% pretty easily. I haven't done an extensive search on the topic, here is one of the first sources I found with the kind of graph I wanted: https://www.nordicghp.com/2017/01/heat-pump-effective-temperature-range/ https://www.nordicghp.com/2017/01/heat-pump-effective-temper... I read about 270% at -4°C (25°F). 1 watt of electricity in, 2.7 watts of heat out, not bad. The same graph reads about 480% at 18°C(65°F), and 150% at -20°C (-5°F). I know that gas heat pumps exist as well, and some systems are hybrid, so it sounds like a no brainer to me. On the flip side, they can be quite expensive, so do your calculations relative to the temperature history, and hourly electricity hours. It would be more efficient to heat when the air is warmer during the day (assuming air pump), and combine that with a well-insulated house. Solar (thermal or electric) is pretty interesting too, I wonder what kind of efficiency one can get with photovoltaic + heat pump (photovoltaic is more versatile, being more useful during summer).

semi-extrinsic 6y ago
I think GP is talking about heat pump water heaters that are working between the air in the room where heater is installed and the hot water. Very different from what you are suggesting. It could still make sense if it is installed in a room with lots of waste heat, e.g. next to a clothes dryer and other appliances.
- war1025 6y ago
  > I think GP is talking about heat pump water heaters that are working between the air in the room where heater is installed and the hot water. Correct. Every heat pump water heater I've ever seen has the heat pump directly on top of the water tank. It's possible they make ones that tie into an external heat pump, but I've never seen one.
  - fomine3 6y ago
    In Japan, separated heat pump unit / tank is pretty ubiquitous and never seen union unit. https://echonet.jp/introduce_en/gz-000505/ https://echonet.jp/introduce_en/gz-000505/
  - MayeulC 6y ago
    Huh? That would indeed be pretty dumb. I would have at least expected a duct that pumps air from outside... Heat pumps make little sense if you don't have access to a thermostatic source, or indeed any "free" heat source.
    - war1025 6y ago
      The trouble with water heaters is you want hot water year round. Heat pumps are pretty ineffective once you get down close to freezing. So having an exterior air source heat pump would only make sense for maybe half the year in a lot of locations. Generally utility rooms in houses have a decent amount of waste heat anyway from being near the furnace, so it probably does work out decently a lot of the time. My in-laws have a heat pump water heater and it makes their basement noticeably colder in the summer, which is pretty nice.

extrapickles 6y ago

This is because a bunch of the units on the market source their heat from directly around the water heater (likely a conditioned area of your house). They have options for ducts to dump the cold air they generate outside during the winter, but this doesn't help as you then need to draw in outside air to make up the pressure differential (otherwise good luck opening doors). Units that have the radiator/compressor outside are 3-5x the price, and are hard to find.

choeger 6y ago
These "hard-to-find" units are pretty much the standard offering for heating new houses here in Germany. Where are you living?
- war1025 6y ago
  Home heat pumps always have an exterior unit here. It's heat pump water heaters that don't [0]. [0] https://www.rheem.com/products/residential/water-heating/hybrid/ https://www.rheem.com/products/residential/water-heating/hyb...