New to Heat Pumps - Looking for advice

Hardware Heatpump
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Hello,

Recently moved into a house that has a heatpump and solar panels, having previous only been in a house with a combi boiler. The heatpump is a Samsung RC160MHXEA connected to two external thermostats, one is a dumb rotating wall thermostat for the underfloor heating and the other is a programmable room thermostat for the radiators.

Trying to understand how to run the heat pump most efficiently, whilst keeping the house warm and minimising electricity usage. I currently don’t have any active monitoring on the heat pump (have ordered a bunch of stuff from here, but its out of stock at the moment), so my current monitoring is from purely reading our electricity meter.

In the 28 days we have been here, we have used around 1200 kWh of electricity, which is about £300! Where we used to live, our combined electricity and gas was around £100 a month. Once we noticed this usage a few days ago, I start keeping a close eye on the electricity meter. After a couple of nights in a row of using 17kWh overnight (between around 21:30 and 0700), on the next night I just turned off the heatpump completely. That night (between the same hours) we only used 4kWh. So the heat pump is obviously the biggest draw.

I understand some of the theory of heat pumps and that you want to try and minimise the amount of times they start / stop as this process uses a lot of electricity (when I turned the heat pump back on after it being off all night, it used around 5kWh in less than an hour which makes sense). But what does being “on” actually mean? With the heat pump constantly making a noise? (i.e with the compressor on and the fans spinning). On our heat pump controls, we have two icons, one is for when the compressor is on (when the outdoor unit turns on and is making lots of noise) and the other is for the “water circulation pump” being on. Do we just want the “water circulation pump” to be on, but not the compressor? or both to maximise efficiency / energy usage?

I think I also understand the theory that you want to have the “flow” temperature as low as possible, whilst maintaining a comfortable temperature. Our heat pump runs in weather compensation mode and there are bunch of different numbers and offsets I know I can configure for this.

I have read through this article: 3 Steps to Maximise Your Heat Pump (or Boiler) Efficiency  - HeatGeek which explains a “method” of determining what the correct flow temperatures need to be for your own house. Does this make sense? I had a go at following this yesterday evening. I opened up all the radiator valves to full and set the radiator thermostat to 23c. I reduced the minimum flow temperature on the weather compensation setting and the heat pump came on and stayed permanently on (making noise, compressor on etc) and during the evening the heat settled at 21c and didn’t rise, despite the thermostat being set to 23c. I left this running overnight (heat pump constantly going, which it has not done before since we moved in, its always on for a bit, off for a bit etc) and when I woke up, we had used around 20kWh. So more than before I started messing with it. The house was warmer than we would like (22.5c, target would be 21c) overnight, so I probably need to make some more adjustments to the flow temperature to find the “ideal” settings, however with the increased usage, I panicked and set the thermostat back to 21c and the heat pump turned off. It’s now back to the compressor not being on all the time. Was that the correct thing to do? Is having it constantly on (24/7 365 days a year?!) the more efficient / least energy consuming way to use it or? or should I just be setting the room thermostat to 21c and let it turn it on and off as it deems necessary?

I hope the above makes sense! Please shout if I can provide any other info that would be useful. Just trying to understand if ours is running really inefficiently or if these sorts of energy usage is expected (I was under the impression that heat pumps were supposed to save you money on your energy bills!) Any advice / assistance is greatly appreciated!

EDIT: I have bolded my specific questions, in case anyone can jump in on those. But very thankful for any advice / input at all!

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Hello @microadam and welcome!

All good questions!

That’s great, what was the flow and return temperature during this period?

What make of heat pump do you have and what capacity of heat output in kW?

What size property do you have and how well insulated is it? Do you have an idea for what the annual heating demand should be?

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Hello,

Thanks for responding!

So its a Samsung RC160MHXEA 16kW heat pump. Unfortunately as I mentioned, I have no way of measuring anything about the heat pump at the moment, so I couldn’t tell you with any accuracy what the temperatures were. With regards to the return temperature, I don’t think anything on the display tells me that, but making an educated guess based on the weather compensation settings, perhaps the flow temp was somewhere between 36c and 40c?

In terms of property size, its about 1700sqft and its pretty well insulated I believe. When we moved in we got an energy performance cert and it was rated a B. To give some idea, when I turned the heatpump off the other night, it was around 21c when I turned it off and about 19.5c when it hadn’t been on at all for the whole night. Unfortunately I have no idea what the annual heating demand for the property is, as we have only just moved in.

Sorry I can’t give you any more specifics! Any thoughts on my specific questions at all? Am I close to understanding how this all works (following that article I linked) or am I waaay off? :slight_smile:

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Moving to a heat pump from a combi can be a bit of a shock, and it can take a while to figure out what works best. 42 kWh per day does seem a bit high, depending on the size of your heat pump, heat loss of your property, and time of year. You should be able to find the EPC for your property, which will give the annual heating and hot water demand. Comparing this to your old property may be enlightening.

For comparison, my 1400 sqft detached house with mediocre insulation is rated D, and demands 20,000 kWh of heat each year. I have a 11 kWh heat pump which consumed more than 20 kWh per day during January, a little less during February.

The RC160MHXEA looks to be a 16 kW heat pump, which is pretty beefy. That will produce up to 16 kW of heat, but the maximum input power is 3.8 kW [Edit: I finally found the right spec sheet]. This should be matched to the “peak heat load” of the property at sub-zero temperatures, but I suspect it is oversized for that property.

The main power consumption will be when the compressor is on in the outside unit. Think of the compressor bit being like the flame burning in a gas boiler - you’re only generating heat when it’s on. The water pumps take comparatively little power, maybe a couple hundred watts, though you probably don’t want them on 24/7.

Weather compensation is definitely the key to getting the most out of your heat pump, to keep the flow temperatures as low as possible. Running at a low temperature for longer is better (more efficient) than running hotter for short intervals. Typically you might run at 45 when it’s freezing outside, and 35 when it’s (say) 7 degrees. If your insulation is very good, then you might get away with even lower temps. If the heat pump controller is sufficiently smart, it will turn itself off when it deems necessary. Your mileage may vary.

Heat Geek do have some very good videos about heat pumps for consumers, which I would recommend for any owner or prospective owner. You do need to work out what’s best for you though, especially given how large your hear pump is.

Keeping your heat pump on all the time is recommended by many, but this doesn’t mean it is producing heat constantly. Think of it more like “on standby” 24/7, relying on its internal logic and/or your room stat to control when it actually heats. I’d expect the water pumps to come on at the same time.

If the house is occupied all day, then I’d recommend setting the programmable room stat to have a constant temperature for the day time and a -2 or -3 degree ‘set back’ for night time. If the house is vacant during the day then maybe reducing the temperature by a couple degrees is worthwhile, but make sure it turns back up a couple hours before you get home. Be aware the underfloor heating can be particularly slow to warm up / cool down.

Be wary of the type of room stat you have - modern TPI based ones will try to “modulate” the heat source by calling for short bursts of heat, causing the heat pump to “cycle”. (I’ve been bitten by this). You can sometimes override this in the settings.

It would also be worth figuring out how the thermostat for the underfloor heating interacts with the room stat for the radiators. Are these considered two separate zones with two separate pumps? Or will turning on one cause the other to heat up? You probably want balance these carefully so they don’t compete with each other, or end up taking turns calling for heat. (I’m just guessing, I have no experience here).

Have a look at some of the other posts in the Heatpump forum here; there are quite a few useful contributions from other users. I’ve only had a heat pump for 3 months, and have learned so much from this forum.

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Hi Tim!

Thank you very much for the detailed response!

I think you are right, in that this heat pump is likely oversized for this property :see_no_evil: I just found the EPC document, and it states its 1420sq ft, rated a B and demands 13850kWh a year for space heating.

Keeping your heat pump on all the time is recommended by many, but this doesn’t mean it is producing heat constantly . Think of it more like “on standby” 24/7, relying on its internal logic and/or your room stat to control when it actually heats. I’d expect the water pumps to come on at the same time.

So with regards to the definition of “on” we don’t want the compressor on 24 / 7 then? (perhaps in winter, when it’s very cold it will need to be). Using your analogy of the compressor being like the flame in a boiler, the boiler is always “on” but the flame isn’t. Running that flame constantly is a bad idea, same with the compressor? (so that heat geek article I linked to is actually a terrible idea, as that has the compressor running constantly, as the thermostat is essentially constantly calling for heat, but the heat its providing, will never be enough to reach the target temperature as we have turned the flow temperature down). I feel like I am missing something here though, as I can’t see why they would possibly recommend doing that if it’s a really bad idea.

if the house is occupied all day, then I’d recommend setting the programmable room stat to have a constant temperature for the day time and a -2 or -3 degree ‘set back’ for night time.

This is exactly our situation, me and my wife both work from home. So in this case, with it set to a constant temperature, it will work just like the traditional gas boiler. Temperature drops below target, signal to heat pump to turn on the compressor, temperature in house rises until it hits target, signal back to heat pump to turn off compressor. Does that sound right? I thought this constant “on / off” cycling is what everyone refers to as being inefficient? if thats the “normal” efficient way to do it, when people say “don’t turn it off” what does turning it off mean? As I thought the “off” signal from the thermostat was essentially “turning the heat pump off”. I think I must be mistaken on this point.

You raise a couple of very good points around how the two heating zones interact. I am not 100% sure, so that is definitely something I need to determine. There is only one unit outside, but there are two different “pumps” in the house (little black device with some flashing numbers on) which work independently. How the flow works between each, I do not know, so something I will try to work out for sure!

I will definitely have a read through all of the stuff on here, I am very much hoping once I have some decent monitoring setup I will be able to get a bit of a better understanding :slight_smile:

EDIT: That spec sheet is great thank you! I had been struggling to find something that comprehensive!

EDIT 2: I should also say, that 1200 kWh usage figure is whole house electricity usage. I don’t have any way of measuring specific heat pump electricity usage yet. Having said that, other than the usual fridge / freezer and a couple of servers that run 24/7 there shouldn’t be anything else using masses of power constantly. Anything I do have running, is the same that was run at our old house where the electricity bills were much less extreme!)

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Heat Geek’s article is based on the assumption that the heat pump is able to modulate down to a lower power draw to keep the temperatures level. An oversized heat pump in a well insulated house will struggle to get rid of the heat it creates if it can’t modulate down low enough and has to cycle instead. So, this approach may not work well for you, but it doesn’t mean it’s a bad idea generally.

Yes, this would be about right. Gentle cycling throughout the day is fine, as long as the heat pump compressor is on for a decent amount of time for each cycle, and is not starting more than 3 times-an-hour. So-called “short cycling” is when the heat pump runs for 5 or 10 minutes, many times an hour - this should be avoided as it could shorten the life of the compressor. (From what I understand).

They mean leave the system on stand-by so it can turn on whenever it needs to be on, i.e. when the room stat calls for heat. This also means leaving the thermostat set at a constant temperature throughout the day, rather different temperatures at different times. Or, to put it another way, it’s better to keep the house warm than let it go cold and have the heat pump work hard to make it warm again. (Though a gentle set-back temperature at night time is generally considered best for comfort)

Ah, that’s not so bad if we assume 10 kWh consumption by the rest of the house, though still not insignificant. The solar panels will help offset that, though obviously your demand for heat is typically highest when there’s little power from the sun.

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Heat Geek’s article is based on the assumption that the heat pump is able to modulate down to a lower power draw to keep the temperatures level. An oversized heat pump in a well insulated house will struggle to get rid of the heat it creates if it can’t modulate down low enough and has to cycle instead. So, this approach may not work well for you, but it doesn’t mean it’s a bad idea generally.

Not sure I fully understand this. So I have just watched their video that the article is based on: https://www.youtube.com/watch?v=SfgTOcclbCY and I think I get the logic. When I ran their test, the compressor and pumps were just constantly on, all night. The way I thought it would work, is that the compressor would turn on to reach our desired flow temperature and then automatically turn off again once it was reached (i.e like a thermostat that controls room temp), but would keep pumping that water around the system, so the water pump would be on all the time, but the compressor would not be on all the time, until the flow temperature dropped and needed to be brought up to the target flow temperature again. Is that not how it works? (no problem at all if you have no idea by the way!) So, if that is correct, then perhaps my flow temperature was just set too high? (the house did get a bit too hot overnight) and so the compressor just had to be constantly on to maintain that flow temperature? Does that make any sort of sense? In my head the room thermostat is what “turns off” the pump COMPLETELY so nothing is flowing around the radiators and nothing is being heated. So by turning the stat up way high, but reducing the flow temperature, that shouldn’t mean the compressor needs to be on constantly, just that the water pump should be and the compressor should then come on and off when it needs to, in order to maintain that temperature being pumped around the system. So I think either my understanding of how this works is wrong (most likely) or the flow temperature was too high. I guess my question is when the room thermostat calls for heat, does that essentially mean turn on the compressor until the room thermostat no longer calls for heat? OR does it mean now start pumping water through the radiators and if that water isn’t at the correct flow temperature, use the compressor whenever needed to keep it at the desired flow temperature? The former is what I observed, but the second is how I think / hope it should work, which if true, just means I need to work out why I observed the former (most likely due to flow temps being set too high).

Apologies, this feels like a bit of a ramble, but I think I have got to the crux of where my lack of understanding is :joy: Any input would be greatly appreciated, but please feel free to say if you don’t know either way!

Yes, this would be about right. Gentle cycling throughout the day is fine, as long as the heat pump compressor is on for a decent amount of time for each cycle, and is not starting more than 3 times-an-hour. So-called “short cycling” is when the heat pump runs for 5 or 10 minutes, many times an hour - this should be avoided as it could shorten the life of the compressor. (From what I understand).

This makes good sense, short cycling is bad. What about if it only cycles once every few hours? Is that highly inefficient? or is that in affect the holy grail? as it only comes on once every few hours but has to do minimal work, because the temperature has only changed very little?

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That is how I would expect it to work (though I’m not familiar with your specific model), and that is how my heat pump behaves (though it does not always behave as I expect it to). So, in theory, you can just leave it on and it’ll do it’s thing, but in practice (even with my modern one), it’s sometimes necessary to override what the heat pump brain is doing (i.e. with a simple room stat).

See this thread by MrForest about not running the heat pump all the time.

It’s a combination of both. My system, when there’s a call for heat, will turn on the pumps and look at the flow temps. If the temps are below the target temp, then it’ll start the compressor. However, if the room stat stops calling for heat soon after (which mine does, annoyingly), then my HP will continue running at its lowest possible power (1 kW) for 20 minutes before shutting off. Your system is likely to have it’s own behaviour model, which will be easier to determine once you have some metering on it.

Once every few hours (or even once an hour) would work well, assuming it’s running for a decent length of time.

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This is all really good information! Thank you! The fact that yours works like I thought it should, means I am not way of base, and chances are, mine should do something similar, despite it being slightly older, it should still work in a relatively similar manner.

I think I am going to leave it alone for the night to see what happens with it just being controlled by the stat, with relatively low flow temps (all be it that I have guessed at!) with a slightly lower set back temperature. Then tomorrow, see if I can replicate the heat geek process and figure out if the compressor does ever turn off. I believe I have seen a setting that determines whether or not an external stat is used, so I think worst case scenario, I can just disable it and control it all via the main heat pump controls.

Thanks again for all your info, its been extremely helpful!

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Lots of great stuff in your replies but where did that “leaving the thermostat set at a constant temperature” come from? I mean, don’t let the house go stone cold unless you can tell it to start heating back up many hours before you return, but being too fussy about “constant temperature” will cost more than letting the temperature vary by a degree or two depending on time of day (can usually get away with slightly cooler when people are moving about more), and whether it’s forecast to get colder (so grab more heat now while it’s cheaper!) or warmer (wait if we can).

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We have a 12kW Vaillant ASHP with weather compensation (WC) enabled and the sensoComfort (Vaillant) thermostats. House is 300m2 / EPC C. Heat loss is around 11kW @ 21C at -3C.
We keep things as simple as possible … but try to make use of cheap electricity overnight. The systems is running 24x7 but we setback the temperature (mentioned above). Max flow is 35C with a mix of rads and UFH.
0630->2130 18.5C
2130->0200 16C
0200->0630 20C (Octopus Go Faster)

For March we used 487kWh (measured by emonTx) and had a CoP of 3.95. We do have a 4kW PV with immersion diverter which has I’m sure improved things as it’s been quite sunny.
Hope this helps in terms of usage pattern.

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Thanks Steve! This is really interesting info. You mention you have a max flow of 35c. At what temperature is that? -3c? We have our max flow set to 50c at -2c (was like this when we moved in, haven’t changed this yet) and min flow set to 27c at 15c (this was 37c when we moved in). We do have it set to a minus 5c offset though (was +5c when we moved in) so I believe that just does minus 5c to whatever the temp it initially decides to do, so I think it’s weather comp is essentially max 45c at -2 and min 22c at 15c. Do these seem high at all? (Obviously every house is different, but just in general I guess!)

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I think max flow here is set to 55c but only at -10 ambient or something like that. I suspect the frequent defrost cycles needed at that temperature might prevent it ever actually achieving that in our climate!

I think minimum is in the mid 30s because anything lower resulted in the heat requirement being below the pump’s minimum and resulting in lots of cycling and low efficiency

You’re right about how offset works, in my limited experience.

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Do you have the design documents for the system? The heat loss calcs?

From there you should get an idea what flow is needed at -3 to achieve 21C indoors and then work backwards from there?

Let the weather comp curve do the heavy lifting?

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Unfortunately no design docs! We inherited it when we moved in, so not much info on how it’s setup!

Thanks Steve for the additional details. All very interesting!

Monitoring equipment was shipped from the store yesterday so hopefully have that soon!

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Bit more on the flow temps. Vaillant allow the max and min flow temps and a heat curve to be set per zone. We have the curve set to adaptive which allows the controller to adjust it automatically (currently rad zone is 0.3 and UFH 0.5.
I’ve set the max flow to be 35C, which is keeping us warm enough. The rads were sized (all type 22) for 40C @ -3C … but we seem to be able to better this.
55C flow temp would mean the SCoP would be lower and more frequent defrost needed. Temps <5C seem to cause freezing.

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Am partial to hybrid system - this one always intrigued me, but my climate is too harsh for one of these.
If they made it in GSHP then I would consider it.

in combination of evacuated solar panel and mass storage

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This seems reasonable to me. You can keep tweaking it downwards (or leftwards) until the house isn’t warm enough, making note of how well the system performs at different outside temperatures.

Gradually turning down the main room thermostat is another way to save…

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thats my next plan! I have wanted to get it stable to see if I can figure out whats its doing before changing too many variables. We did try a whole day at 19c, and I found that too cold :joy: lowering the flow temp is the best option I think :smiley: