Boiler overheats

[E36bob]

Hi all, new to this handy site.
I have an old gloworm space saver boiler, which has started to overheat the water. Hot tank stat is 50, rad 20 degrees .
When running either the radiators or the hot water, or both, the boiler and the circulation pump run continually, until turned off by either stat.
Before, the water temp was acheived by the pump running continually, and the boiler cutting in when required.
I removed the boiler front cover and the small copper wire which runs from the boiler water temperature control housing around the back to the boiler is broken. The copper pipe carries air, is this how the boiler senses the water temperature?
I would like to know what exactly this does, and is it an easy fix?.
My DIY skills are reasonable.
I have tried to get a diagram off the net , but without success.
Thanks for replies in advance

 

[kevplumb]

no it dosen't carry air
renew it

 

[chrishutt]

What has broken is the thermostat. This is a vital control device and you should not use the boiler without it! It is not an expensive spare and quite easy to fit, so shouldn't be too expensive.

Before it broke, the tube would have contained a volatile liquid which would expand or contract according to the temperature in the boiler heat exchanger (where the probe end of the tube is located). This expansion / contraction would operate a switch which would open or shut the gas valve.

 

[kevplumb]

thermostat.

knew it was called summat

 

[Paul Barker]

Isn't it sommat te do with earth bonding?

 

[E36bob]

Thanks to all for the replies.
I shall replace the thermostat asap, we can use the hot water tank electric element in the meantime.
If I cannot fit it myself, I will get a plumber to do it.
Thanks again