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Wiring in a RF Thermistat to a Combi Boiler

Postby DarrenBolton » Sun May 04, 2008 6:48 pm

Hi all, looking for abit of advice.

I've just purchased a Salus RT500RF thermistat to wire into my Halstead Ace combi boiler and would just like some general advice getting my head round the wiring diagrams.

I can under stand that the RF receiver needs to go across connections L1 and L3, so that it then control when the power is given.

Whats got me confused is the instructions in the RF manual regarding either 230v or volt free. Can someone enlighten me on this please and point me in the right direction of what I need to connect where.
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Postby ericmark » Mon May 05, 2008 11:20 am

I note the boiler instructions call for volt free contacts and thermostat instructions tell you to remove link for volt free I am not sure if double insulated or not I can't find any reference in instructions. Double insulated will have a double square sign and does not require earth otherwise an earth will be required. It would be normal to power from same supply as boiler and a 4 or 5 core cable will be required between the two. Often two twin and earth cables are used one for power other to the L1 and L3 I will agree it does seem odd to say no volt contacts are required when it shows on plan that L1 and L2 are internally connected together and a three core cable with earth would do the job and no real need for two line cables I would suspect they are playing safe in case some one gets cores swapped but since it says no volt and the thermostat can work no volt I can not see why you should not connect as no volt.

I will now try to explain no volt. It means there is no voltage connected to the contacts from the supply of the thermostat and is normally used where the boiler switches a 24volt supply for the thermostat but the thermostat supply voltage is 230 volts. Many old thermostats had little heaters built in to compensate for the reaction time and could not be used in no volt mode.

The fitting of thermostats outside has reduced costs I believe I will assume that is what you are doing. I don't have a modern system with TRV's on all radiators so I still use a room stat for main control but my daughter has a combi boiler and we were considering fitting an external thermostat to turn off central heating in the summer months and would be interested to hear how it works. Of course with TRV's there is no point of fitting a thermostat inside it would mess up the system.

Postby sparx » Mon May 05, 2008 10:46 pm

Hi, the RF stat can work in two modes, it needs 230v to operate, then if your boiler has a control supply and return then they go to 1&3 and no link must be fitted between L & 1 or3 .
some boilers only require a switch line to control return which lets you link from L to 1 or 3, saving one core in cable ie only needs 3core + earth instead of 4 core + earth.
'Volt free' is a bit of a misnomer it just means the contacts are operating on a seperate circuit not derived from the stats 230v direct.
I fitted a Salus stat recently and I think it shows a dotted line between L & term 3 (common) this is your optional link.
As for where to fit it, part L of bldg regs REQUIRES a stat INDOORS in an area without a TRV usually in a passage or similar so that when all TRV's are up to set temp and shut off the stat then shuts the boiler off when passage also up to temp. set on stat, to prevent boiler constantly 'cycling' on/off,
Hope this helps ,SPARX
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