I have an 18 month old 24hxi Glow worm boiler configured with a
Honeywell 3 port valve, Llandis control panel, Honeywell wall
thermostat and Honeywell cylinder thermostat. The system is configured
as the Honeywell Y plan, and can supply hot water only, heating only
or hot water and heating combined. It is a conventional system, not a
Whilst under warranty (Year 1) the system kept dropping to F1 (fail to
ignite) at apparently random times. A Glow worm engineer called and
told me that there had been a fault with some of these models due to
some spade terminals being poorly fitted. He re-crimped the terminals
and all seemed OK.
The fault recurred very occasionally after this, but not sufficient to
justify a call-out.
The only change from standard that I have done since then is to change
the 3 terminal(+earth) - (with accellerate) Honeywell wall stat to a
two wire programmable stat. I jumpered a wire from the Live terminal
to terminal 4 in the panel behind the system programmer as per
Honeywell's instructions for upgrading a T6360 wall thermostat to a 2
The system happily supplied HW throughout this summer, but when we
started requiring HW+Heating combined a few random F1s started to
appear. More recently we got an F4, and our installer called,
struggled to find a solution, but after a call to Glow worm he flicked
a switch near top left of boiler, and the F4 disappeared. The
installer seemed to think that this was just a random fault.
All seemed fine until we started to leave the heating and HW on in the
night, when we began to wake to a flashing F1 again.
I have replaced the 3 port valve, the wall thermostat, the cylinder
thermostat with identical but new items in the hope that this would
fix things, but the dreaded F1s continue.
I have swapped back the original wall stat and removed the link, but
the same problem persists.
Today - a partial breakthrough. The systems fails to F1 when there is
still a demand for heat (via the wall thermostat) but the cylinder
stat decides the cylinder is up to temp and switches off.
The 3 port valve correctly positions for HW only, both HW and CH, AND
I guess that the night period with low consumption of hot water allows
sufficient time for the H/W thermostat to cut out, thus causing the
F1, but I have no knowledge of why this might happen and a way to
The system is quite happy to run in Hw only mode or CH only mode, or
combined mode until the cylinder stat switches off.
I really would appreciate some advice. The only good news is that with
the boiler etc downstairs and the cylinder etc upstairs I have
probably exercised sufficiently today to allow me to eat the entire
turkey myself this year! :>))