Not Hot Water In Shower, Taps Are Ok

[CW1]

Hi,
Started a couple of weeks ago, the hot water just in the shower was getting cooler, had to keep turning the end of the shower bar (/valve) to get it hotter. Got to the point I couldn't turn it any more, was at the maximum, & water was barely warm.
Happened to have "a friend of a friend" (a plumber tho' he doesn't really do it full time now) round to change a damaged tap for me so asked him about it. As soon as he turned it on he said the bar was corroded & needed replacing. I was surprised (it was 5 years old) but agreed to change it. 2 days on it's doing exactly the same.
All the taps are fine, I had turned the water temperature down at the boiler tho' the water in the taps was still scalding, but have since turned that back up.
If it's the boiler, wouldn't it affect the taps too, but can't be the shower as that part's brand new.
Any suggestions before I spend a[nother] small fortune on a plumber (or heating engineer ??) ?

[The Builder]

Combi boiler or immersion cylinder CW?

[davebro]

We don't know what your system is. A mixer shower supplied from hot & cold feeds? An electric shower?

What was the "bar" that was corroded?

[CW1]

Combi boiler, mixer shower.
Bar (valve) as in
https://www.screwfix.com/p/mulga-exposed-thermostatic-mixer-shower-valve-fixed-chrome/117fv

[davebro]

Maybe the flow through the shower is not enough to activate the diverter valve in the boiler (which turns on the gas heater) but the flow through the taps is. Could be a damaged (split) diaphragm on the diverter valve. (I'm not a CORGI !)

[The Builder]

Don't you just hate faults like this CW?
It doesn't make any sense if you hot taps are ok.

The only thing I can offer is that there is a partial bung-up in the hot feed to the bar. It only takes a small bit of debris to affect a thermostatic mixer........... unlike taps which are often less affected.

Thinking about Davebros's diverter valve. It's a good thought, but since the taps are fine, then the valve must surely be diverting ok.
Beats me.....................

[davebro]

my diverter valve was damaged but would operate with a full flow of water - with up to 1/2 flow it did not trigger the burner.

[CW1]

The boiler is firing up davebro ... is it a boiler engineer I need then ?

Int it just, Builder :(( One thing after blinkin' another ... is there an easy (ha !) way to flush it ? Annoyed I spent £50 needlessly already, told the guy I thought the valve (bar !) was ok *sigh*

[davebro]

I'm saying "diverter valve" that may not be the correct term - the thing I mean is a diaphragm operated switch that triggers the gas ignition for the hot water.

[CW1]

But the boiler fires up, davebro.

[calmck]

Is the boiler firing up for central heating

[davebro]

ok - go with the builder.

[The Builder]

I think Dave may be referring to the flow valve that fires up the hot water when it senses flow (something turned on.)

To flush......... turn off boiler, remove bar, turn on boiler, and let it run for a while to clear the pipework.
The further problem could be that some debris is already in the body of the valve though.

[CW1]

Sorry Builder, am I being thick ?
How does turning the boiler off / on pump water thru' the shower pipes if the bar's off the wall (ie. I can't turn shower on) ? Would hope no debris in the new bar (yet ?) ...

[CW1]

Must admit, haven't tried the central heating, calmck.

[The Builder]

Sorry.i didn't explain that well.
Turn off the mains water to the boiler to stop it coming out when you take off the bar.
Disconnect bar.
Turn mains water back on to give it a good flush.
Disconnecting the bar will act as though you had turned on the shower.
I'm not convinced, but it's worth a shot.

[CW1]

Ah, that makes more sense. Think I may have sussed it tho'. I saw this in one of the reviews,

"I notice that a previous reviewer referred to the hot input being on the right - well it's on the left as it should be. He must have been trying to fit it upside down!!"

Mine have always been on the right ... & that's how the plumber fitted this new one ! There are no instructions, & I did notice he hadn't screwed it flush to the wall like the old one was.

Builder, can I just reinstall it the opposite way round ?

[The Builder]

That's interesting CW.
I've just watched a Youtube of this shower bar.
The HOT is clearly on the side where the outlet is.............. that's the LEFT as you look at it.

Curiouser and curiouser :o)

[CW1]

It is very strange Builder, mine's definitely aways been on the right.
Had a closer look last night, it's actually the right way round, with the hot input on the right, but the "+" & the "-" are the opposite way round to what I'm used to, so where I would turn up to make hotter I now need to turn down. To turn temperature UP. Cack handed or what ! LOL
One of those 2 + 2 = 5 where the same problem was totally unrelated, but thank you so much for your advice, & everyone else too :D