Hot water too hot with gas boiler, cylinder thermostat seems to stay cool.

[scots_jon]

Apologies if this has been asked before, I did try a search.

We have juts moved into a house that has a Baxi 100E gas fired boiler feeding the central heating and hot water cylinder. The hot water is always far to hot, dangerously so. I have checked the valves and these seem to correctly select between the heating and hot water loops.

There is a water thermostat on the side of the cylinder in a cutout in the insulation. This seems to act only as a switch, i.e. with it set below 20c the water never gets hot, with it anywhere above 20C the water is far too hot. The thermostat seems to work because I can here it click when the knob is turned and that in turn controls the hot water valve. However even with the tank brim full of very hot water the surface that the thermostat sits on is never warm! So it seems the thermostat isn't getting to detect the water temperature. Further evidence is that the there is a immersion heater in the cylinder (not wired up). The 'stat on that clicks when turned to about 75C, while the 'stat in the cutout on the side of the tank is still clicking at 20C and the tank is cool behind it.

Is the tank possibly twin walled??? is the side 'stat just in the wrong place? I can't think what on earth is going on.

Many thanks in advance for some help.

Cheers,
Jon.

 

[diamondgas]

Hi jon & :welcome: to the forum 🙂

A picture saves a thousand words. Could you paste a photo of the hot water cylinder by any chance? You are right however, the cylinder thermostat does need to be in contact with the cylinder wall about a third of the way up the cylinder. If the wall behind the cyl stat is cool then I'd say it's in the wrong place.

 

[scots_jon]

Pictures below, not sure what more to say! I was going to simply rewire it using the immersion heater 'stat rather than the one on the side, but the one side seems to be three wire one. I've included a picture of the immersion heater just to prove that it isn't that heating the water! Any help would be greatfully appreciated, the water is scalding and I'm worried about the missus and kids.

Cheers,
Jon.

 

[SafeGasInstall]

welcome to the forum Jon ,

if competent to do the job then this use this

honeywell cylider stat - Google Search

 

[scots_jon]

welcome to the forum Jon ,

if competent to do the job then this use this

honeywell cylider stat - Google Search

Thanks for the advice. If the cylinder wall behind the 'stat is cool to the touch, how would this help? The existing stat seems to be ok, it's juts not getting heated by the water in the cylinder.???

 

[diamondgas]

Thanks for the pictures Jon. I cannot explain why the cylinder wall is not getting hot??? It should ...!

You'll not be able to wire the cyl stat into the imersion stat if you have a 3 port valve as you need 3 wires, C-NO-NC ... However if its a 2 port then you'll be okay!

 

[tamz]

Either reposition the stat higher up or turn your boiler stat down to around 60º. The water in the cylinder can only get as hot as the water heating it.
If 60º water is too hot for you then consider having a thermostatic mixing valve fitted and set it as low as you like.
The temperature of the stored water should be above 60 to kill legionella bugs.

 

[stevetheplumber]

move the cylinder stat up to where it can get a reading usually one third up is good but yours appears to be there so go up higher only problem could be the water turning of before there was sufficent in the tank to fill a bath but that looks like quite a big cylinder
there can be 10 degrees of difference in a cylyinder from normal stat position and the top

 

[scots_jon]

Hi All,
Thanks for your help. I've moved the 'stat further up the tank, to about half way, and it seems to be working better now. I'll leave it for a few days to see how it settles.

Cheers,
Jon.