Tepid water from hot taps unless another is running

[BillyWillySilly]

We have an odd issue where our shower room (closest to the boiler/hot water tank) does not turn hot unless there is another hot tap further down the system running. Daughter went for a show last night and said there was no hot water and I checked if the tank was warm to the touch - and it was, and tried running water from various taps. I assumed she'd been draining the system and we left well alone, waiting for the night to come and early morning heating. I did notice that all the surorunding pipework was hot though ie rads. With the hot weather, none of our radiators would come on so I'm not sure if there is heat leaking.

I tried taps at 6am and got hot water in the shower when on full whack so I assumed all was ok. Just tried to get in the show now and noticed that to was back to being barely tepid. If I run the further bath tap to a trickle, shower gets hot though.

Coincidentally we had a boiler service yesterday but Im not sure if that was the cause as my wife is reporting tepid showers since the hot weather.

What could it be? Is this the diaphragm and venturi issue?

 

[Alec_t]

Boiler make and model?
Faulty flow sensor?

 

[Deleted member 267285]

If you have a tank, check the cylinder thermostat is to correct setting.

 

[BillyWillySilly]

I did check the cylinder thermostat and that is part of the problem. Just had an engineer out who noticed the boiler temperature was lowish. When he turned it up the problem disappeared. Apparently, this is to do with the relative temperature between the heat that the boiler sends, and that of the cylinder. Essentially, the cylinder was saying it was hot enough.

However, the engineer noted we had a shower pump driving the hot water from the cylinder and not only to specific showers/bath taps. And it is not kicking in on all taps so hot water delivery is variable at best. He suggested a new tap might help. But to sovle the bigger problems is a rethink on the entire system apparently.

 

[BillyWillySilly]

Two engineers in and there is a bit of confusion. The second engineer thinks the boiler and tank are a red herring and spent far more time here looking at different options. Cylinder temperature is @70 now, and the boiler seems to be doing fine at heating it up. When hot water is called, the outlet pipes are very hot but some of the taps not.

Apparently, the problem is one of, or a combination of:
1. The pump: It's a 2013 made Monsoon Universal 2.0 bar single impeller. It's not kicking in on all taps e.g. the shower mostly does kick it in, but recently not enough of a flow, but if I added a sink tap the shower then works well.
2. A hidden valve: he isolated the pump and water still comes out of the hot tap. The strength of the flow varies in the house but there is free flowing water regardless. Potentially there is an anti-scauld valve somewhere but it is not visible and I suspect under the tiles of our shower room.

He suggested I try replacing the pump as that is the only thing that can resonably be done right now. However, that won't be a cheap fix at around £200. The call out was £99 so this punt that may not cure it is a bit of a risk.

 

[BillyWillySilly]

This did not fix the problem - new pump fitted and whilst more powerful it does not always trigger. But I think I've figured out why and for which I will start another thread.