Hi All,
Hope you're well.
Quick recap of my shop boiler woes. (18 year old non-condensor)
2 years ago it broke so I took out the 6 months "guarantee fix"
insurance for £300. After about 10 visits and half a dozen different
engineers who changed primary and DHW Heat exchangers, diverter valve,
pump head, insulation etc it was back working but making a hell of a
noise but back to working again.

So this year We've had the random F.22 (dry fire) error sometimes
continually, sometimes the boiler would run for most of the day so we're
back on the process of elimination. Replaced NTC sensors which
temporarily appear to fix things but they were e-bay cheapies so I
replaced them again with proper Valliant ones which still didn't make a
difference.

First thing I noticed was significant pressure swing so ordered up a new
pressure vessel, filling loop valve (which has been leaking for years),
Pressure relief valve, new Flu kit, FLow/Return service valves.

oh and a vortex 300 filter.

Expansion vessel was shot, flue also mutilated by original install or by
"engineer" levering it off to remove primary HEX. The other symptoms
have been the boiler working but then going into burner anti-cycle so
the water gets hot and then the burner shuts off and the pump pumps
water around for 10 or 20 minutes even though the system temp has
dropped to 30 degrees or so.

So having put on the new expansion vessel, and valves etc it was running
virtually silently but still giving F.22 intermittently or every few
seconds. So... I've been running the system with a chemical flush and it
appears to be improving having run continuously all day yesterday.

Now, what I'm thinking is the automatic bypass valve might be blocked
open giving rise to the premature burner shut-down and excessive cooling
of circulating water as the bypass pike gets hot very quickly when the
burner starts up.

So... when I drain and flush out cleaner I'll inspect auto bypass valve
but there are 3 versions listed Presumably it's to do with the pressure
at which the valve opens and I'm guessing is relative to the head the
pump needs to pump to highest radiators?
150240 - AUTOMATIC BYPASS VALVE 2.5M
150242 - AUTOMATIC BYPASS VALVE 1.7M
150243 - AUTOMATIC BYPASS VALVE 3.5M

Both shop and flat circuits are opened/closed by 2 port valves but the
shop circuit is permanently "open" and can't be shut down with
call-to-heat, likewise the flat circuit has several radiators that can't
be closed down by TRV's so I'm thinking the auto-bypass valve could
actually be blanked off completely without any problems?

Obviously there's the questionable diverter valve which if the actuator
is connected and someone turns on the hot tap the boiler instantly drops
beck to F.22 but we don't need hot water and as the valve is
ridiculously unreliable I'd rather just leave it disconnected.

Sorry for the long lost but if anyone has any other suggestions to try
before venturing into the realms of changing the main PCB I'm all ears.

Cheers
Pete