The mystery continues

[conny]

During these 30 seconds, is the water still hot or has it turned cold?

Are any of your 'upstream', (i.e. near the boiler), taps, mixer type which may have a built in TMV that is not obvious without investigation.

 

[lockett]

The water runs hot gradually turning cold, then suddenly stops after the 30 seconds.

 

[oldbutnotdead]

Yes, with the boiler cold water supply isolated, hot tap water runs for 30 seconds, then just stops, with no more water flowing.

Is that water from the hot tap or hot water from the hot tap?

EDIT Slow browser, sorry- you've already answered that one.
Just to confirm, when you did this test only cold water was coming from the hot tap UNTIL the cold water supply to the boiler was isolated?

 

[lockett]

The sequence was
Turn off the cold water supply to the boiler,
Run the hot tap, water runs hot gradually turning cold, stops after 30 seconds no further water coming from the tap.

 

[oldbutnotdead]

What sort of shower is it? What exact part of it was replaced? Who replaced it? If it's easier, take a pic.
Bath mixer taps- can you get a pic of what's under the bath?
Plan C is some major bodging on the cold water supply so you can investigate properly before having to lift floorboards.
Again, when was the place built /bathroom last refurbed (if it was before 2010 there's unlikely to be a TMV installed).

 

[lockett]

I fitted a brand new Mira Atom EV Thermostatic bar mixer. The bath tap is a none mixer. I have attached pictures from under the bath. The bungalow was built in 1980. We have only lived in the property for just over a year, so can't say when the bathroom was installed.

 

[oldbutnotdead]

I fitted a brand new Mira Atom EV Thermostatic bar mixer. The bath tap is a none mixer. I have attached pictures from under the bath. The bungalow was built in 1980. We have only lived in the property for just over a year, so can't say when the bathroom was installed.View attachment 301864View attachment 301865View attachment 301866

Can you point your phone a bit further up and down so the supply pipes are visible. Waste pipes don't look 40 years old....
How much of a pain would it be to remove the shower bar and replace with some temporary stopends or bits of pipe with ball valves? Is the shower the only mixer tap in the house?
Primary job for TMV is to prevent scalding in the bath so it should (it it was there) be in the bath supply pipework

 

[lockett]

Unfortunately I have resealed the bath panel (maybe to premature) but I can follow the pipes from the floor to the tap inlet, and it's a straight run without any other pipe work attached. I don't have any fittings to blank of the shower. Do you think it's likely that a brand new shower from one of the top makes would have bypass that would allow the hot tap next to the boiler to run for 30 seconds at 5 sec/1L when the water supply to the boiler it turned off.
I have two other mixer taps in the house, but the engineer in his test ruled them out, isolated them for the test.

 

[oldbutnotdead]

Has the shower been installed backwards? (Relying on the old shower being installed correctly might not be wise).
How good is your cold water pressure- has it increased recently?
Have you tried running with the main stoptap partially closed?
IF all other mixer taps have been eliminated then the shower is suspect until proved otherwise- we know the shower exists, the TMV is an unproved hypothesis.
Remove the shower bar, acquire some fittings so you can have water on without it firing continually out of the shower supply pipes, try it.

EDIT Ideally your temp shower lash-up would allow you to individually turn on hot and cold at the shower position. This will also let you positively identify hot and cold at the shower

 

[lockett]

When I fitted the shower it was obvious which the hot and cold supply pipes were, l could feel the temperature difference quite easily.
My water pressure is very good, so the stop cock is throttled back.
I will look into getting some fittings for the shower test.

 

[oldbutnotdead]

When I fitted the shower it was obvious which the hot and cold supply pipes were, l could feel the temperature difference quite easily.
My water pressure is very good, so the stop cock is throttled back.
I will look into getting some fittings for the shower test.

Fair enough. If water pressure is high (like 5 bar plus high) you might want to look at a pressure reducing valve (throttling at the stoptap has a slightly different effect- reducing the flow rather than the pressure). If your combi is standard, the dhw heat exchanger may be causing a significant pressure drop in the hot supply, if the pressure differential is significant at the shower mixer (or any stealth TMV) it can cause problems.

 

[lockett]

No reduction in the hot water through the boiler, really good flow from taps and shower.

 

[Johntheo5]

Most if not all combi boiler with a preheat function come with a small expansion vessel on the HW supply but this should only give a slight increase in pressure when you initially reopen a hot tap almost immediately after just closing it, a litre or two at a higher pressure.

 

[lockett]

Unfortunately the hot tap runs for 30 seconds
At a flow rate of 5 sec / 1L = 6 Ltrs which is 3 to 6 times the volume of the pre-heater vessel.