Glowworm flexcom 15hx overheating

13 Apr 2012
Reaction score
United Kingdom
Hi there,

Limited DIY'er however would appreciate constructive advice.

Recently had hot water cylinder replaced due to leaking indirect connection, after a week or so, instead of boiler (open vented system) stopping at 82oC when fired up, temperature would rise to 99oC....and occasionally cut out with fault code f22 or f25 appearing indicating restriction in system, faulty pump, air in system etc.

Summary: All radiators are working and hot water is ok.

1) Bled all rads and no air came out - clear water.
2) Checked F&E tank and water present above feed pipe and below overflow pipe, however orange scum present on surface of water indicating air in system (?) which I removed.
3) All stop valves within airing cupboard (adjacent to cylinder) open albeit one feeding cylinder was not fully open which was opened.
4) Pressure release valve (?) adjacent to cylinder bled and fresh water squirted out.
5) Bled screw on pump - clear water came out - tested pump for operation by increasing/decreasing speed setting = flow of bleeding increased/decreased accordingly.
6) Switched all trv valves off for 5min then opened each one independently to hopefully push any blockage (?) our of system by water pressure.

Boiler is 3yo and recently serviced - problem has only occurred since hot water tank replaced - any suggestions where next to look?

Thanks in advance,

Sponsored Links
S or y plan system? In other words, how many motorised valves do you have?

If it is 2 x 2 ports, there must be a bypass, preferably of the spring type, or the pump over run wont function.

Unfortunately you haven't given any detail of the nature of the fault........does it happen on ch, hw or both? Does it happen after the boiler has been on 20mins or straightaway?

Checking the pump runs on three speeds doesnt really prove much I'm afraid. Did the installer change any wiring when the cylinder was changed?
Hi Simon,

Thanks for response.

Only 1 motoised valve visible in airing cupboard.

System overheats/goes to fsult code after 20mins or so and intermittent in all formats, i.e. CH, HW & CH+HW.

Works were restricted to direct like-for-like replacement of HW cylinder including new surface mounted thermostat on tank & immersion coil.

System went into fault mode again today. Switched off system and attached hose to bleed valve directly above boiler as gloworm techie indicated this may bleed air from system.

Held ball-cock in position in F&E tank then opened valve and put hose into bucket with all electrics off etc. Clear water flow came out however as hose became submerged, bubbles appeared (trapped air?) on occasion with simultaneous sound like low gurgling noise. Checked header tank & found water level dropped also.

After about draining about 8 pts of water, sealed valve then replaced drained water into header tank to retain sentinel levels and avoid ingress of fresh water then released bal-cock.

Fired up boiler however still overheating as rising above 82oC.
Sponsored Links
Are there trvs on every radiator? This would cause a flow restriction after a time period.

Failing that, as it Sounds like a flow problem, i would change the pump before looking at other options.
Thanks Simon,

Yes TRV's on each radiator.

Pump was replaced at same time as boiler, i.e. 3 years ago.


In which case you have a system where, when on CH mode only, water flow through the boiler is effectively terminated when every room is up to temperature (ie: TRVs have shut).

This will cause nuisance lockouts due to overheating.

Remove one TRV head from a radiator where you need the heat, for example, the lounge. Or if you have a room stat, you should remove the TRV head of the radiator nearest to it.
Thanks again Simon,

However, surprised TRV's coud be the cause as lock-outs have only occurred since HW cylinder was replaced in last month. No issues previously in 3 years of operations.

Also as all rads are operational, would it be fair to assume the pump is working (circulating heat) and no airlock in system? Apologies if wrong, however just an assumption.

One possibility that nags is me that when HW system was installed by Heating Engineer/Plumber, perhaps some debris became dislodged and got into system although was leaning ehilst he carried out operations seemed a clean easy-ish job undertaken, albeit via an experienced guy.

Daresay will be giving him a call this week to see what can be fone, just tryinh to get time off from work to accomodate is a prob....

Thanks again,

Remove a TRV head and see if the problem disappears.

I think you may be 'overthinking' the cause, if you see what I mean.
Hi Simon,

Opened up TRV valve fully in hallway/stairway where house thermostat is (re-setting it to 20oC from original 30oC where TRV controlled temperature) as an interim measure and run the system.

After now running for several hours , temperature on boiler for all options i.e. CH, HW and CH+HW has maximised at 87oC, not perfect 82oC, however a lot better than 99oC as previously.

Hence it seems that the opening up of the TRV valve has reduced the restriction and reduced the extent of the boiler overheating.

Next job is to purchase a lockhead valve for this radiator and install fully open relying upon the room thermostat I guess. Trust by doing so there will be no issues with radiators unbalancing as that is an entirely new field of ops for me?

Thanks again Simon, much appreciated for you time and efforts. Just a little bit disappointed mate's father who is an experienced plumber with own business who installed boiler/some rads and trv's on all rads put the trv on that radiator, especially considering what I now know to be best practice.

Best regards,

Not sure if you have sorted this but
This happened to our boiler, which is the same as yours , same error codes

It was still under warranty, so they came out to it

The problem was that the float in the condensate chamber had somehow stuck, took it out cleaned both chamber and float and re fit, been ok since
I should just take the TRV head off, Scott.

There's going to be no improvement in fitting a lockshield - unless you want it to look nicer.

The less times you drain down a system, the better the water quality will be.
Thanks to all for assistance.

Had issue with similar error codes re:frozen condensate pipe last year, however had checked condensate outlet which was drippig freely, i.e. ok.

2 weeks now and no overheating so TRV valve is coming off in hallway.

Many thanks again to all.


DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links