Thermostatic mixing valve issue

[Gossamer]

Hi,
Wasn’t getting hot water out of a hot tap where there’s a thermostatic mixing valve so assumed it had failed.
Replaced it and still have an issue.
I only get hot water when the cold tap is on as well as the hot, not just the hot.
There are separate hot and cold taps, not a mixer.
Help!
Ta.

 

[Johntheo5]

If both hot and cold are on and presumably you are getting a mixed flow at ~ 50C?, if you then suddenly shut off the cold only, does the hot water continue to run for a number of seconds and then shut off?.

 

[CBW]

What type of mixing valve? What type of hot water system? Do you have hot water everywhere else? Also need to answer John’s question.

 

[Gossamer]

If both hot and cold are on and presumably you are getting a mixed flow at ~ 50C?, if you then suddenly shut off the cold only, does the hot water continue to run for a number of seconds and then shut off?.

Thanks.
The taps are all separate hot and cold.
If I run the hot tap its cold.
If I turn off the cold feed (via the stop tap) the water gets hot. Likely 50 degrees but I don’t have a temp gauge.
Strangely if I run the hot tap normally (with the cold feed on) it’s cold but if I then also run the cold tap it runs at say 40 ish degrees.

 

[CBW]

Sounds like you have an open vented conventional system, and sounds like a check valve has failed, or there’s a situation where the cold is pushing the hot out of the way. Have you had any other work carried out, for example a new kitchen sink tap?

 

[Gossamer]

What type of mixing valve? What type of hot water system? Do you have hot water everywhere else? Also need to answer John’s question.

Thanks.
It’s a toolstation made 4 trade.
I think the water is gravity fed.
The bathroom hot tap and shower are cold BUT if I then turn the kitchen hot tap on all of them get warm, say 40 degrees which is what I expect.

 

[Gossamer]

Sounds like you have an open vented conventional system, and sounds like a double check valve has failed, or there’s a situation where the cold is pushing the hot out of the wa. Have you had any other work carried out, for example a new kitchen sink tap?

I can’t see a check valve.
No work other then swapping thr kitchen tap glands but there was this issue before.
I did think the cold pressure might be overpowering the hot but I throttled the cold feed back and it did nothing until it was off and hot water came through the valve unmixed.

 

[CBW]

Do you have hot water to everywhere else? Are you able to bypass the mixing valve for now and see if that works?

 

[Johntheo5]

So looks like gravity hot and mains cold? Would it be possible to install a PRV somewhere ? If it got the cold down to say 1bar then with hot at ~ 0.4bar, mixing might be allowed.
Or, one of these, pressure equalizing valves.

Deva PEV002 Pressure Equalising Valve​

 

[Gossamer]

Do you have hot water to everywhere else? Are you able to bypass the mixing valve for now and see if that works?

Not in isolation, they only work strangely when the kitchen tap is on. Should be a simple system but just isn’t working. It’s an old system so swapping out the valve should have sorted it.

So looks like gravity hot and mains cold? Would it be possible to install a PRV somewhere ? If it got the cold down to say 1bar then with hot at ~ 0.4bar, mixing might be allowed.
Or, one of these, pressure equalizing valves.

Deva PEV002 Pressure Equalising Valve​

Yes I think so.
Maybe could fit a prv but I would have thought throttling back the cold would have got it working to an extent but it did nothing. It’s an old system so an issue like this would have materialised by now I would have thought.

It’s a head scratcher!

 

[Johntheo5]

I don't think throttling is much use because until you actually get a (mixed) flow then the cold pressure will remain exactly the same as unthrottled, you may/can prove it by shutting off the cold water completely to get the hot to flow like previously then just crack (barely) open the cold and see if it then mixes with the hot to reduce the temperature.
Its a old system but maybe the mains pressure has increased or, more unlikely, there is a PRV installed somewhere that has now gone faulty?.

I have never seen any numbers re the maximum cold/hot differential pressure that will still allow a thermostatic mixer to work but would have thought maybe 0.5 to 1bar.

 

[Gossamer]

I don't think throttling is much use because until you actually get a (mixed) flow then the cold pressure will remain exactly the same as unthrottled, you may/can prove it by shutting off the cold water completely to get the hot to flow like previously then just crack (barely) open the cold and see if it then mixes with the hot to reduce the temperature.
Its a old system but maybe the mains pressure has increased or, more unlikely, there is a PRV installed somewhere that has now gone faulty?.

I have never seen any numbers re the maximum cold/hot differential pressure that will still allow a thermostatic mixer to work but would have thought maybe 0.5 to 1bar.

Thanks. Prompted by these posts I’ve been playing with the cold feed and it’s pretty high pressure so small changes seem to make a big difference.
I’ve found a sweet spot whereby the TMV ‘seems’ to be doing its job. It kills the cold tap pressure though.
Thinking I might install a PRV on the cold feed into the TMV so I can leave the stop tap open and fine tune the cold feed into the TMV.
I suspect the pressure differential is more than you suggest.
There are no other valves hidden away so it must either be a faulty TMV (which I might have proven isn’t the case?) or I need a PRV on its cold feed.

 

[Johntheo5]

Ensure that you can get a PRV that can reduce as low as possible, say to 0.5 to 1 bar, also ensure that its "drop tight" which means it is completely leak past proof even with no demand, otherwise you might be back to square one if no demand for hot water for even relatively short periods, don't think its a faulty TMV.

 

[fixitflav]

I can’t see a check valve.
No work other then swapping thr kitchen tap glands but there was this issue before.
I did think the cold pressure might be overpowering the hot but I throttled the cold feed back and it did nothing until it was off and hot water came through the valve unmixed.

Where is the mixing valve? Can you post a sketch?

 

[dilalio]

Do you have an airlock in the hot supply? Or even a foreign object that has been sucked down from the cwsc.

Something has changed in the pressure differential. Either hot has reduced or cold has increased or valve to equalise has failed.