5 ms·

Gas heating, electric AC.

nicolaslem 5y ago
As AC units are basically heat pumps why aren't people running them in reverse in the winter? Except in very cold climates, it should still be cheaper/more efficient than gaz. What am I missing?
- qeternity 5y ago
  > Except in very cold climates, it should still be cheaper/more efficient than gaz This isn't true at all. Gas is much cheaper per joule than electricity.
  - i_am_proteus 5y ago
    This. Bear in mind also that even with the efficiency gain of the heat pump in warmer places (much of the US lives in places that are not warm), there are efficiency losses from generation and transmission of electricity to the end user.
  - agurk 5y ago
    A key point to understand about a heat pump is that it can move a lot more (heat) energy than the equivalent it consumes. One joule of pumping could get up to six joules of heat moved[0] - so it could be represented as being 600% efficient. Burning gas can only generate a maximum of one joule of heat for every joule consumed (i.e. 100% efficient). Getting six joules requires favourable conditions and a good setup, but the normal expected efficiency is considerably above 100%. Gas is (normally - I'm on a variable tariff and being paid to consume electricity at the moment) cheaper than electricity per joule, but not so much that it works out cheaper than a heat pump to achieve the same effect. [0] https://www.eec.org.au/for-energy-users/technologies-2/heat-pumps https://www.eec.org.au/for-energy-users/technologies-2/heat-...
    - zbrozek 5y ago
      Much depends on where exactly you are and who you pay for electricity and when. For me, under PG&E pricing, the running cost for heat pump heating ends up higher than with gas. And the equipment cost is much higher. I'll end up getting it anyway when I get an AC system someday, but it'll have to run off solar to be worthwhile.
  - throw0101a 5y ago
    > Gas is much cheaper per joule than electricity. There is actually a tip-over point. For a given price for gas, and a given price for electricity, each will be more economical at different points because the efficiency of heat pumps (for heating) is a function of the external temperature. There are calculations that can be done to determine this point: * https://c03.apogee.net/mvc/home/hes/land/el?utilityname=eversource&spc=hcc https://c03.apogee.net/mvc/home/hes/land/el?utilityname=ever... * https://masslandlords.net/heat-pump-vs-furnace/ https://masslandlords.net/heat-pump-vs-furnace/ It's not as clear-cut as you make it sound: * https://en.wikipedia.org/wiki/Coefficient_of_performance https://en.wikipedia.org/wiki/Coefficient_of_performance
  - thehappypm 5y ago
    Heat pumps change the equation a bit. Yes, 1 kWh of resistive heating is a lot more expensive than 1 kWh of natural gas. But with high-efficiency heat pumps you can produce a few kWh of heat in your house with 1 input kWh, so it comes down to just how much cheaper natural gas is per kWh.
- erik_seaberg 5y ago
  A heat pump needs a reversing valve, and goes through a cycle that defrosts the outside coils. A cheap air conditioner might not have those.
- mrfusion 5y ago
  In Florida I think they wanted to charge me $1000 extra for a heat pump instead of just an AC. for the five cold days a year it didn’t seem worth it. The air handler just has resistive coils for heat.

As AC units are basically heat pumps why aren't people running them in reverse in the winter? Except in very cold climates, it should still be cheaper/more efficient than gaz. What am I missing?

qeternity 5y ago
> Except in very cold climates, it should still be cheaper/more efficient than gaz This isn't true at all. Gas is much cheaper per joule than electricity.
- i_am_proteus 5y ago
  This. Bear in mind also that even with the efficiency gain of the heat pump in warmer places (much of the US lives in places that are not warm), there are efficiency losses from generation and transmission of electricity to the end user.
- agurk 5y ago
  A key point to understand about a heat pump is that it can move a lot more (heat) energy than the equivalent it consumes. One joule of pumping could get up to six joules of heat moved[0] - so it could be represented as being 600% efficient. Burning gas can only generate a maximum of one joule of heat for every joule consumed (i.e. 100% efficient). Getting six joules requires favourable conditions and a good setup, but the normal expected efficiency is considerably above 100%. Gas is (normally - I'm on a variable tariff and being paid to consume electricity at the moment) cheaper than electricity per joule, but not so much that it works out cheaper than a heat pump to achieve the same effect. [0] https://www.eec.org.au/for-energy-users/technologies-2/heat-pumps https://www.eec.org.au/for-energy-users/technologies-2/heat-...
  - zbrozek 5y ago
    Much depends on where exactly you are and who you pay for electricity and when. For me, under PG&E pricing, the running cost for heat pump heating ends up higher than with gas. And the equipment cost is much higher. I'll end up getting it anyway when I get an AC system someday, but it'll have to run off solar to be worthwhile.
- throw0101a 5y ago
  > Gas is much cheaper per joule than electricity. There is actually a tip-over point. For a given price for gas, and a given price for electricity, each will be more economical at different points because the efficiency of heat pumps (for heating) is a function of the external temperature. There are calculations that can be done to determine this point: * https://c03.apogee.net/mvc/home/hes/land/el?utilityname=eversource&spc=hcc https://c03.apogee.net/mvc/home/hes/land/el?utilityname=ever... * https://masslandlords.net/heat-pump-vs-furnace/ https://masslandlords.net/heat-pump-vs-furnace/ It's not as clear-cut as you make it sound: * https://en.wikipedia.org/wiki/Coefficient_of_performance https://en.wikipedia.org/wiki/Coefficient_of_performance
- thehappypm 5y ago
  Heat pumps change the equation a bit. Yes, 1 kWh of resistive heating is a lot more expensive than 1 kWh of natural gas. But with high-efficiency heat pumps you can produce a few kWh of heat in your house with 1 input kWh, so it comes down to just how much cheaper natural gas is per kWh.
erik_seaberg 5y ago
A heat pump needs a reversing valve, and goes through a cycle that defrosts the outside coils. A cheap air conditioner might not have those.
mrfusion 5y ago
In Florida I think they wanted to charge me $1000 extra for a heat pump instead of just an AC. for the five cold days a year it didn’t seem worth it. The air handler just has resistive coils for heat.