Buffer Tank Hot when Heating Mode Turned Off

[James1234]

Hello all, moved into our extended/refurbished house a couple of months ago. We have ASHP (Daikin unit), hot water cylinder and buffer tank. The space heating has 4 zones in the house (upstairs/downstairs, old house/extension).

 

The system was designed by the Daikin rep at the local plumbers merchants, install by local plumber and signed off by separate MCS certifying company. I am not 100% convinced that everyone in that chain has done what they are supposed to as when I quiz one they refer to the others. If I was doing this again I think I would have taken a different route.

 

We are getting hot water and in the colder months heating was working fine. Now we are into summer I have put the master controller into 'heating off' mode. My understanding was the buffer tank was just associated with the space heating and would therefore be cold when the space heating was off, but it has remained hot. I quizzed the plumber who installed everything it who says that the buffer tank is used when the DHW tank had got up to temperature to allow the ASHP unit to wind down, as it couldn't turn off straight away. Seems wildly inefficient to have a tank of hot water sitting there for 6 months of the year.

 

Interested in other people's thoughts and suggestions.

 

Plumbing drawing and photo of installed system below.

 

 

 

Many thanks.

 

James

 

 

 

[SteamyTea]

Your 'buffer' looks more like a volumiser, if it is getting warm when heating is off, but DHW on, then there must be a path if least resistance through it.

Are the pipes to the UFH manifolds also getting warm?

[IGP]

1 minute ago, SteamyTea said:

Your 'buffer' looks more like a volumiser, if it is getting warm when heating is off, but DHW on, then there must be a path if least resistance through it.

Are the pipes to the UFH manifolds also getting warm?

Looks like a buffer, the drawing suggests a volumiser but the photo looks like it has four ports being used. 
 

Looks like it could do with the buffer being removed (or at least bypassed with the aid of a bit of piping and valves). Also sounds like it’s over zoned and can see third party controls which generally mean weather compensation isn’t been used. Could be wrong but it looks like it’s been setup very boiler-esque. 

[Nickfromwales]

Needs more questions asked, and some clarity on plumbing arrangements, but I think it simply needs a 2-port zone valve on the return to stop convection heating from the cylinder. 
 

It’s defo a buffer as it’s piped up as a 4-port setup.

 

My guess is that heating flows through the buffer by design, when heating mode is selected, as does DHW primary flow. The thing I can’t see is the 3-port diverted valve that sends flow to each discipline individually.

 

Can we have some better pics plz, show plumbing from different angles, all valves etc, insulation removed wherever possible?

 

What temp is the buffer sat at when only DHW is on?

 

Do you turn heating off on the ASHP control as well as on the 3rd party heating controls of the UFH setup? 

[SteamyTea]

28 minutes ago, IGP said:

Looks like a buffer

Didn't look at the picture.

Seems to have at least 3 pipes connect, so yes, a buffer.

 

Need some insulation on the pipes.

Get the installers back and get them to correct it to the drawing.

[James1234]

Pipework to the manifolds is not getting hot, just the buffer tank itself.

[Nickfromwales]

1 minute ago, James1234 said:

Pipework to the manifolds is not getting hot, just the buffer tank itself.

 

6 minutes ago, Nickfromwales said:

Do you turn heating off on the ASHP control as well as on the 3rd party heating controls of the UFH setup? 

?

 

Are you telling the heat pump itself that heating is off? 

[James1234]

4 minutes ago, SteamyTea said:

Didn't look at the picture.

Seems to have at least 3 pipes connect, so yes, a buffer.

 

Need some insulation on the pipes.

Get the installers back and get them to correct it to the drawing.

So would you say the drawing is a correct/proper setup? But the plumber hasn't installed what has been designed?

[James1234]

7 minutes ago, Nickfromwales said:

Needs more questions asked, and some clarity on plumbing arrangements, but I think it simply needs a 2-port zone valve on the return to stop convection heating from the cylinder. 
 

It’s defo a buffer as it’s piped up as a 4-port setup.

 

My guess is that heating flows through the buffer by design, when heating mode is selected, as does DHW primary flow. The thing I can’t see is the 3-port diverted valve that sends flow to each discipline individually.

 

Can we have some better pics plz, show plumbing from different angles, all valves etc, insulation removed wherever possible?

 

What temp is the buffer sat at when only DHW is on?

 

Do you turn heating off on the ASHP control as well as on the 3rd party heating controls of the UFH setup? 

Will get some more photos when back home this evening.

 

Heating is turned off at the 4 thermostats and also selected on the black controller which I am told talks to the ASHP (you can select to turn off the DHW tank, set times that the DHW is heated to suit lower electricity prices etc.

[SteamyTea]

36 minutes ago, James1234 said:

would you say the drawing is a correct/proper setup

Well it is a setup.

As @Nickfromwales points out, probably needs some isolation on the heating return.

 

 

[James1234]

Thank you all for your help on this, it is really appreciated, as is the wider Buildhub, I have used it a lot for ideas, guidance and support during my build. 

 

Sorry to take a while to reply, I was keen on getting the best photos and making them up clearly to make things as simple as possible. Below are a set of marked up photos showing closer up images of the system (one which shows before the insulation and solar inverter went in). The temperature of the buffer tank is over 42 degrees (my Boots thermometer stopped working at that point because it thought the patient was suffering sever heat stroke).

 

[JohnMo]

On 25/05/2025 at 15:16, James1234 said:

We have ASHP (Daikin unit), hot water cylinder and buffer tank. The space heating has 4 zones in the house

For me would look to eliminate the buffer all together and move the heating to a single zone. You have so many pumps, all need power and will eat through electric.

 

If you can eliminate all the zones and run out loop.

1. Heat pump will just supply the UFH directly no pumps or mixers needed. Good chance you can run the whole system cooler, as no mixing will occur. So you get a better CoP. You run cooler, because mixers are gone and without buffer mixing in there doesn't occur either.

2. You eliminate at least 2 pumps. They will pull approx 60W each, run most of the time over the heating season, so 0.060Wx2(pumps)x24 (hrs)x180(days)x£0.25 is £130 cost savings a year for pumps alone. 

3. You will have to do some loop tuning to get system balanced.

 

but could save around £200 per year overall

 

Things you need to sort out. 

External insulation where you enter the ground. Then Seal the conduit end before you get mice living there.

 

The ideal place for your controller isn't where it is, it should be in your hallway or somewhere that represents house temperature.

 

Insulation needed on hot water circulation from cylinder. Look to add 25mm wall thickness insulation to those pipes. I would look to add a timer to the pump also if one not there already.

 

In fact whole cylinder pipes insulation. 

[James1234]

Thank you @SteamyTea, @Nickfromwales, @IGP and @JohnMo

 

I clearly have some things to unpick. Are there any industry standards or Daikin best practice that I could refer to when opening up discussions with the plumber? There seems to be such a wide range of opinions out there. If I was to get someone in to review the system who would you think would be best, Daikin Technical, Daikin Approved installer, Heat Geek approved installer etc?

 

If I go back to my initial question "should the buffer tank get hot when the space heating is turned off?" It sounds like no, and the plumber's explanation of "the buffer tank is need for the ASHP to wind down after heating the DHW cylinder" is incorrect.

 

The selection of the heat pump was done by the Daikin rep at the local plumbers merchants. He calculated a 6kw unit was needed but as they didn't have any in stock supplied a 9kw unit at the same price. I had a lengthy phone call with him at the time questioning this and he said the 9kw would be able to work at a lower temperature and therefore work as efficiently as the smaller unit. Interested in your opinions on that as well.

[IGP]

I would suspect like a lot of heat pumps in that output range, they’re actually the same physical units but firmware limited top output. Would need to check the model numbers etc to confirm, but usually the dimensions and the refrigerant charge etc between the units would be identical. If that’s the case, then there would be no harm done whatsoever by putting the 9kW in. 
 

If I was in your position I would be reaching out to a suitably trained plumber (heat geek or equivalent) rather than Daikin Technical, just because the focus is on the system as a whole and all the bits that aren’t the ASHP itself whereas a Daikin guy might be focused on the ASHP itself rather the rest of the system (hence, from a manufacturer perspective - buffers are fantastic as the rest of the system is now hydraulically separated and definitely not their problem, irrespective of the issues they an cause)

 

 

 

[JamesPa]

7 hours ago, James1234 said:

The selection of the heat pump was done by the Daikin rep at the local plumbers merchants. He calculated a 6kw unit was needed but as they didn't have any in stock supplied a 9kw unit at the same price. I had a lengthy phone call with him at the time questioning this and he said the 9kw would be able to work at a lower temperature and therefore work as efficiently as the smaller unit. Interested in your opinions on that as well.

Red flags here.  The 9kW unit is actually (sofaik) the 16kW software derated, whereas the 6kW is the 8kW derated.  Given this there is absolutely no way should he have fitted the 9kW.  The fluff about lower temperature is Bow Locks.  The temperature you operate at is determined principally by the size of emitters not the heat pump.

 

Given this I'm not surprised he fitted a buffer tank, which is almost always technically unnecessary (and frequently compromises performance) in a domestic environment.

 

Taking all this together you are, I am sorry to say, quite possibly paying 30% more for your heating than you need to.

 

Is there any prospect of getting them back to do the job properly, it's notoriously difficult as MCS is designed to protect installers not consumers, but some others have had a success.  If you have records of consumption/heat delivered you may be able to prove it's oversized.  Since they know they supplied the wrong one that may be a starting point for a claim that it's unfit for purpose. Daikin, unfortunately, don't make it easy to access detailed technical data on performance Vs load (or indeed anything else), which will make it difficult for you to prove detriment.  What sort of COP/SCOP are you getting?

 

I think a step back from the immediate problem and a quick exploration of these matters might be a good course of action.  You may well conclude that nothing else can be done, but it might also be silly to do something before thinking it all through.  

 

Is the system diagram you posted the actual one as installed (and if so where is the buffer tank - BT3 is shown as a 2 port volumiser but your photos appear to confirm there is a 4 port buffer present), or did the plumber do something different to what was designed for some spurious reason?  What's the rationale behind 4 zones?

Edited May 28 by JamesPa

[SteamyTea]

1 hour ago, JamesPa said:

Red flags here.  The 9kW unit is actually (sofaik) the 16kW software derated, whereas the 6kW is the 8kW derated

If that is the case, then the part load i.e. most of the time, performance is probably compromised quite badly.

 

It is a real shame that all heat pumps don't have a decent datalogger on them. Data shows so much.

[sharpener]

On 28/05/2025 at 07:33, James1234 said:

The selection of the heat pump was done by the Daikin rep at the local plumbers merchants. He calculated a 6kw unit was needed but as they didn't have any in stock supplied a 9kw unit at the same price. I had a lengthy phone call with him at the time questioning this and he said the 9kw would be able to work at a lower temperature and therefore work as efficiently as the smaller unit. Interested in your opinions on that as well.

 

The key point here is that the turn-down ratio of a heat pump is usually about 3:1. So the 6kW unit would happily run continuously at 2kW output.

 

Hence on the face of it the 9kW would not be too bad, at 3kW. But if as @JamesPa says it is a 16kW unit derated by software then the min output is going to be over 5kW which is 2 1/2 times what you should have had. So it is going to spend most of its life cycling severly which is poor for efficiency and also poor for the compressor lifetime.

I think you might usefully consider a Small Claims Court case for "not fit for purpose", the threat of this might miraculously cause a 6kW unit to be found!

[James1234]

On 28/05/2025 at 14:52, JamesPa said:

Is the system diagram you posted the actual one as installed (and if so where is the buffer tank - BT3 is shown as a 2 port volumiser but your photos appear to confirm there is a 4 port buffer present), or did the plumber do something different to what was designed for some spurious reason?  What's the rationale behind 4 zones?

The plumber has put in what is shown in the photos but has not followed the drawing which calls for the volumiser.

 

The logic behind the 4 zones is upstairs and downstairs, old (less well insulated) part of the house, new (better insulated) part of the house. It was a discussion the plumber and I had but it sounds like he was no better informed than me in that conversation.. 

 

I will have some discussions with the various parties and will do some more digging and get back to the thread with progress.

 

Thanks once again for all the advice, very much appreciate.

 

[James1234]

On 28/05/2025 at 14:52, JamesPa said:

The 9kW unit is actually (sofaik) the 16kW software derated, whereas the 6kW is the 8kW derated

Just looked on a supplier website and indeed the 9kw - 16kw units all look the same with the same 3.8kg refrigerant charge but the 7.5kw and below are a different looking unit with 1.35kg refrigerant charge. So yes, looks like you are correct @JamesPa

 

On 28/05/2025 at 14:52, JamesPa said:

What sort of COP/SCOP are you getting?

It has only been running for 3 months, from the Daikin controller 114kWh electricity used, 351kWh heat produced = 3.07

[JamesPa]

12 minutes ago, James1234 said:

The plumber has put in what is shown in the photos but has not followed the drawing which calls for the volumiser.

 

The logic behind the 4 zones is upstairs and downstairs, old (less well insulated) part of the house, new (better insulated) part of the house. It was a discussion the plumber and I had but it sounds like he was no better informed than me in that conversation.. 

 

I will have some discussions with the various parties and will do some more digging and get back to the thread with progress.

 

Thanks once again for all the advice, very much appreciate.

 

Good plan.

 

If you could get it to be instalked as designed ie a volumiser (not buffer tank) and the 6kW (or even the 8kW) model, that would be a excellent advance.  All the secondary pumps will need to go as a consequence and they should balance the zones on the zone control valves.

 

Most (not all) domestic heat pump installations should be run 24x7 as a single zone without buffer tank, a correctly or only modestly oversized heat pump, with no or minimal controls other than the heat pump native controller, and any room thermostat used only as a temperature limiter not a temperature controller.  Weather compensation should be adjusted to the lowest possible operating temperature consistent with heating the house.

 

It's pretty much the polar opposite of how we typically operate boilers in the uk and some installers just don't 'get it'.  Unfortunately the industry has got out of the habit of designing systems over the past decades, preferring instead to overspecify everything and let the controls sort it out (to the detriment of the consumer in terms of running cost and comfort).  Heat pumps aren't so tolerant so now installers have to do it right, some are lagging!

[JamesPa]

1 hour ago, James1234 said:

Just looked on a supplier website and indeed the 9kw - 16kw units all look the same with the same 3.8kg refrigerant charge but the 7.5kw and below are a different looking unit with 1.35kg refrigerant charge. So yes, looks like you are correct @JamesPa

 

It has only been running for 3 months, from the Daikin controller 114kWh electricity used, 351kWh heat produced = 3.07

I've heard worse, and it could have been much worse, but its not going to win the 'Top of the SCOPS' award!  

 

Three months take us back to March.  Over 4 would be a reasonable expectation at this time of year depending on your operating temperatures and how Daikin arrange their trade offs, so there is very likely improvement to be had.  For comparison my 7kW Vaillant, operating at ft 42C at design OAT of -2, reports 4.2, 4.7, 4.7 respectively for Mar, April and May.  8m in the South of England.