Help please with a room thermostat.

TUVOK

Hello,
My son has a Worcester Greenstar Si condensing boiler fitted about 7 years ago.
The program is situated right by the boiler and he has a mechanical type room thermostat in his lounge.
The query is that the room thermostat is now not working correctly, it is some considerable years old, can I just fit a replacement mechanical type thermostat to replace it or should it be a different type?
Thank you for any/all replies

Le_Kirk

TUVOK said:

Hello,
My son has a Worcester Greenstar Si condensing boiler fitted about 7 years ago.
The program is situated right by the boiler and he has a mechanical type room thermostat in his lounge.
The query is that the room thermostat is now not working correctly, it is some considerable years old, can I just fit a replacement mechanical type thermostat to replace it or should it be a different type?
Thank you for any/all replies

What about it is not working correctly?  How does the "not working correctly" manifest itself?

TUVOK

It will only operate at 20-22 degrees, in effect does not respond to room temp.
i.e if you came in and measured room temp to be 15 degrees and turned the thermostat to 18 degrees nothing would happen if you put stat at the 20-22 degree mark, boiler will fire.
Hope this helps?

grumbler

Yes, you can replace the thermostat with the same of similar one. It's worth unscrewing it first, taking and possibly posting here a picture of wiring. Also, you can make sure that it's the thermostat that is faulty by manually connecting/disconnecting the wires (caution - it's mains voltage!)

knightstyle

Older thermostats had an extra, neutral, connection which made them more sensitive. I wonder if this has come loose or if the small resistor, I think, has failed?
As others have said almost any old style thermostat will work, electronic ones are more sensitive and some can be set to give different temperatures for morning and evening.

Mutton_Geoff

knightstyle said:

Older thermostats had an extra, neutral, connection which made them more sensitive. I wonder if this has come loose or if the small resistor, I think, has failed?
As others have said almost any old style thermostat will work, electronic ones are more sensitive and some can be set to give different temperatures for morning and evening.

The neutral connection actually makes the old style thermostat less sensitive so it doesn't keep cycling around the set temperature (histerysyis).

I agree, replacing the stat with a modern programmable stat will help save energy (you set different temperatures for different times of day according to your needs). Better still a geo based system that turns the heating/HW off when you are out regardless of the presets.

If fitting any of these, then the boiler timer must be set to be on for 24 hours so the new device has full control over the boiler.

Jeepers_Creepers

Mutton_Geoff said:

The neutral connection actually makes the old style thermostat less sensitive so it doesn't keep cycling around the set temperature (histerysyis).

Hmm, I think Knightstyle is more correct on this.

These old electro-mechanical stats have terrible lag due to the tiny movement of the bi-metallic strip reacting to small fluctuations in temp; on their own, they probably wouldn't move enough to 'click' the contacts with a more than 2 degree room temp variation - far too uncomfortable.

Hence the accelerator resistor which is wired to also come on with the boiler; this heats the bi-met strip and 'exaggerates' the warmth of the room - when the boiler comes on to heat the rads, so does this little resistor which them warms the bi-met strip more than it would otherwise, so the stat clicks off again sooner than it otherwise would. Ditto when the stat turns the boiler off - the heater also goes off and the bi-met cools more rapidly than just from the room's falling temp.

Crude, very crude - but also clever and effective!

Electronic stats are too sensitive in themselves, so have 'hysteresis' built in to its program. On some models, this can be adjusted. 

Jeepers_Creepers

TUVOK, is this a combi boiler? 

Anyhoo, to answer your question, you can fit almost any room stat there, electronic or even Programmable. 

Being an 'old' electro-mechanical stat, your friend's unit will almost certainly have a neutral connection too, so that means you also have the choice of mains or battery operated replacements. 

And, as Knightstyle says, they could even go Programmable type - which I'd thoroughly recommend. These will do the job of the timer and stat together, so that you can tell it to, say, come on ast 21oC at 7am, go down to 16oC at 8.30am, back up to 20oC at 4pm, up further to 21oC 6pm, and down to 15oC at 11pm for the night-time. And you can override it any time you wish.

If you friend does fancy going 'Prog Stat', then this strikes me as an amazing bargain: https://www.ebay.co.uk/itm/Danfoss-Randall-TPOne-M-087N785200-Programmable-ErP-Class-4-DHW-Thermostat-New/143424279353?epid=14021664577&hash=item2164c0d339:g:EJ8AAOSwjYldtbGc  but your friend should check it out to make sure it is what they want. 

If you friend comes up with some choices, you can always come back on here and ask for advice, including on wiring. 

TUVOK

grumbler said:

Yes, you can replace the thermostat with the same of similar one. It's worth unscrewing it first, taking and possibly posting here a picture of wiring. Also, you can make sure that it's the thermostat that is faulty by manually connecting/disconnecting the wires (caution - it's mains voltage!)

Mains voltage  is one thing that I am worried about as I am sure that you have to be aware of that with this particular boiler a Worcester Greenstar.
I think that an ordinary mechanical thermostat would be suitable for him, and I am happy about disconnecting existing thermostat and replacing with a new one (assume internal wiring connections of stat would be the same as existing one , apart from wire colour changes)
It's the mains voltage at play I worry about? 

TUVOK

Jeepers_Creepers said:

TUVOK, is this a combi boiler? 

Anyhoo, to answer your question, you can fit almost any room stat there, electronic or even Programmable. 

Being an 'old' electro-mechanical stat, your friend's unit will almost certainly have a neutral connection too, so that means you also have the choice of mains or battery operated replacements. 

And, as Knightstyle says, they could even go Programmable type - which I'd thoroughly recommend. These will do the job of the timer and stat together, so that you can tell it to, say, come on ast 21oC at 7am, go down to 16oC at 8.30am, back up to 20oC at 4pm, up further to 21oC 6pm, and down to 15oC at 11pm for the night-time. And you can override it any time you wish.

If you friend does fancy going 'Prog Stat', then this strikes me as an amazing bargain: https://www.ebay.co.uk/itm/Danfoss-Randall-TPOne-M-087N785200-Programmable-ErP-Class-4-DHW-Thermostat-New/143424279353?epid=14021664577&hash=item2164c0d339:g:EJ8AAOSwjYldtbGc  but your friend should check it out to make sure it is what they want. 

If you friend comes up with some choices, you can always come back on here and ask for advice, including on wiring. 

Yes, it is a combi boiler about 7 years old, room stat is 25+ years old, so I assume that is the problem allied with that the stat does work but only in a 2 degree band, i.e. 20 to 22 degrees fire the boiler!

Jeepers_Creepers

Any chance of a photo showing the existing connections to this 'stat? With this we can guide you - literally 'take this wire from x and put it in y'.

That is assuming that someone - either your friend or yourself - is savvy enough to wire a plug? 

If an old mechanical stat goes wrang, it just seems a waste to me to simply replace it with another crude device. For the same outlay, they can have an electronic prog stat  - really, they make life so much more comfy, and will possibly save you energy.