Central Heating Thermostat Advice

[Julie Lee]

Hi - I wonder if anyone can help before I throttle my partner who is wanting to replace our central heating thermostat. He keeps telling me the problems but won't come on here himself to get advice ... so here goes ..

• The old room thermostat is a 3-wire type and he wants a new digital one with LCD screen.
• He doesn't want battery powered so needs to be mains powered.
• He doesn't mind if it has a backup battery that is "float charged" to keep stores settings under power failure but it's not critical.
• To match the rating of the old stat the new one needs to be 16A rated switch contacts.
• He has a separate programmer so only needs basic room stat functions that shows set temperature and current room temperature - if the new one happens to have a built in programmer that's ok.

He wil be installing the new stat in a difference location and needs to run new cabling. The heating system is wired to the "S Plan". His understanding is that he will need 16A 1.5mm flat 4 core (+earth) cable wired as follows -

1 - Permanent Live to power the stat

2 - Permanent Neutral to power the stat

3 - Live feed from the programmer to the stat switch contact

4 - Control wire from other side of the stat switch contact to carry the programmer Live back to the zone valve and on to the pump and boiler.

His 3 questions are - 

• Is his understanding of the wiring correct?
• Where can he buy the 4 core (+earth) cable from, or should he run two flat twin and earth cables?
• Which room stats are mains powered as all he has found are battery powered?

I hope this makes sense and someone can help - thank you, much appreciated

 

[steptoe]

What sort of boiler is it?

16amp is a massive load for a boiler!!!

 

[ProDave]

You won't need a 16A thermostat to switch a heating system. Most are rated much lower.

Use two twin and earth cables. Fed from a source from a 6A MCB, then 1mm should be fine.

Check the specifications of whichever one you choose to ensure it retains it's program settings in the event of a power cut. Not all of them do.

 

[SLIPSHOD & SLAPDASH]

Use two twin and earth cables. Fed from a source from a 6A MCB, then 1mm should be fine.

I doubt its fed from a mcb direct. More likely from a  FCU (Spur)  which should have a 3 or 5A fuse.

 

[ProDave]

I doubt its fed from a mcb direct. More likely from a  FCU (Spur)  which should have a 3 or 5A fuse.

True. but my point was only use 1mm if something protects it, don't use 1mm if it's fed directly from a 32A MCB (but you wouldn't feed a thermostat directly from that anyway)

 

[Richard-the-Ninth]

1 - Permanent Live to power the stat

2 - Permanent Neutral to power the stat

3 - Live feed from the programmer to the stat switch contact

4 - Control wire from other side of the stat switch contact to carry the programmer Live back to the zone valve and on to the pump and boiler.

Only a 3 core and earth required.

Item 1 and 3 are the same thing  So you can link from 1-3

 

[ProDave]

Only a 3 core and earth required.

Item 1 and 3 are the same thing  So you can link from 1-3

No, The live feed from the programmer will only be on at the times the programmer is set to have the heating on. so it's a switched live feed.

 

[Steve3948]

Agree with PD, depending on what thermostat is purchased.

 

[SPECIAL LOCATION]

I wouldn't use flat solid T&E or 3C+E to a thermostat...

it can be to ridged to terminate into the connections easily.... 

Just get a long enough piece of 5core flex...

You will have all the conductors you need and its easier to terminate...

http://www.tlc-direct.co.uk/Main_Index/Cable_Index/Flex_White_3/index.html

Guinness

 

[Steve3948]

I wouldn't use flat solid T&E or 3C+E to a thermostat...

it can be to ridged to terminate into the connections easily....

Just get a long enough piece of 5core flex...

You will have all the conductors you need and its easier to terminate...

http://www.tlc-direct.co.uk/Main_Index/Cable_Index/Flex_White_3/index.html

Guinness

Maybe with 1.5, but never had a problem with 1.0 to be honest, after doing 12 a week for 7 years.

 

[SLIPSHOD & SLAPDASH]

I wouldn't use flat solid T&E or 3C+E to a thermostat...

it can be to ridged to terminate into the connections easily.... 

Just get a long enough piece of 5core flex...

You will have all the conductors you need and its easier to terminate...

http://www.tlc-direct.co.uk/Main_Index/Cable_Index/Flex_White_3/index.html

Guinness

Said by a true plumber 

 

[Julie Lee]

Thank you all ... the boiler is a Baxi solo WM 50/4 PF which may not itself be 16A but the old thermostat was 16A and the load on the thermostat switch contacts would be a combination of the boiler, pump, and zone valves. 

I checked the link for 5 core flex and see there's a 10 amp 1.0mm flex ... so this flex would comply with IEE regulations? He thought it had to be flat cable.

 

[SPECIAL LOCATION]

Maybe with 1.5, but never had a problem with 1.0 to be honest, after doing 12 a week for 7 years.

Taking into account the OP is a DIY....    (not regular day-in day-out spark) 

and the thinking they need 16A supply / switching...

so  quite probably not very familiar with wiring / terminating / test gear etc... 

2x 1.0mm T&E greater chance for getting cores confused...

No regs against using flex providing it is not exposed to extreme stresses etc..  (521.9.1)

There is NO reg that says flat T&E must be used.

Cheaper 1x single 5core flex... than 2x 1.0mm T&E...

Some heating controls have really crappy screw terminals that aren't the easiest to get a good grip on the cable...

Bit different from a regular electrician who may have a coil of 1.0m T&E on the go anyway -vs-

someone buying a one-off length of cable / flex..

So based on the original question..

my previous answer still stands IMHO.

Guinness

Thank you all ... the boiler is a Baxi solo WM 50/4 PF which may not itself be 16A but the old thermostat was 16A and the load on the thermostat switch contacts would be a combination of the boiler, pump, and zone valves. 

I checked the link for 5 core flex and see there's a 10 amp 1.0mm flex ... so this flex would comply with IEE regulations? He thought it had to be flat cable.

16A is over 3kWatts...    (16A x 230v = 3680watts)

You will be hard pushed to exceed 3A on heating system control..

Pumps/valves etc..

Guinness

 

[steptoe]

Id go with the 5core flex,

I have a friend with his programmer/stat wired in 5core flex.  :innocent

its absolutely fine.

 

[Julie Lee]

That makes sense, even I'm starting to understand it now. So thank you all very much ... will go with the 5 core flex .. problem solved.