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

Hardware Heatpump
222

Hi that’s more or less what I did.

Set all the TRV’s to max then balanced the radiators to get even temps throughout the house.

I had to leave all the TRV’s lower in the bedrooms or everyone sweats to death.

The design temps for those rooms were 18 degrees as per the Octopus radiator schedule so they should be properly sized for that temperature in some of the rooms the kids computers etc provide enough heat and are so well insulated they stay warm. They are only sort of 3x3m’ish.

I did explain to Octopus that the boiler I had installed had weather compensation and had been running for 12 months between 27.5 - 42.4’ish depending on weather and that produced comfortable temps throughout the house.

When they sent me the radiator schedule with a design flow of 50 that did not seem a million miles away from what my boiler was doing I did question the radiator replacements but they insisted they needed to increase the size of six.

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223

The radiators in the bedrooms are obviously bigger than they need to be compared to those in the rest of the house, otherwise you wouldn’t be exceeding the design temperature, sweating to death whilst the rest of the house is comfortable.

They clearly put out too much heat, otherwise you wouldn’t be turning them off.

If you want to correct it, you either need to reduce the radiator sizes in the bedrooms where you have too much heat or increase the sizes of other radiators so that each room is at the desired temperature with no trvs in operation.

224

Sorry, that’s my mistake I just pasted the other table again.

Here is the correct table I must be getting tired.

image

I will see if I can correct the other post.

Here is the full radiator schedule.

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225

That is 5,381w of radiators open at dt 30c.

That is 10.5kW at dT 50c

So at dT 15C they will put out about 2.2kW of heat.

Nowhere near enough.

That is why you have problems.

226

It may be even worse.

I included your dining room twice at 1,363w but I think that is the total?

Obviously I don’t know the layout of your house but there is no lounge or kitchen?

Is the dining room everything, your lounge, kitchen and dining room?

I think the output of your permanently open radiators is only 3.544w at dT 30c

That is 6,800w at dT 50c so at dT 15c it will only be 1,428w

The heat pump will cycle like crazy with that radiator capacity I would imagine.

227

Unless I am just not getting this that makes sense as the heat pump at below 5 degrees outside runs continuously at about 900-1000W no cycling perfect.

The numbers from the MMI are reporting a COP of ~3 in the cold weather.

So in theory it’s delivering about 3kW to the house.

Not the 7.8kW Octopus said was needed.

It goes up to maybe 1100 to 1200W at -2 so about 3.6kW

If the house is warm then the radiators and the flow temps are sufficient to heat the house so they are fine.

The issue is that when it’s above 5 degrees the heat pump can’t modulate down any further and starts cycling.

Or am I missing something?

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228

I don’t know what you are missing.

You did your own heat loss calculations and came up with 6.8kW

You have it massively wrong too?

What flow temperature where you running when it was -3c outside?

I was at 33c and I could barely run at 1kW input most of the time.

I thought you were at 40c+

Where are you getting 1kW electrical input from, the MMI?

I have the same heat pump in a smaller house, mine does not cycle at all at any temperature.

My house was warm with radiators that were way too small running at 28c flow at times, it worked and kept us warm but the COP was poor.

229

This is correct, and a common symptom of houses with oversized heat pumps. Welcome to the club! If weather compensation has reduced the flow temperature, the radiators won’t be outputting enough heat, so heat pump cycles. You either need to increase the flow temperature (i.e. flatten the curve), or increase the size of the radiators.

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230

No there are two radiators in the dining room.

The Dining room and kitchen are sort of open plan.

It’s not a perfect representation but here.

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231

But the heat loss of that room is 1,1121w according to Octopus and the total radiator out put is 1,363w (512w and 851w)

That means that the radiator output from those highlighted in green on your chart is only 3,544w at dt 30c

It nothing like enough.

Going back to your heat loss, Octopus only have it at 1,121w but you have 2,334w

They do a funny thing when a room has more than one radiator, they include the heat loss multiple times but the radiator outputs individually.

You cannot get the heat loss Octopus used or calculated from the radiator schedule as it only includes heated areas, it is not necessarily the full heat loss they calculated and used to determine the size of the heat pump.

232

-3 to -4 leaving water temp was about 42-44 degrees varies a bit due to the overshoot.

I have a CT clamp on the power supply to the HP it’s not perfect and includes power for all the electronics and pump etc.

I also have a grid meter for my battery storage that monitors all power use on my property that monitors all power use and it seems to agree with the readings from the CT.

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233

At -3c I needed about 1,400w minimum to run at 33c Average was about 1,600w during a cycle between defrosts and my defrosts are not so frequent at the low flow temperature.

I cannot see any way that you were running at 42c to 44c flow temperature and using 1kW of electricity.

It is the same heat pump and just not possible.

Mine is monitored with billing grade heat and electricity meters that are certified and beyond doubt.

I was just at a COP of 3 with 33c flow, you cannot be anywhere near a COP of 3 at those flow and outdoor temperatures, it’s impossible.

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234

Well, all I can say is whatever output they are they are providing enough heat for the rooms.

Temperatures are about as perfect as I could ask for at the flow temps I have set.

The damn heat pump just won’t lower its output enough at higher outside temps.

5 degrees or below bring it on it loves it runs fine temperature is lovely.

COP of 3 according to the MMI I’m happy I have no doubt that’s exaggerated.

Temperature goes above and it’s cycle city runs for 20 minutes then shuts down.

COP of 2.5 according to the MMI.

Needless to say, there are a lot more days above 5 degrees than below.

TBH I am less worried about the efficiency it’s the constant cycling and the wear and tear that will put on the heat pump plus a lower COP means wasted energy we are supposed to be installing heat pumps to save energy.

235

Its great that you were warm enough when you needed it.

I too have no complaints on that score.

I know what your problem is and you need to go down the radiator shop to fix it.

Well sort of, it will never be perfect because once you stop the cycling you will be too hot.

There is not much you can do other than fit bigger radiators and perhaps consider changing the heat pump for a similar size unit from another manufacturer with a wider range of operation.

Octopus only fit Daikin as far as I am aware and I don’t think there is anything more suitable in their range that would work with your current radiators and your heat loss.

I hope somebody else has better ideas than me though.

236

@Rob-G,

If yours is an Octopus installation I would be going back to them and flagging it. If it is a different installer then go back to them and see what they say.

Octopus replaced my EDLA09DA3V3 with the EDLA08E2V3 on the 10th of January. The new unit is giving me a COP of between 4.4 - 4.8 when it is ~8°c so a big improvement.

Pre the replacement I also got an independent heat loss survey from a local heat geek, he provided confirmation of actual heat loss and confirmed that 4 of the radiators were the wrong size and needed to be replaced (similar situation to yourself). We needed flow temperatures of 28°c upstairs and 43°c downstairs (hint, no way that will work) so we changed 5 (4 new, 1 swapped) and as of next week we should be all good at 40°c and hopefully make it into COP 5.

I know all the radiator stuff is a pain to understand and get your head around (it took me a few months) but Matt is right here. To get the temperature down for the weather compensation to work you need much bigger radiators or to get used to the cycling and poor COP.

I did also offer for Octopus run a set of primaries to the neighbours house and run a small district heat network to allow the 09 to work efficiently but apparently they preferred to change it for the smaller unit. :joy:

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237

Yes it’s an Octopus installation, I am in discussions with them about the system.

I will probably end up getting an independent heat loss report if I can to make sure everything is correct before any changes are made but in the mean time I did my own using Heatpunk.

Heat pump - design report.pdf (323.7 KB)

I entered the house as is with the radiators currently installed and let Heatpunk calculate the output based on flow temp.

All of the radiators below have the heat output values from the radiator schedule the ones highlighted in green I have removed the TRV’s. I have set the control mode on the heat pump to LWT not the room stat so it is running pure weather compensation.

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Those are the heat output of the rads at 50c.

No matter what I do I just can’t see how I will get this heat pump to stop cycling other than making the house hotter than is bearable or opening windows to increase the heat loss it just wont modulate down low enough to provide the amount of heat needed to keep the right amount of energy in the water.

238

Hi Robert,

You have included the Dining Room radiators twice.

You only have 1,363w of radiators in total in that room.

Your total radiator output at dT50c is 7,885w from your Octopus radiator schedule.

It should be printed at the top right corner of that document?

I had just under 9kW of radiators at dt50c and I needed to run at a flow temperature of 42c to make the heat pump work. I just used a fixed lwt as you are doing.

So you will need to run at 43c to 44c I would imagine with all radiators working, if you close any then the flow temperature will need to be higher.

This means you just have to run the heat pump in short bursts when it is warmer outside.

I think it is a good idea to get your heat loss assessed by an independent third party.

If it is much lower than Octopus calculated then you can make a case that the heat pump is oversized.

If the heat loss is actually correct then your heat pump is the correct size.

Increasing radiator sizes will help considerably, mine is now much easier to live with than it was. Even then I had more radiators than you have in my smaller house.

It will be very difficult to make it work well with what you have at the moment.

My heat pump data is here

https://emoncms.org/app/view?name=MyHeatpump&readkey=bbd1cd04728bd8c9f7acfb2ee51936d2

It is the same heat pump that you have in a smaller house than you have and with the same or, most likely, lower heat loss.

It is possible to make it work.

I haven’t been opening windows this year to control the heat, it has been cold enough to not need that.

@squarepeg77 has the same heat pump and has a similar amount of radiators to you in a similarly sized house as you. Jonathan’s data is here

https://emoncms.org/app/view?name=MyHeatpump&readkey=0e9ab50e73d8ae59d9e6dcc70e5dd5f7

Have a look, maybe it will give you some clues of how to make things work.

239

Bottom line I believe the heat loss survey is wrong. I will have to try and get an independent in.

I think as you pointed out they counted the Dining room radiators twice.

Also I think they used the default ACH (Air change per hour) value.

The whole property was assessed on the basis of it being a 1970’s bungalow and takes no account of renovation or the build dates of the extensions.

However the whole top floor was a 2010 loft conversion which should have a much lower ACH.

If I adjust the top floor to reflect it is post 2006 it halves the heat loss for that floor.

In the software you can select the date the property was built and that adjusts the ACH which impacts the heat loss.

These are the figures for exactly the same property just with ACH set according to year built.

Pre 2000 =6615W @ -3
Post 2000 =5826W @ -3
Post 2006 =4464W @ -3

If I adjust the post 2006 for heat loss at +5c (just before the heat pump starts to cycle) I get a heat loss of 3130W so more or less the amount of heat the MMI/COP is telling me is being delivered.

240

@Rob-G

Please have a look at the spreadsheet attached, it seems like most your system is ok but some rooms are quite a way off.

Rob-G_SystemOverview.xlsx (14.1 KB)

The spreadsheet (provided I have not fat fingered anything) shows you should have a design flow temperature of 54°c and a realistic minimum without cycling in the range of 40°c. (on the Madoka that would be 54@-2/3 and 40@15). You cannot really go much below 40°c as this is very close to the pumps minimum heat output of 4kw with your radiators.

Even at the design temperature of 50°c and with a slightly cooler bathroom some of your radiators would need to be balanced to a DT of 15-20. On a low temperature heating system this is not realistic and I have not been able to get above a dt10.

If you replace the heat pump (looks like you should have the EDLA08E2V3 personally) you would also need to look swap some of the radiators due to the unbalanced system (could discuss with Octopus if you want to argue it).

Providing you wish to have a 50°c flow temperature based on the octopus heat loss I would personally do the following.

  1. bathroom design temperature lowered to 20°c
  2. bedroom 2, store and WC radiators to be decreased in size to allow for correct system sizing and balancing.
  3. Office radiator to be increased or lower room design temperature to 20°c

Decreasing the radiators will impact the low end but unless you want to have the upheaval of re designing the system to 40°c or lower and replacing all of your radiators your options are somewhat limited.

you are welcome to copy this and re try with your heatpunk calculations but I ran out of time and wanted to get you something to have a look at.

Please remember I am just a system owner with access to excel and a load of automated mathematical formula, This spreadsheet is designed to be a helpful guide and I am not a hydronic engineer / system designer.

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241

Thanks for taking the time to do that.

I will have a look later to see if I can work it out.

I will see what Octopus say and if I can’t get anywhere with them I will see if I can get an independent heat loss survey from a heat geek engineer.

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