Help! Hot water far too hot!

dram9564

We have an indirect hot water cylinder that seems to be ignoring the thermostat and is producing very hot water. The oil-fired boiler is turned off for the summer and so water heating is by the immersion heater only. The bottom of the cylinder, where the cylinder surface thermostat was fitted, feels cold but the top of the cylinder feels warm even through the insulation - so I guess the cylinder is kettled (this is a very hard water area). I cut a new hole in the insulation near the top of the cylinder where it feels much warmer and installed a new surface thermostat there - but the water is still too hot. If I turn the thermostat I can feel it click but this makes no difference to the immersion heater - I can hear the water heating regardless of the thermostat and it only stops if I switch off the power to the immersion (isolator switch). Is the cylinder thermostat somehow being over-ridden by the boiler settings, even though the boiler is switched off? Or is it possible that the system is wrongly wired and the cylinder thermostat circuit has never worked correctly (I just replaced the old thermostat like for like)? How could I check for that?

Le_Kirk

You installed a new surface thermostat. To where did you connect it? These are normally used for connecting into the boiler to provide a signal to say "whoooa, water hot enough turn of boiler please" The (presume electric) immersion heater would have its thermostat in its head. Turn off power, remove head cover from immersion and have a look inside. You should see an adjustable dial. Report back

dram9564

Hello Le_Kirk and thanks for your response. I connected the new surface thermostat as a direct replacement for the old one (same model - Honeywell L641A - same 2 wire positions C and 1) but just relocated it by cutting into the foam insulation higher up the cylinder, on the side but near the top. I should have mentioned that when we were using the oil-fired boiler we had the same problem - the water got too hot and tripped the boiler to shut-off CH and we had to run off the hot water before the boiler would switch back on for CH. (That, of course, was with the immersion heater turned off at isolating switch).
I'll empty the airing cupboard and remove the shelving to get access to the immersion heater head and report back asap!
Thanks, again.

cajef

It does sound like the immersion heater thermostat may have failed, easy to replace as it just slides in, remember to switch off the power to it before taking the cover off.

dram9564

The immersion heater element is Heatrae Sada Gold Dot 27" 3kW. The thermostat part is labelled Pullin H8C18 15amp. It was set to 60C and 'clicked' when turned either way. I've turned this thermostat down to minimum and run off hot water until it feels hand-hot. Surface thermomstat is now clicking at 40-50C. Turned mains back on. I can hear the water heating up. I need a new immersion thermostat?

HappyMJ

dram9564 wrote: »

Hello Le_Kirk and thanks for your response. I connected the new surface thermostat as a direct replacement for the old one (same model - Honeywell L641A - same 2 wire positions C and 1) but just relocated it by cutting into the foam insulation higher up the cylinder, on the side but near the top. I should have mentioned that when we were using the oil-fired boiler we had the same problem - the water got too hot and tripped the boiler to shut-off CH and we had to run off the hot water before the boiler would switch back on for CH. (That, of course, was with the immersion heater turned off at isolating switch).
I'll empty the airing cupboard and remove the shelving to get access to the immersion heater head and report back asap!
Thanks, again.

Or you could do my cheap fix...just set the hot water to come on for just 20-30 minutes twice a day. The water won't get hot enough in that time.

dram9564

Thanks for the tip, but as it is an easy fit and only £10 from local Screwfix I might as well replace it.

dram9564

New immersion thermostat seems to have solved problem. Many thanks to all.

Le_Kirk

dram9564 wrote: »

New immersion thermostat seems to have solved problem. Many thanks to all.

Wow, that was quick, well done and glad it is sorted.