HOT WATER TEMP. INCONSISTENT

[Paul Raymond]

Hi,
Firstly, I am neither plumber nor heating engineer, just a willing diy, have a go guy.
I have a Grant 26 oil fired combi boiler which started continually leaking water from the prv to the outside. The pressure was at 1 bar until the heating system cut in, when it would increase to 2.5 and then blow the prv.
I depressurised the system, checked the EV standing charge was correct at 1 bar [as per manual] also that it held the pressure in the tank and at the valve.... a.o.k.!
I decided to replace the plate heat exchanger, thinking this was the only cause of the constant build up of water pressure in the ch system [i.e. maybe an internal leak/split between hw & ch etc.]
Presto! no more prv leaks or excessive pressure build up, so far!
However! When installed, new [3 years ago] the water temp. on both dhw and chw systems worked fine, until this year [well before the prv leaking issue started] when running the bath, [which we do every day] the hw starts off hot then reduces to just warm, after a short while, basically not holding a constant temp. after running for a while!
Is it advisable to fit a mains water regulator, in case it's above the recommended 8 bar max.? Is there a boiler problem? Is it a service issue? Can the actuator have anything to do with it? Any answers or advice would be greatly appreciated.
p.s. The central heating is working fine.
Thanks for any help guys

 

[just pumps]

Thinking it sounds like a boiler issue and having it serviced might actually save you some fuel costs.

 

[oilhead]

What is the flow rate through your bath tap?
If you run the basin tap, does the same thing happen?
What setting have you got on the hot water?
Is the HW pump running and does the burner fire whilst running your bath?
A split in theDHW heat exchanger would have increased the system pressure all the time, not just in use, so doubt if that was your problem. Does tha pressure now increase when the heating is on? Is there any water going into the expansion vessel during heating on periods? You may have a blocked pipe from the boiler to the vessel.