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It’s possible to heat things like return water or cold water directly and then use heat exchangers.
Temperature differentials also doesn’t matter much because temperature doesn’t directly behave like voltage, if your hot source is too hot you simply use less hot water (lower flow). You’d likely end up with multiple heat pumps throughout a larger home, but each one can be smaller as they cooperate.
Here in Sweden we already always distribute to homes at 70-100°C and then at 50-55°C internally (again, with heat exchangers to minimize energy loss). And that works just fine. We just don’t (yet) have many systems which feed back thermal energy.
You can easily feed back to a central hot water tank, then let the central heater rest as another heatpump returns energy to it.
The different temperatures are not a problem because they’re difficult to achieve or anything. Most devices that need heat would have a pretty wide tolerance. The main problem is efficiency, when that heat is provided by a heat pump.
If you have underfloor heating, that can typically work with water temperatures as low as just 30°C. If you have a heat pump that needs to heat up water to 30°C, with outside temperature being at about 0°C (realistic winter temperature here, and it makes the calculation easy), then a good heat pump typically reaches a CoP of 4 (sometimes even slightly more).
If you instead would use an internal heat grid of 65°C (a pretty typical practical maximum of conventional heat pumps, and a pretty typical temperature for a heat pump powered clothes dryer to run at), then that CoP typically drops to only just about 2.
So, if you let your heat pump run at 65°C and use a heat exchanger to run your heating off of it at 30°C, you use nearly twice as much electricity for every watt of heat you add to your house compared to just running the heat pump at 30°C and using the output directly.
Typically the heating in a home is the application that demands the most amount of heat by far. But it’s also the application that is usually the most tolerant to low temperatures (especially if you have underfloor heating or forced airflow radiators). So you save a huge amount of electricity by having the circulating water temperature as low as possible, but that also makes that heat not very useful for most other devices (a clothes dryer operating at only 30°C would be highly ineffective).
Heat grids work great if they are supplied by industrial waste heat. But if the heat has to come from a heat pump they’re a terrible idea because of how much heat pump efficiency decreases with increasing temperature delta.
Enhanced Vapor Injection (EVI) is the engineering feature that separates capable cold-climate heat pumps from standard units. By injecting refrigerant vapor mid-compression, EVI compressors achieve two key benefits:
Higher discharge temperature: Enables delivery of 60–80°C water even at -20°C ambient, without staging.
You assumed 0°C? They solved it at winter temperatures most of Sweden sees.
Any COP above 1 eventually pays off, as long as it does so before the equipment is due for replacement you’re good.
Your comment makes the error of assuming “not peak efficiency isn’t good enough” even if a ratio just above 1 pays off if other energy sources are more expensive. Such as when you need more heat than the burned waste can provide. Or when you need to maintain the temperature of the hot water tank after filling it. You just need it to pay off before you need to replace the equipment.
Literally just see the video you responded to - some of these tanks can even maintain a thermal gradient in a single water tank and doesn’t have to heat it all to max at once, and some can even provide water out at multiple temps this way. If you get that kind of machine that delta is irrelevant. And also, as the video shows, the top-up heating efficiency is actually high - when it starts at a high temperature and raises it a few more degrees.
You’re also forgetting the per-machine overhead - a home having multiple heat pumps helps in large buildings, but you do not want one for every tap, etc. You want them where you have large thermal energy flows.
It’s possible to heat things like return water or cold water directly and then use heat exchangers.
Temperature differentials also doesn’t matter much because temperature doesn’t directly behave like voltage, if your hot source is too hot you simply use less hot water (lower flow). You’d likely end up with multiple heat pumps throughout a larger home, but each one can be smaller as they cooperate.
Here in Sweden we already always distribute to homes at 70-100°C and then at 50-55°C internally (again, with heat exchangers to minimize energy loss). And that works just fine. We just don’t (yet) have many systems which feed back thermal energy.
You can easily feed back to a central hot water tank, then let the central heater rest as another heatpump returns energy to it.
The different temperatures are not a problem because they’re difficult to achieve or anything. Most devices that need heat would have a pretty wide tolerance. The main problem is efficiency, when that heat is provided by a heat pump.
If you have underfloor heating, that can typically work with water temperatures as low as just 30°C. If you have a heat pump that needs to heat up water to 30°C, with outside temperature being at about 0°C (realistic winter temperature here, and it makes the calculation easy), then a good heat pump typically reaches a CoP of 4 (sometimes even slightly more).
If you instead would use an internal heat grid of 65°C (a pretty typical practical maximum of conventional heat pumps, and a pretty typical temperature for a heat pump powered clothes dryer to run at), then that CoP typically drops to only just about 2.
So, if you let your heat pump run at 65°C and use a heat exchanger to run your heating off of it at 30°C, you use nearly twice as much electricity for every watt of heat you add to your house compared to just running the heat pump at 30°C and using the output directly.
Typically the heating in a home is the application that demands the most amount of heat by far. But it’s also the application that is usually the most tolerant to low temperatures (especially if you have underfloor heating or forced airflow radiators). So you save a huge amount of electricity by having the circulating water temperature as low as possible, but that also makes that heat not very useful for most other devices (a clothes dryer operating at only 30°C would be highly ineffective).
Heat grids work great if they are supplied by industrial waste heat. But if the heat has to come from a heat pump they’re a terrible idea because of how much heat pump efficiency decreases with increasing temperature delta.
https://www.sidite-solar.com/cold-climate-performance-cop-data-air-source-heat-pump-performance-at--25c-real-cop-efficiency-curves-compared-2026-guide-
You assumed 0°C? They solved it at winter temperatures most of Sweden sees.
Any COP above 1 eventually pays off, as long as it does so before the equipment is due for replacement you’re good.
You clearly did not read the full comment chain, nor my full comment.
Your comment makes the error of assuming “not peak efficiency isn’t good enough” even if a ratio just above 1 pays off if other energy sources are more expensive. Such as when you need more heat than the burned waste can provide. Or when you need to maintain the temperature of the hot water tank after filling it. You just need it to pay off before you need to replace the equipment.
Literally just see the video you responded to - some of these tanks can even maintain a thermal gradient in a single water tank and doesn’t have to heat it all to max at once, and some can even provide water out at multiple temps this way. If you get that kind of machine that delta is irrelevant. And also, as the video shows, the top-up heating efficiency is actually high - when it starts at a high temperature and raises it a few more degrees.
You’re also forgetting the per-machine overhead - a home having multiple heat pumps helps in large buildings, but you do not want one for every tap, etc. You want them where you have large thermal energy flows.