Hot water on/off combi boiler

[David Lomas]

Hello,
I hope you can help, this is driving me mad, everything's been fine for 4 years in new bathroom, few months ago, shower started to go cold, can hear boiler turning off (it's in the bathroom). Now it's every shower multiple times, if you don't have the hot water on 'full' or if it's mixed then the boiler starts stop/starting.
I've already cleaned the mains filters to the boiler, checked and cleaned the tmv in the shower and I've just removed and flushed the crap out of the heat exchanger plate but it no avail.....it's a Baxi 105he by the way.
As I say hot water on full, no problem so have I got a flow/pressure problem? Would United utilities be at fault or do I need a pump or is it something else on my boiler I can check????

Thanks in advance!!

 

[CBW]

It’s not the United utilities’ fault and you don’t need a pump. So it’s just the shower affected?

 

[David Lomas]

It's any 'mixer', hot water on full and it's fine, start mixing and it gets intermittent, I'm only suggestion United utilities as if my pressure has gone down(didn't always do this) then do I raise it with them as a potential issue in the area.

 

[CBW]

If it’s at all mixers, you may have a passing check valve.

 

[David Lomas]

What does that mean??
PS I'm no plumbing expert

 

[CBW]

If you have a check valve, commonly fitted to mixers, especially showers, then there could be cold passing on the hot supply.

 

[David Lomas]

It also affects the mixer taps on the sink too, this isn't as noticeable as normally you just want hot water not mixed, only noticed when trying to narrow the shower down.

 

[ianmcd]

hydraulic venturi, lots of posts about it on here

 

[gas112]

yep hydraulic venturi has fell apart turn cold off and change it . 24mm ring spanner req
https://www.ebay.co.uk/itm/353337386205?epid=1505645137&_trkparms=ispr=1&hash=item52448ca4dd:g:IZ0AAOSwrVJghoXD&amdata=enc:AQAFAAACgBaobrjLl8XobRIiIML1V4Imu%2Fn%2BzU5L90Z278x5ickkWp

 

[David Lomas]

Thanks all, part ordered....I'll let you know when fitted!!

 

[David Lomas]

You were right....Venturi disintegrated....took longer to fish the bits out than fit the new one, all fixed.

Thanks everyone