boiler doesn't fire up when hot tap is on

[skipster1978]

when my bath/shower mixer is turned on the boiler doesn't kick in
all the other taps in the house work fine
changed the mixer tap about three months ago and has worked well until today any help please.

 

[Netdragon]

It would be the flow rate through the shower unit.

The boiler is designed on average to have at least 4 l/m to activate the flow switch. it could be blocked.

Try this jst to give your self an idea of the problem turn shower on fully for hot water, turn the bath hot tap on and see if the water gets hot if so ok turn bath hot tap off then turn the cold on (this steals it from the shower).
if the shower gets hot the valves gone in the shower and cold water is seeping into the hotwater and blocking it. and at the same time u get a cold shower.

 

[skipster1978]

changed back to a normal taps and boiler kicks in when open
must be the mixer shower taps i fitted or boiler slowly packing in
can i fix the mixer taps or do i need to fix the boiler
it seemed to work fine for around two months.

 

[Netdragon]

firstly check if the mixing taps are made for combi boilers some are not that may be why its failing, also the mains is a higher pressure then the dhw supply due the fact it has all the boiler to work its way through.

A way to resolve it is to install a combi approved mixing tap, replace the mixing valve cartridge that could be about 200 pounds depending on make or as you have the valve could try a pressure reducing valve and set it so both hw and mains water is at the same pressure / flow rate and it should work.

Most decent plumbing shops if you take your mixing valve to them explain what you have had happen will be able to advise you on a solution they would use and also the parts to do it.

a Weir cup to check flow rates from your taps could also help to see if you have the correct flow through the boiler as per manufactures manual

 

[skipster1978]

Thanks for your help Netdragon
i have checked the mixer is suitable for combi boilers
i removed the mixer and put a set of normal taps on, and the boiler fired up,
so looks like the shower mixer taps ,
The mixer is fed from a 22mm copper pipe going into a 10mm tap flexi hose then into the mixer which then opens up again to 22-25mm so might be a pressure drop,
the only thing is it worked with that set up for 4 months so not entirely satisfied its the taps
might try getting a boiler service might be clogged as i live in a hard water area.

Thanks again for your help

 

[Agile]

Quoting the boiler make and model would help us!

Measuring the min litres per minute flow rate will identify if the boiler is working correctly.

Tony