Heat Pumps are just Air Conditioners with a reversing valve. They're not some special magic.
this post was submitted on 11 Mar 2024
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Increasingly, they also tend to have variable-speed compressors, which offer further efficiency benefits. I think the distinction between A/C and heat pump is useful for consumers.
Just got a Bosch Inverter (variable speed) last summer, and the remainder of the summer and through the winter, my electric usage is down by about 30% over last year's even though I lowered the thermostat temp in summer and increased it in winter.
Old system was 20 years old. I was hoping to see some ROI after this surprise expense, and it has exceeded expectation so far.
You could add end-of-life solar panels and an automatic transfer switch and save a lot more.
If only! I have a condo in an HOA neighborhood, so no outdoor goodies.
Get on the board and make everyone do it.
Hey, pedant here.
Air conditioners are heat pumps too, and it's not the reversing valve that differentiates them. Heat pumps move heat, reversing valve let's you decide which way to move it.
That's exactly what I said. Air Conditioners are heat pumps which only pump one way.
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I like pedants. Is it correct that they're not necessarily equally efficient in both directions? "Air conditioner" to transfer heat away, vs "heat pump" to transfer heat in? Even though both are heat pumps.
Sort of... It's not so much down to it just not working as good at transferring heat, because the rules of thermodynamics applies... moving heat is moving heat.
But the devil's in the details. If it's below freezing the radiator will frost up and won't work very good. But that problem is solved by temporarily reversing it to heat of the radiator to melt the frost off of it. These systems do this automatically. Freezing temperature is 273 Kelvin, so there is heat outside even when it's below freezing so there's always heat that can be pumped, but there are limits to it.
You don't want to be dependent on a heat pump as the only source of heat for your house. But they build electric heaters into many models to handle those conditions. But obviously on really cold days that it needs to supplement the heating with the electric heater it's not going to be all that efficient, because you're running an electric heater on those days.
But most days it's not going to need to turn on the electric heater, and on your cool spring and fall days it won't even need to defrost. So when you consider it over the course of a year, the heating cost is way lower.
Thanks! I like you space cowboy.
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Can you convert an existing AC system to do both and save money on getting a whole new system installed?
It's probably not worth it. If you have a system more than 10 years old, then you're probably going to have to replace the accumulator as well if it uses a different kind of refrigerant. It's likely cheaper to buy a whole unit and furnace than messing with it.
What about a new system? Had the AC compressor replaced last year but not the gas fired furnace due to cost lol
I believe so but you need a new compressor - the heat exchanger in the house can stay the same I think.
You have a furnace that provides heat, air handler that moves the air, and compressor that forces heat in a certain direction (inside to outside in the case of AC) with coils in the air handler to make use of that (re)moved heat.
Heat pumps have several features that make them a bit more than backwards AC, like defrost systems, VFDs and often dual-fuel controls. If it snows where you are, you'll also want it off the ground. So, best to get a new system.
As another said, you might be able to reuse the coolant lines and coils in the air handler. It might not be a bad idea to keep the furnace for backup when it's extra cold.
so why do they have to cost 4-5x what it costs to get an Aircon installed?
I posted this elsewhere in this thread, but here seems good too:
Obligatory Technology Connections ( @[email protected]): Old HVAC industry practices are holding us back and costing us money. But we can fix it.
TL;DW (but you really should, it's a great video): "Recommended" heat pumps are often oversized and overpriced compared to what is actually needed, and homeowners need to be aware of this.
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Heat Pumps are just Air Conditioners with a reversing valve. They are some special magic.
Cool yeah, uh I'll do that as soon as I have a bunch of that money stuff.
Obligatory Technology Connections (@[email protected]):
Old HVAC industry practices are holding us back and costing us money. But we can fix it.
TL;DW (but you really should, it's a great video): "Recommended" heat pumps are often oversized and overpriced compared to what is actually needed, and homeowners need to be aware of this.
Same with Solar Panels, Home Battery Storage system, and honestly just completely redone insulation along the perimeter and siding.
You know what, on second thought, I would rather just move tf out of this place.
Because it's 3-500% efficent duh
I have recently upgraded from a furnace to a condensing furnace, so in the winter my house now emits room temperature soda water rather than hot steamy smoke.
My air conditioner was brand new when I moved in and is in early middle age; when it is ready for retirement I'm replacing it with a heat pump system keeping my current furnace as emergency heat.
Does the paper have any results that say they're still cleaner on a dirty grid? As far as I can tell it's only cleaner in the future after at least a 50% decarbonization. Which is reasonable, even in my fairly conservative city most of our power is low on carbon.
The thing is heat from the outside gets moved inside of the house using a heat pump, and to facilitate this movement you need somewhere between 1/2 and 1/4 of the energy you end up moving. E.g. a heat pump with 
(coefficient of performance) of 4 would move 4kW of heat into your house and use 1kW of electric energy to accomplish this. Gas by comparison moves 4kW of gas to your house and burns it there to get 4kW of heat.
So you could burn a bit more than 1kW of gas in a modern gas electric plant, turn it into electricity and use it to run a heat pump and you would end up emitting less CO2, the real world grid might skew that worse because generally you don't end up burning coal to heat housing but you might still use it for electricity. So generally even though it might be unintuitive the more complicated and lossy way to heat your home (the heat pump powered by fossil powered electricity) , is the more effective one compared to burning the same fossil fuel directly because you use the heat pump to capture heat from the environment.
Maybe not in the article, but I've heard in other places that a carbon heavy grid still gets enough energy to the heat pump that the heat pump's efficiency can offset that increase.
You're also installing a system that is easier to decarbon in the future, which isn't the case for natural gas.
It is really not hard. Heat pump coefficients of efficiency floor at 1, but typically range between ~2.5 and 7. That is, for every joule of energy they consume, they pump 2.5 to 7 joules of heat into the conditioned space.
So you have to just look at efficiencies involved.
- The electric heat pump (theoretically ~250-700% efficient at warming a space)
- The efficiency of energy transmission on the electrical grid (maybe around 90% ish? Depends on a lot of factors)
- The fossil fuel furnace (theoretically something like 90-100% efficient)
- The efficiency of the residential fossil fuel delivery system (last I read, something around 5% of natural gas ends up just leaking)
- The efficiency at conversion of chemical to electrical energy by fossil production plants (and this one is rough -- I see numbers like 40-60% efficient)
Still, we're just summing stuff. And while I won't pretend any napkin math here is the same as a formal research project, we can plainly see that the HUGE energy efficiency of heat pumps can easily eclipse the inefficiency of fossil electrical production, all else being equal. Of course, whether it actually WILL be better than a fossil furnace will depend on local factors, but these places are increasingly becoming edge cases. And then, on top of that, you unlock future potential to seamlessly switch fuel sources from fossils to renewables, which becomes very important in lifecycle cost analysis.
This is the same reason electric cars beats ICE (gas driven) cars even when charged on coal power. Big coal plants and the distribution grid are more efficient than small scale car engines.
You need combustion engines to be BIG to get past 30-40% efficiency ranges, but the really big power plants can just perform efficient burns and heat water to drive efficient turbines, which is impossible in cars. And the rest of losses in electric cars are either minimal or equivalent, so you get a big net benefit.
Even better than that is an electric bus and other public transit!
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I think the point is to compare the heat pump with an electricity heater, there may be other ways that generate less carbon footprint of course
The point is to compare a heat pump with a natural gas furnace.
The efficiency of getting electricity to a house may be less, but a heat pump has an energy efficiency greater than 1.
I've done energy models for houses here in Saskatchewan (~560 tCO2e/GWh) and at the moment, they are not cleaner than heating with natural gas, which is the typical primary heat source. Obviously, it would depend on grid carbon intensity, so there is a level of grid 'cleanness' where heat pumps would become cleaner, but that tipping point depends on a number of factors.
You could do a rough estimation with the seasonal heating efficiency of a heat pump based on the heating-degree-days of your location versus a certain efficiency of natural gas furnace. Burning natural gas is about 0.18 kgCO2e/kWh. So, if you have a heat pump that's 200% seasonally efficient, you'd need the grid carbon intensity to be about 0.38 kgCO2e/kWh (380 tCO2e/GWh) to be equivalent to a 95% efficient natural gas furnace.
The 200% seasonal efficiency is a bit off, Nordic models, measured with the "colder" European climate zone, get 300%+ and have guaranteed output at -25C / -13F. Example model from Mitsubishi:
It's worse than 5.5x or 4.3x in warmer areas but the right model air source heat pumps work fine down to pretty damned cold. Norway and Sweden have a ton of them as they spend a ton of energy on heating and this saves homeowners a ton of money every year.
Best models optimized for average climate now reach 5.5x or better in the green, moderate zone, SCOP of 4.3 is actually pretty terrible but this one is built to be ice proof.
Example latest bestest heat pump with 6+ seasonal COP:
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Notable, but outside of very cold climates (which I think I feel safe describing Saskatchewan as being), heat pumps are a LOT more than 200% efficient. In mild climate, they can be 2-4X that.
Definitely, that's why I say the seasonal heating efficiency is based on heating-degree-days of the location. I'm not sure they'd get to 2-4x 200% efficient, though. 350% might be more reasonable.
It gets hard to say because COP varies with climate. But even in SEER ratings, 17-20 are pretty much the norm for modern systems and I have seen as high as 23. That translates to a 4-4.5 COP in an average climate.
But those COPs get higher the more mild your climate -- I am somewhere with quite mild winter where we only get a hard freeze once or maybe twice a year, and generally winter low temps are in the 40-50F range.
I believe the theoretical max efficiency for a heat pump is something like 8.8 COP. In a mild climate like mine, where most of the time if your heat is running it's to heat to ~70ish from an ~50ish outdoor temp, you're should be getting a lot closer to 7 than you are to 2.
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