Halstead Finest Gold faulty C/H boiler

[Knorry]

Problem

Boiler fires up on Central Heating Mode, Pump runs and system heats up.

Boiler reaches temperture and thermistor shuts boiler burner down.

Pump runs on, then..

Pump and boiler shut down. for no reason! (approx 45 seconds after boiler gas goes off)

Boiler gradually overheats as there is nowhere for the water to go.

Boiler pushes hot water and steam through Pressure Relief Valve.

Boiler cools down and once recharged to 1 Bar starts all over again.

The Boiler works fine on Hot Water Mode, and has done all summer

Its as if the boiler goes to sleep whilst trying to heat the system up, then starts to come back on and go through the same actions again.

Boiler was installed in 1999 and has been serviced annually

Any ideas out there please

 

[MANDATE]

If its anything like the Ace High then In HW mode there is only a small amount of system water that circulates through a heat exchanger. So cold water is heated and out though taps hot.
In CH mode the whole system is heated and the water expands.
The extra volume has to be accommodated in the expansion vessel.
If there is insufficient air space in the vessel the pressure will rise beyond what is acceptable and open the PRV.
My opinion is that the vessel has lost some of its air charge.
With a correctly charged vessel the water pressure should only rise about 1 to 1.5bar so the PRV usually set about 3bar will never open.
Note also PRVs once opened are not likely to seal again correctly

 

[Knorry]

Thanks for your reply

The problem seems to be the boiler shuts down,(fan off pump off) and when the water in the heat exchanger cools, the boiler, and pump then starts up and goes through the same procedure again.

I cant understand why the whole thing shuts down after the boiler stat comes in.

The PRV seems to be ok and is doing its job properly.

 

[MANDATE]

I'm thinking if the expansion vessel cannot accommodate the extra water, the pressure will rise and open the PRV.
The pressure will then drop and the PRV closes.
If or when boiler stops it can't relight due the lack of pressure, so the solution is not to lose the pressure in the first place.
To do that the PRV needs to remain closed, which means pressure must not reach 3bar. Which in your case it does and that is down to the expansion vessel.
The vessel needs the diaphram to be in a position so the vessel has no water in it. only 'air'.
It's this volume of air that gets compressed and increases the pressure.
If this initial volume is halved then the design pressure is doubled.
So instead of going from say 1.2bar to 2.2 (1bar increase) it goes from 1.2bar to 4.4bar. Well in theory it would but the PRV operates at 3bar.
So there is little point delving any deeper until the expansion vessel has been checked out.