Boiler kettling periodically

[Dan Buc]

Got an old, 15-20yr old Glow-worm 24hxi
(open-vented system) that's had a weird deeprumbling noise for years. It’s not the usual “kettle boiling” whistle, it’s more like a low jackhammer vibration you can feel through the house. Happens 20 mins into a hot-water cycle, lasts circa 40 secs, pitch rises, then stops. Seems to improve everytime I've ran a cleaner throught the system, but I've only every drained and refilled. Never flushed out for hours.

This weekend I went full DIY:

Ran 2× Sentinel X800 cleaner at 75 °C for 24 hrs

Fitted a Sentinel Vortex 250 magnetic filter as there was no filter.

Flushed for 4 hrs till water went from black to light brown (not a powerflush)

Dosed with X100 inhibitor


Noise already improved but still there a bit.

So I’m wondering:

1. Rough cost for a proper service (burner out, heat exchanger cleaned, electrodes checked)?


2. Does that usually cure this kind of kettling, or is the heat exchanger likely done?


3. If replacing boiler,, what’s a good like-for-like heat-only swap (flow/return at top, minimal pipe mods, reuse pump and zonevalve)?


4. Ballpark supply + fit price?


5. Keep open-vented or go sealed?



Got a Drayton Wiser controller and decent pump/valve, so just boiler work needed.
Would love a reality check from anyone who’s dealt with these old Glow-worms. I've had absolutely no bother with this boiler (other than kettling) in the last 5 years and the previous owner had not bother for at least 10 year since it was fitted.

 

[dilalio]

When was it last serviced properly?

 

[fixitflav]

Interesting layout. Usual way is boiler - open vent - cold feed - pump. Yours could be cold feed on the boiler return, which was common decades ago. Where is your open vent? If it's after the pump maybe there is overpumping from the vent pipe, which would aid corrosion by bringing in oxygen.
I wouldn't be 100% sure the pump is OK. I used to get kettling towards end of heating cycle. Didn't suspect the pump, Grundfos similar to yours, but then it started to vibrate badly. On dismantling, the stainless ring pressed into the casing at the eye of the impeller had come adrift. The impeller, 2 discs about mm apart, was clogged with crud. Replaced the pump and the kettling disappeared.

 

[Dan Buc]

Open vent looks like its on return side like you've suggested. Plumber came today while I was at work (mrs was in). I'd asked them to clean out the heat exchanger and give it a service. They told her that they would never manage to get seals for it or any spare parts sow wouldnt touch it. I've also bought a replacement grundfos UPS 3 pump and they refused to fit it saying their was nothing wrong with the pump yet hadnt actually taken a tool out the toolbag yet.. Instead they spent 2hrs assessing the whole system to quote me for a combi - something i never requested. Not impressed. I did notice that when I changed the pump speed to the fastest setting it improved the kettling so Im swapping out the pump tonight to see if that improves flow and hopefully helps with the kettling. oh, and I've found several seal kits online so they were talking nonsense. I'd buy one but not sure if a plumber will use it if supplied by myself.

 

[Notch7]

a kettle boiling is better than a boiler kettling.....

 

[Notch7]

They told her that they would never manage to get seals for it or any spare parts sow wouldnt touch it.

thats a bit naughty, a quick google shows a load of parts for that model including seals-not sure if its the same??

https://www.heatingspareparts.com/datasheets/glowworm/ultracom-spc-24-spc-hxi

"lets sell him a new combi".................

 

[Dan Buc]

its not the ultracom its just 24hxi, but thanks for that link. I've still managed to find the correct model on that website https://www.heatingspareparts.com/boiler-spares/glowworm/glowworm-spc-24hxi

 

[Johntheo5]

Open vent looks like its on return side like you've suggested. Plumber came today while I was at work (mrs was in). I'd asked them to clean out the heat exchanger and give it a service. They told her that they would never manage to get seals for it or any spare parts sow wouldnt touch it. I've also bought a replacement grundfos UPS 3 pump and they refused to fit it saying their was nothing wrong with the pump yet hadnt actually taken a tool out the toolbag yet.. Instead they spent 2hrs assessing the whole system to quote me for a combi - something i never requested. Not impressed. I did notice that when I changed the pump speed to the fastest setting it improved the kettling so Im swapping out the pump tonight to see if that improves flow and hopefully helps with the kettling. oh, and I've found several seal kits online so they were talking nonsense. I'd buy one but not sure if a plumber will use it if supplied by myself.

Your old Grundfos selectric is certainly a vetern alright so no harm in replacing it, beware though, the UPS 3 is a giant of a pump but has plenty of modes to calm it down for use with a OV system, do not run it at CC3(constant speed) which can create havoc in a OV system with pump carry over, air ingress etc as it will pump at a constant head of 6.4M right up to a flowrate of ~ 1.12m3/hr, 18.7LPM, suggest CC1 at 4.2M for a start and if getting carry over (look into the F&E cistern especially while starting and stopping the pump) try CP1 (constant pressure) at 3.0M.

 

[fixitflav]

I've also bought a replacement grundfos UPS 3 pump and they refused to fit it saying their was nothing wrong with the pump yet hadnt actually taken a tool out the toolbag yet..

You could check whether the impeller on the existing pump is blocked like mine was without disturbing the pipework, just undo the 4 Allen bolts. I cleaned my old impeller and keep it as a spare. Kettling lower at higher speed suggests the output is low, usually OK on minimum speed, as mine is.