General > General Technical Chat
What is the real story around heat pumps?
nctnico:
--- Quote from: Zero999 on February 25, 2024, 12:36:21 pm ---This one from Strathclyde shows slightly lower COPs.
https://www.esru.strath.ac.uk/EandE/Web_sites/10-11/ASHP_CO2/ccl-potential.html
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/
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.
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In the end nothing beats getting information about hands-on experiences with the equipment. On a forum you can get a good list with prospects with some pros & cons but from there you have to start reading reviews about the various pieces of equipment out there. A good source may be looking for a forum where installers of heatpump systems hang out (likely a forum for AC installers as these are related) to get some info about maintenance problems for various models / brands.
zilp:
--- 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
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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.
--- 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?
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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?
--- 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?
--- 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.
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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.
--- 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.
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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.
pcprogrammer:
--- Quote from: Siwastaja on February 25, 2024, 10:18:55 am ---
--- Quote from: pcprogrammer on February 25, 2024, 07:00:25 am ---I never claimed the wood to be free, but do still have about 12m3 from one of our own trees. It was a hell of a job to take it down
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Now the question really is, do you want to burn the results of your hard work in one winter, or maybe use it in a hybrid solution for the next 5 winters?
--- End quote ---
We have a small wood burner in the kitchen/living that is going to consume that wood. That space has to much volume to be heated with only the underfloor heating also due to the chestnut floor we have on top of it. Ceramic floor tiles would have been better, but we like the wooden floor. :)
Inexperience with underfloor heating when designing the system made this happen. Should have added some radiators, but in stead there is the wood burner to lift it from ~17 to >20.
--- Quote ---Oak is quite expensive. It might have been more economical to sell the wood and use the money to by pellets, or scrap offcuts, than burn that tree!
--- End quote ---
Only when you buy it as ready made planks from a DIY store. :-DD
No sawmill owner over here will come to only take a single oak tree.
jmelson:
--- Quote from: pcprogrammer on February 25, 2024, 07:00:25 am ---The thread was started to find out if sales people are lying about air to water heat pumps having far worse performance then ground to water heat pumps, and what the noise levels are.
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Many years ago my wife had a house that had an air to air heat pump. It also was a "brick frame" house, not "brick veneer", meaning the walls were a single course of bricks with no air space or insulation from exterior to interior. It had tons of insulation in the attic, but that was pretty useless. In the moderate winter climate of Missouri, the place was impossible to heat!
Where we live now, some neighbors have air-air heat pumps. When I am outside near their property, I can hear the unit running, and hear the valves changing over for defrost, but it is no louder than a typical air conditioner. But, NO QUESTION, a ground-source heat pump will do the job on 1/10th the energy. A friend did have a modern log cabin out in the woods with a ground source heat pump, and he used to brag about the CRAZY low energy bills!
Jon
Siwastaja:
--- 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.
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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.
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