Author Topic: Air source heat pump experiments  (Read 3488 times)

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Offline woodchipsTopic starter

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Air source heat pump experiments
« on: October 20, 2023, 09:54:55 am »
I bought at an auction a swimming pool air source heat pump, a Hydropro Premium 14, output 11kW.

The control module had got wet and the LCD and functions failed, but it did come with a replacement module that didn't work, ust came up with communication errors. Asked the supplier why but no help, or any assistance as to get it working. Seems the message there is be warned that you can pay £3000 for a heat pump and 5 years later it is scrap.

Anyway, found that ignoring the module and just operating the relays can get the compressor and fan to run, also the 4 way valve to swap from cooling to heating but that had to be changed after the compressor had been running for a minute or two. Did post a query about this on this forum and apparently it is critical that this valve is driven correctly. Seems ok to me with this delay.

With an air temperature of 15C and running in heat mode I was able to get output water temperature into the 60'sC.

We have just had a few colder days, air temp down to 2C so had another try. With air at 9C, water at 8C, 237V and 9.5A the heatpump was running fine. Leaving it running and after just 10 minutes or so water was at 57C, evaporator output at 5C, current 14.1A. I think this is really quite impressive, that water temperature can be used in standard radiators, no need for underfloor heating. The cost is the current consumption rising from 9.5A to 14.1A. Power consumed in the heat pump, the water pump was separately powered, is 3340W. The water was just 10l so how much will that be raised by 3.3kW in 10 minutes? That is 2M Joules, divided by Joules to calories, divide by 1000 to kcalories divided by 10 litres gives 64C, I measured about 50C so close-ish. The amunt of water wasn't measured, just the plastice tray size measured and calculated so could be well out.

Conclusions are that, yes, an air source heat pump can raise water temperatures to far above what the word on the street suggests is possible. But in so doing the compressor is ending up working much harder, current consumption rises by about 50% to possible life threatening levels. It is also quite clear that the heat pump isn't designed to raise water temperatures in cold climates because it is next to impossible to insulate the compressor etc to stop significant heat loss. Not what this was designed to do of course, but does limit its usefulness in another application.

Would be very interested in what others think about this, and any other tests made.



 

Online Berni

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Re: Air source heat pump experiments
« Reply #1 on: October 20, 2023, 11:35:47 am »
Yep heatpumps always become much less efficient as the difference in the input and output temperature becomes too large.

A water temperature of 65°C is definitely too high. I have a 12kW Viessman heatpump heating the house and it typically runs the water temperature around 35 to 45°C into the radiators. If you require a larger water temperature then it is worth investing into larger and thicker radiators with more surface area.

The efficiency can further be improved by running the compressor slower when there is low heat output required, for this reason most heatpumps for heating homes have a VFD drive running a 3 phase compressor. But you could still use this as is for heating a house since you typically use a 100 liter water storage tank, letting you cycle the heat pump on and off for reasonably long periods. Compressors are usually not very insulated anyway since the majority of the compressor is in the cold side. The compressor hot side is only the piston and output valve. They sometimes put a jacket around the compressor for audible noise damping.
 

Online Siwastaja

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Re: Air source heat pump experiments
« Reply #2 on: October 21, 2023, 06:02:24 pm »
Of course, when correctly controlled and designed to do so, you can easily use a heatpump to heat 45degC above ambient, and much more.

It's just that investing in bigger radiators, fan coil units or underfloor heating could make financial sense in the long run because COP will get so much better. For 60degC deltaT, practical COP limit for commercial units is somewhere around 1.5 to maybe 2.0 with very well designed units. With 40degC difference, that would be easily 2 to 3.

Swimming pool heatpump is not designed for high output temperatures so it likely performs worse than those designed for heating domestic hot water in cold climates, but it's still not that surprising to hear you having good results, there is no big fundamental difference, just tiny design details. The fact it has the reversing valve (allowing defrosting) indicates it's not the cheapest-ass swimming pool heatpump.

You can insulate the compressor for cold climate (usually they use some heavy form of mineral wool mat that doubles as acoustic and thermal insulation) but don't overdo it because you don't want to overheat the compressor during summer. It's also important to add compressor heating, something like 50W, in cold climates so that the oil gets heated up before starting the compressor. This can be turned off later when the thing runs.

Even if you don't insulate the compressor and some internal pipework, it still has so little surface area that you are not losing more than some tens of W. Compare that to the power savings in thousands of W and it's not a big deal. Some people like to modify their heatpumps to get tiny efficiency improvements but it usually isn't worth even the insulation material cost.

Below about +5degC, defrosting algorithm becomes quite important for real-world efficiency.

If you can use large reservoid of water (say, more than 300 liters or so), then an inverter compressor does not have much benefit over the good old on-off; but if you are driving directly into radiators, then being able to run at reduced power is priceless. Very short operating cycles not only strain the compressor, but also reduce efficiency as the process always runs suboptimal for a minute or so when starting. But even inverter machines have to operate in on-off mode at very low loads because they have minimum power which can be surprisingly high (e.g., 40% of maximum for my heatpump). I have 1200 liters of water and I have very long operating cycles, just a few cycles per day.
« Last Edit: October 21, 2023, 06:12:10 pm by Siwastaja »
 

Offline woodchipsTopic starter

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Re: Air source heat pump experiments
« Reply #3 on: October 25, 2023, 12:33:10 pm »
Thank you for your replies.

Strange but all the books and articles I have read on heat pumps just say that maximum temperature is about 40C, never thought anything more about it and just assumed that there was some sort of limit. As has been said, what changes is the COP, gets worse as temperature goes up, obvious really when in front of your nose.

This raises the question, why is it better value for money to put in underfloor heating that work the heat pump harder? It is very expensive to do, to say nothing of the mess and disruption, a lower COP might be a good deal instead.

In the UK we don't now seem to have frosts, a few days per year, so ignoring the icing up is an acceptable alternative, just not use it when outside temperature is below about 6C.

Even not bothering with the crankcase heater is probably ok, if only used above freezing. If you blow the compressor up then still much cheaper than underfloor heating.

Apparently a swimming pool, or sauna, will be about 40C so pretty warm. Can't get too much airflow over the radiator otherwise drafts, but something is possible, again to save on renewing radiators, money.

Think the optimum use for this heat pump is to get a genset in an insulated building, my garage, and use it to keep the genset cool. Will need to duct in the combustion air and exhaust but that is pretty simple. Extracting the heat from the genset should raise the overall efficiency to 80-90%, really not bad. Keeps the noise down as well.
 

Online Siwastaja

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Re: Air source heat pump experiments
« Reply #4 on: October 25, 2023, 01:02:57 pm »
Strange but all the books and articles I have read on heat pumps just say that maximum temperature is about 40C

Never heard such in any book or article. It would be obviously total bullshit. You have managed to come across some very strange resources. For example, a completely random Google search result states maximum temperature of 121 to 135degC for discharge (refrigerant in gas form after the compressor) temperature: https://www.sciencedirect.com/topics/engineering/discharge-temperature . Around 120degC is where heatpumps usually error out. Now there is some superheat in this so condensing temperature (temperature at phase change from gas to liquid) would be somewhat lower, plus losses in heat exchanger - for example right now at ambient of -1degC, my heatpump is running with discharge temperature of +65degC, producing +34degC water. The usual highest recommended water temperature for these types of heatpumps (designed to heat domestic hot water) is around +60degC (which is pretty useful against legionella in DHW systems), at which point the discharge temperature would be around +100degC. Much higher than that suggests flow rate problem e.g. clogged water line.

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This raises the question, why is it better value for money to put in underfloor heating that work the heat pump harder? It is very expensive to do, to say nothing of the mess and disruption, a lower COP might be a good deal instead.

Of course, retrofitting underfloor heating would be extremely expensive and lower COP instead is probably a good tradeoff. Then again, if you are building new, hydronic underfloor heating is something to take seriously as it's most flexible for heatpump use, and at the time of construction not that expensive. For retrofits, swapping a few radiators for bigger ones is not usually too expensive; fan coil units are even better if you can accept the tiny amount of noise. For example, I retrofitted two new radiators and swapped some larger old ones in places of the smaller old ones, and it was a 500EUR DIY project. Swapping a radiator is half-an-hour job for a professional plumber, per radiator. Compared to old, small radiators designed for gas/oil heating you easily get 10-15degC decrease in water temperature. I have heard people making decisions of buying a more expensive (say 7000EUR instead of 4000EUR) air-to-water heatpump with purely the mental impression* that the more expensive unit probably has better COP at extreme temperatures - with zero data to back it up. Instead of spending that 3000EUR for this premium model, they could have spent 1500EUR to radiator swap and would have had significant COP improvement, guaranteed.

*) heatpump manufacturers operating in Nordic regions play this marketing game advertising that their product produces heat even at -30degC or so, but they never say what the COP is. It's of course 1.0. My cheapest-brand ES/Amitime heatpump runs without a hitch, producing 50-degC output, at -30degC, no problem. This is what they do, ignoring a few brands which have arbitrary cut-off limits.

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In the UK we don't now seem to have frosts, a few days per year, so ignoring the icing up is an acceptable alternative, just not use it when outside temperature is below about 6C.

This is a sensible thing to do, actually even in colder climates, if your goal is to just replace part of fossil fuel heating and keep the old system running in parallel. This enables you to get some absolute cheapeast ass swimming pool heatpump that doesn't even have reversing valve (and thus cannot defrost). Just remember that defrosting is already needed above 0degC (maybe as high as +5, or as you say, +6degC, and I'm sure this is more common in UK than just a few times a year, yes?). Although very humid air actually protects against frosting, I have seen my unit running non-stop without frosting up at all even at temperatures so low as +1degC with >95% humidity. It's actually raining under the evaporator which is running super efficiently given all the energy released from this phase change!

Heatpumps make biggest annual savings during the conditions which are cold enough so that amount of heating power is non-negligible, but still not too cold so that COP plummets. In Finland, even though -30degC is pretty "normal", it could be only for a week per year or some years not at all. At +7degC, all heatpumps perform well, but given that houses are insulated to survive that -30degC, heating power needed at +7 is so small that again it doesn't matter if the COP is 4 or 4.5 or 5 or whatever. Therefore, it is most important to see how well the pump does at -5degC or so, that's where the number of days * energy needed per day is the highest. I guess the equivalent "point of importance" in the UK would be +7degC or so, making it fully possible to use a non-defrosting super cheap design and still get nearly the same savings as with high-performance expensive heatpump.
« Last Edit: October 25, 2023, 01:12:12 pm by Siwastaja »
 
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Offline coppice

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Re: Air source heat pump experiments
« Reply #5 on: October 25, 2023, 01:11:23 pm »
Strange but all the books and articles I have read on heat pumps just say that maximum temperature is about 40C
Are you sure they said the maximum temperature is 40C? Unless you are looking at extreme temperatures, heat pumps are usually about differential temperatures, not absolute ones. So, maybe you were looking at things which talked about a maximum raise of temperature of 40C. That hasn't been the maximum for a long time, but its not long since that was the maximum that made much sense. Getting a lift of more than 40C used to bring the COP so low it was a dumb idea. Possible, but useless.
 

Online Siwastaja

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Re: Air source heat pump experiments
« Reply #6 on: October 25, 2023, 01:24:38 pm »
Are you sure they said the maximum temperature is 40C? Unless you are looking at extreme temperatures, heat pumps are usually about differential temperatures, not absolute ones.

Both are important. Going much above say +65degC (output water temperature; equivalent discharge would be over 110degC) would require specialized refrigerants, compressor oils etc. On the other hand, the difference is important from the COP standpoint.

The commonly seen idea that compressor needs to "work hard" with large differences is not very accurate; what strains the compressor is the absolutely high output temperature. This is what stresses compressor oil, motor winding insulation etc.

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So, maybe you were looking at things which talked about a maximum raise of temperature of 40C.

Yeah, probably. Although even that is quite conservative. There is also this widespread idea that running a compressor at, say, COP=1.5 does not "make sense". But compared to direct electric heating at COP=1, it makes sense, and once the compressor is there and running, turning it off makes little sense either, unless you have some significantly cheaper alternative energy source to switch to. Large temperature differences are not that demanding for the compressor either, unless the absolute output temperature is close to the upper limit.
« Last Edit: October 25, 2023, 01:26:33 pm by Siwastaja »
 

Offline coppice

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Re: Air source heat pump experiments
« Reply #7 on: October 25, 2023, 01:42:19 pm »
Are you sure they said the maximum temperature is 40C? Unless you are looking at extreme temperatures, heat pumps are usually about differential temperatures, not absolute ones.

Both are important. Going much above say +65degC (output water temperature; equivalent discharge would be over 110degC) would require specialized refrigerants, compressor oils etc. On the other hand, the difference is important from the COP standpoint.

The commonly seen idea that compressor needs to "work hard" with large differences is not very accurate; what strains the compressor is the absolutely high output temperature. This is what stresses compressor oil, motor winding insulation etc.
It seems you just restated my point about extreme temperatures in long form. If you want to expand further, heat pumps have more complex issues at excessively low source temperatures. Their self heating means they can work from quite low temperatures on the source side, but may need a resistive heater to initially get warm enough to start. The thing about working hard is that when the COP is low it takes a low more cycles of the pump to shift the same amount of heat. Therefore the mechanical wear and tear is considerably higher.
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So, maybe you were looking at things which talked about a maximum raise of temperature of 40C.

Yeah, probably. Although even that is quite conservative. There is also this widespread idea that running a compressor at, say, COP=1.5 does not "make sense". But compared to direct electric heating at COP=1, it makes sense, and once the compressor is there and running, turning it off makes little sense either, unless you have some significantly cheaper alternative energy source to switch to. Large temperature differences are not that demanding for the compressor either, unless the absolute output temperature is close to the upper limit.
Books aren't generally that up to date. A COP of 3 under good conditions, falling below one without the differential being huge was quite normal not that long ago. If the COP sometimes drops to 1.5 that's fine. If its like that day after day you probably wouldn't recover the cost of a heat pump over a resistive heater, and you would definitely be losing against a gas heater.
 

Online Berni

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Re: Air source heat pump experiments
« Reply #8 on: October 25, 2023, 03:09:22 pm »
For a brief period i did see internal combustion engine powered heat pumps.

Since a diesel engine will run on heating oil, you can use one to directly spin the compressor, then use the combine heat output of both of them to heat the house. But the price of heating oil has went up considerably since way back then. Plus the whole thing is likely more expensive when it has to include a whole engine, plus it is louder, requires more maintinance...etc

But yeah COP is the name of the game with heat pumps, there is no hard limit on temperatures since you can go to any temperature provided you can run with the correct refrigerant gas and have the correct pressures, it just won't be efficient. The whole point of having a heatpump versus a electric heater is the higher efficiency. And by running heatpumps in a more favorable operating range you get more useful heat out of it. Things like in floor heating help reduce the hot side temperature while using a ground heat exchanger or ground water well on the outside helps keep the cold side temperature higher. All of these are an extra investment that might or might not make sense for the COP gain it provides, since the difference in the cost of consumed electrical power has to make up for it.

Something like in floor heating makes little sense to add into a finished house, but it is something to consider when building a new home. This kind of heating also has other comfort benefits, it feels warmer to humans since people walk on the floor not on the ceiling, meaning you can have a lower average room temperature, it also helps with drying out the air less in winter (some people don't handle dry air very well). In places like bathrooms it feels nice to step from the shower onto a warm floor, also helps dry the floor and floor mat.
 

Offline coppice

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Re: Air source heat pump experiments
« Reply #9 on: October 25, 2023, 03:10:37 pm »
For a brief period i did see internal combustion engine powered heat pumps.
They still exist. Lots of trucks have them. The first heat pumps were powered by steam engines.

(Large number of cars and truck have a heat pump driven from the engine, as a supplementary function of the engine, but when I said trucks I was thinking of the dedicated ICE engine powered heat pumps for refrigeration.)
« Last Edit: October 25, 2023, 03:16:16 pm by coppice »
 

Online Siwastaja

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Re: Air source heat pump experiments
« Reply #10 on: October 25, 2023, 03:42:06 pm »
The thing about working hard is that when the COP is low it takes a low more cycles of the pump to shift the same amount of heat. Therefore the mechanical wear and tear is considerably higher.

Considerably higher compared to what, and why?

What you say is technically 100% true, but taken literally that would mean seriously overdimensioning the heatpump, expecting longer-than-usual lifetime due to it almost never running at full speed (which could be unwarranted assumption, too), and then getting surprised if it sometimes runs at full speed. But that's not sensible heating system design.

In reality, to avoid investing in excess unused capacity, and to avoid inverter machine running too much in on/off mode (which people also think causes some wear - which could be true especially in cold conditions even with compressor heating!), one should choose a heatpump power rating so that it runs at full speed while still not close to COP1 (e.g., at COP1.5), and use auxiliary resistive heating to support the power need in extreme conditions, where the "marginal COP" (compare to concept of marginal tax) is so close to 1.0 that having more compressor power makes no financial sense whatsoever. Resistive is small, cheap and reliable!

Quite interestingly, this leads to a design where low distribution temperature (underfloor, fan coils) directs to choose a larger heatpump, the opposite to what people intuitively do. (What they think is: "I have small radiators, I need high temperature, therefore I need a bigger heatpump." But in reality, they would be hitting that marginal COP = 1 point at much lower power, so actually a smaller machine can do that compressor only, and rest can be done with resistive heating. Larger machine does not have better COP given the same conditions (Tambient, Twater), in fact it can be worse if the machine runs at minimum speed (or even worse, on/off) with larger bottom tray heater running at bigger power constantly, or a compressor heater running during off-times trying to reduce the wear caused by frequent cold starts).

Heat pump dimensioning is weird and unintuitive because for building heating, required power goes up linearly as outdoor temperature lowers, but thermal power generated by heatpumps goes down at the same time, so the curves of demand and response go opposite and cross somewhere. For compressor-only design, the required machine size would be ridiculously high, with very little benefits, and very real downsides.

And for air-to-water designs using small radiators, heatpump output power (given constant input power) does not only go down, it goes down faster than linear, nearly quadratically!

Remember that a heatpump is roughly a constant input power thing; output power varies based on temperature difference, because the amount of harvested free energy varies; but you always get the (constant) input power back.

For example, the compressor on my machine uses 2.2kW electrical at full speed, and produces 2 - 9 kW of heat, inversely depending on how much power is needed to heat the house. When I need 1kW, I can get 9kW; when I need 6kW, I can get 2kW. Compressor only can provide the heat required down to -15degC, but due to that inverse relationship, already at -4degC or so the machine produces too much heat for minimum inverter speed (0.8kW electric) and has to cycle on/off. This is completely normal and not "harmful" per se, but if I were to buy a 6kW electrical machine, which could supply the maximum 6kW heating power needed in -30degC conditions, then I would have an expensive (15000e industrial grade) and large outdoor unit sitting there mostly generating brief pulses of excess heat only to turn off in 15 minutes, most of the year. Efficiency also suffers in on/off mode. Optimum efficiency is usually somewhere near full power, say 80%. Near full speed is also the only way to guarantee proper circulation of compressor oil, which is why at lower speed these machines periodically cycle to higher speed for a minute. There is no harm in running constant high speed (say 80%) all the time instead. These machines are designed to run full power and I'm skeptical about the lifetime increase you could get from derating. It's well possible the downsides of derating reduce the lifetime more.
« Last Edit: October 25, 2023, 03:54:21 pm by Siwastaja »
 

Offline nctnico

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Re: Air source heat pump experiments
« Reply #11 on: October 25, 2023, 03:53:40 pm »
Something like in floor heating makes little sense to add into a finished house, but it is something to consider when building a new home.
It depends a bit on how a home is build. Concrete floors typically have a soft layer of mortar on top where you can route grooves into (using a wall chaser) for in floor heat piping. In my case this was a relatively easy upgrade to do on the ground floor. Top tip: make underfloor heating in the bathroom a priority.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline coppice

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Re: Air source heat pump experiments
« Reply #12 on: October 25, 2023, 04:02:21 pm »
The thing about working hard is that when the COP is low it takes a low more cycles of the pump to shift the same amount of heat. Therefore the mechanical wear and tear is considerably higher.

Considerably higher compared to what, and why?
Considerably higher than a system which doesn't need such a big temperature differential.
What you say is technically 100% true, but taken literally that would mean seriously overdimensioning the heatpump, expecting longer-than-usual lifetime due to it almost never running at full speed (which could be unwarranted assumption, too), and then getting surprised if it sometimes runs at full speed. But that's not sensible heating system design.

In reality, to avoid investing in excess unused capacity, and to avoid inverter machine running too much in on/off mode (which people also think causes some wear - which could be true especially in cold conditions even with compressor heating!), one should choose a heatpump power rating so that it runs at full speed while still not close to COP1 (e.g., at COP1.5), and use auxiliary resistive heating to support the power need in extreme conditions, where the "marginal COP" (compare to concept of marginal tax) is so close to 1.0 that having more compressor power makes no financial sense whatsoever. Resistive is small, cheap and reliable!

Quite interestingly, this leads to a design where low distribution temperature (underfloor, fan coils) directs to choose a larger heatpump, the opposite to what people intuitively do. (What they think is: "I have small radiators, I need high temperature, therefore I need a bigger heatpump." But in reality, they would be hitting that marginal COP = 1 point at much lower power, so actually a smaller machine can do that compressor only, and rest can be done with resistive heating. Larger machine does not have better COP given the same conditions (Tambient, Twater), in fact it can be worse if the machine runs at minimum speed (or even worse, on/off) with larger bottom tray heater running at bigger power constantly, or a compressor heater running during off-times trying to reduce the wear caused by frequent cold starts).

Heat pump dimensioning is weird and unintuitive because for building heating, required power goes up linearly as outdoor temperature lowers, but thermal power generated by heatpumps goes down at the same time, so the curves of demand and response go opposite and cross somewhere. For compressor-only design, the required machine size would be ridiculously high, with very little benefits, and very real downsides.

And for air-to-water designs using small radiators, heatpump output power (given constant input power) does not only go down, it goes down faster than linear, nearly quadratically!

Remember that a heatpump is roughly a constant input power thing; output power varies based on temperature difference, because the amount of harvested free energy varies; but you always get the (constant) input power back.

For example, the compressor on my machine uses 2.2kW electrical at full speed, and produces 2 - 9 kW of heat, inversely depending on how much power is needed to heat the house. When I need 1kW, I can get 9kW; when I need 6kW, I can get 2kW. Compressor only can provide the heat required down to -15degC, but due to that inverse relationship, already at -4degC or so the machine produces too much heat for minimum inverter speed (0.8kW electric) and has to cycle on/off. This is completely normal and not "harmful" per se, but if I were to buy a 6kW electrical machine, which could supply the maximum 6kW heating power needed in -30degC conditions, then I would have an expensive (15000e industrial grade) and large outdoor unit sitting there mostly generating brief pulses of excess heat only to turn off in 15 minutes, most of the year. Efficiency also suffers in on/off mode. Optimum efficiency is usually somewhere near full power, say 80%. Near full speed is also the only way to guarantee proper circulation of compressor oil, which is why at lower speed these machines periodically cycle to higher speed for a minute. There is no harm in running constant high speed (say 80%) all the time instead. These machines are designed to run full power and I'm skeptical about the lifetime increase you could get from derating. It's well possible the downsides of derating reduce the lifetime more.
Other than heating water for washing, why do so many people have this weird obsession with using heat pumps to heat water? Until I returned to the UK 6 years ago I spent 25 years using heat pumps to heat and cool my home. I was comfortable. I never heated any water. I never created any large temperature differentials. I never had a bad COP. Bad COP values are an option you choose when you choose to use a weird setup.
 

Online Siwastaja

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Re: Air source heat pump experiments
« Reply #13 on: October 25, 2023, 04:24:40 pm »
Considerably higher than a system which doesn't need such a big temperature differential.

Would you explain why do you think a large temperature differential is causing considerably higher wear? I'm not aware of any such mechanism, as long as the absolute output temperature is not close to the limits. To my understanding, running at extreme cold conditions at relatively high temperature differential is not harmful to compressor life, and there is quite a lot of experience about this in Finland.

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Other than heating water for washing, why do so many people have this weird obsession with using heat pumps to heat water?

Several reasons:
1) retrofits to existing hydronic heating systems, such that all rooms can be heated. Air-to-air unit offers better COP than a radiator based air-to-water retrofit, but for many rooms, many units would be required, or some rooms would be resistively heated, bringing total COP down again.
2) with underfloor heating, COP is as good as with air-to-air units, with the benefit of comfort, silence, and again, even distribution to the whole house
3) (maybe a weird edge case, but relevant for some, me for example): water allows energy to be stored quite easily. With current volatility of spot price markets, this opens interesting optimization opportunities.
and, finally, as you mention, washing: some people use quite a lot of DHW. The fact that a single unit heats up your house and heats your DHW at increased efficiency all around year, especially in summer, is useful.

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Bad COP values are an option you choose when you choose to use a weird setup.

Yeah, I do agree that it's not always a good idea to force an air-to-water unit when a much cheaper air-to-air room air conditioner, or two, or even three, would do much better job, at better COP and smaller investment. And also summertime cooling without condensation issues. This simple solution is sometimes underrated.
 

Offline langwadt

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Re: Air source heat pump experiments
« Reply #14 on: October 25, 2023, 04:33:08 pm »
what kind of sensor are on the system? I'd expect a control that uses temperature and pressure to control a valve for effieciency 
 

Offline coppice

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Re: Air source heat pump experiments
« Reply #15 on: October 25, 2023, 04:43:43 pm »
Considerably higher than a system which doesn't need such a big temperature differential.

Would you explain why do you think a large temperature differential is causing considerably higher wear? I'm not aware of any such mechanism, as long as the absolute output temperature is not close to the limits. To my understanding, running at extreme cold conditions at relatively high temperature differential is not harmful to compressor life, and there is quite a lot of experience about this in Finland.
If the temperature differential is big enough to make the COP lousy the pump is just doing a lot more work. Its wear and tear has to be higher. The idea of it being strained is bogus, but every turn it takes is another unit of wear and tear,

As for Finland I would like to see some real solid info. People from Scandinavia keep saying on forums that Japan has been full of heat pumps heating their homes for decades, rather than referencing their own experiences. Er, no. Most Japanese homes have had cooling only air cons for decades, and recently a lot have installed heat pumps specifically for water heating. However, apart from the really cold areas, like Hokkaido, Japan is famous as the one developed country in a cool climate where people have not taken home heating very seriously. I guess its cultural, but they seem content with very limited heating facilities.
 

Online Siwastaja

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Re: Air source heat pump experiments
« Reply #16 on: October 25, 2023, 04:58:16 pm »
If the temperature differential is big enough to make the COP lousy the pump is just doing a lot more work. Its wear and tear has to be higher.

I already explained why this is practically nonsense and am not going to reiterate it. Heatpumps are designed to work at 100% compressor RPM, work optimally at 60-100% range, probably suffer below 40% range due to more cycling. Maybe between 60% and 80% average there would be a 20%-ish lifetime difference, but that's it.

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As for Finland I would like to see some real solid info.

I can assure you that old units (most popular being Mitsubishi Electric) installed in the first boom of 2006-2007 and running since then are pretty common, and the Finnish usage pattern is buying one undersized unit for larger than intended space and running it full power the whole winter, keeping it running during extreme cold. This is of course the best bang-for-buck approach, and these things last 15 years easily despite running at full power.

Most demanding condition would be with temperature at maximum, fan at minimum, and indoor unit not cleaned for years, so that air flow is hindered and discharge temperature is higher than usual. Large difference from outdoor temperature alone does not much harm; unnecessary cycling in winter is definitely worse, and that would result in if you try to avoid "running at full power".
« Last Edit: October 25, 2023, 05:47:44 pm by Siwastaja »
 

Offline coppice

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Re: Air source heat pump experiments
« Reply #17 on: October 25, 2023, 07:47:03 pm »
[I can assure you that old units (most popular being Mitsubishi Electric) installed in the first boom of 2006-2007 and running since then are pretty common, and the Finnish usage pattern is buying one undersized unit for larger than intended space and running it full power the whole winter, keeping it running during extreme cold. This is of course the best bang-for-buck approach, and these things last 15 years easily despite running at full power.

Most demanding condition would be with temperature at maximum, fan at minimum, and indoor unit not cleaned for years, so that air flow is hindered and discharge temperature is higher than usual. Large difference from outdoor temperature alone does not much harm; unnecessary cycling in winter is definitely worse, and that would result in if you try to avoid "running at full power".
If you are running these heat pumps at such a constant rate, does that mean that large parts of the Finnish winter are at a fairly steady temperature?
 

Online Siwastaja

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Re: Air source heat pump experiments
« Reply #18 on: October 26, 2023, 05:35:14 am »
If you are running these heat pumps at such a constant rate, does that mean that large parts of the Finnish winter are at a fairly steady temperature?

No, it means the heatpump is used as supportive heater and tops out already at mild weather (say -5degC), and keeps topping out during colder times (say -25degC). This is cost-effective because, let's say the system is dimensioned so that compressor-only power suffices at -5degC, then:

* at -5degC, even this smaller unit running at 100% provides the same COP as a bigger alternative
* at 0 degC, small unit probably has better COP running at 70% power and smaller tray heater, than oversized unit on/offing short cycles with tray&compressor heaters constantly on
* at -25degC, both undersized and oversized units have COP near 1, even if the larger one does it all by compressor.
* at say -15degC, larger unit (compressor only) might have COP of 1.6 while the combination of smaller one + resistive aux would have 1.3. The difference is here; but is this worth the extra investment?
« Last Edit: October 26, 2023, 05:37:44 am by Siwastaja »
 

Offline woodchipsTopic starter

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Re: Air source heat pump experiments
« Reply #19 on: October 29, 2023, 09:57:42 am »
Thanks for all the comments.

Book I am reading is Heat Pump Technology by von Cube and Steimle, 1981. Have others but this is the more technical book. Goes into explaining that all the losses drop the COP, and there seem to be lots of losses once the electricity reaches your house.

The maximum of 40C temperature comes from all the newspaper etc articles about how to use a heat pump. When you know nothing you assume the writers of these articles are experts, why else would they be paid? Along with all the other absolute fact statements you soon realise you have to do the work, they don't.

Why does a compressor need an oil heater? You jump in your car, start the engine in -5C and drive away, surely modern oils can cope? I am not saying that start and immediately drive is a good idea, will ruin the engine in time, but if you run the compressor off load for a few minutes to get the oil moving and then put on load?

Other thing you discover is the Carnot limits.

A compressor just needs a rotating shaft from an electric motor, why not a wind turbine or similar? My heat pump uses 14A so 3.5kW so going to be a large turbine.

Have a few of these heat pump assemblies out of old computers that cool the CPU, are they any use in a heat pump?

 

Online Siwastaja

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Re: Air source heat pump experiments
« Reply #20 on: October 29, 2023, 10:36:57 am »
but if you run the compressor off load for a few minutes to get the oil moving and then put on load?

What "off load"? Once the compressor runs, it is immediately working (creating pressure). Or are you suggesting adding an electrically controlled bypass valve so that it can run "idle"?

Compressors are not cars. Oils have to be compatible with the refrigerants (and last decades with no chemical reactions between them), and of course they would engineer the oil to perform best at the normal operating temperatures for minimum possible friction.

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Other thing you discover is the Carnot limits.
So what about it? Real heatpumps are far away from this theoretical limit. Just calculate it for yourself, the formula is simple.

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A compressor just needs a rotating shaft from an electric motor, why not a wind turbine or similar?
Compressor has high pressure (tens of bars), shaft coupling which doesn't leak is difficult to do. Much more difficult than shaft seals in car engines, where small leakages are not catastrophic, one just adds a bit of new engine oil.

And what for? Turns out electric motors are highly efficient just like electric generators are, so transmission of electricity from wind turbine is just so much simpler than transmission of mechanical power. And don't forget the RPMs would never match to what you need so you would need some variable gearboxes in the way, too, generating losses!

This is also why ICE-driven heatpumps as mentioned by Berni remain niche. Electric compressors where the motor is integrated within the pressure vessel are just so cost-effective, especially when manufactured in large scales. Better turn whatever mechanical power you have to electric power first and use it to turn the highly efficient integrated low-cost motor.
« Last Edit: October 29, 2023, 11:22:04 am by Siwastaja »
 

Offline johansen

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Re: Air source heat pump experiments
« Reply #21 on: October 29, 2023, 05:24:19 pm »

The maximum of 40C temperature comes from all the newspaper etc articles about how to use a heat pump.

Why does a compressor need an oil heater? You jump in your car, start the engine in -5C and drive away, surely modern oils can cope? I am not saying that start and immediately drive is a good idea, will ruin the engine in time, but if you run the compressor off load for a few minutes to get the oil moving and then put on load?

The discharge of my embrako 10,000 btu r134a compressor on my hot water heater is 90C when the water is 50C.  Nothing wrong with that, but its cop is probably only 2.5.

The oil heater is needed when liquid refrigerant is allowed to flow into the compressor and soak into the oil, while the compressor is off. When it starts up, it foams, the compressor is then starved for oil.
 


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