Bath/Shower Mixer Taps driving me mad, please help!!!

[Berni967]

I currently have a single lever mixer on my bath being run off a combi boiler which is driving me spare. When using the tap I can get a reasonable temperature but when I switch to shower its either scalding hot or luke warm, after about 2-3 minutes the hot water cuts out and its freezing cold again, this happens several times during a shower. I have a Worcester Greenstar 24i Junior combi mkIII boiler and at the moment the pressure gauge is quite low at less than 1 bar, the pressure keeps dropping after topping up. Thought a larger bore hose would help but not sure which size to get or a low pressure shower head. Any help or suggestions would be greatfully received, my son is doing my head in keep shouting about how woeful the new shower is, had a push on to taps shower before which kept going cold as well thought the mixer would help what with the connectors blowing off the taps all the time.
Thanks
Berni

 

[SilentMike]

low pressure will cause intermittent hot water

 

[LegionellaSlave]

Sounds like a faulty 'stat in the tap, assuming the tap is thermostatic and suitable for a pressured system. :welcome:

 

[jdstratton@btin]

It could be a problem with the boiler plate heat exchanger . Does this happen on your basin or sink taps ? I do not know this particular boiler but it could be a blocked heat exchanger if it happens on other taps around the house . If the plate heat exchangers are blocked the boiler burner will keep going on and off as it thinks its overheating .
Not something to mess with . Need a Gas Safe Engineer . Hopefully its not this but it could be if all taps are the same .

 

[Berni967]

The taps are a bath/shower mixer no thermostat.

The hot water only cuts out when using the shower the other taps work ok.

The pressure is not very good in the bathroom as the hot water is from combi and cold water from tank in the roof space of flats, I'm on the third floor.
Thanks for the help

 

[LegionellaSlave]

Berni has it always been like that?

 

[Phil]

Pressurised hot and tank fed cold? imbalanced feeds = poor shower

whats the temp like if you run the shower with the head off?

 

[jdstratton@btin]

Try running the shower and listen to the boiler to see if the burner keeps going on and off . Worth a try .
Having water into the shower head at different pressures will cause an inbalance problem as advised but cannot see why your water would go hot / cold . Does it stay hot if you just open the hot tap ?

 

[Berni967]

With my old boiler the water pressure was worse so was unable to have a shower. With this new boiler a shower has always been a problem because of the problem with getting a usable temperature and the boiler cutting out all the time.

 

[Berni967]

The temperature through the tap is ok and the boiler doesn't cut out, the temperature through the shower hose with the shower head off is ok and the boiler doesn't cut out. The only time I have problems is when the shower is on the wall and running, then there are temperature problems and the boiler cuts out.

 

[mountainman]

Definitely sounding like a pressure/flow imbalance.
Dunno if I missed it in one of your posts but are you sure that the cold supply is off the mains?

If it is then you may need to fit a pressure reducing valve to provide a balanced supply of cold to the combi and shower.

 

[Masood]

When using the tap I can get a reasonable temperature but when I switch to shower its either scalding hot or luke warm, after about 2-3 minutes the hot water cuts out and its freezing cold again, this happens several times during a shower ... at the moment the pressure gauge is quite low at less than 1 bar, the pressure keeps dropping after topping up.
Berni

The hot water only cuts out when using the shower the other taps work ok.

The pressure is not very good in the bathroom as the hot water is from combi and cold water from tank in the roof space of flats, I'm on the third floor.
Thanks for the help

temperature through the shower hose with the shower head is ok and the boiler doesn't cut out. The only time I have problems is when the shower is on the wall and running, then there are temperature problems and the boiler cuts out.

Sounds like a combination of two factors - partially blocked plate to plate heat exchanger causing boiler to overheat and cycle on/off when hot water is run. This would also explain the system pressure dropping after topping up - the sludge would cause the system to over-pressurise and blow out of the pressure relief valve.

Under any other conditions than the shower being used, there is enough flow for the boiler not to overheat, but when the combination of A) the shower head being on the wall (added height, greater gravitational force to overcome), and B) the smaller bore of the shower hose, the flow of water is reduced enough to make the plate to plate heat exchanger so hot that the boiler cuts out.

Having mains hot water and gravity-fed cold, and being on the 3rd floor, so having very little gravitational head will not help matters, as the pressure imbalance will make it very difficult to control temperature.

 

[Berni967]

My cold supply is from a tank in the roof space of flats, i'm on the third floor

 

[mountainman]

My cold supply is from a tank in the roof space of flats, i'm on the third floor

Problem solved, pressure imbalance.

 

[Berni967]

Is there anything I can do to improve the pressure imbalance, would a large bore shower hose help or a shower head for low pressure. Is there anything a plumber from the council could do to improve matters. Thanks to everyone for all your help its much appreciated.

 

[Mike Jackson]

Ideally you need to plumb the bath cold to the cold main so that you've got balanced pressures. You could possible put a pressure reducing valve on the hot but this could cause further issues with the boiler.