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Discuss WB Greenstar 37cdi combi in the Central Heating Forum area at

  1. Wirdy

    Wirdy Member

    Just out of interest, has anyone delved into 'scoping the pwm signal from the board to the ddpwm grundfos pump on this series of boiler (3 thin wires-white/black/purple from board to pump electronics)?
    WB techline say they had no info on what the pwm waveforms should look like for the different pump modes (fixed & variable) available.
    I know this is pretty deep stuff.
  2. DuncanM

    DuncanM Plumber GSR

    Wow. I usually ask has it got 240v? Firstly, WHY? You might find info on it in the Grundfos installation manual. This is not really relevant to us mate. We don't make the bits just figure out if they working or not.
  3. Wirdy

    Wirdy Member

    Good question. It was for a fix on a 9 yr-old boiler. Was chasing a [email protected] of a fault, intermittent dhw (once every few weeks, boiler overtemped), even after every one of the usual dhw suspects had been tested or replaced. Reset fine every time, no fault codes & was never there when I tested it:- normal operation, performance mode, min mode, dhw on/off loads of times, service menu-ntc sensors, dhw flow rates, fan rpm, fan pressure, everything tested fine.
    The only thing left was the pump, but it was less than a yr old (I had fitted it) & ch worked great, all the time. Got the fault to occur just once while testing, and could hear a very quiet (almost imperceptible) ticking noise from the pump.Just out of desperation I swapped out the pump electronics module from a seized pump and now all seems well for over a month.
    I did notice that when the boiler was initially powered up & the pump runs for a couple of seconds during the start-up sequence, with the old module fitted the pump only ran once every 3-4 power-ups, with the replacement module it ran the pump every time.
    Throughout all this time (6 months), there was never a problem with the CH function, just on DHW.
    This one had me pulling my hair out & bacame my nemesis. If the fault ever comes back again, I think it's time to recommend a new boiler.. Or I might just be away on holiday, permanently!! :D
    • Like Like x 1
  4. Chuck

    Chuck Well-Known Member

    Since it was an intermittent fault you could have captured the working waveforms during the period it was, err, working. Unless you can reliably provoke the non-working state, however, knowledge of them would not have helped your diagnosis much.

    Once you did manage to get the boiler non-working low-tech diagnostics (stop, safety, look, listen, think) got you to the answer. Good catch!
    • Like Like x 1
  5. Rob Foster

    Rob Foster Top Contributor!!

    I do not know if it’s relevant and on the edge if my comfort zone. Uk grid rarely supplies 240v it’s mostly below this and most appliances are designed to work off 220 . Continuously running electrical appliances can be tuned to save electricity eg the co op do this with their shop fridges centralheatking
  6. Wirdy

    Wirdy Member

    Yes cheers, good logic Chuck.
    At this moment my missus would say to me, TMI.. (Too Much Information), so ignore the rest of this post if you're not interested.

    From info found online. Three small wires to the pump are pwm in, pwm out & signal Ref (0v earth). Voltage can be anything between 5-24v. From docs I've seen wb use 5 volts and Honeywell use 10v. The pump module doesn't seem to mind & adjusts itself. It's the duty cycle that's important and the output goes to a simple triac circuit within the module to control the pump speed.
    It's quite a smart system, especially the feedback (pwm out) that uses pump current draw to estimate system pressure/flow & vary the pump speed to compensate for variations due to size of heating system, summer/winter temps & closed/open trv's. Clever boffins at wb & grundfos. Much cleverer than me :(
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