Heat Pump Energy output over a 24hour period as a function of different variables

[matt-drummer (Matthew Gregory)]

@Henry_Fletcher

That’s really interesting to know, thanks for sharing @matt-drummer, maybe I’ll ask them to quote for ours too! Out of interest, was it an “Octopus” team (Octopus labelled van, Octopus logo on their shirts etc) or did they contract to the best installers in your area I wonder?

They were all Octopus employees. They arrived in Octopus liveried vehicles and wore Octopus branded clothing with their names on.

There were four of them, three from within about a fifty mile radius and the lead installer came from the other side of the country, he spent five nights in a hotel in Ipswich.

The installer who came to check everything this week said asked who I had and when I told him he said they were some of the best they had.

I think their aim is to get a consistent high standard of install and they certainly go out of their way to try to achieve that. They got a couple of visits from a manager during the install to check on their progress.

[TrystanLea]

Im going to try a couple of ideas later for analysing defrosts a bit better. If we broke out cooling energy in the heat pump dashboard that would help I think. That way we could at least identify days or periods with the most energy lost to defrosts.

[Henry_Fletcher]

Ah very nice, that does sound exceptionally positive.

[Henry_Fletcher]

that’s a good idea @TrystanLea

[Henry_Fletcher]

Out of interest @TrystanLea, for this house that you mentioned where you improved the COP by removing the zoning, how were the zones controlled and what emitters did they have?

I’ve noticed a lot of zoning employs on/off valving rather than proportional control of the flow rate, I don’t imagine on/off zoning valves play well with the preferred more gradual response profile of a heat pump?

The only products that do proportional control of the flow rate (to the best of my knowledge) are on radiator digital TRVs like the Honeywell Evohome system. I imagine this approach would work well with a heat pump as it deals with room based variations i.e. solar gain, occupation levels, cooking etc etc that a single thermostat can’t manage (although admittedly an MVHR might do a good job of smoothing out these variations across the whole house)
It also allows for reduced heat to unoccupied rooms on a timed basis…personally from a comfort perspective I don’t like the idea of a house constantly at one temperature, I prefer the bedroom cool for sleeping and warm for getting up in the morning, the living room can be cool during the day and needs to be a bit warmer in the evening, my little office bedroom needs to be warm during working hours but can be left to cool off at night.

This is probably deviating a bit off the original topic but I like the idea of proportional flow control to different rooms balanced against using the lowest possible flow temperature from the heat pump.

[matt-drummer (Matthew Gregory)]

Obviously my thoughts on the subject of heating our house with a heat pump are all theoretical at the moment.

Our house had three heating zones when we bought it. I never operated it like that though, I just set them all to the same temperature. Our gas boiler wasn’t massively over sized so it all worked really well.

I noticed that when I walked into the house on a cold day, even into the conservatory, that I could really feel the heat from the radiators even though they were barely warm.

That is why I am quite confident that our house will work at the low flow temperatures as the radiators are all the right size.

We only had to change two, one in the hallway that was an original from the 1980’s and one in a bathroom that they didn’t know the heat output from, a designer radiator. It had to be changed in order to get the BUS grant.

Achieving high COP values is great but it is not the whole story. We need to remember that the ASHP is just a source of hot water for us and the aim is to be as comfortable as we want to be at the lowest cost possible. Chasing COP but spending more overall on electricity is not the correct answer.

We and our homes are all different.

I think the key to achieving the highest efficiency (COP) and the lowest overall costs is in the correct sizing of the radiators in relation to the calculated heat loss of each room.

We want the heat downstairs where we live and not so much upstairs when we are sleeping so our radiators are sized appropriately. Our downstairs radiators are pretty large, those in the bedrooms no so much. In our heat loss survey all our radiators upstairs were bang on the correct size, not too big and not too small. The ones downstairs are slightly bigger than required.

Time will tell, but I think this will allow us to run on pure weather compensation with the trv’s wide open and achieve a balanced heat around the house with a high COP and similar monetary costs to our gas system.

I think problems come and comprises become necessary when you have some rooms too hot and some too cold. I just want the water to come from the heat pump, go around the radiators and go back to the heat pump. Valves opening and closing, changes in heat demand all seem to lead to inefficiencies from what I read and make it more difficult to achieve the primary aim…

I would always choose a heat pump that was slightly too large rather than the other way around as I would choose being warm over having slightly more money.

[matt-drummer (Matthew Gregory)]

My plan is to concentrate on making the downstairs of our house comfortable.

If I find that upstairs isn’t warm enough I will just change the radiators in the four bedrooms. They are currently T11’s and I will just swap them for T22’s of the same size. It will be quite cheap and easy to do.

I am convinced that I will be happiest if I can just have the heat pump running for as long as possible at a constant flow temperature.

We had a brand new air to air heat pump and I found that it’s energy use is very modest when running for long periods at a constant temperature. It is a Fujitsu 5kW model with a single indoor unit. Initally it draws up to 2kW but it settles down after a short time and consumes around 500W for hours on end.

[matt-drummer (Matthew Gregory)]

@Henry_Fletcher

Out of interest @TrystanLea, for this house that you mentioned where you improved the COP by removing the zoning, how were the zones controlled and what emitters did they have?

I’ve noticed a lot of zoning employs on/off valving rather than proportional control of the flow rate, I don’t imagine on/off zoning valves play well with the preferred more gradual response profile of a heat pump?

The only products that do proportional control of the flow rate (to the best of my knowledge) are on radiator digital TRVs like the Honeywell Evohome system. I imagine this approach would work well with a heat pump as it deals with room based variations i.e. solar gain, occupation levels, cooking etc etc that a single thermostat can’t manage (although admittedly an MVHR might do a good job of smoothing out these variations across the whole house)
It also allows for reduced heat to unoccupied rooms on a timed basis…personally from a comfort perspective I don’t like the idea of a house constantly at one temperature, I prefer the bedroom cool for sleeping and warm for getting up in the morning, the living room can be cool during the day and needs to be a bit warmer in the evening, my little office bedroom needs to be warm during working hours but can be left to cool off at night.

This is probably deviating a bit off the original topic but I like the idea of proportional flow control to different rooms balanced against using the lowest possible flow temperature from the heat pump.

I am no expert but I agree with you.

My feeling is that valves that are either open or closed will mess with the heat pump’s desire to just run at a constant flow rate and temperature. Lots of valves opening and closing change the volume of water flowing and the heat demand.

I don’t know how good these heat pumps are at responding to quick changes in heat demand and what it does to their efficiency but my logic (probably flawed!) tells me that something operating in a steady state for long periods of time is probably the most efficient mode of operation.

[TrystanLea]

@Henry_Fletcher

Out of interest @TrystanLea, for this house that you mentioned where you improved the COP by removing the zoning, how were the zones controlled and what emitters did they have?

Agreed that zoning is context specific. In this case the house was a 100m2 detached, stone construction with internal wall insulation, form factor of the house, square, two storey rather than e.g extended long house. Most of the rooms had no doors between them, so high air movement potential. E.g kitchen open to the livingroom, open to the stairs to 1st floor. Zones where on/off underfloor heating control, heatmister and neostats. Upstairs radiators were also undersized.

I’ve personally sized the radiators in our house in such a way that with a single zone and all radiators fully open you still get +1-2C in the livingroom vs the bedrooms, which works for us in terms of comfort.
We could theoretically, based on room by room calculation, save energy by zoning up and down stairs but I haven’t experimented with that practically. I think we’d be hard pressed to realise noticeable savings with only 77m2 and family of four we occupy most of the house now, so I think open loop, single zone is a strategy that makes sense for us, but I agree that would change depending on context.

@matt-drummer

I think the key to achieving the highest efficiency (COP) and the lowest overall costs is in the correct sizing of the radiators in relation to the calculated heat loss of each room.

Agreed.

[TrystanLea]

Here’s cooling/defrost energy vs heat delivered on our system for the last 3 years:

Zooming in on the peak around December 10, 2022:

4.1 kWh lost to defrosts 61.9 kWh of heat initially delivered, 57.8 kWh of heat net

Detailed view for December 10, 2022:

2579 W gross, 2408W net

Flow temperature is very low 31C, outside temp is above zero at 1C. Average room temp quite low (comfort preferences - we could have run the system hotter), peaks at 19C at end of the evening.

[TrystanLea]

Looking at the Ecodan datasheet for our unit, it doesnt list a COP that matches the 31.1C flow temp and 1C outside temp exactly.

• At -7C outside and Mid range output the COP should be between 3.85 (25C flow) and 3.25 (35C flow).
• At 2C outside and Mid range output the COP should be between 4.9 (25C flow) and 3.54 (35C flow).

I’ve got a COP calculator that interpolates this table as part of https://openenergymonitor.org/heatlossjs/ and it suggests that the COP should be 3.74. Im actually getting 3.37. Without the defrosts I would have got 3.6…

That’s probably quite a good example of reduced performance compared to the datasheet.

[glyn.hudson]

@Henry_Fletcher

As heat pumps get older their output falls? Does anyone have any data on this?

I’ve never heard of this, I can’t see any reason why this would the case? Unless the evaporator or fan gets dirty, but it can easily be cleaned.

@Henry_Fletcher

When it is cold and wet, heat pump rated output cannot be sustained over a 24hour period due to defrost cycles etc? How much is this reduction? If I can run my 5kW rated heatpump at a 4.2kW output on average for 24hours I can get 100kWh. I think houses need to be pretty big/ poorly insulated before they require more than 100kWh in one 24hour period, so I’m not sure why anyone is fitting anything other than 5kW and smaller rated units!

It’s useful to have a bit of capacity in reserve, to account for the occasions when you need to re-heat the house when it’s cold e.g coming back from holiday. In this case you need to input more heat than you would normally need to maintain the temperature. I’ve managed to get 85kWh of heat from my 5kW heat pump re-heating my house when the outside temperature was below 0C, this is way more than I need to maintain the temperature. But if you have a wood stove you probably don’t need to worry too much about this.

@Henry_Fletcher

Small heat pumps running hard are noisy compared to large heat pumps running at a low modulation?

I don’t think this is always the case, in general higher rated heat pumps are noisier than lower rated heat pumps e.g my 5kW Gen6 Samsung is nice and quiet, but larger Samsung Gen6 heat pumps have a reputation for being noisy. However, the noise of the heat pump is proportional to flow temperature, there’s a good chance that people who have issues with a noisy heat pump is because it’s running at a high flow temperature. I think the quietest heatpump is the Vaillant Aerotherm+, although the new Samsung HTQ is supposedly pretty quiet. Quieter heatpumps tend to be larger physically since the larger the fan the quieter the noise. My outdoor unit is wall-mounted so having a compact unit was a big plus.

@Henry_Fletcher

Fear from installers that customers will complain their house is too cold if they install too small a heat pump…

This is totally understandable, a cold customer is not a happy customer! However, I think this is the main reason that heatpumps are commissioned to run a higher than needed flow temperature. The over sizing of the heatpump is maybe due to incorrect assumptions made during the heatloss calculation, or not doing a heatloss calculation at all and sizing the heatpump based on EPC data, this is actually pretty common!

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The single thing which has the greatest effect on heat pump performance is flow temperature, if you want to maximise SCOP the focus should be on getting the flow temperature as low as possible, ideally under 40C at design temperature. Getting a low temperature will probably require radiator and possibly pipework upgrades but will be worthwhile long term.

From my experience and looking at monitoring data, flow temperature and control strategy has a much greater effect on performance than heat pump than sizing and cycling. I can think of examples where an oversized heatpump can get good performance even while cycling because it’s running at a low flow temperate with optimised control setup.

@johncantor has made a good video on the topic of heatpump cycling

Heat Pumps Cycling (switching on and off)

[johncantor]

@Henry_Fletcher

As heat pumps get older their output falls? Does anyone have any data on this?

Just to pick up on a few points. As of many years, compressors are scroll or rotary now, they almost never wear out or ‘degrade’. The old reciprocating (piston) type had reed valves that suffered wear & tear, so performance could drop. Refrigerant is very unlikely to ‘slow leak’ from a factory-filled system. Water condensation and defrost tends to keep heat-exchangers quite clean. Water heat-exchanger should be fine, but I guess there is a possibility of some blockage over time, and this could reduce the COP, but not really drop the heat output. It would be hard to tell performance without a heat meter, but input power and refrigerant discharge temperature might give some idea.

[johncantor]

@Henry_Fletcher

Small heat pumps running hard

Its a common notion that a small heat pump can ‘run hard’ as a small car might. However, they are quite different to cars. My old 5kW Ecodan was fitted (in 2008) alongside a 30kW boiler. It ran for months mid-winter without stop (apart from defrosts) at around 35C. No ‘straining’, just a lot of steady-state running

[Ian_Calderbank]

@Henry_Fletcher

The only products that do proportional control of the flow rate (to the best of my knowledge) are on radiator digital TRVs like the Honeywell Evohome system. I imagine this approach would work well with a heat pump as it deals with room based variations i.e. solar gain, occupation levels, cooking etc etc that a single thermostat can’t manage (although admittedly an MVHR might do a good job of smoothing out these variations across the whole house)
It also allows for reduced heat to unoccupied rooms on a timed basis…personally from a comfort perspective I don’t like the idea of a house constantly at one temperature, I prefer the bedroom cool for sleeping and warm for getting up in the morning, the living room can be cool during the day and needs to be a bit warmer in the evening, my little office bedroom needs to be warm during working hours but can be left to cool off at night.

just wanted to comment on this. as I have an evohome system and a self installed Samsung16kw. When we were on Gas, I made heavy use of the evohome system to run the house with “the least possible gas energy input I can get away with” consistent with “just about acceptable comfort in the rooms we were using”. Letting unused rooms run cool. This gave us a gas consumption significantly below the EPC (14kkwh vs 23kkwh, 210m2 house) at the cost of always wearing two jumpers (apart from our 1 cosy, the evening TV room)
Now that I have an ASHP (but only since March), I removed the TRV heads from all of downstairs as part of the commissioning. With the TRV heads in place there were a number of issues (which I was expecting, to be fair): 1. when rads partially shut down, not a large enough effective total emitter area available to the heat pump thus cycling , can’t sustain needed output; 2. not enough flow rate to meet the rate need to sustain the heat output required at the DT thus again cycling, can’t sustain needed output; 3. Loud Harmonic noise (like your pipework becomes a church organ) due to reed effect of nearly-closed TRV with water being pushed through it at at least 4x higher flow rate and several Meters higher head than previously with the gas systems circulating pump.

solution is all TRVs removed downstairs, balancing with lockshields only. With tweaking of flow temp (tweaking WeatherComp curve until you get a stable comfort level), and doors left mostly open (they were mostly closed before) rooms don’t overheat. Effective total comfort level of the house became much higher because all rooms are heated to 20-21C. Once the ASHP in, we didn’t wear jumpers in march or april inside, even in the cold snaps. A side effect is that bedrooms end up warmer (19C ish) than we were used to (we ran them at 15C usually) purely by the heat coming up from all rooms constantly heated downstairs. I had trouble sleeping for first month due to the warmth.

Total annual heat input into the house with the ASHP will I have no doubt be higher than the best I did on gas with the evohome system fully turning down unused rooms. Total energy input will I expect course be lower due to COP, but I don’t have the data for that yet.

[Henry_Fletcher]

Dear all, some really really valuable experience being shared here, such a fantastic community of helpful, interested and well informed people! Thanks for sharing that video by @johncantor @glyn.hudson, and thank you for making it John!

Trying to summarise:

• De rating/ reduced efficiency with age is pretty negligible for modern heat pumps
• A larger heat pump running at a lower % of it’s rated output is not necessarily less noisy than a smaller heat pump running at the same ouput but a higher percentage of its rating. However, a larger fan on a heat pump will typically result in a lower noise for a given output.
• Modern heat pumps modulate down very effectively, so modest oversizing is pretty safe from a COP perspective. The focus should be on getting the flow temp as low as possible.
• First hand experience from Ian would suggest the Evohome digital proportional radiator flow valves can’t effectively regulate a heat pump system and maintain a decent COP…weather compensated flow temperature is a better option. Get the radiator sizes right for the different rooms and don’t expect to adjust the temperature up and down quickly in any of the rooms.

[Zapaman (Dominic Daniel McCann)]

In terms of tracking performance then these 2 charts are a good way to track current performance. In my case I am planning some minor upgrades to my radiators. So looking at these graphs once the next heating season starts should reveal whether there has been any noticeable change in the heat output and COP etc from our ASHP resulting from those changes.

[johncantor]

Sorry, a bit slow responding. So, a 10kW unit running at 50% should be considerably quieter than a 5kW at 100%. Re size. Ideally, the unit will operate between say 100% and say 30%, with quite a lot of operation around 60-70% where COP should be best (though only marginally best). An oversize unit will spend more time at lower speeds, and more time off/cycling. It may tend to ‘rev up’ when you dont really want it too. Again, efficiency differences not great

[AndrewC (Andrew Cunningham)]

Hi All
This is my first post here so…HELLO.ooooo
I have collected some data from the lovely open energy files to compare power usage on the coldest day of the year, December 14 2022.
Taking the coldest 24 hours energy use, dividing by 24 and comparing that figure with the capacity of the heat pump on that day. I have learnt that Heat Pump capacity changes depending on ambient temp and also flow temp. So the capacity figures have been adjusted.
Hope it is of interest
Yours
Andrew Cunningham @suburbanpirate

[Ken_Bone]

Larger units with large evaporator coils with have advantages in our moist winters.

I’m currently running an old Samsung 16kw R410a Unit, which is way over sized for my property.

But I runs and modulates smoother than my Newer Grant 10kw Unit R32, this is a really small unit and I found it suffered with frequent defrost cycles over the winter.

Our system is fully open loop, rads and UFH, no buffer/hydronic separation.