Boiler Y plan wiring question

Discussion in 'Plumbing and Central Heating' started by david_uk, 6 Nov 2009.

  1. david_uk

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    Hi, before I ask the question, just to let you know I am a qualified electrician and am only doing the electrics part of the system.

    My boiler is Myson floor standing lump-of-cast-iron boiler approx 20 years old. Model number is Myson Marathon 800b. It originally had an internal mechanical timer. This was disconnected a few years back and replaced with a Lifestyle programmer by our local (trusted) heating engineer, and has been working OK for a while.

    We got it serviced recently, and asked them to look at the three port valve as it was only supplying hot water, and you had to move the valve to manual to get heating. Unfortunately, because the thing is in a difficult to get to place the guy couldn't do anything with it at the time, so it was left that we'd use manual until I could dismantle the cupboard and rebook the call.

    I dismantled the cupboard and took it into my own hands to replace the head of the valve myself as I have done it before. I took the front off the boiler to wire in the thing, and noticed all of the wiring was brittle and the chock strip connectors were all very brittle and fell apart. I decided to get a proper wiring centre and relocate all of the cables to that, leaving just the switched supply to the boiler only in it.

    I did that last night, and turned it on. As far as I can see, the electrics are working ok, but after a short while there was a noise in the loft, and the heating was boiling up the overflow into the header tank. Turned it off PDQ!

    Where I THINK I might have gone wrong is that the boiler has a thermostat on it. I couldn't find a wiring diagram for it online, so decided to leave it out. I know - dumb! i'm guessing that without the stat shutting the boiler off when it got to temperature allowed the water in the system to boil. I think reconnecting the stat will probably be the answer. Would I be right in assuming that to connect this stat, all that is required will be to have the switched live coming from the wiring centre on one terminal, and the other terminal of the stat to the live to the boiler? I.E. place it in series between the switched feed to the boiler and the boiler itself. It's a mechanical dial that has numbers 1 - 8 and off, with a thickish wire running into the boiler cast iron lump.

    Also, a second unrelated question. The old wiring of the controller didn't have a cable from the hot water off terminal of the controller to the tank stat / valve. It used L and N plus HW and heating on only. I guess the tank stat provides the hot water off signal to the valve. Is there a reason they might not have used this cable? I have wired it in as I followed the honeywell y plan diagram.

    Many thanks!
     
  2. kevplumb

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    boiler wiring ;)
     
  3. stem

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    You were very lucky. The water in the boiler would have boiled. When it expands into the header tank if it's plastic they can soften and collapse and then the scalding water cascades through the house. People have been killed by this. Touching radiators at 100 degrees C would also cause a nasty burn.

    You need to reinstate the boiler wiring as it was originally and connect the supply from the controls strainght into the LN & E connections without changing the internal boiler wiring.
     
  4. stem

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    The three port valve requires a live on the grey wire when "hot water is not required" This live comes from either the hot water satisfied on the cylinder stat, or hot water off from the programmer. The live on the grey wire in conjunction with a live on the white from the heating, allows the valve to wind fully across to the heating only position.

    It wouldn't have caused you a problem unless you had tried to set your programmer to have heating "on" with the hot water "off" at the same time. If you had, the valve would have sat in the centre position supplying both water and heating but without thermostatic control on the hot water side. You have now installed it correctly.

    It is possible at sometime in the past a timeswitch was used to control the system that would not allow the "hot water off and heating on" combination. If so, a hot water off wire from the programmer would not have been required. Infact, there probably wouldn't have been a connection for it.
     

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