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Next steps on my ASHP journey: Radiators and Pipework

 
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Air Source Heat Pumps (ASHPs)
Last Post by bobflux 59 minutes ago
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JamesPa
 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 5339
02/09/2026 7:21 pm  

Posted by: @rob-of-york
↑

Do you know if adjusting the flow rate used is possible in that way

I don't for certain.  There are a couple of possible installer parameters namely

conf heat build pump, unit %pwm, default auto, but can be set 50%-100%

This is the most likely 

There is also max rem feed head, unit mbar, default 900, can be set between 200 and 900.

 

I imagine one of these does the trick but you would need to check with valiant tech support or a vaillant installer to be sure.


This post was modified 1 hour ago 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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bobflux
 bobflux
(@bobflux)
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Joined: 6 months ago
Posts: 168
02/09/2026 7:45 pm  

Posted by: @rob-of-york
↑

What I think I would need is the constant flow rate used turning down to about 0.25l/s, which corresponds to 7kW at DT7. That looks like it would then work with my mix of 22mm and 15mm pipework. Do you know if adjusting the flow rate used is possible in that way?

Flow rate is set by pressure created by the circulator versus resistance from pipes, valves, etc. On most heat pumps, circulator speed is adjustable, so you can always lower the flow rate. However, the max flow rate with the circulator set at maximum speed depends on your pipes, and there is no way to go beyond that, except either bigger pipes or a stronger circulator. But resistance (pressure drop) is proportional to speed squared, and circulator power consumption to pressure drop times flow rate ie a power of 3... so when the pipes are too small, fitting a bigger circulator doesn't really help.

If you have pipe lengths and diameters I can run the numbers.

Posted by: @jamespa
↑
Some heat pumps modulate the flow rate to achieve a constant DT (no idea why) but not Vaillant, at least not by default.

Mine (LG) can be configured in this mode. I think the idea is to reduce circulator power consumption when it runs at low modulation. On my installation, it doesn't work: when deltaT is too low, it reduces circulator speed to increase deltaT... but the flow reduction causes output temperature to rise (power = flow x deltaT) so the regulation lowers compressor rpm to keep output temperature on set point. So heating power decreases and... deltaT is too low again. It goes in a loop and ends up meeting the requested flow temperature while getting stuck at minimum heating power and very low circulator speed. So if the heat pump is configured to regulate based on flow temperature, this mode is completely useless.

It has a mode that regulates based on air temperature, so perhaps it would work there.

 

 

 



   
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