2 ms·

Heat pumps are amazing things, they are more than 100% efficient, since they use only n watt-hours to move >n watt-hours of energy from one place to another. Un

Heat pumps are amazing things, they are more than 100% efficient, since they use only n watt-hours to move >n watt-hours of energy from one place to another. Under ideal conditions they can move 3n while consuming only n. It's far more efficient to use a heat pump in reverse to heat a space than it is to use a traditional electric heater, which is "only" 100% efficient. (Perhaps I should have said joules, not Wh? Units are confusing.)

hunter2_ 5y ago
When cooling a dwelling, the trick is figuring out where to move the heat. Typically we dump it outside (waste) but we should be heating our hot water tanks, swimming pools, or anything else that needs concentrated heat at the same time that we're removing it from the indoor air. Alec from Technology Connections recently made some videos about heat pumps on YouTube that touch on this sort of thing.
- mikepurvis 5y ago
  This has always made me super grouchy— I even have my house plumbed for hydronics! I wish I could tie the AC units into that, put all the rads in bypass during the summer, and use the hydronic water to preheat shower water before it hits the gas-powered on demand system.
- amluto 5y ago
  In very hot climates, swimming pools get too hot, and you might prefer to chill the pool water.
  - asdfasgasdgasdg 5y ago
    I'm sure this might happen somewhere, but I grew up in Florida and it never happened in Orlando. I would always have been happy for the pool to be warmer.
- nfriedly 5y ago
  > Typically we dump it outside (waste) but we should be heating our hot water tanks, swimming pools, or anything else that needs concentrated heat at the same time that we're removing it from the indoor air. My house actually does that! I had never heard of the idea before moving here, but we have a geothermal system and in addition to the ground loop, it also dumps heat into the hot water heater when cooling the house. (Until the water reaches some maximum temperature.)
  - rstupek 5y ago
    In order to get the energy credit on install of the geo system the install had to include a hot water heater as part of the install
    - BizarroLand 5y ago
      And the cool thing is that the spare "coolness" of the water is released into the air, providing free Air Conditioning while also cheaply heating your water.
- extraduder_ire 5y ago
  > we should be heating our hot water tanks, swimming pools, or anything else that needs concentrated heat That comment about swimming pools nearly made me leap out of my chair. I was already convinced to get a heat-pump, but I was just planning on putting excess heat under the frost-line to save it for winter. To think, if it was insulated and disinfected properly, you'd have both a cheaply heated pool, and a huge thermal-battery.
alisonkisk 5y ago
It's only >100% efficient because % efficiency is defined in a goofy way: benefit/cost, instead of benefit/(cost+benefit). (Moving 3n requires moving 4n and wasting n: 75% real efficiency). This difference only matters for A/C, when cost is not much larger than benefit.
- mjmahone17 5y ago
  This doesn’t make sense to me. You’re saying a water heater that uses N joules to heat water by 0.5N joules is 33% efficient? That would be difficult for most people to translate: I use 100 wh to get 50 wh of heat, which should imply 50% efficiency. As the cost to gain the end state I want is what I care about.
  - quickthrowman 5y ago
    The poster you are responding to was referring to heat pumps, not a resistive electric water heater like you assumed.
- johncolanduoni 5y ago
  But only n has to be low-entropy energy, while the 3n is ambient heat that you’re trying to get rid of. There’s no scarcity of ambient heat when you’re just trying to move it anywhere else but inside.
M2Ys4U 5y ago
>(Perhaps I should have said joules, not Wh? Units are confusing.) 1 Wh = 3600 J, so you're not wrong. It's the same thing as talking about using tonnes instead of kilograms, only the conversion factor here is 3600 instead of 1000.

hunter2_ 5y ago

When cooling a dwelling, the trick is figuring out where to move the heat. Typically we dump it outside (waste) but we should be heating our hot water tanks, swimming pools, or anything else that needs concentrated heat at the same time that we're removing it from the indoor air. Alec from Technology Connections recently made some videos about heat pumps on YouTube that touch on this sort of thing.

mikepurvis 5y ago
This has always made me super grouchy— I even have my house plumbed for hydronics! I wish I could tie the AC units into that, put all the rads in bypass during the summer, and use the hydronic water to preheat shower water before it hits the gas-powered on demand system.
amluto 5y ago
In very hot climates, swimming pools get too hot, and you might prefer to chill the pool water.
- asdfasgasdgasdg 5y ago
  I'm sure this might happen somewhere, but I grew up in Florida and it never happened in Orlando. I would always have been happy for the pool to be warmer.
nfriedly 5y ago
> Typically we dump it outside (waste) but we should be heating our hot water tanks, swimming pools, or anything else that needs concentrated heat at the same time that we're removing it from the indoor air. My house actually does that! I had never heard of the idea before moving here, but we have a geothermal system and in addition to the ground loop, it also dumps heat into the hot water heater when cooling the house. (Until the water reaches some maximum temperature.)
- rstupek 5y ago
  In order to get the energy credit on install of the geo system the install had to include a hot water heater as part of the install
  - BizarroLand 5y ago
    And the cool thing is that the spare "coolness" of the water is released into the air, providing free Air Conditioning while also cheaply heating your water.
extraduder_ire 5y ago
> we should be heating our hot water tanks, swimming pools, or anything else that needs concentrated heat That comment about swimming pools nearly made me leap out of my chair. I was already convinced to get a heat-pump, but I was just planning on putting excess heat under the frost-line to save it for winter. To think, if it was insulated and disinfected properly, you'd have both a cheaply heated pool, and a huge thermal-battery.

alisonkisk 5y ago

It's only >100% efficient because % efficiency is defined in a goofy way: benefit/cost, instead of benefit/(cost+benefit). (Moving 3n requires moving 4n and wasting n: 75% real efficiency). This difference only matters for A/C, when cost is not much larger than benefit.

mjmahone17 5y ago
This doesn’t make sense to me. You’re saying a water heater that uses N joules to heat water by 0.5N joules is 33% efficient? That would be difficult for most people to translate: I use 100 wh to get 50 wh of heat, which should imply 50% efficiency. As the cost to gain the end state I want is what I care about.
- quickthrowman 5y ago
  The poster you are responding to was referring to heat pumps, not a resistive electric water heater like you assumed.
johncolanduoni 5y ago
But only n has to be low-entropy energy, while the 3n is ambient heat that you’re trying to get rid of. There’s no scarcity of ambient heat when you’re just trying to move it anywhere else but inside.

M2Ys4U 5y ago

>(Perhaps I should have said joules, not Wh? Units are confusing.) 1 Wh = 3600 J, so you're not wrong. It's the same thing as talking about using tonnes instead of kilograms, only the conversion factor here is 3600 instead of 1000.