My heat pump is too big, what should I do?

Hardware Heatpump
41

The conservatory is not an issue for me, we can just not use it or use the electric under floor heating if it is too cold. But it never is too cold as the heat loss is not what they say.

But that is really not the issue.

The main house, the bit we need to heat loses 5.1kW at 21c indoors and -2.3c outside.

That s according to their heat loss calculations with the MCS requires air changes per hour based on a 1960 to 2000 build house with an epc of D

We have almost an epc of A so we actually lose less heat, about 4kW

So on a day like today the whole house including the conservatory needs about 3kW of heat, maybe a little less.

The house needs 2.5kW and the conservatory needs/can only take 500w

The heat pump produces a minimum of 4.6kW so the conservatory gets 500w and the house gets the remaining 4.1kW

As the house only needs 2.5kW or less it gets too hot.

The only way to solve it is to let the heat pump run and let the house get hotter and hotter or run shorter cycles at a much lower efficiency.

The colder it gets the less of a problem I have as the house needs more of the heat pump’s output.

My problem is that I don’t need 4.6kW of heat when it is 7c or above outside.

The minimum heat output of the heat pump is about 4.6kW and I can’t turn it down without losing efficiency.

The shorter cycles use the most electricity as it is most inefficient during the first 30 to 60 minutes of each heating cycle

I wanted a heat pump that can just run as continuously as possible putting in about the amount of heat that the house requires.

I can’t do that with this heat pump.

Bigger radiators will help with any heat pump I have, I will be able to run at lower flow temperatures even when it is really cold outside.

The heat pump we have will be great when it is colder, I don’t need to wait to find that out, I can already see that.

The problems are when it is between 5c and 15c outside when I need a bit of heat but not all that this heat pump produces.

That is where the part of the country I live in spends most of it’s time, it is rarely that cold here for long.

1 Like
42

This is what is actually happening in my house right now.

According to the heat loss survey I need about 3kW of heat, 2kW for the house and 1kW for the conservatory.

The heat pump is producing 4.5 kW of heat and the conservatory radiator is taking 500w.

The other 4kW is going into the house when it only needs 2kW

If my heat pump could produce only 2.5kW I would be fine, 500w to the conservatory (which is adequate) and 2kW to the house.

But I can’t do this.

43

Why are those the only choices? Why not let it run at cheap rate and set back at peak rate?

I have a 8.5kW pump from another maker with a minimum output in the range of 3.5kW but used it to put average 1.55kW of heat in over the last 24h at an estimated efficiency of 425%. I can see your pump is a bit too big but I still don’t see how replacing radiators other than the conservatory helps much. You’ll have lower flow temperatures but a bigger volume so will still be putting too much heat into the house. I must just be too thick to follow this.

44

I am not sure what my choices are.

Most posters say that my radiators are too small.

My heat pump has a minimum electrical input of about 900w, it cannot go any lower.

It was tested at 35c flow and 7c outside and gave COP of 5.0 so that is 4,500w of heat.

If I want less heat then the COP will go down.

If I want the COP quoted I need to run at 35c.

If I increase the flow temperature the electricity consumption increases.

My current radiators cannot deliver 4,500w of heat at a flow of 35c.

So if I run at 35c the COP suffers, I get 3.2 to 3.5 I would say.

If I run at 42c I get the same really 3.2 to 3.5.

I don’t have any peak or cheap rate electricity, I run entirely from batteries so I don’t care when the heat pump runs. I just charge my batteries as necessary, they have enough to run the house and heat pump on any day.

Replacing the radiators helps me to deliver all the heat produced by the heat pump at its minimum electrical input to the house at the best possible COP.

The only problem is that the house gets too hot if I allow the heat pump to run for long periods.

If I run the heat pump for shorter periods and keep the house temperature lower then the initial electrical consumption of the heat pump at the beginning of a heating cycle is so high that the COP is poor.

So I see my choices as a hot house or poor COP, or possibly somewhere inbetween but I won’t know where the inbetween is until I finish the radiator upgrades.

In summary, the house needs about 3kW of heat but the heat pump uses too much electricity to deliver 3kW of heat averaged out over shorter periods of operation due to the large energy consumption on initial compressor start up and the restricted lowest electrical input.

This is because it has the same compressor as the 16kW version of the heat pump as well as the same circulation pump, they are designed for much larger homes.

So that is my summary.

I am interested to know how I can get it to use less electricity to do the same job but so far I can’t see a way, everything I have tried ands up with the same result.

The only thing I have done that has seen any improvement in COP and less electricity use is the partial radiator upgrades I made on Saturday.

If I had a heat pump that could deliver 2.5kW whilst using 500w of electricity continuously then I think I would be better off than with one that uses three times as much electricity to produce 5kW of heat for half of the day.

I think I am using 50% more electricity than I need to if I had the correct size heat pump.

You are not too thick, you are spot on, I will be putting too much heat into the house with the bigger radiators but the lower flow temperature will save me money over running it at the higher flow temperature I have to now.

45

I have exactly the same situation, with roughly the same numbers. My only options are either a) get a smaller heat pump, or b) open some windows. The latter has zero capital cost. :laughing:

3 Likes
46

Hi Tim,

My heat pump is massively too big and it is because the installer made mistakes in their initial heat loss calculation.

I can probably have a smaller heat pump at zero or possibly even negative capital cost.

So tha question is, if you could have a smaller heat pump at no capital cost, would you?

47

Is there an option to get the slightly smaller unit that can modulate lower and to get octopus to put in wet UFH in the conservatory for you as after all they did say it could be fully used and in your situation that would likely be the best / only way without 8 radiators in there. :rofl:

48

I would also add that if I doubled (or slightly more than doubled) the amount of heat going into our conservatory to comply with the heat loss survey then it would be a sweat box, it is already warm enough in our conservatory, it’s 20c in here at the moment and the rest of the house is 22c, it doesn’t need to be any hotter.

The heat loss survey is just a guess and takes no account of reality, It’s my house and not the MCS’s!

1 Like
49

Hi Zak,

I don’t need or want wet UFH in our conservatory, it’s not cold in here, it is just that the heat loss survey says it should/must be.

The conservatory is fine, it is just the rest of the house that isn’t.

The 8Kw Daikin is not slightly smaller, the 8kW and 9kW are in different leagues to each other, The 8kW is half the 9kw as the 9kW is really a 16kW heat pump.

I feel it is deception and we have been mis-sold this heat pump, we would be in no worse a position had they installed the 16kW version in our house as they all perform the same at the lowest level.

1 Like
50

Oh, most certainly, though ask me again once we’ve had a cold snap.

Running in shorter cycles should be way to keep a heat pump operating at lower flow temperatures when the radiators cannot keep up. Most systems are able to do this with better COP than if they ran longer and hotter.

Your system seems to be peculiar in that it appears to perform less well for the first 30-90 minutes.

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No idea what’s going on here, but I see the same pattern all over your feed. Sometimes related to the change in flow rate, but often not.

Even if performance is worse, running with some cycling and a cooler house should cost less in electrical consumption. COP isn’t everything…

3 Likes
51

Thanks Tim,

I am fully aware that I am using more electricity than I could be at the moment.

I am just chasing COP right now, I just want to get it as good as I can, I am not worried about the cost.

Once I have the COP as good as possible I can concentrate on making it the least worst I can if that makes sense.

My theory is that if I get the COP as good as possible then I will know for sure that the house isn’t impeding the heat pump and then all I need to do is tune the heat pump settings and my operating schedule to get the best out of it in terms of cost. I feel that if I don’t get it to the best COP first then I don’t have a good base to work from, I will never know if the house was holding it back in some way.

The first 90 minutes scares me and is my biggest concern with it.

I will have a big chunk of my radiator upgrade complete on Tuesday and it should be enough to turn the flow right down and see what I can do with it.

I really don’t want to change the heat pump unless I feel there is no option, I want to make it work if I can.

52

I wonder if anybody can explain this as Tim has highlighted.

Why does it take 90 minutes to get to this, I don’t understand why after the initial heat up my heat pump takes so long to settle down?

It is why I am so reluctant to change anything at the moment, I just want it to keep running.

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53

[Thinking as I type. I’m not an expert]

The same phenomena can be seen when heating up the hot water cylinder: link

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I like to look at DHW cycle because it removes the radiators from the equation. One would usually expect COP to start higher and gradually decrease as flow temperatures rise. Your’s is backwards.

It worth taking a step back and look at what the real measurements are here: electricity consumption, flow rate, flow and temperatures. Remember, the measurement of heat produced is derived from flow rate and dT, so best look at where those come from.

  • Electricity is gradually rising, as it works harder. I’m don’t know if that’s a normal profile; worth comparing to some other units. We can probably assume the meter is accurate?
  • Flow rate is constant, so can be ignored.
  • Flow temperature rises at a constant gradient throughout, with a slight speed up in the middle. I’d expect some correlation between gradient and heat.
  • Delta between flow and return is about 2K, and is higher towards the end. This is what drives the measured heat.
  • Heat does not match the Carnot COP simulations at all (using heatpump factor 0.4). We ought to be seeing a constant heat output of 6kW (with a slight upwards slope) and an average COP of 2.9.

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I’m still not convinced that there isn’t a metering issue here, perhaps on the flow temperature sensor. Maybe it only gets a good reading when it’s hot? Or when it’s been warm for long enough. It might be worth putting another thermometer on the same pipe, check you’re getting the same reading.

2 Likes
54

Interesting. I do wonder if there is a sensor issue too. (ESPAltherma comparison of data would be useful here as shows additional data that may be useful in troubleshooting @matt-drummer !)

I have a different tank (Mixergy with PHE), but am getting a very different profile with essentially the same Daikin unit (the 11kW rated version however):

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Link to time window above

2 Likes
55

Hi Stephen and Tim,

I have all i need to get ESPAltherma running, I am sure we have talked about it before.

Maybe Stephen you could message me with how you made it work, I’m sure you did it without HA?

Tim, I have a Bosch infra red thermometer, I guess that would be a guide, good enough?

I have applied the Carnot simulation to my graphs and in the initial heat up phase I fall beneath it but at the end of the heating runs I am above the Carnot simulation.

I don’t really know what any of it means but I am below it to start with and end up above it???

Is it possible that the flow temperature sensor is faulty?

You have seen my placement, there’s not a lot you can get wrong?

1 Like
56

Interesting.

My flow is showing as 33.7c on my feed.

My thermometer shows 24.5c

Nothing is running though so I will check again when it is running

57

That may not be a reliable reading - infrared thermometers don’t work well on shiny metal. You might need to add some black paint or something.

58

The metal is not particularly shiny, it’s dull copper and brass.

I have a clamp type thermometer for measuring radiator flow and return temperatures, in fact I have two of them so I will give them a go.

Flow 30.3 and return 27.2 on thermometer - feeds are 32.1 and 28.9

1 Like
59

Let’s look at how much energy it would take to heat 200 litres of water (excludes the coil) from 34 C (return temp at start) to 50 C (end temp?) using an online calculator… {buttons pressed, cogs turn, lights whirr} => 3.72 Kwh. While this is a massive oversimplification of the water heating process, and ignores any thermocline that may exist inside the cylinder, but it gives us a ball park of how much it could take.

Meanwhile, your metering says 2.3 kWh of heat was put into the tank to heat the water up to that temperature. :man_shrugging:

1 Like
60

Thanks Tim,

This is not the first time we have questioned the metering in my quest to make my heat pump work.

I think I have mentioned this before too, the COP in producing hot water varies a lot and at first i thought it was to do with the starting temperature but I am not so sure. It doesn’t really follow that pattern.