Pulsacoil 2000 can't seem to stop heating up - help!

I came across your website a few months ago, while trying to find out what was wrong with my pulsacoil which just required topping up the feed & expansion tank.

It's really great you have this forum and I've learnt a bit more about the pulsacoil reading all the problems/issues other users have encountered, so it's not just me then !

The problem I've been having seems to be that the thermal unit can't seem to stop heating up. The off-peak switch is on, but lately the thermal unit appears to heat up till it gets quite hot and the manual overheat button trips, and the unit gets extremely hot.

However, I do have a supply of hot water, and it is only when it trips due to overheating and all the hot water goes, that I wait for the unit to cool down and reset the manual overheat button. One minute it appears to be normal and then later on in the day/evening, it seems to heat up to the point it can't seem to stop.

The off-peak, I thought should run between a certain time only, but it seems that when the unit is switched back on, it heats up and it can be anytime during the day/evening, not always overnight or early in the morning. Don't even want to think what my electricity bill will look like!

I have checked that the standalone feed & expansion tank above the unit is at the correct level which it is.
The 2 thermostat immersion off peak & on peak seem to be quite warm, not sure if that's ok when the unit is switched off.

Now because it doesn't seem to stop heating up, I am switching off the off-peak button instead of coming home from work to see the red fault light lit up.

Any ideas what might be going on and what possibly needs to be replaced/repaired and roughly how much should I be expected to pay to call an engineer out to repair the problem ?

Thank you very much for any advice you can offer me as I am at my wits end with my pulsacoil.

Hi Jinnie,

This sounds like simple thermostat failure in the lower, off-peak heater element.

Will take about 45 mins to fit upgraded thermostats to both heater elements. The cost will depend entirely on where you are and who you use!

Cheers, Mike

It could be the overheat stat that is the issue. We tend to only use the boost for an hour or two but the off peak for around 7 hours. Try the stats first, if the problem persists then replace the overheat stat.