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What is the relationship between flow rate and system balancing?

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 2 hours ago
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 AndrewJ
(@andrewj)
Estimable Member Member
Joined: 10 months ago
Posts: 86
Topic starter 16/11/2025 12:21 pm  

I've been thinking about this over the last few days, following the latest Podcast where flow rates are discussed - this was mentioned in passing but not followed through unfortunately.

If the flow rate is critical, then how is that affected by balancing the system by turning up/down the LSVs?  Or turning down the TRVs which could prevent any flow through the radiator?  Does it require an adjustment to the flow rate to compensate or does that just unbalance the system again?  What happens if the flow rate is negatively impacted - how do I tell?

Perhaps a heating engineer can discuss this in some detail.  Incidentally, I have no access to my system's flow rate so I can neither see what the current value is nor change it.  But the system does need balancing as upstairs is running 1.5=2.0c warmer than downstairs, and even downstairs room temps span 19.5 to 22.2c.

 



   
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 JamesPa
(@jamespa)
Illustrious Member Moderator
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16/11/2025 12:40 pm  

Im noit a heating engineer, but I will have a go anyway (I think you are overthinking a bit)

 

Posted by: @andrewj
↑

If the flow rate is critical, then how is that affected by balancing the system by turning up/down the LSVs?

If you turn down all the LSVs then you will restrict the flow.  If you turn some up and some down the main effect is to redistribute the flow (thats what you are trying to do).  Overall you want the flow as unrestricted as possible consistent with the system being in balance.  So at least one LSV (often the one furthest from the heat pump, unless that's a towel rail) should be fully open.  Towel rail LSVs should, I believe, always be restricted otherwise its virtually a short circuit.  

Posted by: @andrewj
↑

 Does it require an adjustment to the flow rate to compensate or does that just unbalance the system again?

Not normally.  The water pump will just keep going, the flow rate may change a bit, DT may change a bit and the system will simply reach equilibrium at a slightly different point.  Flow rate is critical, but only in the sense that it has to be high enough to transfer the energy required with a reasonable dt across the emitters, and not so high as to cause noise in the system.  It doesnt have to be a precise figure.

Posted by: @andrewj
↑

What happens if the flow rate is negatively impacted - how do I tell?

Your heat pump will normally monitor flow rate, if it doesnt then you cant easily tell whats happening.  Almost all, probably all, heat pumps will complain and stop working if the flow rate goes too low.

Posted by: @andrewj
↑

But the system does need balancing as upstairs is running 1.5=2.0c warmer than downstairs, and even downstairs room temps span 19.5 to 22.2c.

There is an article on balancing radiators elsewhere on the forum https://renewableheatinghub.co.uk/radiator-balancing-lockshield-valve-guide/


This post was modified 2 hours ago 2 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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