Best place on hot water cylinder to mount DIY thermostat ?

[picbits]

Spent the day troubleshooting my old heating system. It has a sealed hot water cylinder (no tanks in attic) and a 3 way diverter valve to switch between the tank heating and radiators.


Found that the thermostat on the cylinder is knackered. It is effectively a SPDT switch so I was thinking of retrofitting a new temperature sensing system to the cylinder. The old thermostat is well and truly corroded on and has had a leak in the past (not leaking at the moment so I'm not messing about with it). The whole central heating system will be replaced in a few months anyway so I dont want to throw any real money at it.

I am an electronics engineer so can whip up something that will measure the surface temperature on the cylinder and trigger a SPDT relay when the temperature is reached (and have a window of a few degrees between switch off and on). I can get a 0.1 degree accuracy from my sensors by the way

Biggest question is - where on the cylinder is best to mount it? I'm thinking around the area the existing thermostat is mounted but away from any pipes which might give a false reading due to heating up the surrounding copper.

Cheers for any advice
Dom

 

[JohnD]

is it a rectangular metal box, about 75mm x 50mm x 50mm, maybe with an adjusting screw, fastened to cylinder with a metal strap?

Called a cylinder thermostat.

You can buy a new one for about £10

As it is mains electricity in the presence of copper pipes and water it would be preferable to buy an ordinary one rather than make your own.

Usually fitted about two-thirds down from the top (it must be higher than the lower entry of the primary water from the boiler)

There is no reason why it should leak - it is in contact with the outer surface of the cylinder, not with water.

Cable should be heat-resistant.

 

[gas4you]

Or 1/3rd up from bottom Don't put it between the flow and return on the cylinder.

 

[picbits]

Existing thermostat is screwed onto/into the cylinder. The cylinder itself has a foam insulation from factory and the pipes protrude from this so fitting a "strap on" isn't really an option as there is no copper exposed.

There will only be 5 volts with a couple of milliamps going to the sensor and it will be run through a double insulated power supply anyway (fused and regulated). The sensor will be epoxy mounted with thermal conducting epoxy on a bare part of the cylinder (i.e. a part that I've made a small hole in the insulation). The epoxy itself is also electrically insulating so even if I had a couple of thousand volt surge through the supply which got to the sensor nothing would happen anyway (apart from a small puff of magic smoke).

The system was here when we moved in last year and is all on its last legs - boiler is a Prima F50 and the Cylinder has more leaks than Wales round all the joints. Its only around 14 years old but like everything else in this house it has never been looked after.

 

[gas4you]

For standard cyl stats you have to cut out the foam insulation to slot them in against the copper cylinder

 

[JohnD]

the strap-ons are fitted by cutting away a rectangle of foam for the stat to pass through and press against the cylinder. There are probably 20 million fitted like this in the UK (of which only one is in my house) so it is far from impossible.

however I can see you want to fit your home-made one so I will leave you to it.

 

[gas4you]

Well said John

 

[picbits]

Hey - if I can pick up a strap on stat for a few quid and save myself the hassle of making one then I'm all for it

Will go on a search tomorrow for a cheapish one to get this back up and running nice and quickly.

I learned my lesson with my homebuilt networked computer controlled washing machine - became the most famous washing machine in the world once I built it and it even had its own "Winwash 2000" operating system. Only problem was the missus couldn't use it so went out and bought a new one. That was a months worth of work down the drain lmao

 

[gas4you]

Should be able to get one for around £10-£15.

 

[picbits]

Found some in Screwfix for £8.50-£14 inc VAT. Got a store 20 minutes away so will pick one up tomorrow.

Cheers guys - you've helped a lot

 

[picbits]

Picked up a cheapo Horstmann cylinder thermostat for £8.50 and its all fitted and working

Should keep the old system going until I can find a decent plumber to fit me a new combi boiler at a reasonable price.

 

[Agile]

If you were to have designed the stat you would need to have had some feedback in the circuit or a Schmidt trigger to provide a switching hysterisis of about 3 C to prevent short cycling of the boiler!

Tony

 

[picbits]

I'd have used a PIC Microcontroller with an LCD display and a small keypad for data entry. You would be able to set the turn on and turn off temperatures digitally and it would have had an automatic 65 degrees overheat initially to kill anything nasty in the tank dropping to 60 degrees for the rest of the day.

I priced it up yesterday and could have done it for around £15 in parts but banging on a boggo stat has done the job nicely.

Now somewhere in a box in the garage I've got a digital central heating timer ...........

 

[Onetap]

NOOOOOOOOOOOOOOOOOOOOOOOOO!

FFS. You bunch of muppets. Go and sit on the naughty step, the whole lot of you.

Spent the day troubleshooting my old heating system. It has a sealed hot water cylinder (no tanks in attic) and a 3 way diverter valve to switch between the tank heating and radiators.


Dom

It is an UNVENTED hot water storage cylinder.
Now he's going to put on a strap-on?

The thermostat is rusted in because it's a special.
He shouldn't be buggering about with it if he isn't certified competent (CITB or IoP) and that is why.

The thermostat must be the manufacturer's replacement or you invalidate the BBA certification and possibly your house insurance when it bursts.

The existing thermostat will have the control immersion thermostat and a second, manually re-settable overheat thermostat.

He's going to fit a strap on? What happens if it falls off?
Why do you think it was screwed into place?

So what else have you buggered about with on it?

 

[JohnD]

shame