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Radiator Valves Bore

Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

Last Post by Mars 5 minutes ago

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BrightSpark

(@brightspark)

New Member Member

Joined: 7 months ago

Posts: 1

Topic starter 31/07/2026 3:13 pm [#3119]

Hi All could do with a little help, we have had a Valiant Heat Pump installed all working ok, no buffer tank. We now are ready to fit a radiator in our newly completed bathroom, we have bought the radiator but need help with the valves as i understand Heat Pumps require maximum flow with little restriction, on looking at most modern 15mm bore valves they actually only have an inlet hole of about 5-6mm if you look into the pipe.

Have asked our original installer and they said any valve is ok just not TRV in Bathroom due to Heat and moisture.

Asked manufacturer of radiator, who makes valves, they say you need a wide bore valve with a Heat Pump but they do not make them.
I am confused if Heat Pumps should be used unrestricted what type of valves are people using as the wide bore valve is not common, is it correct to use the standard valves with a 5-6mm inlet.

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 5227

31/07/2026 3:27 pm

Posted by: @brightspark

↑

am confused if Heat Pumps should be used unrestricted what type of valves are people using as the wide bore valve is not common, is it correct to use the standard valves with a 5-6mm inlet.

Normal valves on mine no problem. You will likely end up restricting quite a few on the lsvs to balance anyway.

Recommend to ignore.

Btw you don't want to be using trvs in more than a minority of rooms, if at all. No harm in fitting them, just remove the heads or turn up to max then balance on the lsvs. Or replace the trv heads with decorators caps and adjust to balance (easier adjustment than with many lsvs). Remember that the objective is low and slow, operating at min ft consistent with heating your house.

This post was modified 56 minutes ago by JamesPa

This post was modified 53 minutes 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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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4781

31/07/2026 4:27 pm

I may be oversimplifying it, but I don’t think "heat pumps need maximum flow" necessarily means every radiator valve has to be completely unrestricted.

My understanding is that the important thing is maintaining enough flow through the system as a whole. Individual radiators are still normally balanced, which means some restriction is introduced deliberately so one radiator does not take more than its share.

I also wouldn’t assume the small opening you can see inside the valve automatically makes it unsuitable. The actual flow capacity of the valve is probably more important than the visible bore, and a bathroom radiator may not need a particularly large flow rate anyway.

The point about avoiding a TRV in the bathroom may be less about heat and moisture and more about making sure one radiator remains permanently open. With no buffer tank, the installer may be relying on that radiator as part of the minimum-flow route, especially if other radiators have TRVs.

I’d probably go back to the installer with one very specific question: Is this bathroom radiator intended to remain permanently open to help maintain minimum flow through the heat pump system?

If the answer is yes, then ordinary manual valves would seem the obvious choice, with the lockshield used for balancing. If the answer is no, then I’d ask them to confirm what valve specification they actually want rather than relying on the visible hole size alone.

So I’m not convinced you automatically need a special wide-bore valve, but I would want the installer to confirm whether this radiator has an important hydraulic role before fitting anything.

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