Mexico Super 2 - Kettling when heating hot water only?

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HughPugh

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Hello
I have a rather old, traditional but thus far reliable heating / hot water system. It has evolved over the years and now consists of an Ideal Mexico Super 2 (awaiting its annual service), 11 radiators (2 storey house), a copper hot water cylinder in the airing cupboard upstairs, cold water tank and F&E header tank in the loft, a Honeywell V4043H motorised zone valve, a Honeywell L641A cylinder thermostat set at 60deg, a Honeywell ST699 that now controls the hot water only and is left on the 'Constant' setting and a Honeywell CM67 for the heating only. The system is dosed with Sentinnel X100 and X200.

At this time of year, I have the central heating turned off on the CM67.

Just recently, the boiler has been firing up more frequently than usual and stays on for a few minutes, Obviously the only thing that can be calling for heat at the moment is the cylinder thermostat.

The problem is that the boiler is kettling during these short periods. Interestingly, if I turn the central heating back on and set it to manual so that the pump runs, the kettling stops.

Any ideas, pointers or suggestions welcomed.
Thank you.
 
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you may have a blocked cold feed, the point where the F&E tank in the loft connects to the system its quite common for this to become blocked, this is how the system is kept full of water, an easy check is take a container to the highest radiator and open the bleed vent and see if the water runs freely and constant if it does then the feed is fine if it doesnt then it is blocked and will need sorting
 
Thanks Ian & denso13.
There is water in the F&E tank and at the correct level. The other day I manually held the ball valve and drained a few pints from the system. The level dropped as expected and then refilled once the ball valve was released.

Does that prove/disprove the same thing as you're suggesting?
 
That test should prove there is water in the system, and the cold feed isn't blocked. Kettling is usually a result of the water getting a bit hotter than it should, within the boiler, which can be down to lack or water or poor circulation. The cylinder stat will call for heat if the water within the cylinder is not up to a temperature to satisfy the cylinder stat, boiler will also fire if the temperature within is below that set at the boiler stat.

Problem I would suggest is, the heat is not getting transferred as efficiently as it has been in the past, hence cylinder is taking longer to heat, and boiler is firing more often. Could be scale/muck in the pipework/boiler, or possibly scale on the coil within the cylinder, slowing down heat transfer.

The noise will disappear when the CH is switched on, as the pump will be shifting the heated water from within the boiler, around the radiators, hence the core temp of the boiler will remain slightly lower when CH is being called for.
 
Dump half a bottle of fairy liquid into the system and boiler will be as quiet as a mouse.
 
Probably the boiler is coming to the end of it useful life. The boiler will reach it core temperature then it will kettle as you have a gravity primary flow and return which is slow.

Of course it will stop kettling when radiators is switch on and pump is running but have you run it to core temperature ( when it is at peak ) and all is hot, does it kettling?

Kettling only happened if there is hot spots inside the heat exchanger, probably caused by lime scale, sludge and derbis inside.

Daniel.
 
The chlorides in detergent will feck the rads after a while, but at least the boiler will be quiet. :D
 
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