Portrait photo of Mike Bryant, AKA Mike the Boilerman

Mike the Boilerman -

Your Gas Safe Registered boiler 

and central heating repair technician in west Berkshire. Willing to travel :)



I am still carrying out house visits. Full details explained


(Last updated 21st April 2021)

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 these days.

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 on my mobile 07866 766364.


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Copyright Michael Bryant 2021

Site first published 16th January 2004

Last updated 26th April 2021

Gas Safe Register 197499, CIPHE registration number 56207

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