Glow Worm CXi
24CXi, 30CXi, 38CXi
This boiler is the combi version of the Glow Worm SXi. (A combi is the type
of boiler that lights when a hot tap is turned on, and heats the water on its
way to the tap rather than it being stored in a hot tank.) Very popular and
competitively priced when introduced about 15 years ago, but discontinued now.
I'm already I'm getting a trickle of callers telling me they've been told theirs
can't be fixed and ought to be replaced as it's a rather old model, which is
surprising as its a high efficiency condensing boiler and I find parts freely
available over the counter in any boiler spares merchant.
When new this boiler seemed quite a good design but over the years a number
of faults have turned out to be commonplace.
Here are the common faults and failures:
1) Circuit board failure. The boiler stops working and the user display is
either blank, or blinks with an 'F' code, e.g. F3, F4 etc. Nine out of ten calls
I get to this boiler are circuit board failure so I keep two of three in the van
2) Burner door seal failure. The original door seal was neoprene and
embrittled with age, leading to combustion gas escaping from the inner
combustion chamber to the outer, where the gas valve, fan etc reside. Combustion
gas is approx 50 water vapour so this results in all the components in the outer
combustion chamber sweating with condensation. Not good for electrical
components and if not caught promptly can also result in terminal corrosion of
the outer combustion chamber, or a dead boiler or at least a very expensive
repair. Glow Worm promptly introduced a graphite seal that must be fitted at
first service. If your CXi hasn't been serviced for many years I strongly
recommend a service now to inspect and probably replace this seal.
3) Overheating. The boiler has an overheat protection thermostat inside which
trips when the flow pipe gets too hot and shuts the boiler down, presenting as a
breakdown to the user. Sometimes this is a random 'nuisance tripping' and
re-setting it is all that is required. Other times it is a symptom of corrosion
deposits contaminating the heat exchanger. If the latter, a new heat exchanger
will be required as flushing it with chemicals just doesn't work. The heat
exchanger comprises about 12 thin stainless steel annular tubes all connected in
parallel. The first tube or two to clear then takes all the chemical flow, and
the other tubes remain blocked no matter how long one flushes for.
4) Fan failure. The fan blows combustion air through the heat exchanger and
the speed is controlled by the main control circuit board communicating with a
small board in the fan. When the electronic communication fails the fan or the
PCB needs replacing, hard to tell which but rarely both. I've yet to see
mechanical failure of the fan.
That's about it really. Virtually all calls I get for this boiler fall into one
of these categories.
If you'd like me to repair yours, feel free to contact me here.
Page first published 20th June 2017
Last updated 20th June 2017