Feeder tank boiling over at night

Hi, the last 2 nights the feeder tank on top of the pulsacoil unit has been bubbling and steaming away. I thought yesterday it might have been topped up too high but I removed some water and the same thing happened last night.

It only happens on the off peak supply, in fact nothing seems to happen at all on the boost supply which is a pain as it means I can only test at night at the moment!

Last year I fitted the upgrade kit with the combined overheat rods, as it would occasionally trip at night, and it's been working fine for 6-8 months since then. The unit is around 8 years old and I'm wondering if the immersion needs replacing? We're in London so scaling is probably quite bad and there does appear to be some around the lower rod, which can't be good. All the electrics look fine.

I'd be grateful for any thoughts!

Have you also fitted the required Thermostats with the upgrade kit? The fault could be only at your Off Peak thermostat, they come sometimes out of calibration , so get a new thermostat fitted , this should be solving the problem, also look for any air in the cylinder, there is a little bleeding valve on the small 15mm inlet pipe between your F&E Tank and the cylinder ( similar to a radiator bleeding valve, with the same key)
hope this helps
Mario at Mario's Boiler Service

Thanks Mario, I fitted this 6 months ago and has been working fine up until the last 2 nights:

"MIP080 Retro-fit conversion kit, replaces existing rod stats and overheat stat with a dual combined overheat and rod stat."

didn't know about the bleed valve so thanks for the tip i'll make sure there's no air in the cylinder. any ideas why the boost isn't working either? would that suggest it is the thermostats?

thanks for chipping in.

first, the top of the boiler is always the hottest, so if there is a substantial temperature in the top, the boost will not start, but this also could be a: the thermostat on the boost or the immersion, hard to say without testing.
regards Mario

By the way, never use both immersion at once, always use the boost only if needed.
regards Mario

MIP080 can be purchased via this link: Gledhill Pulsacoil 2000 Spares

Do not buy any other thermostat as the temperature has to reach around 70oC