Immersion Stat. Fails to On?

[Sam Norris]

Hi,

We use the immersion heater on the hot water cylinder sometimes. The other day the 18" thermostat broke and I replaced it this morning. What interested me was the old one failed to `on', so the heater heated lots until it was making such a noise that we noticed. I'm surprised the failure state isn't off. What would happen if I didn't turn it off?

Also, on removing the old one, the plastic casing around the terminal for one of the wires is distorted as if it had melted slightly. Perhaps that occurred at the same time as the stat. failed.

Both new and old are 16A on a 3kW heater.

Sam.

[Christian McArdle]

Failure modes of thermostats allow failure to occur to the on or off position.

When installing an immersion, it is a good idea to select one with two thermostats. One will be a normal one set to the required temperature. The other should be a manual reset one set 10 degrees higher. This way, the consequences of thermostat failure are reduced. Unvented cylinders are required to have such immersions, but they are a good idea for any cylinder.

Christian.

[Ed Sirett]

The water would boil, the space where the cold tank would fill with much steam/condensation/humidity the cold tank would heat up.

Eventually if the cold tank was inadequately suppported and/or missing its lid it might collapse and drop scalding water below. This is the way someone was killed recently.

However the noise is enough to make any resonable curious person switch off the heater.

[Ed Sirett]

What's more the second thermostat is required to be of the manual reset type.

[Christian McArdle]

Isn't that what I said?

Christian.

[Ed Sirett]

Sorry, my fault.