Re-ranging a British Thermostat

Very Important Warning: This is a record of how I converted my thermostat to switch at different temperatures from those available on a standard thermostat. It could be unsafe to rely only on this modified thermostat for your temperature control. You should include a secondary backup thermal switch to protect you and your equipment against thermostat failure.

DO NOT use a modified thermostat for domestic water heating control, it could cause serious burn injury or could cause the tank to explode if the water boils.

A common British water heating thermostat normally covers 30 to 70 degrees C
Here's a picture of the one I have modified ...

To remove the cover, you need to gently prise the clips on each side of the thermostat. Careful not to crack the cover.

Here you see the internals.
The black thing in the middle is the adjusting knob, which you can pull off of its splined shaft, rotate a few degrees and replace, to shift the temperature range. It's that easy! To the left of it you see a white stalk, this is the manual reset shaft, which MUST be correctly positioned when you re-assemble the cover.

Here's a picture of the adjusting knob from a different angle...
You'll find that you can move the knob to get a range of adjustment anywhere from [-10 to +30 degrees], up to [+50 to +90 degrees].

Use a thermometer to check your trip points, you'll hear the actuator click as you go below and above the trip point.

Don't try to set a range which goes higher than 90C. The thermostat has a secondary over-temperature sensor installed (the round metal disk on the back face of the the thermostat) which will remain tripped if you go above 90C. To reset this, you must wait for the temperature to drop by around 10 degrees, then push the manual reset shaft firmly. You'll hear a sharp click when it resets.

The manual reset shaft MUST be fed back thru the hole I've outlined in red. This is tricky, but I found that if you poke a needle thru the hole, from the ouside, and have the point resting on the shaft end, while re-assembling, you can easily guide the shaft thru the hole as you replace the cover.

Now, make sure you record your new temperature range on the scale plate of the thermostat. It is easy to scrape off the original numbers. Use a permanent marker to write in the new range.

Mark the thermostat as not suitable for use in domestic heating.

Now test your assembled thermostat at the lowest and highest temperatures of the range, to make sure it works OK. Use a resistanc emeter or continuity tester to confirm correct action of the contacts.

Now pop the whole plastic head into boiling water and the manual reset trip should operate. The resistance between the terminals should be infinite, open contacts.

Allow the thermostat to cool down and measure the contact resistance - should still be open.

Press the manual reset stalk firmly until you hear a sharp click. The contacts should now be closed.

Thoroughly shake any water out of the plastic head and leave the whole thermostat in a warm place, such as on top of a radiator or in an airing cupboard to dry, before you install it in your system.

Any questions or comments, please contact me at grahamlaming at hotmail dot com



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