Hot water thermostat positioning

[simm42]

Background: I've owned this place for 18 months - as far as I can tell it was refurbed in the 90's by well meaning amateurs and then neglected for the last 10 years by the subsequent owners. I'm doing a combination of major upgrades and fixing what isn't working to last long enough until it gets replaced as part of the appropriate major upgrade. The heating system is getting done next year.

With that out of the way, I don't believe the hot water thermostat is entirely reliable. It seems extremely variable if it will turn off, and the control box appears to give priority to hot water over central heating, so if I don't keep it set low enough, there house gets cold. This leaves me with not quite enough hot water for a full bath.

One option is just to replace it - its £20 and a minor wiring job so no issue there.

However I can't find any documentation on how close the metal plate on the back of the thermostat should be to the tank? Its spring loaded to push it back, but the plastic sides of the thermostat keep the plate a good 2mm away from the exposed wall of the tank where the insulation has been cut away.

Is that normal? Or is it incorrectly fitted? My instinct is to say it should be up against the copper in order to have an accurate reading.

If it is wrong, is there a standard way of fixing this?

 

[JohnD]

show us a photo of the stat you mean

and the pipes, pump and motorised valve too, please

 

[simm42]

I'm not at home right now however the stat is one of these (perhaps an older variant): https://www.screwfix.com/p/drayton-hts3-cylinder-stat/95630

Motor is mounted in the tank cupboard along with a fairly standard 3 way valve. Boiler is in a different room, potterton. Header tanks in the loft. They all work as expected (though not well, hence the planned replacement) - the only part behaving unexpectedly is the stat on the tank which turns on fine, but doesn't always turn off reliably even when the water appears to have reached temperature. The temperature is set to much lower than what the boiler is capable of heating to.

 

[JohnD]

there's hardly anything in that stat to go wrong. I suppose if you tipped a sugary drink into it, it might stick. It's OK to press the casing of the stat against the metal of the cylinder, it should have a belt or stretch cord holding it tight. If the cylinder has insulating foam it must be cut away. if it has a red jacket the jacket can go over it.

It has to be a bit higher than the lowest circulating pipe from the boiler that goes into the side of the cylinder, but the lower it is, the more hot water you can store, It should be set round about 50C (the top of the cylinder will be higher) and the boiler should be about 10C hotter. You will need a thermometer or other device to check accuracy.

Photos will help.

 

[John D v2.0]

Is that normal? Or is it incorrectly fitted? My instinct is to say it should be up against the copper in order to have an accurate reading.

No you are correct it should be touching otherwise how can it know the temperature correctly.
You may need to dig a space for it in the insulation and strap it in place properly

 

[dilalio]

He said that the insulation IS cut away and the stat IS against the metal surface... he is asking if the metal plate of the stat should protrude beyond the plastic housing of the stat casing to touch the cylinder so it is metal against metal!
This would obviously be ideal - unless it is done purposely to aid electrical safety! Can’t say I’ve ever really looked at them that closely... I just strap em on and wire em up!
OP you could get a magnetic thermometer and place it next to the stat to monitor if it is correctly opening and closing the switch - or just replace the stat for new for the sake of a few quid and see if that improves things. Also consider repositioning it but photos of where it currently is would help us see if it’s in a bad spot!