Bath hot tap no hot water

[Plumb Tidy]

Hi all i have an odd one, i have had one like this before but would like to know the reasoning behind it if there are any explanations.
My customer has had a bath installed a year and a half ago but has had no constant hot water through the tap since it came out of the box. She has put up with it for this long as the bloke who installed it got angry on the job and scared her so she never asked him to come back. When first opening the hot tap scalding hot water comes out for around 10 seconds, then it goes cold, it has really good flow over 5 Its/min i ran off to see what the 3 year old worc combi was doing and it was not firing up. All other taps in the house are working good with hot water.

The pipework under the bath tees off from 22mm and reduces to 15mm for around 2 meters in push fit and has an isolation valve half way in the most awkward place known to man. Then goes back to 22mm to the taps.

I had this problem before where the pipework went from 22mm-15mm-22mm. Everyone said the usual thing pinhole in the diverter although after getting Mr Gas safe to change the diaphragm the problem was still there. In the end we changed the pipe work changed the tap as the customer wanted new anyway and it worked. Im not sure if it was the tap or the pipework.

I was wondering if the sudden burst of flow when the tap first opens triggers the boiler to kick in but because of the 22-15-22 the flow and pressure gets quite turbulent and does something to cause the boiler to stop heating? Could it be a faulty new tap? or could it be the boiler playing up?

Access is difficult as ever to diagnose pipework and remove the taps so if there any advice on the matter so i don't waste the customers time and money that would be much appreciated.👌

Also worth a mention if you close the hot tap down so its about 1 -2ltrs a minute you get scalding hot water again

 

[Aquarius]

Is it a bath mixer? When you take it down to 1/2 litres per minute, I take it this is firing the boiler?

 

[Plumb Tidy]

Hi Aquarius, yes mate its a water fall mixer from watersmith. I closed the isolation valve to the cold incase it was passing internally but your right when i barley open the tap the boiler fires up and she gets a hot bath in around 45 minutes

 

[Aquarius]

Obviously something amiss with tap or boiler. When you isolated cold was this at the tap or boiler? Did it fire up as normal?

 

[Plumb Tidy]

i isolated the cold. disconnect the flexi hose to the tap chucked a short length of push fit on aimed into the bath and the boiler fired up and the water was considerably warmer not hot though. I connected all back up as normal, opened the basin hot a fraction to fire up the boiler and opened up the bath hot tap all hey presto hot water?!? Shut off the basin tap and the bath hot goes cold again. I am repiping soon and taking out the push fit elbows and restrictions and shall update the results. If that doesn't work its got to be the boiler

 

[Riley]

If there a faulty TMV

 

[Plumb Tidy]

If there a faulty TMV

Hi Riley, no i completed this job a while back now. I upsized the pipe to 22mm and cut out around 4 x15mm push fit elbows and the 500mm flexi tap connector. I used 22mm gate valve with a pulled bend up to the tap and she got a decent flow of hot water back. Magic.