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Likely Gas Combi-Boiler P.C.B. Damage.

Mornin' all,

                         I hope that you are all keeping your spirits up during these trying times.


Yesterday I made a mistake. I can only attribute the reason to the work environment and me being pre-occupied with self distancing on site and being distracted by the house occupants, plus stupidity.


The owner had apparently struck a wall thermostat due to it being unreliable in operation, and he having a bad temper. The dial part was cracked. He asked me to change it for a new one. The wall thermostat is a Honeywell make. I have wired up many of these in my time. I did not have a Honeywell replacement but did have another make of a similar design. So I fitted that. After I had fitted the new one I noticed that I had no mains supply showing at the wall stat. I checked the switched fused connection unit at the boiler and it was displaying a glowing neon, so the 3 Amp fuse there was intact. I was asked to refit the original wall stat, which I did, and a gas engineer is coming out to look at the boiler. I did not see the boiler working initially, but the owner and his wife assured me that it was working that morning.


I must have wired the new wall stat up wrongly and shorted between L and N. That probably blew the 1, 2 or 3.15 Amp fuse on the boiler P.C.B. I did not dismantle the boiler to look.


What is the chance of irreparable damage to the boiler P.C.B?


Unfortunately I did not note the make and model of the boiler.


Bye,


Gloomy Z.


  • Zoomup:




    Chris Pearson:

    Hang on!


    Isn't the thermostat just a temperature controlled switch? Does it have any significant resistance when the contacts are closed?




    The old faulty one did. The new one should have just minimal contact resistance. What's your point Chris?



    That if the thermostat has negligible resistance when closed, shorting out the cable cannot have done any harm.

  • Have you never replaced a programmer with a light switch to get the heating back on a temporary basis?


    True, it's not a good idea to connect the live and neutral together instead of the live feed and switched live. 


    Andy Betteridge

  • Isn't the thermostat just a temperature controlled switch? Does it have any significant resistance when the contacts are closed?



    Traditional wall thermostats are a bit more than that - yes a temperature controlled switch, but also a resistive heating element (usually actually a resistor) connected between the switched L and N - to provide a little extra heat to the sensing element when calling for heat - to anticipate heat from the boiler and reduce hysteresis. Shorting L-SL won't be a problem of course but shorting L-N or SL-N when cold will draw a very large fault current from the supply ... typically via the boiler PCB.

       - Andy.

  • Chris Pearson:

    Hang on!


    Isn't the thermostat just a temperature controlled switch? Does it have any significant resistance when the contacts are closed?





  • Does the boiler have a built in time clock? You are very unlikely to have damaged the boiler, if you havent been near the pcb, unless you L N short, but that will usually just take out the 1 amp fuse on the pcb.

  • Typiod:

    Does the boiler have a built in time clock? You are very unlikely to have damaged the boiler, if you havent been near the pcb, unless you L N short, but that will usually just take out the 1 amp fuse on the pcb.




    Yes there is a time switch set to 24 hours on. It is an older style with a rotating dial and small tappets which can be physically pushed to ON or OFF. It is permanently ON.


    Z.

  • Some years ago when I was wiring several boilers and heating system controls a week for a heating company I went to Maplin and stocked up on all the common fuses used in boilers, I cannot remember if I still have any or not, it’s now a few years since I used any and I don’ know if there’s any in the spare fuse box in the bottom of my van or not.


    They came in very useful when drying boilers out after after major water leaks, including one on Boxing Day one year, a hot air gun and a packet of fuses were always handy and did not cost a lot.


    Andy Betteridge


  • Thank you - I see what you mean. More than just a switch!
  • https://www.theheatxchange.co.uk/worcester/fusing-element-set-t0-5-t1-6-t2-5-t3-15a/87445030100/sku_WOR87445030100
  • However a bit of shopping around is wise

    https://uk.rs-online.com/web/p/cartridge-fuses/0563633/