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Relay Help!! 24v to replace 18v?

Discussion in 'Troubleshooting and Repair' started by sammas, Dec 30, 2016.

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  1. sammas

    sammas

    3
    0
    Dec 30, 2016
    Hello Everyone,
    I am trying to repair my vaillant motherboard (711451) out of my combi boiler, the fault is simple, one of the relays on the circuit board is worn out, the problem I have is trying to find a relay that I can replace it with!
    The relay on the pcb is a Schrack RP489018 18V 8A 250V PCB 18VDC non-latching relays, I cant seem to find these online or anywhere,

    I have managed to purchase a Schrack RP438024 24V 12A / 250V Does anyone know if this relay would be ok to use instead?
    if not can anyone help me as to where I can purchase a suitable 4 pin 18v relay

    any advise would be appreciated,

    Sam
     

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    Last edited: Dec 30, 2016
  2. hevans1944

    hevans1944 Hop - AC8NS

    4,626
    2,159
    Jun 21, 2012
    It is unlikely that the relay you purchased with a 24 V coil will operate reliably in place of a relay with an 18 V coil.

    What makes you think the existing relay is "worn out" instead of something else being the culprit? Relays hardly ever wear out, but the contacts can be damaged, sometimes "welding" together because of an overload fault or failure of arc suppression components. If this is the problem, sometimes it can be repaired by carefully prying the contacts apart to break the "weld" and then carefully filing the two contacts smooth again with a very fine jeweler's file or an emery board file. This is only a temporary "fix" until you find a replacement.

    Sometimes a relay coil will open, usually because excessive voltage has been applied to the coil because of a fault somewhere else. The only reasonable repair is to replace the relay with a new one having the same rated coil voltage... 18 V in your case.

    There are lots of reasons for a relay failing to operate, but most of them do not involve the relay itself because relays are very rugged electro-mechanical components. Make sure you troubleshoot sufficiently to be absolutely sure the relay is actually defective before going off on a wild goose chase, and then finding out the goose you caught didn't solve your problem.
     
    davenn and sammas like this.
  3. sammas

    sammas

    3
    0
    Dec 30, 2016
    Thanks for your reply, yes I believe it is definitely the relay that is at fault due to the relay being all black inside and it is a common issue on this pcb, I think that the contacts are worn/ burnt, that's a good idea to clean the contacts I will give it ago if I cannot get a relay,
    Thanks
    Sam
     
  4. Minder

    Minder

    3,119
    670
    Apr 24, 2015
    You can get qty10 18v relays for just under $9.00 on ebay, you may have to get creative and shoe-horn it in.
    If it is just using the N.O. contacts you can cut the N.C. pin off.
    M.
     
  5. hevans1944

    hevans1944 Hop - AC8NS

    4,626
    2,159
    Jun 21, 2012
    Worn contacts you could probably live with, but burnt contacts indicate something is wrong with the circuit (or circuit design) of the load being switched, and an arc is forming when the contacts open to disconnect the load. Sometimes arcing also occurs when contacts close, because of contact bounce. What load are the relay contacts switching, voltage and current, inductive or resistive?

    It is possible that the relay contacts are simply not adequate for the service for which they are being used. Inductive, resistive, and heater loads all have different contact ratings. For example, when switching a heater load, the resistance of the "cold" heating element can be only a tenth of the "hot" resistance, leading to a huge inrush current. I don't know what your board is switching, but given the small size of the relays it is probably an inductive AC load, a mains-suppied solenoid-operated gas valve perhaps. Please describe the load this relay is switching.
     
  6. sammas

    sammas

    3
    0
    Dec 30, 2016
    yes it is switching a solenoid operated gas valve! the solenoid was buzzing like crazy because the relay was not operating it correctly, the relay on the board is 8amp /250v~AC1
     
  7. hevans1944

    hevans1944 Hop - AC8NS

    4,626
    2,159
    Jun 21, 2012
    Is the solenoid valve a typical low-voltage AC, with the coil voltage supplied from a nearby step-down (doorbell) transformer? Or is it perhaps supplied from a step-down transformer mounted on the Vaillant circuit board?

    There may be a mis-match between the solenoid voltage requirements and what the relay contacts are providing, aggravated if the relay contacts are damaged. You could test this by temporarily bridging the relay contacts by hand with a short length of insulated wire (tinned on each end if stranded wire) to see if the solenoid valve actuates without buzzing.

    These gas valve do wear out eventually and they do buzz when that happens. I think it is caused by years of accumulation of "stuff" that gums up the works. IIRC, I was able to take my gravity furnace gas valve apart after thirty-something (or forty-something) years of service, clean it, and put it back in service again. That stopped the buzzing, which is apparently caused by the plunger in the solenoid not fully retracting (and completing the magnetic circuit) when the solenoid coil is energized. I can't recommend this sort of maintenance for the average homeowner because of the inherent hazards of working with natural gas valves. You don't want to get it wrong! Best to have a qualified professional change out the gas valve if it still buzzes with proper voltage and current applied.
     
  8. JoseGiu

    JoseGiu

    1
    0
    Jan 22, 2017
    Hello Sammas,

    Too I am having problem with the relay RP489018 18V 8A 250V AC1;

    You've found someone who sells them, if so, please let me know where and who.

    Thank you very much.
    Giu.
     
  9. alberto_gulmini

    alberto_gulmini

    1
    0
    Dec 8, 2017

    ciao sammas, greetings from italy. you are 100% right . my Vailant boiler , vmw282it has the same issue. "body of evidence, when boiler tends to " cough" and then tries to start some " sweet" " knocks" over the main board or even better on the relais case , solves the issue. temporaryly obviously. it's " smoked" inside too. then the question is : where have you , if any , found a place to buy thse relais? i just emailed here
    http://www.impexron.it/RP489018-Italia_Vendita_Prezzi-Schrack-477449-1573

    i think is the italian branch of a german company that hopefully will sell these parts. any other source?
    thanks in advance.

    if is more comfortable for you to use what's app, feel free to ask my mobile phone via pm. thanks and ciao
     
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