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

Hardware Heatpump
202

Hi,

So if my understanding of the MMI is correct then at 10c the set point for the flow is 33c.

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I have all the TRV’s in the bedrooms set to 20c(18c at night) otherwise they get too hot all the common rooms bathroom, kitchen, dinning room, living room are set 24c and all sit at 21c quite happily.

The reason I set the TRV’s a couple of degrees above where I want the temp is the TRV’s seem to be optimised for GAS CH so start shutting off the valve as they approach the set temp but I have found this does not play well the lower flow from the heat pump.

203

Indeed, to avoid starving the heat pump of water flow (which also drives up the flow temp if you hit minimum output) you really want to try and set the downstairs TRV’s to the maximum setting (or remove the head entirely) - similar to what you’ve done.

Any differences in heat output/room temperature can be dialled in through balancing (assuming the radiators are generally able to meet the peak heat demand for the room)

It looks that way - assuming you have a heat curve defined? I’ve found going lower than sat 32C or so doesn’t really work that well from an efficiency perspective on my system. YMMV.

Switching to LWT mode and the 4C overshoot may result in some overheating, but you want to then reduce the weather curve (but not too far) - and it should allow balancing to be done if you can get to a steady state.

If rooms are not getting to temperature, ensure the valves (not the TRV as that should already be at max) are fully open.

Any rooms that get hot too quickly or overheat, start closing the valve little by little.

Unfortunately, most valves only start restricting flow rate at a certain point, and then they are very agressive, so you can turn and turn with nothing happening, and then all of a sudden the flow stops…its at that point you’ve found the “authority”, and then its’ trial and error:

room too cold, open slightly.
room too hot, close slightly.

There is a thread specifically on balancing here as well…:

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204

OVO and heat Geek?

205

Exactly this.

They are chimpanzees.

You can’t trust them to do anything unsupervised but if you do hold their hand and point at exactly where the bits need to go to get their banana then they can join pipes.

They have money.

The well operated energy part of their business is rich enough to bankroll losses incurred by the heat pump division so you can get a discounted installation cost.

Unfortunately they’re dishonest and they have lawyers.

So they’re very happy to lie to your face unless you have overwhelming evidence of where they didn’t do things correctly and don’t get a banana.

Treat them the way that they treat you in their terms and conditions (as an adversary and with contempt) and can work.

Treat them the way they would like you to (trust in the brand) and they’ll run rings around you.

Here’s a fun idea:

Refuse to pay the final installment (release your tax rebate/BUS grant) until the job is demonstrably complete.

With “demonstrably complete” being it performing to design as measured by open energy monitor and MID measuring instruments.

MCS and DESNZ should be onboard with that. It’s the reason that you as the consumer must be happy with the installation before the government pays the installer on your behalf. :slight_smile:

206

If you have access to the upfront capital required - and there’s actually any availablity on the part of the installers - that does seem to be the peace of mind option for heat pumps today.

207

My 9kW is going, I have tried everything and I can’t live with it.

It will be replaced with an 8kW Daikin by Octopus.

No arguments now, it is obvious that my heat loss is a little over 4kW at the design temperatures, just as I have always said.

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208

Keep us posted on how it goes Matt, it’s been an interesting, if somewhat trialling, journey for you. I’m sure you’ll put the new unit through its paces.

I’m am interested in the 8kw unit for myself. In particular what sort of saving it would give me per year to justify the change. My heat loss for yesterday with average outside temp -2.5C was 4.3kw and the worst day in November when it was more humid was still only 5kw. It’s a shame OVO wouldn’t give me access to the high res data from the meters to prove this in their 2 year heat pump trial, I tried but it didn’t work. It’s great that you and Zak have succeeded with Octopus.

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209

Hi Matt,
I remember Octopus offered to replace your 9kW unit with 8kW model a while ago and you declined, at the time.

Can I ask what changed your mind? More data? Exhausting the available options, including a visit from a Daikin engineer?

I’m pleased you will get the 8kW unit. I think we know that will perform very well in your property and be easy to live with.

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210

Hi Jonathan,

You are correct with everything you say.

Over the last couple of weeks I have been able to confirm beyond doubt what my heat loss actually is around the design temperatures.

Although it was obviously nothing like the heat loss surveys produced it is never easy to convince anybody without absolute proof, now it is beyond doubt.

I don’t think there is anything wrong with the 8kW but I feel that the 9kW is a more substantial and maybe a higher quality unit, I didn’t want to change really, I would have preferred to keep what I have.

But, the last couple of weeks have shown me how comfortable it is living with a heat pump that isn’t way too big.

I am dreading going back to the warmer weather, it’s just unpleasant.

I can’t continue like I have been and it has to go.

I now understand as fully as I can what is going on with the house, the heat pump and the Madoka, they will never work together above freezing.

I don’t like giving up and I think I have got the best I could out of this heat pump in our house. I think I have exhausted every way possible to make it work in our house when it’s warmer at a reasonable level of efficiency.

It isn’t just about efficiency though, I always had to make a choice between running the heat pump continuously and controlling the temperature with windows and doors, or running with the Madoka controlling the room temperature but living with no heat for 3 hours at a time. Neither worked very well, either it is too hot and uses too much electricity or parts of the house get cold for a while.

The Madoka is designed to load compensate by modulating the heat produced to maintain the set room temperature, it can’t modulate down when it is already at the bottom and this heat pump in our house is always at the bottom of its operating range.

I also had a problem with Octopus, although they offered a swap it didn’t come easily the first time, it wasn’t offered in good spirit and it came with a certain amount of intimidation, basically they didn’t believe I had a problem with the heat pump and they felt it was the correct unit for our house.

That seems to have changed now, they appear to accept that the heat loss is not what they calculated.

It makes a difference to me, it’s now amicable and not confrontational.

I will try and get the best out of the 8kW as you might expect but in all honesty I just want a heat pump that I can leave alone to heat the house.

I would expect my annual electricity consumption to be somewhere between half and two thirds of what it is with the 9kW.

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211

Hi all, thx again for all the info no matter how long I have had this heat pump there seems to be more to learn.

I found a suggestion either in this thread or another to try a website called Heatpunk.

It seems very detailed you can enter the U values for your floors, walls, ceilings, roof etc.
Create a 2d map of your house with windows, doors and radiators.

Having a somewhat complex house with an extension and loft conversion I had to do a lot of digging around on various websites like Celotex for U values and cross-referencing that with my extension plans.

After lots of measuring and data entry, the site came back with a heat loss of 6672W as opposed to Octopus’s 7753W(I’m still not sure why they insisted on installing the 9kW heat pump).

The total area of the building is 129.30 m2, giving an average heat loss of 51 W/m2 at -3°C.

I’m still not 100% happy that these values are accurate as it states the radiators I have installed will not be powerful enough to keep the house at 21deg at a flow of 40 at -3deg but I have run the heat pump at that temp so it’s either overestimating the heat loss or underestimating the output from the rads.

It’s difficult to be sure though as I have the overshoot set to 4 degrees to try and reduce the cycling so my flow is oscillating by 8 degrees which is not great for thermal comfort at times.

I found once the temps increased outside my hallway would warm to 23 degrees and shut the heat pump off for hours and some rooms would drop 4-5 degrees before the heat pump would switch back on. So I have switched the control method to LWT with WD curve set to 40 at -3 and 20 at +18 all of the rooms are sat at a steady comfortable temperature despite the large overshoot setting.

I have put the two sets of heat loss calculations next to each other in the chart below and you start to see the problem.

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Due to them overestimating the heat loss in many of these rooms, I paid to replace radiators that were perfectly adequate. The largest difference is a whopping 81.6% in the hallway(where the thermostat is).

Some of these I queried at the time I wish I had stuck to my guns now.

Anyway, with the temperature now back above 6 degrees we are back to constant cycling of the heat pump every 20-30 minutes I have no idea how long it is going to last like that.

I did try getting in touch with Daikin tech support to ask about the minimum power for the 4,6 and 8kW but all they seemed to be able to do was send me the tech specs in the brochure.

I will go back to Octopus with this data and see what they propose.

212

Hi Robert,

When was your heat pump installed?

Although the percentages on some rooms look big they are not actually large amounts.

Maybe the Madoka location is causing an issue as that area of the house heats up faster than the rest of the house and does not cool down so quickly. Maybe you need to move it or limit the heat delivered to the hallway.

The overshoot is not +/-4c, it is only +4c so you cannot have a flow temperature that fluctuates by 8c because of the overshoot setting.

Even at a heat loss of 6.7kW you are not that far away from the 9kW range.

My heat loss at design is more like 4kW and @KnightPhoenix is similar. The heat loss of my house is actually less than 3kW at design, it is just our conservatory that loses a little over 1kW that they now insist must be heated that caused all the trouble.

I have a heat loss that is much less than yours and I have no cycling at all. Apart from defrosting my 9kW Daikin will run for six hours at a time (the maximum) quite happily.

The minimum input for the smaller Daikin models is around 285w

213

Hi,

Yes, the thermostat location was part of the problem but I have removed that variable by setting the control method to leaving water temp. The heat pump just tries to deliver a constant water temp based on the WD curve.

It’s 9C outside so the LWT is currently set to 28C.

It’s constantly cycling as you can see from the power draw below measured with a CT.

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I have smart TRV’s on most the radiators so I can show you the house temps as well. Most of the bedroom TRV’s shut off to stop the rooms from overheating but all the common rooms are set to 24 and are wide open.

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Those rooms seem to sit at a constant 20-21 degrees without the TRV’s needing to interfere.

EDIT: The heat pump was installed Oct 22.

214

Hi Robert,

If your heat loss is really 6.7kW at -3c and you have to run at 40c flow then you will need the 9kW

Defrosting in those conditions mean your heat pump will barely put out enough to maintain the house temperature.

Do you know how much heat was required in those temperatures we have recently encountered?

My heat pump was installed in June 2023, the only heat pumps available at that time were the 9kW or the 7kW.

As my heat loss was calculated at over 7kW I got the 9kW heat pump. Maybe it is the same with you?

Nothing else was available.

As I can demonstrate that my heat loss is barely half of what they calculated then I can make a good case that my heat pump is too big.

Based on your own heat loss calculations I would say that yours is not too big, it is just barely big enough taking into consideration the flow temperature you need to run.

I have no trvs on my radiators, it is the last thing you want with this heat pump when it is warmer outside, you need somewhere for the heat to go.

I dare say that would be contributing to your short cycling. The bedroom trvs closing will cause your return temperature to rise and the heat pump to cycle.

As I said, my 9kW does not short cycle and my house is smaller than yours with a lower heat loss.

Unfortunately, your heat pump isn’t too big when you need it, it is just that it won’t modulate down to what you need when it is warmer outside, it is the nature of the 9kW Daikin.

I have sized all my radiators to match the heat loss of the rooms and the desired room temperatures, that way none of them ever turn off, they can’t anyway as I have no trvs.

The radiators need to be in proportion to the heat loss of the room to get the best out of this heat pump, trvs and zones are undesirable.

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215

I do.

Here is data from the 19th pretty much the worst case for the heat pump.
-3 degrees outside
Between midnight and 04:00 it chugged along at 1kW
Between 04:00 and 05:00 it was heating hot water so the house cooled down a bit.
It does draw a good chunk of power to heat the house up between 05:00 and 06:00

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Here is another sample 15th to 16th drops to -4

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I guess the spikes about 6AM are defrost cycles.

216

Might not be relevant, as you may have tested this. Could the cycling be due to system volume - i.e. the TRVs getting shut off and thus only heating a single room.

We had this and removed all TRVs and actuators (UFH downstairs). Then balanced the relevant bits down to control temperatures in each room.

Also the LWT of 28 will cause the machine to cycle (I think you said you had the 9kw), the minimum temperature seems to be about 32…

Oops snap @matt-drummer

On days like today, I simply turn off the HP for a number of hours and then switch it back on at a given point. I’m nearly at the point of switching back to thermostat control but haven’t quite made the plunge - I’d love for LWT to just work…

Also we have a single Tado, which I don’t trust.

I’m awaiting Octopus to replace with Madoka which is proving harder than I expected…

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217

This heat pump is great when it’s cold. It’s not the worst case for this heat pump, it is the best.

The problems come when it is warmer.

My radiators are massive, I don’t think I could possibly go bigger if I wanted to and I wouldn’t dream of fitting trvs.

If rooms are too hot then the other rooms that are ok need bigger radiators and the flow temperature reducing.

When I had to run at 40c at -2c outside the heat pump was defrosting every 20 minutes, it was all it could do to get to 40c in 20 minutes.

The heat output defrosting like that is a maximum of about 6.3kW.

Octopus insist on designing at 50c flow at -2.3c outside.

My experience is that this heat pump will never get to 50c as designed, it just isn’t possible.

This ends up with radiators that are too small usually, mine just about worked.

My radiators are now three times the output they were at installation.

It now works pretty well, so well that even though it is grossly oversized I am still a little nervous about changing it and I could live with it if I had to.

Cycling will be caused by either ice or the returning water temperature being too hot.

This heat pump isn’t really happy much below 35c flow temperature, most of the time this results in the minimum electrical input and reducing the flow further just results in less heat, a poorer COP and an unhappy heat pump.

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218

I have to use the TRV’s on the bedrooms otherwise they overheat.

Here is the data from the radiator in my daughters room.

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The TRV is programmed to 18 during the day, 17 overnight, 20 in the morning(Purple line).

The blue line above is the temperature that never drops below 21.

Her bedroom door is always closed yet stays at a comfortable temperature.

The TRV’s only impact the bedrooms the rest of the TRV’s are set to 24 and never shut off I could completely remove them and it would make no difference.

I have about 5kW of radiators permanently open to allow flow around the system.

I have been monitoring the flow and return with a heat meter with K clamps and the dT is currently 5deg.

Flow rate is normally about 10l a minute when it’s running.

219

Hi Robert,

When you say 5k of radiators permanently open what do you mean?

Is that 5,000w at dt50c as they are sold in this country? If so 5,000w of radiators at dt 50c only put out 600w at dT10c, 1,050w at dT 15c and 1,500w at dT20c. Nowhere near enough to take 4,000w from your heat pump. If that is what you have, then that’s why it is short cycling.

Or maybe that is from your Octopus radiator schedule at dT30c? In which case they are about 10,000w at dT 50c and you will get 3,000w at dT 20c and only 2,100w at dt 15c, still not enough for your heat pump.

In my house I have about 45kW of radiators with no trvs

It allows me to run at 33c (about 30c mean flow) and produce about 4kw of heat at a dt of around 7c between room and mean flow.

trvs and heat pumps don’t really go well together, if a room needs a trv then the radiator for the room is too big or more likely the others in the house are too small.

Of course, you can do this with your house, it’s your house, but it will impact how your heat pump runs.

If your heat pump is cycling because the returning water temperature is too high then shutting radiators will only make it worse.

Better to open windows if the room is too hot and you want the heat pump to run for longer.

Or, you just have to live with it.

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220

Hi, so in the spreadsheet below are all the radiators.
The ones in green have either a standard TRV set to 5 or a smart TRV set to 24(The rooms only get to about 21 so the smart TRV’s stay 100% open.

In theory, I could remove them all to make sure they are not interfering in any way.

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EDIT: The wrong table above here is the correct one.

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EDIT: The second table has the value from the Octopus radiator schedule all the green ones have TRV’s set to max to only close to prevent over-temp.

It seems crazy to open windows to get an expensive heat pump to run efficiently after spending thousands on insulation and the heat pump.

Why design a heat pump that only runs efficiently 2 days of the year when the temp drops below 0 for 3-4 hours?

221

I am sorry Robert, I can’t see radiator outputs, that is the same table with room heat losses.

If the radiator outputs are similar to the room heat losses then that is only 4,795w of radiators.

And if that is from an Octopus design at 50c flow then at 40c lwt they can only put out about 2.1kW of heat if your rooms are 21c and the mean flow temperature is about 37c.

It is about 50% of the heat output from the heat pump.

It is why it will short cycle when it is warmer outside and the heat loss of your house is much lower. If some or all of the other radiators shut off then there is nowhere for the heat to go and the returning water temperature will increase until the heat pump stops.

I think there are some other brands do this too, but having the smallest Daikin in the 9 to 16kW range comes with the narrowest operating window in terms of heat output, around 4kW to 9kW.

It has the same compressor and circulation pump as the `larger’ models and is basically software limited at the upper end.

All four in the range operate at the same lowest level.

In a smaller house this is more of an issue, in fact it is a massive pain.

I know this only too well, opening windows is more efficient than letting the heat pump short cycle.

Or, just get used to being warmer.

Or replace the bedroom radiators with smaller ones, not really advisable.

Or replace the other radiators with bigger ones so that the heat in the house is more balanced and you can run lower flow temperatures, what I did.

Maybe it is possible to limit the heat output of some radiators using the lock shield valve.

But persisting with trvs will just reduce your radiator output and cause the heat pump to cycle more often.

I hate to be the bearer of doom and gloom but that is how it is, it is my experience and many others go through the same.

Maybe somebody else has other ideas to make it work better with small radiators and trvs but I don’t and have yet to encounter anybody that does.

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