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semi gravity upgrade question ? please help

Discussion in 'Plumbing and Central Heating' started by mprice109, 12 Feb 2013.

  1. mprice109

    mprice109

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    hi everybody

    I currently have a old semi gravity system with no cylinder stat ! I cant set it to have hot water only ! when i do the heating comes on aswell doesnt seem right to me was wondering is this upgrade possible ?

    i only have one zone valve which seems do be on the heating circuit .
    been like this for to long now surely im wasting energy and money .

    any thoughts / comments on image are much appreciated !

    thanks in advance

    mark
     
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  3. stem

    stem

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    That is because you have a live being fed from the boiler to the pump via the 'C' & 'NC' contacts in the motorised valve when you have the hot water on.

    Getting rid of the wire between 'NC' at the motorised valve and terminal '9' will stop this happening, however, the pump will be started as soon as the motorised valve starts to open rather than when it is fully open. (Because of the link between teminals 5 & 9)

    The 'C Plan' is designed for use where the motorised valve is used to stop gravity flow to the hot water cylinder and the pump is used to control the heating. You are trying to use the pump and the valve to control the heating and it's not really suited, as there is nothing to stop the gravity flow to the hot water cylinder when the heating is on.
     
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  4. mprice109

    mprice109

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    Thanks for your reply

    Ive wired to the c plan drawing on the left image and eveything seems to work
    Until cylinder stat is satisfied then it closes the motorised valve and the pump seems to over heat ! Is this because my valve is on the heating side of the system ? Is there a workaround for this as i originally had no cylinder stat and a room stat wired up wrongly so temperature was controlled by boiler thermostat
    i checked all connections as per honeywell guide and all fine bit stumped now thought right image could be a work around ?

    Could i possibly have a system that the pump runs when water is wanted on its own and when heating is required ? I only have three pipes to my boiler and i have read somthing about an injector tee ?

    Mark
     
  5. D_Hailsham

    D_Hailsham

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    Yes: with the valve closed there is no circuit for the water to be pumped round!

    No: the only solution is to move the valve so it is in the HW circuit. Then wire as in the original C Plan circuit but using the DT 200. The programmer should be set to fully pumped. (That's not a mistake!)

    You must ensure that, when the valve is closed, there is an open path from the boiler to the vent pipe.
     
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  7. mprice109

    mprice109

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    is this a suitable wiring diagram to make my old system the most effecient it can be ?

    mark
     
  8. stem

    stem

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    Probably not, depending upon how your system is piped up. As you call it "semi gravity" I assume that this means the hot water cylinder is heated by natural gravity circulation and the central heating only is pumped.

    If so, with your diagram, when just the hot water comes on, it will turn on the boiler and start the pump. However, the motorised valve will still be closed, so the pump will be working against a closed motorised valve.
     
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  9. mprice109

    mprice109

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    thanks for your reply

    what if i attach the pump to the room stat switched live would that work ?
    position number 5
    mark
     
  10. stem

    stem

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    That would work. The only negative is that the pump would be started before the valve was fully opened. Having said that, it's only a few seconds delay, so it might not hurt it.

    The usual way to wire up a pumped heating gravity hot water system is much simpler, because there is not a motorised valve involved. Like this.


    My guess is that the valve was added possibly because there was some gravity circulation around the radiators when the pump was stopped. The general concept can be adapted to incorporate the motorised valve and using the microswitch to start the pump when the valve was fully open.


    I assume that you are aware of the major limitation of gravity hot water systems is that in the winter when the central heating is 'on' the hot water will also be heating up if required or not.The cylinder thermostat just allows the boiler to be turned off when the heating is up to temperature, or not required. (This would be avoided if you took D Hailsham's advice above and modified the plumbing to allow a 'C Plan')

    The hot water will work on the cylinder thermostat fine though in the summer when the heating is off.
     
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  11. DIYnot Local

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