Thermostatic mixing valve problem

A while back a friend asked me if there was any way he could have a warm water supply outside for washing his dog.
As there was easy access to hot and cold supplies inside I fitted one of these. https://www.bes.co.uk/thermostatic-mixing-valve-tmv-2-3-15mm-12162?
Before purchase I checked with the manufacturer that it was suitable for a mixed mains pressure cold feed and gravity fed HW.
It worked fine for several months but then water stopped coming out of the tap. Sometimes if you waited the flow would start and it would produce premixed warm water. Eventually it stopped working altogether.
Thinking that he’d just been unlucky my friend opted to buy a new valve and now a month after fitting the same thing is happening. All rather annoying.
Before fitting the new valve I blew through the non-return valves and discovered that they needed a fair bit of pressure to persuade the valve to “crack open” the first time, after that they seemed to open reasonably easily.
It seems likely to me that the HW non-return valve is sticking on its seat, perhaps because it’s being exposed to reverse mains pressure cold water when the outside tap is off.
Now I’m sure I could disassemble the things and blow through the valve again to free it but I would really rather not have to keep doing this. One thought that occurs to me is that if I could think of a simple way to produce an upstream “pressure pulse” above the non-return valve I could pop it off it’s sticky seat.
Long term answer may be to just go back to external H & C taps but he does like the pre-mixed warm water.
Any suggestions?
Tim
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Please don't feed the trolls

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On Thursday, 10 October 2019 10:08:08 UTC+1, Tim+ wrote:

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So fit a mixer tap without the tmv.
NT
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Prefer not to have a mixer tap outside exposed to the risk of frost. I imagine any internal blender valve (thermostatic or not) will have non-return valves.
Tim
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On Thursday, 10 October 2019 11:15:50 UTC+1, Tim+ wrote:

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is it somehow worse than a single tap exposed to risk of frost?

and any internal blender valve will scale & fail. Make your choice :)
NT
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We don’t get scale, or at least very very little of it. (SW Scotland).
Tim
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I was thinking about scaling. Is it a particular hard water area? Brian
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But is that not the whole point of this exercise, so that you don't need to worry about the temperature? Brian
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On Thu, 10 Oct 2019 09:08:04 +0000, Tim+ wrote:

Fit a shut off valve to the cold feed so that the hot side can start to flow before the cold pressure is added.
That is, turn the mains pressure cold off, turn the mixer on, then slowly crack open the cold mains.
Possibly only have the cold mains on when the shower is running, turning it on after the hot is running then off before turning the mixer off.
This might stop the non-return on the hot side having to open against mains pressure.
A simple gate valve with a handle should do the trick.
Cheers
Dave R
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That *might* help but from having felt how tightly the one way valve can stick to its seat I think the HW pressure is insufficient to overcome the stickiness.
The other problem is that it’s more than a tad inconvenient to have to keeping going inside and outside the house to fiddle with taps.
Tim
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Pressure regulator in the cold feed to the mixer? Something like one of these http://tinyurl.com/y24eahz4 There's even an H&C pressure equaliser there.
--

Chris

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That might be the easiest option. Thanks, I’ll think about it.
Tim
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Tim+ has brought this to us :

What is the height of the header tank, above the tap?
I fitted one on a downstairs toilet wash basin 8 years ago and its been absolutely fine. Header tank is in the loft of my semi. I recently adapted it to provide an outside shower facility for the dogs on my drive. All works great..
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1960s two storey house, header in the loft. All very conventional. The pressure should be right range for the valve and when it’s working, it works fine. It just seems to be the non-return valves causing issues.
Tim
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So remove and eviscerate them, then replace them. Who's to know?
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The homeowner would when his loft header tank overflows. The fact that one seems to be getting jammed, probably due to mains pressure cold water, suggests that without it back-flow through the HW system would occur.
Tim
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