4 ms·

How do they get the efficiency number above 100? Are heat pumps that efficient at cooling as well?

gregable 5y ago
It depends on your definition of efficiency. Electric heat is considered 100% efficient as 100% of the watt hours that go into the device eventually get turned into the same amount of heat. A heat pump moves heat from outdoors to indoors. The outdoor heat lost is ignored in the efficiency calculation (as it should be), and thus the efficiency question is how many watt hours of electricity does it take to move a watt hour of heat (joules) from outside to inside. Depending on the temperatures involved, it's usually a multiple, for example, 1 wH of electricity moves 4 wH equivalent of heat indoors, ie: 400% efficient. If you consider the whole system, much of the electricity for a heat pump may come from burning natural gas elsewhere, which involves losses along the way. This is still usually better than 100% overall efficiency. Direct natural gas heating is less than 100% efficient too. Most furnace systems are about 20% loss in exhaust, though they can be much better. This is still usually better than burning the gas at a plant, converting to electricity and then heating with resistive electrical in your home.
- syshum 5y ago
  Most modern gas furnaces are 95% AFUE, the minimum in northern US states is 90% And comparing AFUE efficiency to HSPF is not really a thing, you can not look at a gas furance that is 95% AFUE, and a Heat Pump that is "400%" claim that the gas furnace is must less "efficient" that is not how any of those works. To be clear a heat pump at temps above 25 degrees Fahrenheit is more efficient but comparing the metrics in this manner is a bit misleading Further heat pumps are good to 25 to 30 degrees Fahrenheit, under that a gas furnace is normally used to supplement a heat pump in colder climates. Most heat pump manufacturers do not recommend operating a heat pump in below freezing conditions
  - dieortin 5y ago
    Then how would you say it should be compared?
  - rudedogg 5y ago
    > Further heat pumps are good to 25 to 30 degrees Fahrenheit, under that a gas furnace is normally used to supplement a heat pump in colder climates. Most heat pump manufacturers do not recommend operating a heat pump in below freezing conditions Just wanted to chime in that they do make a lot of low-temp heat pumps, going down to -30f now. The efficiency drops dramatically at temps that low, the best site to look at cold-weather models is https://ashp.neep.org/#!/product_list/ https://ashp.neep.org/#!/product_list/ If you look though, the "COP @ 5 degrees F" for a lot of models is above 2.0, which from my understanding means you'll be using 1/2 the electricity someone with baseboard (resistance) heat would. And above those temps the COP gets much higher.
dghlsakjg 5y ago
Heat pumps are actually exactly what the title says. Using compressors they are able to extract heat even from relatively cold air and transfer the energy inside. They can use 100 watts of electric energy to move 400 watts of heat from outside to inside. The downside is that the colder it is outside the less efficient they get (since there is less heat outside) so it only works in relatively temperate climates.
- hattmall 5y ago
  What I don't get is the comparison to gas though. Like how can burning natural gas to generate electricity then using that electricity on a separate device be more efficent than just burning the natural gas directly for heat. The essential output of natural gas is heat to generate electricity if the heat pump were 4x efficient then could a natural gas plant generate electricity, run a giant heat pump and then use the heat to generate 4x more electricity?
- XorNot 5y ago
  This does only apply for air-sourced heat pumps though. Unless you are literally living on an ice-sheet, a ground-sourced heat pump can be efficient all-year round and beat out an air system easily.
  - dghlsakjg 5y ago
    Fascinating. I’m in the PNW where I don’t think it gets cold enough for this to matter

It depends on your definition of efficiency. Electric heat is considered 100% efficient as 100% of the watt hours that go into the device eventually get turned into the same amount of heat. A heat pump moves heat from outdoors to indoors. The outdoor heat lost is ignored in the efficiency calculation (as it should be), and thus the efficiency question is how many watt hours of electricity does it take to move a watt hour of heat (joules) from outside to inside. Depending on the temperatures involved, it's usually a multiple, for example, 1 wH of electricity moves 4 wH equivalent of heat indoors, ie: 400% efficient. If you consider the whole system, much of the electricity for a heat pump may come from burning natural gas elsewhere, which involves losses along the way. This is still usually better than 100% overall efficiency. Direct natural gas heating is less than 100% efficient too. Most furnace systems are about 20% loss in exhaust, though they can be much better. This is still usually better than burning the gas at a plant, converting to electricity and then heating with resistive electrical in your home.

syshum 5y ago
Most modern gas furnaces are 95% AFUE, the minimum in northern US states is 90% And comparing AFUE efficiency to HSPF is not really a thing, you can not look at a gas furance that is 95% AFUE, and a Heat Pump that is "400%" claim that the gas furnace is must less "efficient" that is not how any of those works. To be clear a heat pump at temps above 25 degrees Fahrenheit is more efficient but comparing the metrics in this manner is a bit misleading Further heat pumps are good to 25 to 30 degrees Fahrenheit, under that a gas furnace is normally used to supplement a heat pump in colder climates. Most heat pump manufacturers do not recommend operating a heat pump in below freezing conditions
- dieortin 5y ago
  Then how would you say it should be compared?
- rudedogg 5y ago
  > Further heat pumps are good to 25 to 30 degrees Fahrenheit, under that a gas furnace is normally used to supplement a heat pump in colder climates. Most heat pump manufacturers do not recommend operating a heat pump in below freezing conditions Just wanted to chime in that they do make a lot of low-temp heat pumps, going down to -30f now. The efficiency drops dramatically at temps that low, the best site to look at cold-weather models is https://ashp.neep.org/#!/product_list/ https://ashp.neep.org/#!/product_list/ If you look though, the "COP @ 5 degrees F" for a lot of models is above 2.0, which from my understanding means you'll be using 1/2 the electricity someone with baseboard (resistance) heat would. And above those temps the COP gets much higher.

dghlsakjg 5y ago

Heat pumps are actually exactly what the title says. Using compressors they are able to extract heat even from relatively cold air and transfer the energy inside. They can use 100 watts of electric energy to move 400 watts of heat from outside to inside. The downside is that the colder it is outside the less efficient they get (since there is less heat outside) so it only works in relatively temperate climates.

hattmall 5y ago
What I don't get is the comparison to gas though. Like how can burning natural gas to generate electricity then using that electricity on a separate device be more efficent than just burning the natural gas directly for heat. The essential output of natural gas is heat to generate electricity if the heat pump were 4x efficient then could a natural gas plant generate electricity, run a giant heat pump and then use the heat to generate 4x more electricity?
XorNot 5y ago
This does only apply for air-sourced heat pumps though. Unless you are literally living on an ice-sheet, a ground-sourced heat pump can be efficient all-year round and beat out an air system easily.
- dghlsakjg 5y ago
  Fascinating. I’m in the PNW where I don’t think it gets cold enough for this to matter