Replacement cylinder thermostat

[WillC1423]

Our builders have installed a 250L horizontal Everflo cylinder with a thermostat that shuts off the hot water when the cylinder is up to temperature. With the thermostat set to 45 and the boiler flow temp set to 65, the thermostat never switches off, even though the water in the cylinder is over 60 degrees. To get the thermostat to switch off I have to turn down the temperature to around 40, but then it doesn't turn on until the cylinder temperature is <40, which sometimes leads to cool baths/showers. There appears to be a ~20 degree threshold between the on/off state of the thermostat. I assume this is to reduce boiler cycling, but it makes the thermostat more or less useless unless I turn the flow temp on the boiler right up, which I don't want to do for efficiency reasons.

So, my question is are these cylinder thermostats replaceable? and if so, can you recommend a brand I should replace it with?

 

[oldbuffer]

1. The range of the thermostat is 45 to 65 degrees.
2. They are replaceable, but need to be replaced with the Everflo supplied thermostat. Ordinary ones won't fit.
3. Only people with a current "G3" accreditation are allowed to work on these cylinders. They are not a DIY proposition.
4. They are guaranteed for 2 years, but read the terms and conditions.
5. If it's a recent installation, and your builders supplied and installed it, I'd suggest getting them back to correct it.
6. Installation of an unvented cylinder is notifiable to Building Control. You should have a completed benchmark and certificate for G3 installation.