Frost Stat Wiring

22 Oct 2009
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United Kingdom
I have just moved into a house thats fitted with a Greenstar 25Si central heating boiler thats about 5 years old. I'd like to fit a frost stat because I'm away for weeks at a time and like to bring the heating on automatically when the temperatures tumble. The diagram for the boiler shows a very simple 2 connection circuit, ha ha thats easy I thought so went out and bought a Tower RFS Frost Stat from my local supplier. But when I opened it up, it has a small circuit and seems to need both live and neutral. T1 is live, T3 and T4 are nuetral connections. T3 shows an M inside a circle. T3 and T4 has a neon lamp across them. Have I bought the wrong stat, or can I still use it ? Advice on how to wire it will be greatly appreciated.
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The TFS has an Integral accelerator heater so you'll need 3-core and earth cable to provide
neutral and the switched live back to the wiring centre.
hey thanks TTC, your right about the accelerator heater. But still can't see how to wire the terminals. The boiler has FR and FS allocated for the frost stat so I assume one of them is live (FS ?). With there being only 3 terminals on the stat I can't see how to supply the heater and make the switched return as its showing T3 and T4 as neutral ?

Hangin on !
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T1 is live, T3 and T4 are nuetral connections.
No there aren't 2 neutral connections, T1 is live, T3 is the live return, and T4 is neutral.
Not all frost stats have have an accelerator or lamp that require a neutral, so the Greenstar manual may not show the neutral connection for it.

Based on the information you have given, I would expect at the boiler connections that FS is "frost supply" and FR is "frost return". If so the wiring would be as follows:

FS (frost supply)-----------Live------------- T1
FR (frost return)------Heating Output---- T3

N----------------------------Neutral---------- T4

Terminal T1 is the live supply, when the thermostat calls for heat, it makes T3 live that goes back to the boiler switching it on, and also puts a live on the lamp (x), the other side of which is connected to neutral.

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