Immersun Overheating Hot Water

phsci

In April 1999 a new European Standard, EN60335-2-73, replaced the old BS3456 Part 2, introduced new requirements and was itself updated in 2009.  Any new immersion thermostat must have both a thermostat and an overheat safety cut-out.   This is because the thermostat can fail with the contacts closed and no longer restrict the temperature of the water in the cylinder.  This may be the case with your 30-year old installation.  The increased immersion running time that the solar diverter may cause makes it very important to have a safe modern thermostat with full boiling protection. Even the immersion heater itself should ideally be changed whilst the installation contractor is present as they may not last long with the increased usage; especially in hard water areas old elements can be quite degraded.

At worst, a plastic cold-water storage cistern (to which the hot water cylinder vents) can become full of very hot water, fail, and scald anyone unfortunate to be sleeping in a room below.  See a warning from the Health and Safety Executive following a couple of fatalities:
https://www.hse.gov.uk/services/localgovernment/hotwatersystems.htm

michaels

I've never seen an immersion without a thermostat, can you send a picture of yours, typically there is a plastic cover with a small nut to unscrew to take the cover off and then there is a thermostat, very often covered by a sticker that you have to remove then use a screw driver to adjust, I have to take my glasses off to read the numbers cos they are small.

tim_p

this very thing happened to my immersion / thermostat. The thermostat failed on and the iBoost just kept pumping in more power. I heard the tank boiling! Replaced the thermostat and all well again.  Similarly the immersion was fitted many years ago and removal would be likely to split the tank given the force needed to remove it. If my immersion fails I’ll replace it with a bigger, more modern version with two short side mounted immersions.