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

DavidB · 2026-02-06T16:10:13Z

I have a concurrent thread running re improving COP. This is a spin off question which I hope is more appropriate as a new topic. i am slowly getting to know my system, installed Sept. last year. During a call to Vaillant today, the tech support indicated that something odd appears to be happening in that the flow and return DT is mostly <5C. My hand on the UFH flow and return suggest the flow is as expected but the return is ambient - cold to touch. The circuit has its own pump, which has a green light so seems to be running. The pic below shows the flow meters - with which I am unfamiliar. My first impression was that the flow is actually zero - which would explain the cold return. Have I interpreted the picture correctly?? Apologies for the poor lighting and hope it is enough. Both meters have the red disk in the same upper position. Any advice would be appreciated.

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Air Source Heat Pumps (ASHPs)

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DavidB

(@davidb)

Trusted Member Member

Joined: 8 months ago

Posts: 71

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)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 5456

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 8 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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