Posted by: @brightspark

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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.

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.

@mars my understanding is that it was the radiator manufacturer that suggested large bore valves not the installer

Posted by: @brightspark

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

@jamespa, you’re right. I misread that. It was the radiator manufacturer recommending the large-bore valve, while the installer said any standard valve would be acceptable, provided it wasn’t a TRV. Good catch.

Posted by: @brightspark

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looking at most modern 15mm bore valves they actually only have an inlet hole of about 5-6mm if you look into the pipe.

Hole diameter is small but hole length is also very small. So there is flow restriction, but only over a short length, resulting in much less pressure drop than a long pipe of the same diameter.

Caleffi 401402 Right angle 1/2" TRV has Kv=2.7 m3/h which is plenty, no trouble with that. With 1 mH2O pressure drop, when fully open it will pass 0.85 m3/h, and you don't need this much for a radiator.

It is difficult and expensive to make a valve that has both low pressure drop when wide open (which requires a wide valve opening) and good control when partially closed (which requires a small and precisely adjustable opening). If the valve is too big for the application, the range of knob positions that actually does useful flow control becomes tiny and it's difficult to adjust, it tends to behave as either full-on or off.

Besides, pipes have pressure drop too and that adds up to the valve's pressure drop. Once the valve has much lower drop than the pipes when fully open, there is no point in using a bigger valve...