switchable or permanent live to froststat??

Discussion in 'Electricians' Talk' started by Ron H, Oct 31, 2023.


Should froststat be permanent live or deactivated by boilerswitch? Please explain in text!

  1. Permanent

  2. switchable

    0 vote(s)
  1. Ron H

    Ron H Member

    Hi, House/boiler is mid-90's. I am updating my froststat (it physically broke and has been coming and staying on) and I have found that it is on a PERMANENT live (on voltage checking) despite the boiler switch overriding its effect, (ie fully turning off boiler), so I presume that the boiler is on the switchable live.

    Is this set-up wired correctly? I see no obvious reason the froststat should be permanently live, as the boiler switch definitely deacticates it (I have clearly shown this).

    Should I rewire froststat to be switchable live? Should boiler be switchable (as it seems to be) - or permanently live? Thank you!
  2. The Happy Builder

    The Happy Builder Screwfix Select

    A permanent live to a normally open frost stat then onto a normally closed pipe stat on the heating return, then a connection to fire the boiler, turn the circulator on and open any valves to allow the boiler to fire and send heated water around the heating system with the pipe stat turning it all back off again when the system is warm.
  3. Ron H

    Ron H Member

    but why is mine permanent - as the boiler switch overrides it anyway? It serves no useful purpose being permanently live?
  4. The Happy Builder

    The Happy Builder Screwfix Select

    Leave it alone if you don't understand how it is wired.

    The frost stat works completely independently of the central heating and boiler controls, it has to have a permanent live to it taken from the same switched fused connection unit as the rest of the heating and boiler controls.

    A frost stat wired from a timer or any other central controls is useless.
    HarryL1234 likes this.
  5. Bob Rathbone

    Bob Rathbone Screwfix Select

    The frost stat is to prevent the water in the system freezing in the event of very low temperatures and the CH programmer being set to off. It will over ride the programmer and turn the system on just enough to prevent freezing.
    HarryL1234 likes this.
  6. Ron H

    Ron H Member

    Thanks all, I couldn't leave this as it has to be fixed and heating was coming on unnecessarily. All is working OK now with new froststat. The main boiler switch is essentially on all time anyway, but the curiosity remains that in the case of switching off, say for a long holiday etc, this does appear to deactivate the froststat.
  7. The Happy Builder

    The Happy Builder Screwfix Select

    The boiler should NOT be turned off if you go on holiday, you leave the boiler turned then uses the holiday setting on the programmable thermostat if there is one or turn the room thermostat/s down to a suitable temperature such as 10 degrees and turn the hot water off.

    Deactivating the frost thermostat by turning off the frost protection on the heating system may invalidate your insurance completely or result in a reduction in any pay out for repairs due to water leaks being cut drastically.

    I have known several old ladies who turned their heating off over Christmas whilst they went to stay with friends or family over the holidays and their homes were wrecked when they returned; and have done remedial work in some of them, arguing with the insurance companies on behalf of the customers to get the full cost of the work covered.

    You need to review how the frost thermostat is installed and how you are operating your heating system; in particular you should be absolutely clear in your head that you DO NOT TURN THE FROST PROTECTION OFF.
  8. MGW

    MGW Screwfix Select

    The basic idea is the whole of the central heating is supplied from one FCU or plug, and in my case that is in turn supplied from an inverter, battery, solar panels and grid so loss or grid power will not stop central heating from working.

    It also means one switch on the FCU kills all central heating power so safe for the boiler man to work on it. Also any fuse inside the boiler must not be by-passed with any thermostat, so often there is a special feed in the boiler to the thermostat so if a fuse blows it is not by-passed.

    Today in the main the frost stat is a thing of the past, the programmable thermostat doubles as both frost stat and main thermostat and often has an away mode built in.

    Yes some people do daft things, but one still can't by-pass the boilers internal fuses with any thermostat.

    To be frank, my boiler was in the garage, and as such it should have had a frost stat, but it didn't and in the 30 years it was installed nothing went wrong as far as freezing goes, this house also boiler in garage but now converted into a flat, so simply use the wall thermostat turned to a low setting. Where a frost stat is required is where the boiler is outside, common around here, but in the main the frost stat powers trace heating and is independent from the boiler, so run out of oil or LPG is still works.
  9. The Happy Builder

    The Happy Builder Screwfix Select

    If the boiler manufacturer provides terminals within the boiler for the connection of an external frost stat use them.

    But don't then be stupid and turn the boiler off at the main switch or switched fused spur, use the heating controls.

    Surely it isn't too difficult to understand the implications of turning the boiler off completely or switching the frost stat supply off?
  10. Ron H

    Ron H Member

    OK, let's all "Calm Down"! I had no plans to turn my boiler off over winter but thank you for your words of advice!! I am actually glad the f/s is powered via the switch as this did allow me to deactivate it when the old one went crazy after breaking. Regards to all

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