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What is the real story around heat pumps?

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Zero999:

--- Quote from: zilp on February 25, 2024, 02:53:06 pm ---
--- Quote from: Zero999 on February 25, 2024, 12:36:21 pm ---It's very difficult to find objective data. Not only does COP, of an air source heat pump, depend on the inside and outside temperatures, but also the relative humidity.

I had a bit of a Google and found lots of contradictory information.

This laboratory test shows a COP of between 3 and 5, at 5°C, but it was done under very dry conditions. I live in a very humid climate and COP does down with high relative humidity, when the air temperature is 5°C.
https://www.nrel.gov/docs/fy23osti/85081.pdf

--- End quote ---

The funny thing is that the effect of humidity is actually both beneficial and detrimental. On the one hand, there is a lot of latent energy to be had from condensing water vapor. On the other hand, if you get to the freezing point at the evaporator surface, the condensed water freezes and clogs up the evaporator, requiring energy to be expended for defrosting. Though it should be noted that the phase transition from liquid to frozen also releases latent energy, so the energy "spent" for thawing the ice was previously extracted from the ice by the heat pump. It's just that the heat pump will release more energy than would be absolutely necessary for thawing the ice, and also, the heat might be taken from a different place than where the extracted energy was pumped, both of which will generally make the freeze/defrost cycle a net energy loss and thus reduce COP. This is probably the primary thing that reduces COP around the freezing point with high humidity.
--- End quote ---
Going from what I've read, humidity can both increase, as well as decrease the COP. If the temperature is above 6°C, high humidity generally increases the COP. At temperatures below 6°C, high humidity deceases the COP, which is worse around freezing, then becomes less of an issue below freezing, since freezing air holds less moisture.

In my specific situation, I expect the heat pump to mostly run when the temperature is around 0°C to 6°C, hence why I believe the humid climate where I live is a hindrance, rather than a help. If I were going to use it for hot water, say to heat a pool in summer, then it would be different.



--- Quote ---
--- Quote from: Zero999 on February 25, 2024, 12:36:21 pm ---A quote which I find concerning.

--- Quote ---The energy consumption due to the defrost cycles has to be considered in calculating the heat pump performance but the calculation methods proposed by the European standards ignore this effect.
--- End quote ---

Given this can I trust data based on European standards, or is this not the case and this is out of date information?

--- End quote ---

I have no idea as to the current requirements in the relevant standards, but I guess it isn't really possible to specify a representative way to include defrost energy in one generic COP number, as that, as you yourself said, depends on humidity, and also on rather specific weather patterns. Specifically: It's not something that is well captured by average values, because the COP (including defrost energy) will often be better at -5°C than at +3°C at the same relative humidity, so, the amount of time during a year spent in the critical region determines the influence of defrosting on COP.

At the same time, more transparency wrt defrost energy certainly would be useful, because there certainly is optimization potential. My own heat pump, for example, defaults to defrosting with a resistive heater. Or more specifically, it takes heat from the storage tank (that's kept at DHW temperatures) for defrosting, but switches on the resistive heater in the tank during the process to re-add the energy it extracts from the tank during defrost. But you can just disable the resistive heater and it will happily take the heat from the tank anyway, which was pumped there at a COP of ~ 2 or so (during winter), thus halving the electricity used for defrosting. (You just have to manually re-enable the resistive heater when it is actually needed, which obviously wouldn't be necessary if the controller were just smart enough to do this automatically ...)

Equally, COP doesn't reflect standby consumption of the heating system. My own heat pump uses 12 W in standby, i.e., all pumps off, compressor off, just watching tank and room temperature, eats 100 kWh in a year. Obviously, that should be possible at sub 1 W, right? Given that the system sits idle ~ 70% of the year ... that should be an easy improvement to make for a ~ 3% reduction in power consumption, shouldn't it?
--- End quote ---

I understand that it's not a simple calculation. It's similar to car mileage, in that it depends on how it's being used.

Yes, your standby consumption does sound high. I would have hoped that minimising it would have been an important design goal.


--- Quote ---
--- Quote from: Zero999 on February 25, 2024, 12:36:21 pm ---Someone on a British forum complaining about a COP of 2.3 for space heating and 2.5 for water.
https://forum.buildhub.org.uk/topic/34775-is-my-cop-rubbish/

--- End quote ---

Well, did you read the thread?

During the time apparently covered by those numbers, they had huge holes in the wall, the flow sensor was broken, the total thermal energy they report is significantly lower than what they used in previous years (which might well be because of the broken flow sensor), ... i.e., for all we can tell, that COP is just complete nonsense?
--- End quote ---
To be honest, no I didn't read the whole thread in detail. I concentrated my efforts on reading objective studies. Whilst forums can be useful and a lot can be learned from them, much of it is anecdotal. The only reason why it caught my attention was the numbers were similar to studies in areas with a similar winter climate, to where I live.


--- Quote ---
--- Quote from: Zero999 on February 25, 2024, 12:36:21 pm ---I think those who estimated a COP of 3.5 for me were optimistic. 2.5 seems more likely, given the data I've seen.

--- End quote ---

I don't think anyone estimated a COP for you? Obviously, noone here knows the specifics of your heating system, so it would be nonsensical to estimate a COP for you based on no information. The 350% mentioned by me and others is a good general assumption about heat pumps on average, i.e., a useful basis for discussing, say, general policies as to how to switch energy supply of a country to renewable energy. But obviously, it would be nonsensical to use such a general average to make investment decisions for a specific house. And it is equally nonsensical for you to just assume 250% instead. 350% is fine for a first back-of-the-envelope calculation. Beyond that, you need to measure the actual properties of your heating systems and look at the COP curves published by heat pump manufacturers to get a reasonably accurate estimate as to what you could achieve with a particular heat pump model in your particular home.
--- End quote ---
I thought reading objective studies about heat pumps in areas with a similar winter climate, to my location would give me a reasonable figure. I accept that it will be different, depending on how I use the heat pump, but I would have thought it would by closer, than a general ballpark of 350%, which doesn't even take into account the local climate.

I'm not sure how much I trust COP figures given by manufactures, given there are numerous different factors and they are likely to be skewed in the manufacture's favour. I've seen this with component data sheets, audio amplifiers and vehicle mileage, although I've also had instances when it's better than expected, such as my motorcycle doing more miles per tank, than expected. I suppose I'm more careful, because it would be a big investment, it's not something I'm overly familiar with and then there's the political side to it, which doesn't help.


--- Quote ---
--- Quote from: Zero999 on February 25, 2024, 12:36:21 pm ---I've also realised I didn't take into account the fact that if I can stop using gas, which would also involve replacing my cooker with an induction hob, I can reduce my standing charge (a flat rate daily connection fee, which independent of usage), but then I have to factor in the fact it probably costs a bit more to run.

--- End quote ---

Chances are it doesn't. Gas stoves are very inefficient as far as heating the stuff you put on them is concerned, they primarily heat the room they are in. Whether that's a loss obviously depends. If you are at the same time heating with gas anyhow ... well, you might as well burn it in an open flame in the kitchen? Though the increased CO (and CO2) concentration might be a reason to open the window, so maybe it's net negative still. But then, during summer, you might increase cooling load and thus pay more for getting rid of the waste heat than you saved from the cheap-ish energy source. Or at least it might make things more uncomfortable if it is too hot for your comfort already.

--- End quote ---
I'm aware of the fact that gas stoves aren't very efficient, I've read 40% is reasonable, but don't forget electricity costs four times as much for me, as gas, so an induction hob will still be more expensive for me to run. I doubt it will make much of a difference since, only a tiny part of my energy usage is for cooking, but it's something to consider.

I doubt it'll make any difference to ventilation. I have a CO detector, which never goes off and I only use the fume extractor when I'm cooking something smelly.

I generally avoid cooking in hot whether and summer is generally cool where I live, so unwanted heat is less of a big deal.


--- Quote from: Siwastaja on February 25, 2024, 07:08:49 pm ---
--- Quote from: tszaboo on February 25, 2024, 02:14:13 pm ---I've quickly looked up air to hot water pumps, they start at 3500. You get a subsidy, but only if they install it.

--- End quote ---

But 3500 is not that much at all IMHO. Sure I'd like to see them start at 2000, but :-//. 3500EUR is quite easily recovered already during halfway of the lifetime of the product. It gets nasty when the heatpump is 7000 (some sort of premium brand name which isn't that much better in reality) and then the install cost another 7000, or even more.

--- End quote ---
I agree. That doesn't sound expensive for the heat pump itself. It's the rest of the installation and associated labour which is expensive. Subsidies are only given if you use an approved contractor and it has to meet certain criteria. I don't even know if it's legal for you to do it yourself. I would expect a licenced refrigeration tech will be required, or at the very least an electrician with the appropriate qualifications.

Siwastaja:

--- Quote from: Zero999 on February 25, 2024, 12:36:21 pm ---It's very difficult to find objective data. Not only does COP, of an air source heat pump, depend on the inside and outside temperatures, but also the relative humidity.

I had a bit of a Google and found lots of contradictory information.
--- End quote ---

I agree the information is not that easy to find. You need to combine bits and pieces and measurements by individuals. One helpful advice that I have repeated and do it once again: differences between heatpumps (cheap vs. premium) are surprisingly small, and effect of condensing temperature (i.e., water temperature you need to get into your radiators/floor circuits/etc.) is just massive, regardless of brand. Therefore, while we roughly know the climate you live in, not knowing your water distribution temperature, we can't say whether your SCOP will be 2.0 or 5.0. Very unlikely anything outside this range. Maybe you have underfloor heating pipes and good insulation and need water temperature of +27degC. Then your SCOP can easily be 5.0. Maybe you have tiny radiators that act as the name suggest, primarily radiating the heat instead of natural convection, and maybe you need +80degC water to feel warm. Then SCOP would plummet to 2.0 or so, as you would run on resistive aux heating at COP=1, large part of the winter.

Then again, if you read a complaint that COP is bad when one has a massive hole in the wall and they are producing steaming hot water at full power 24/7 during winter, I would just ignore the whole discussion and try to find more useful data points. Unless, of course, you have a large hole in your wall, too.


--- Quote ---I live in a very humid climate and COP does down with high relative humidity, when the air temperature is 5°C.

--- End quote ---

I can say from own experience that COP most definitely goes up in high humidity conditions when the air temperature is +5°C. You can easily see this from the fact that Tpipe (evaporator internal temperature) as measured by the heatpump is closer to ambient temperature in such conditions (e.g.: dry air, Ta=5.0, Tpipe=-0.5, ice is forming; RH=100% humid conditions: Ta=5.0, Tpipe=3.0, it rains under the outdoor unit).

More heat can be extracted because turning water vapor into liquid releases a lot of latent heat. At +2 or so things get different - even the high humidity is not able to keep dT low enough, and freezing will start to occur. At medium humidity, freezing can occur already at +5 or so.

Defrosting is fundamentally not a bad thing: if you calculate the total enthalpy you are seeing the sum is actually better than in completely dry lab conditions! You get extra heat by making ice from water vapor in the first place, and during defrost you don't turn it back to vapor, mostly liquid, so you gained more than lost! In reality of course significant losses are involved, e.g. because the compressor is used to transfer the heat round-trip, and because, even with fan stopped, the evaporator (now condenser during defrost) sits hot in the outdoors air, possibly in windy conditions, for a minute or two, so defrosting reduces true COP below the level of dry testing, but given decently working decision algorithms (which they usually are), the effect is much smaller than many assume.

Specifically air-to-water units are very efficient at defrosting because they have better source of stored heat, in form of hundreds of liters of warm water, than air-to-air units which can only utilize the tiny amount of thermal energy stored in the indoor unit coil (copper with aluminum fins) and therefore heat the outdoor unit with worse COP during defrost. This is easy to see from the power consumption of e.g. my heatpump, which, for the 2300W input power unit, is just between 300-600W during defrost, and it only lasts for 3-5 minutes.

But sure, there is a small dent in COP curve around +5degC. It doesn't get progressively worse, as very cold air will be dry. The loss for every defrost cycle is higher when very cold, but fewer are needed, so there is just a small jump of about -0.2..-0.3 near +5degC compared to dry testing which keeps quite constant down to however low temperatures.


--- Quote ---A quote which I find concerning.
--- End quote ---

You can keep being a concerned citizen, but then again, this is how reality works. Your car consumes more fuel than the official numbers, just like every other car in existence, and just like they always did. Your condensing boiler performs worse compared to marketing material, I'm 100% sure about that. This is human nature. The big question is, are you making sensible decisions based on best effort of obtaining true data, or just using your concerns and uncertainties as coping mechanism?


--- Quote ---I think those who estimated a COP of 3.5 for me were optimistic. 2.5 seems more likely, given the data I've seen.
--- End quote ---

SCOP of 2.5 in British weather, which is pretty optimal for air source heatpumping (at least from Finnish perspective), would require some massive blunder. It's not impossible to do that badly, but it would be an outlier. I'm close to 2.5 (don't have exact measurement but very good guesstimates) in Finnish climate which is significantly colder.

However, important fundamental fact to understand is that COP is not a constant over year. We talk about annual COP, SCOP, defined simply as annually produced thermal energy divided by annually consumed electricity. We do this because we are interested about how much money we are putting in the long run. Heatpumps are not suitable for those who only have £100 on their bank account and are struggling to pay a single bill on February, because of the cost nonlinearity, which makes the effect of time-of-the-year on the bill even larger than it already is with linear systems (basically everything else than heatpumps).


--- Quote ---I've also realised I didn't take into account the fact that if I can stop using gas, which would also involve replacing my cooker with an induction hob, I can reduce my standing charge (a flat rate daily connection fee, which independent of usage), but then I have to factor in the fact it probably costs a bit more to run.

--- End quote ---

This is indeed another (albeit small) downside in gas-based systems, you rely on another contract and someone supplying you with this product every day. Compare with wood or oil (I use both as auxiliary heating methods) which can be locally stored for years, and if you mainly do heatpump, the volumes that need to be stored are modest.

Marco:
Wood is clearly the cheapest per kwh if you have the space to have ton's worth delivered by truck.

Though on truly modern homes so little heating is required that it would almost never be worth the effort unless you enjoy fuelling the stove.

nctnico:

--- Quote from: Zero999 on February 25, 2024, 07:30:56 pm ---I'm not sure how much I trust COP figures given by manufactures, given there are numerous different factors and they are likely to be skewed in the manufacture's favour.

--- End quote ---
The nameplate COP / SCOP number is a standarised value at a standard indoor / output temperature. However, there should be more detailed information available. The service manual for the Panasonic airco unit I have, has extensive tables that show COP/SCOP for combinations of indoor / outdoor temperatures.

Zero999:

--- Quote from: Siwastaja on February 25, 2024, 07:38:25 pm ---
--- Quote from: Zero999 on February 25, 2024, 12:36:21 pm ---It's very difficult to find objective data. Not only does COP, of an air source heat pump, depend on the inside and outside temperatures, but also the relative humidity.

I had a bit of a Google and found lots of contradictory information.
--- End quote ---

I agree the information is not that easy to find. You need to combine bits and pieces and measurements by individuals. One helpful advice that I have repeated and do it once again: differences between heatpumps (cheap vs. premium) are surprisingly small, and effect of condensing temperature (i.e., water temperature you need to get into your radiators/floor circuits/etc.) is just massive, regardless of brand. Therefore, while we roughly know the climate you live in, not knowing your water distribution temperature, we can't say whether your SCOP will be 2.0 or 5.0. Very unlikely anything outside this range. Maybe you have underfloor heating pipes and good insulation and need water temperature of +27degC. Then your SCOP can easily be 5.0. Maybe you have tiny radiators that act as the name suggest, primarily radiating the heat instead of natural convection, and maybe you need +80degC water to feel warm. Then SCOP would plummet to 2.0 or so, as you would run on resistive aux heating at COP=1, large part of the winter.

Then again, if you read a complaint that COP is bad when one has a massive hole in the wall and they are producing steaming hot water at full power 24/7 during winter, I would just ignore the whole discussion and try to find more useful data points. Unless, of course, you have a large hole in your wall, too.
--- End quote ---
You raise some interesting points, but I won't quote everyone of them and reply to save space. Note that just because I've not responded to and quoted them all, it doesn't mean they were unhelpful or that I have ignored them.

I know for certain that my current heating system is completely incomputable with a heat pump. The pipes are too thin and radiators too small. The whole lot will need to be replaced, which will be very costly. Although I don't use hot water myself, I still need the option, for when I have guests over and no one else would buy the house, which doesn't have hot water. At the moment it's heated on demand, by the boiler. A heat pump would require a tank and associated plumbing, which would be expensive. I haven't looked at how much it would cost, but I've seen £12 000 mentioned in this thread.

To put it into perspective, my energy bill both gas and electricity last year was £545.30. I used 1472kWh in total (including both gas and electricity) last year. Most of my bill is standing charges at £253.78.

Regarding the hole in wall comment, no I don't take that forum thread too seriously. I do add more weight to the other articles I linked, which involve real data from Scotland and Northern Italy, especially the latter which is very comprehensive.


--- Quote ---The big question is, are you making sensible decisions based on best effort of obtaining true data, or just using your concerns and uncertainties as coping mechanism?
--- End quote ---
A coping mechanism for what?

Given the high upfront cost of replacing my current system, which works perfectly, it's perfectly reasonable I'm going to be overly cautious, especially given it's possible the new system won't necessarily be cheaper to run or more reliable, than the current one. This is why it's just a thought experiment at this stage and I have no intention of changing. I wouldn't expect someone to buy a new car, when their current one works fine. I don't see why this is any different.

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