Low hot water pressure - Firebird oil

[dai pongo]

I have an old Firebird oil boiler (cant find the number) that works fine until the water pipes 'clonk' and then the hot water pressure drops. I switch the tap off and on again it works for a while until the 'clonk'. The cold water pressure is fairly high (normal) but when I reduce it to stop the 'clonk' the heating works but its pressure is not sufficient to have a good shower. The clonk comes from inside the boiler but cannot put my finger on exactly where. Is there a valve to control such events? We have always had high water pressure and its been running fine for years up till now.

 

[SimonG]

Could be the mixing valve in the boiler. It is a combi?

 

[dai pongo]

Yes it is a Combi. Is the mixing valve the one with a black knob centre right?

 

[plumbtech]

they do give trouble those mixing valves. sounds like a combi 90 very common firebird

 

[dai pongo]

Thank you for your help. I am keeping an eye on the cold water pressure, keeping it low enough to have a shower. If it plays up again I will have to get the engineer to check and change the mixer valve. Thanks again.