DGOBS ....you cracked it!
Hi DGOBS,
Thanks for getting back to me!
The issue is fixed!...So heres what I did after revisisting your post(Its quite a long reply as I hope someone else may benefit )
With the boiler system charged to 1.3 bar cold starting pressure, I located the Expansion Vessel (EV) in the boiler ( at the back of the boiler ) I pressed the Schrader valve pin on the EV ( at top ) and and heard air escaping, so I presumed diaphragm in vessel had not split as there was no water coming out out of the valve.
I then went to a well known car/bike shop and bought a mini digital type gauge from the car dept for eight quid.
Back at home, I isolated the CH flow and return pipes, opened the boiler drain cock ( white fitting just behind wilo pump) and drained the boiler. I then re measured the EV air pressure and it was almost nothing. So, at this point I presumed that the EV had lost its 10psi charge over time - this would explain the observed large ( >2 bar ) pressure difference from cold startup pressure (1.3 bar) to when the heating was working- the increase in volume of water (due to expansion as it was hetaed) was not being mitigated by the volume of air air in the EV!
So, With boiler drain cock still open, I pumped air ( with a Bike Pump ) into the EV Schrader valve until I had 10 psi in there ( as measured with the digital gauge ) I then opened the CH and flow and returns, charged the system to 1 bar and fired up the boiler.
So with a cold starting pressure of 1 bar and the boiler temperature stat to max, I cranked up the room stat to 25 and the hot water tank to 60 to see what the pressure differential when system was maxed out. It floated at around 2 bar...... so happy days!. At the same time I cleaned out the magna clean, topped up with inhibitor and bled the air from the rads.
Interestingly, before charging the EV, the cold starting pressure used to have to be 1.3 bar (at least) to get the Flow switch in -and I also had to stagger the hot water and Ch startup times otherwise the flow switch would not come in with both CH and HW valves open.
The situation now is, both DHW and CH valves can open at the same time with only 1 bar cold starting pressure and the flow switch makes and boiler fires up!....I think the reduction the volume of water in the boiler (as a result of pumping up the EV ) means that more flow is achieved at the flow switch when the pump starts....result!
Thanks for you steer DGOBS, as soon as I started having problems agagin I referred back to your original post which mentioned the EV....I guess in the heat of getting the heat exchanger replaced, system cleaned, magna clean fitted and having kids..the EV dropped off the radar....looks like it was a major contributory factor all along....just like your original post said...Happy wife = Happy Days.
Cheers DGOBS..... heres to a warm XMAS!