Can I install an electronic thermostat on my system??

[basill]

Hi folks. I have a Saunier Duval Thema F23E combi boiler with the standard SD thermostat with a round dial temp gauge in the hallway to control the radiator heating. I am wondering if I can simply replace this thermostat with say one of the following all singing and dancing variety:-

http://www.screwfix.com/app/sfd/cat/pro.jsp?id=12157&ts=94623

If it is ok then:-

- is it straight forward enough to swap them, are there any issues to be aware of?

- do I need any new wiring or do I just remove the old thermostat and use its existing wiring (I seem to remember only a couple of wires poking out when the gasman came to install the boiler a few years ago)

- how does the new thermostat override the boiler on/off switches and timer for the heating (that round timer dial with pull out plastic selectors at the boiler)? Do I simply leave the heating on 24/7 and the thermostat effectively becomes the master and controls it firing or not?

Thanks in advance for any help.

 

[Paul Barker]

Yes leave the boiler control on full time.

Depends if the SD thermostat has a neutral connection, check before you buy your programmable thermostat. Some thermostats have a coil which preheats to smooth out demand (predict demand) which requires neutral, so there are three connections, live neutral and call for heat. Others only make a contact, this can be in the form of a live coming in which is passed on in call for heat, but the same stat can be used in no volts mode.

To find the answers for all these questions in your particular case look at your installation and Service instructions which by gas safety installation and use regulations should have been left with you when the boiler was installed.

 

[ArtfulBodger]

previous reply was better than mine!

 

[basill]

Hey thanks for getting cracking so quick!!!

I just checked the SD thermostat and there are two wires, one yellow and one red coming through the back plate into the terminal connectors. there are five terminals in total but just the two in use. The yellow one is going to N and the red one to L. I am guessing that L must be live which is also call for heat and N for neutral?

Job seems to easy to be true not that I am complaining

PS: Checked my SD user manual and their website and couldn't find anything about thermostat installation unfortunately, only how to install and service the combi itself.

 

[chrishutt]

Checked my SD user manual

You want to check the installation manual, which should have been left with you. It will give you details on wiring up a thermostat, but I'd be surprised if it wasn't "volts free" (i.e. low voltage from boiler).

If you know how to use a multimeter you can check voltage at existing room stat. By the way if the existing stat really is wired to the neutral terminal, it's probably not wired correctly!

 

[kevplumb]

External controls
The THEMA boiler is designed to operate at maximum
efficiency at all times. The of a programmable
electronic room thermostat is recommended
for optimum performance, Saunier Duval part
number 40010. Please contact your supplier.
The boiler will work for heating without a room thermostat
and/or timeswitch being connected provided
that the wire link fitted between the two terminals
of the connector (E) is left in place, see diagram
19.
A 230V room thermostat can be used but do not
make any connection to the compensating resistor,
see diagram 19.
ON NO ACCOUNT must any electrical voltage be
applied to any of the terminals of the external controls
plug.
sch 229
sch 228
E
°C
For no external controls
leave wire link (E) in place
When fitting room thermostat,
remove wire link (E) and connect
voltage free thermostat as shown
Diagram 19
sch 227
°C
°C
The mains electrical cable is supplied with the
boiler. It is coiled and tucked inside the boiler
THEMA F 23 E
THEMA F SB 18 E
Cylinder THEMA F SB 23 E

thats what the man says

 

[basill]

Thanks again for the help and quick replies. Turns outthere is only the one manual! Its entitled the user, installation and servicing instructions - if you can call them that! I found the quote that Kevplumb kindly pasted on page 17. Talk about the most concise installation instructions I have ever seen! Maybe they were trying to save on trees

From the installation manual for the thermostat I linked to above it states it is voltage free so it looks like we are in business

I will double check with the multimeter before going ahead just to be on the safe side though as suggested. Thanks everyone.

 

[Paul Barker]

Yes I'm not surprised how poor the manual is.

I'm amaised the system worked at all if there were only two rstat connections one of which was into the neutral. There must have been an incorrect connection inside of the boiler for it to work at all, probably the jumper not removed and the r stat wasn't changing anything.