Grant Vortex Combi - Overheat

[clothahump]

Recently had a 2nd hand boiler fitted, unable to afford new as no income since March.
Initially it all seemed to fire up and do what it is supposed to do, last week it went into overheat mode.
Engineer had told me CH pump was in need of replacement, sure enough the overheat light was accompanied by CH light flashing, got new pump and fitted yesterday, I thought this was the cure.
Turned on the heating this afternoon, all seemed well, then back into overheat mode.
DHW works fine, so that side of things is okay.
HELP

 

[muggles]

Could be blocked boiler pipework or heat exchanger if it's a second hand boiler. Presumably you don't know the condition of the system it was removed from?

 

[clothahump]

Turned it back on after reset, both flow and return are hot but nothing going around the rads, red light on pump flashing.

 

[BoilerBob]

The pcb has gone every time I’ve been to a grant with that sympton it’s been the pcb. Bob

 

[clothahump]

Bugger, now to find out which one it is.

 

[oilhead]

The pcb has gone every time I’ve been to a grant with that sympton it’s been the pcb. Bob

I've had this where the PCB pulsed and bugg***d the pump with the constant switching on and off. Using a screwdriver like a stethoscope, can you hear the pump turning?

 

[clothahump]

Yes, new pump, running properly as far as I can tell.

 

[clothahump]

Turned back on to standby (switches in central position) after 30 minutes rest, pump working, hot water flowing, had some air pass through pump, bled rads and had a bit of air from one. Always seems to be a bit of air in the one right above the boiler.
Fired it up again after pump had stopped distributing hot water, rads are warming up, boiler working as it should for now.

 

[BoilerBob]

What I do if boilers overheating is take off all aavs and make sure the boiler is properly filled with water aavs sometimes block and you get a couple of inches of air which jumps up and down and creates all sorts of bother sounds daft but since doing this I’ve had no bother. Bob

 

[clothahump]

Boiler ran perfectly for nearly 2 weeks, then went into Overheat again, bled rads and away it went, fair bit of air trapped.
Same again today, pressure was nearly up to 2bar so released pressure back to 1, fired up lovely, bled rads, more air.
Turned on hot water, all seemed okay and then it started gurgling and pulsing, not sure where the noise was coming from.
This is slowly driving me mad.

 

[clothahump]

Sound ON.

 

[BoilerBob]

Make sure you have the pump set on right setting that’s a para pump and they need to be set for running on rads not for underfloor. Bob

 

[BoilerBob]

Why drop the pressure at 2 bar my boiler runs at that all the time I’ve 18 rads all doubles and some big ones. Bob

 

[clothahump]

Grant recommend 1 bar max when cold, both pumps are set at high as per instructions.

 

[oilhead]

If you keep picking up air, and the hot water is fine, then there must be a problem in the radiator circuit.
Are all your radiators getting hot?
Is the one above the boiler that gets air the first one in the system?
Have you tried running the heating pump at a lower speed?