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Vaillant Tech Support Flagged Low DT: UFH Return Cold, Suspected Zero Flow

 
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Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 6 months ago
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 DavidB
(@davidb)
Trusted Member Member
Joined: 7 months ago
Posts: 69
Topic starter 09/02/2026 2:11 pm  

@bornagain Thanks for the comments and comparison with your own system.  My understanding is that the flow rate should automatically adjust to provide a DT of 5.  Like you, my system is running ok and perfectly warm.  The reaction I got from the technical advisor suggested all is not well with my system.  I have no comparisons (other than yours) on which to draw a conclusion.  At least, pumping faster than required will consume more power and presumably reduce the COP.



   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 5269
09/02/2026 3:21 pm  

Posted by: @davidb
↑

least, pumping faster than required will consume more power and presumably reduce the COP.

Posted by: @davidb
↑

As I get to know my 3.5kW Aerotherm instal I have had a number of calls with Vaillant tech support.  A recent call turned up info that the flow and return DT was about 2C.

It will consume more power in the water pump but it is more likely to increase COP than decrease it, because it means that you need a slightly lower flow temperature for any given emitter output.  Thus there is a tradeoff.  The folks on openergymonitor reckoned that the optimum for the 3.5kW scenario they modelled was about 2C DT as it happens, but the effect is small as long as you dont go too far above DT5.   DT5 is not sacrosanct, its just a design assumption used to size things at design time, actual operation will always differ!  Significantly higher DT is certainly bad news, lower DT (provided you are getting sufficient output) not.  Max flow rate is also not a problem unless there is excessive noise.

With radiators (which I dont think you have), there is an argument for not modulating the pump speed as the resulting output curve combined with the non linear response of the radiators to DT (rad to room) more closely follows the 'ideal'.

Even though I have a Vaillant I have been unable to determine for certain whether it does bother modulating the water pump or not. 

 

In summary unless any of this is actually causing a problem I wouldn't worry about it!  The Vaillant heat pump is pretty good at sorting itself out so long as there is sufficient flow rate.


This post was modified 6 months ago 5 times by JamesPa

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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