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Discuss Protecting the boiler with a Relay in the Central Heating Forum area at

  1. KettlePilot

    KettlePilot New Member


    I am fitting a Heat only boiler that just serves the radiator circuit, the boiler has its own dedicated block terminal connector on the PCB but I cannot connect the pump to it as we normally do, the pump is a commercial type and Worcester Bosch have stated it will damage the PCB because of the excessive load/current drawn by the pump.

    W-B have advised fitting a relay to protect the boiler electronics. A work colleague has emailed me a picture (see link below) of one a sparky wired for him in similar circumstances but that's where the fun begins, can anyone please help me with a simple wiring schematic on how to install this particular relay to serve both pump/boiler and the other items listed below which may or not be relevant in relation to the relay

    Other items fitted which may be relevant:

    Honeywell RF2 Pack 1 RF Programmer and room stat

    Honeywell T4360 Frost Thermostat

    Honeywell L641B Pipe Thermostat

    Relay link: dav

    Thanks in advance for any help

    Ps. I'm hoping it’s just a simple case of some knowledgeable soul identifying the 'Supply' & 'Load' side of the Relay and how they are wired to the repective heating components !
    Last edited: Dec 2, 2017
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Easier to fit a contactor

    Pump overrun or just come on and off with boiler / load ??
  3. KettlePilot

    KettlePilot New Member

    Thanks for the swift reply Shaun, the Contactor is the Relay, I know some call it by both names.

    Cannot wire pump directly through boiler or extrenal heating controls, has to be routed through a Contactor (Relay) first!
  4. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Not slightly as a relay is for different voltages and contractors for high loads

    And yes do you want pump overrun or just on and off with demand eg same as boiler
  5. KettlePilot

    KettlePilot New Member

    Ahh I see, thought there were 'birds of a different feather'

    I know i need pmp over run, that will be taken care of by the boiler
    just need to know how to wire the heating pump in via the 'Contactor' ?
  6. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Right ok so your still using the boiler pump feed as your switching circuit (eg tell the contractor to operate)

    That makes things easier give us half an hour will knock something up

    Pump load / amp rating ?
    Last edited: Dec 2, 2017
  7. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    is this what you want?

  8. KettlePilot

    KettlePilot New Member

    As requested:

    Pump Link: sdr

    Your schematics make sense. Time to put plan 'A' in to action.

    Thank you very much for taking the time and trouble to help me out Shaun
  9. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Yep that's fine requires no starter and standard voltage and amps my drawing should be fine

    Double check on the elecs forum just incase I've missed something
  10. Chuck

    Chuck Active Member

    Just a comment that the isolating the boiler from its supply should also isolate the contactor and if this is not the case there should be durable warning notice(s) so it is clear that both boiler and contactor supply need to be isolated for the system to be safe . (IET Regs 17th Edn §537.1.6)
  11. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Only if it's inside said boiler eg if you fitted the contractor inside the boiler which i strongly recommended you don't, but label the contractor as dual supplies if you want

    And tbf since the pump load is low and doesn't require its own 5 amp circuit you could combine the two isolators into one and down to 3 amp
  12. KettlePilot

    KettlePilot New Member

    The Contactor is located in a plant room out of bounds to unauthorised personnel and all electrical switch gear is contained within a pre-fabricated housing with clear warning labels/notices attached.

    Good point BTW
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