Shower running hot then cold - hot tap running helps tho

[andyvaughan]

Hi,

I have an (old) Worcester Bosch 230 combi boiler downstairs in the kitchen and a Mira one dial type mixer shower upstairs in the bathroom.

Of late, after turning the shower on, the water heats up for a few minutes (the boiler fires up) and then the boiler shuts off and the water goes cold. The hot water doesn't return unless the shower is switched off and on again after a few minutes.

When the hot tap is run (as a hot dribble) in the upstairs bathroom, the shower behaves itself and the temperature remains constant.

Would anyone happen to know any possible reasons for this? The pump and the hot water/CH valve) on the boiler has been replaced (approx. one year ago). No other plumbing works have been done excepting a recent shower head replacement (not like for like). I don't think the new shower head would benefit from a flow reducing device as it does not exceed the flow test in the free eco kits you can get.

Any ideas?

Thanks in advance,
Andy

 

[WabbitPoo]

Does the same thing happen when you just run a hot tap say, in the sink, without the shower running?

 

[andyvaughan]

Now that's thrown me a bit. The hot tap when run in the bathroom stays hot.

Knowing next to nothing about how boilers work, I logically expected the boiler to shut of regardless of what was drawing the hot water upstairs.

Is the shower heated off something separately to the hot water in the bathroom?

Any ideas?

 

[easyflow]

Take the shower head off so you just have flow from pipe first of all.
See if this makes a difference, just the first step to elimination.

 

[andyvaughan]

Hi, I've tried running the shower without a head on it but it still behaves in the same way.

Someone mentioned a secondary heat exchanger being scaled up. It's a very hard water area here. Could that be plausible?

Thanks,
Andy

 

[Cookingongas]

Check the filters in the shower they will probably be scaled up.

 

[MJN]

Hi Andy,

As the others have said there can be a number of potential causes of this symptom however a common one with diaphragm-driven flow detection mechanisms, as found in your boiler, is that the diaphragm has perished.

When you open a hot tap (or the shower) the resulting flow of water through a diaphragm in the boiler shifts it sideways such that a microswitch is activated (to trigger the boiler) and primary water diverted to flow through the secondary heat exchanger (to heat the water up).

The diaphragm, located inside the diverter valves, can be prone to perishing over time with the resulting holes in it making it less responsive to the flow of water through it. Or, to put it another way, more water flow is required in order to keep the diaphragm sufficiently shifted to keep the boiler running.

A running shower draws only a finite level of water flow hence the perished diaphragm may fail to fully operate. Opening the hot water tap in tandem draws a greater flow hence resulting in a more positive diaphragm shift, and constant boiler operation.

Repair is achieved through either i) replacement of the diaphragm (~£15), or ii) replacement of the whole diverter valve (more expensive but slightly easier and possibly wiser on a boiler that age to minimise risk of eventual failure of that part for other reasons).

Mathew

 

[MJN]

<Duplicate post deleted>

 

[andyvaughan]

Thank you for all the info. It really is much appreciated. I have a chap coming over tomorrow to look at it so at least I'm armed with some useful info to aid diagnosis.

Thanks again,
Andy

 

[andyvaughan]

Hi,

So the plumber came and unfortunately due to the temperamental nature of the problem, he couldn't replicate the issue.

He did go away and consult the Worcester Bosch technical support line to see what he could find out. I wasn't charged for the call out.

I spoke to him today and he has two lines of attack. The first is to replace the diaphragm in the diverter valve. This ought to fix the issue of the shower not firing up the boiler on occasion.

The second line he said is to diagnose the state of the heat exchanger. He said it could be scaled up. When the shower is on, the boiler starts to cycle on and off. The demand light stays on during this cycling. Sometimes the boiler will cycle and then go off even though the shower is on and attempting to draw hot water.

Would a furred up heat exchanger cause these symptoms? Shower on - boiler cycles on and off for a period before switching off for a couple of minutes. Then a hot tap is required in order to fire the boiler up again and the shower gets hot.

Any thoughts/advice much appreciated!

Thanks,
Andy