Thermostatic shower firing boiler up when not in use

[burmesterbob67]

I have a Swirl thermostatic mixer shower bar, running of a Vokera combi boiler, and at odd times the combi boiler fires up as if the shower has been turned on, and the shower bar is 'hissing' and hot to touch. Even though there is no flow through it. Even happens in middle of the night. I notice that if I turn the mixer valve from cold to hot the 'hissing' starts up even though I have not tuned on the flow control.

 

[moggridgeplumbing]

Hope you haven't got a leak behind the hot inlet to the shower valve!!!

 

[burmesterbob67]

No have checked all that, no leaks, and only occurs when mixer is set at over 25 degrees. Cant see why the boiler thinks water is required when the flow valve is still closed ??

 

[moggridgeplumbing]

I can only think that the hot water is flowing back through the cold inlet, which is setting the boiler off. But as they both will be under the same pressure I cannot see why. The cold feed is off mains I assume?

 

[burmesterbob67]

Cold water is off header tank in loft, left there by central heating installer, hot feed is from boiler, sometimes I also get warm water for a while from the cold tap on the sink ?, although i did not alter the feed at all when installing the shower, just took extra feed from the bath taps to feed the shower mixer unit.

 

[moggridgeplumbing]

BINGO! I'm surprised you haven't had the header tank overflowing. Are you sure the cold feed is coming off a header tank or is it a normal 50gallon cold water tank your talking about?

If cold feed still coming from tank then you need to drain down tank and divert mains water to the cold water downfeed.

 

[burmesterbob67]

The tank in the loft is an open top very old metal tank with a ball valve, now you have mentioned it we have had trouble with the overflow recently, and had already replaced the ball valve. Not thinking this was related to the shower, we had a plumber in today and he reset ball valve, but did suggest that we no longer needed the tank due to the combi boiler.
How does this affect the shower though, and where is the water going that I assume is being pumped through the boiler when this problem occurs. Especially as all taps are turned off at the time. You have been extremely helpful already, but would love an explanation.
Also, on a seperate matter, sometimes when using the mixer on shower, it becomes extremely hard to turn back to a cooler temperature, I have tried loosening, but after a few uses its back again ?
Thanks

 

[moggridgeplumbing]

Simple - equal pressure main 3bar - tank 0.25bar water from mains stronger so is forced up the cold side. All shower valves need equal pressure hot and cold.

Get rid of the tank and all your problems will be solved.

 

[moggridgeplumbing]

You cannot turn the shower back to cold because the hot water pressure is stopping the cold coming through.

 

[PEDANTICVINDICTIVEMAN]

Methinks that whoever fitted the combi didn't know what they were doing, cowboy gas fitter springs to mind, have you got a landords gas safety certificate for this place ?

 

[burmesterbob67]

Thanks everyone, seem to have solved my problem, tank is going.

We do have a landlords gas safety certificate, and annual check from British Gas to boot.

 

[moggridgeplumbing]

Great let us know how everything is after tank gone.

 

[burmesterbob67]

hi again, do you think i should have some sort of non-return valve on the cold feed from the tank

 

[moggridgeplumbing]

NO - get rid of the tank.