Just had a call from my daughters who are complaining that the shower in their flat is running hot and cold.
The boiler is a bosch worcester and the shower a mira excell mixer. I told them to turn on the bath hot water tap and see if it's hot and cold as well and it was. However if you turn of the shower and then turn on a hot water tap you get constant running hot water.
The boiler was serviced in june before we bought the flat and has performed ok up until now.
Any ideas or pointers welcomed thanks in advance.

flush the toilet just before you get in the shower it works

But why

it might just be the seals on the mixer, the exact same thing happend on mine, is it the one with the ring round the outside you twist to turn it on then the dial in the center to adjust hot/cold? if it is tell them to press hard on the right most edge of the ring and this helps push the cold water stop further in sealing it up better.
this will at least help until you can get the until replaced.

That all said, this is just what happend with mine, sounds roughly like the same symptoms.

Yes thats the type Gav. I'm well aquainted with them we have 2 in our house like this. So I'll strip them down no probs and try that.

my boiler runs cold when i try to draw hot water too fast. Its just life with a shite boiler IMHO, and replacing it is high on my list of things to do!

There are a few possibilities..........

If they are running the shower with the central heating "on", and running the shower for a long time, every so often the boiler may bias the output to the radiators if the return water temp gets too low.


Another possibility is that, if the shower flow is too low, the hot water circuit can overheat periodically and the boiler will shut off the heat for short periods. If the tap flows more than the shower then that could well explain it. My boiler does this in summer when the radiators are off so there is no other heat exchange other than the hot water.


As Joel says, combi boilers can struggle to keep up at the opposite end of the flow range, but that results in tepid water rather than going hot then cold.

hmmmm, just read your question again (properly!) and I think Gav is right, it's the seals in the mixer.

If it were the reasons I gave, turning on the bath tap would stop it even with the shower on.

Well it'll be the weekend before I can look at it anyway so keep the suggestions coming please.
Jeez I hope it's simple cause parking's a nightmare and at weekends all the wholesalers are shut.

Well I'm just back from having a look and after going through the installation manual and check list I'm not to much sure.
The boiler is heating the water OK and actually faster than the shower can use it,however there is a part during the test that says to reduce the water flow and see if the gas flow reduces and the temp stabilises at 58 degrees. It doesn't seem to do this and through the burner window I can't visually see the flames change height or intensity. At this point it says to check the resistance of the modreg solenoid coil and it's spot on. It also says to check the spindle rotation which I cant seem to do as there is a screw with a red head and a gas pressure set at factory sticker on it (so I left it alone ). The hot water sensor is working within its resistance range as well. So I reckon It's either this gas valve spindle which will need a corgi reg or the main driver board is not telling the gas valve spindle to cut back on the gas to stabilise the temp.
The only thing I ain't done is take the control panel to bits and check all is OK there ( job for next weekend prob).
Would anyone care to pass coment on my findings sorry about the long post folks.

By the way is the spindle on the gas reg valve like a stepper motor type thing and would i be able to monitor/meter it to see if the driver board is sending it a signal.

[Edited on 4/12/05 by omega 24 v6]