Shower problem with combination boiler

[atticus]

I have a fairly new shower (non-thermostatic) and a Potterton Puma 80e boiler. All was working well till the boiler was serviced. The engineer oiled the switch which trips the hot water on/off (it was a bit sticky and sometimes slow to switch on and off). The boiler is now working fine BUT - when using the shower, the hot water keeps switching off, waiting, then switching on again. This hapens even with shower turned to hottest.

I suspect that the shower valve is mixing hot and cold and the boiler is deciding that hot is not needed all the time. (The boiler has a +/- temperature dial for hot water but this still happens whether set to min or max)

What I can do about this. If I had to replace the shower (worst case)

- what should I replace it with. Thanks.

[Martin Pentreath]

Hi Atticus,

I've just dealt with a similar problem with my shower/combi setup. I had at first suspected the shower mixer, but it turned out to be the boiler's secondary heat exchanger which was at fault. It was scaled up on the secondary side, and also clogged with iron oxide on the primary side. This meant that as cold water was flowing through the boiler to be heated for the shower it wasn't carrying heat away as efficiently as it should. The main heat exchanger would eventually overheat and the boiler would shut off for a couple of seconds. All very distressing at

7am on a cold December morning when you've just lathered yourself up.

I solved the problem by removing the secondary heat exchanger (which was not as difficult as I'd thought it might be) and flushing it through with Fernox DS3 on both the primary and secondary sides. I also drained the whole system down, flushed through the radiators, and made sure that the system had plenty of inhibitor in to stop further iron oxide formation on the primary side.

Does the problem with your system only happen when the shower is running (which would point to the mixer being at fault) or does it happen when other taps are running too?

My boiler is an Ariston Microgenus by the way.

Cheers!

Martin

[atticus]

Thanks for the info. I suspect mine is a shower mixer problem because the boiler works normally (i.e. stays on constantly) when running the hot tap only.

[Martin Pentreath]

Hi again Atticus,

Could it be that (via the mixer) the shower is just not taking enough hot water from the boiler to justify it keeping going, which is why it cuts out? This seems unlikely - for the rate of flow of hot water to be below what the boiler should be able to handle sustainably, the shower must be a pathetic dribble! What is the rate of flow through the shower? If this is the problem maybe change the shower head for one which lets more water through (or descale the existing one). I can't see why else what you're describing would happen. Someone who knows more about boilers than me should be along shortly.

I've got an Aqulisa Aquarian thermostatic shower which works fine (now that the boiler had been overhauled).

Screwfix website tells you whether the various showers are compatible with all plumbing systems or not. However, I'm still not convinced that it can be the mixer itself which is causing your problem if it was working OK until the boiler was serviced.

Cheers!

Martin

[atticus]

Thanks Martin. I don't think it's a flow problem, the water volume is very good. Maybe the water pressure is unbalanced between hot and cold? Do I need a pressure balancing valve perhaps - or can I restrict the cold intake??

This is the exist> atticus wrote:

[John Rumm]

Try running the hot tap in the basin at the same time as the shower. If this cures the problem then it is related to the rate of demand for hot water.

[atticus]

OK. With hot water tap on - boiler stays on but shower reduces to dribble. I tried removing the shower head - boiler stays on no problem. So - even though the shower flow is plenty through the existing head - seems it's not enough to keep the boiler on.

Maybe I'll buy another shower head with bigger holes (the existing one is already opened to max). Any other ideas? Thanks for the help.

[John Rumm]

Ehat about if you then turn the basin tap down until you see enough flow through the shower, does it still stay on?

Have you tried turning the hot water temperature limit on the boiler down so that you are taking the bulk of the shower water form the hot?

A new shower head may solve the problem. However don't discount that this may be a boiler fault - especially if it never used to do it.

Some combis seem to have trouble with showers:

You can get the case where a boiler needs a fairly large flow rate throgh the hot in the first place to turn on. You can also get problems with non modulating boilers and lowish flow rates. The temperature rises to beyond the limit temperature set and the boile (being unable to simplyreduce the output power) has to cycle on and off to maintain its output below the limit temperature.

[Martin Pentreath]

Treat yourself to a new showerhead! The cost is peanuts compared to getting people in to tinker with the boiler, and you'll get a better shower each morning into the bargain.

Cheers!

Martin

[atticus]

Yes

Yes - but it doesn't solve the problem.

I think the boiler is OK - it doesn't cut out when running hot water from the taps.

It always turns on OK - just comes on and off when running shower only.

I'm going to try a new head - see if that fixes it. Cheers

[atticus]

Yes - I'm going to try that. Thanks.

[atticus]

OK - the new shower head (which lets through more water) seems to have fixed it.

I have noticed, though, that when the hot water is running, if you turn the cold tap full on at the same time the boiler switches off. Is this normal?

[Martin Pentreath]

Hi Atticus,

Glad to hear you've sorted it out.

The new problem could be because you have a low rate of flow on the incoming water main. If you turn on a cold tap you divert some of the incoming water away from the boiler, so there's not enough water going through it and it cuts out (just like before with the old shower head restricting the flow).

You could solve it by upgrading the water main - I looked into this, but it would have cost thousands because the water company (Thames) charged some huge sum per metre for laying the new pipe.

Cheers!

Martin