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If a heat pump can only heat DHW to 50C then what? Issue with 18KW Heliotherm heat pump

 
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Domestic Hot Water & Cylinders
Last Post by JamesPa 13 hours ago
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 errmm
(@errmm)
Active Member Member
Joined: 1 month ago
Posts: 11
Topic starter 10/03/2026 10:01 pm  

Hi 

After several weeks of pestering, an engineer turned up today to look to fix the issue.  The suggestion is that the intake for cold water was put in at the top of the tank when it was originally installed and understandably that'd mess stratification up so it's now been moved to lower down (not sure exactly how low).  

So I'll monitor the temperature and hw energy consumption closely over next day or two.  If that's the issue why on earth they decided to connect it that way.... and it's been like that for 5 years...

Mike 

 



   
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 errmm
(@errmm)
Active Member Member
Joined: 1 month ago
Posts: 11
Topic starter 12/03/2026 1:03 am  

Hi,

FYI an update of sorts.

So I agree the system doesn't appear to work as a hot water cylinder but instead contains hot water and water is passed through this to be heated up.  There's a heat plate on the side and this is 'somehow' used then to heat the hotwater as it's passed through. 

A visit from the installer suggest that the issue has been due to stratification being weakened, that when warm water from the heatpump is put into the tank to add energy, it was done so at the top of the tank and so disturbed the hot water.  So they have just changed the pipework so instead that water from the heat pump is added at the bottom  However based upon results from today, it's made no difference at all.  

 



   
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JamesPa
 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 4469
12/03/2026 9:43 am  

Posted by: @errmm
↑

Hi,

FYI an update of sorts.

So I agree the system doesn't appear to work as a hot water cylinder but instead contains hot water and water is passed through this to be heated up.  There's a heat plate on the side and this is 'somehow' used then to heat the hotwater as it's passed through. 

A visit from the installer suggest that the issue has been due to stratification being weakened, that when warm water from the heatpump is put into the tank to add energy, it was done so at the top of the tank and so disturbed the hot water.  So they have just changed the pipework so instead that water from the heat pump is added at the bottom  However based upon results from today, it's made no difference at all.  

 

Sorry to hear that but I'm not surprised TBH.  Stratification is effective in a normal unvented tank where the hot water is drawn off at the top.  In a regular thermal store much less so because the water to be heated passes through much of the depth of the tank.  It does have some effect if properly designed and properly plumbed, but nothing like as much as for a normal UVC.  However it sounds like this particular thermal store has is a plate heat exchanger (PHE) on the side, so instead of the water to be heated passing through the tank, the water from the thermal store passes through the PHE via a circulator, with the water to be heated passing through the other side of the PHE.  This is effective for heat transfer but will totally destroy stratification.  This being the case, and assuming I have understood your description of the unit correctly, I wouldn't expect the change that has been made to make any material difference.

Thermal stores are not particularly compatible with the way we typically operate ASHPs.  That doesn't mean that they cant be made to work, but it does mean you have to think carefully about how you operate them, what your DHW demands and whether these can realistically be satisfied.  In practice therefore they are rarely fitted.  In this case it sounds rather like the careful thought has not been done.  It further sounds like the installer doesn't understand what he is doing, which is also no surprise.

Its possible that if this unit is correctly configured and operated it could be made to work - presumably the manufacturers has something in mind when they came up with this (?), but an installer who hasn't a clue what he is doing randomly replumbing things isn't likely to succeed.  There are several things that might be wrong with the plumbing or configuration that possibly could be fixed, but its equally possible that it is just unsuitable for the use case in question.  

The quick fix, if its an R290 ASHP and the controls will allow, is to increase the target temperature in the store to 70C, or perhaps 65.  An R290 ASHP should be capable of doing that and it will more than double the amount of usable water available.  If its an R32 ASHP then you might try a target of 60C which will still be a whole lot better than 50C.  If its an R410 heat pump you are out of luck, because this older refrigerant cant heat hot enough.

Failing that the obvious way forward is to replace it with a normal UVC.  That will require some replumbing and reconfiguration, and a drain to the outside or a suitable existing drain.

Given how much this unit presumably cost, I would have personally hoped for better service than you appear to be experiencing.  I suggest it may be time to get tough assuming of course that the householder's DHW demands themselves are reasonable, ie that they have not got unreasonable expectations.

 

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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