converting from Y plan to S plan?

Discussion in 'Plumbing and Central Heating' started by grommett, 3 Feb 2019.

  1. grommett

    grommett

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    My hot water was way too hot at the tap so I just replaced the cylinder thermostat. While I was at it, I removed the actuator from the mid position valve (Drayton) and tried turning it with some pliers. It seemed very stiff and with only around 45 degrees of movement (not sure if this is normal?) The valve has been in place for around 12 years.

    Now I am wondering if the mid position valve was my problem and should I replace it and if so, should I convert the system to S plan. If I have to drain the system down to change the valve, it wouldn't be much more work to replace it with a pair of 2 port valves. Does this sound like a good idea?

    The system is a pressurised Worcester 24i boiler with a gravity fed 210 litre cylinder and 15 radiators. I also have a large towel radiator in one of the bathrooms that acts as a boiler bypass plus one of the radiators is not thermostatic.

    The only potential problem that I can see would be if I have to run extra cables from the airing cupboard to the controller/junction box. This would not be practical. My airing cupboard contains the mid position valve and the cylinder thermostat with cables running to the utility room which contains the boiler and junction box/programmer. Would the 5 core cable for the existing mid position valve be enough for two replacement 2 port valves?

    Sorry for the long winded questions!
     
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  3. oldbuffer

    oldbuffer

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    The following is my opinion, obviously without seeing what you currently have, and assumes a standard mains voltage Y plan, NOT a low voltage one.

    1. Provided the five core only joins the wiring centre to the 3 way valve, it should have sufficient cores. A Y plan will have one wire for hot water off. This will not be required with an S plan.
    2. The live, neutral and earth for each zone valve can be fed from the same source with the other cores used for the feeds back to the boiler.
    3. The wiring is different, but not significantly so. If you have a reasonable grasp of electrics and can work safely with mains voltages you should be able to do this.
    4. You will need one or more junction boxes to sort out the wiring between the two valves.
    5. A Y plan always has either the heating or hot water or both water circuits open, so residual heat from the boiler when it ceases firing can be dissipated through one or both circuits. An S plan can be fully closed on both hot water and central heating circuits, so needs a bypass to get rid of residual heat. This should be provided by an automatic bypass valve, like a Honeywell DU145, to allow water to cross from flow to return. It should be fitted after the pump but before the motorised valves.
    You boiler may already have an automatic bypass fitted, but even if it does, this may not have sufficient heat dumping capacity. The boiler installation manual should tell you what is required.
     
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  4. grommett

    grommett

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    Hi Old Buffer.

    I've been looking at wiring diagrams and it looks like the cylinder thermostat only uses two wires which means I do have enough cores as you say. I could fit the automatic bypass as you suggest, but would my towel rail not do the job? It is fitted between flow and return about 1.5 - 2.0 meters from the boiler which is another 4-5 meters before the motorised valve and there is no thermostat.

    Another thought is adding some full bore isolation valves incase I ever need to replace the 2 port valves. Is this advisable, and would I fit them either side of the 2-port valves or just before them?

    Cheers
     
  5. oldbuffer

    oldbuffer

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    1. As long as you have a circuit which is guaranteed to be open to dump residual heat you should be OK.
    2. On a clean system, motorised valves should last a good few years. If you get something like Honeywell V4043, the actuator head can be changed without draining the system, and I'd expect the actual valves to last for years. I personally wouldn't bother fitting isolation valves at all, but if I were asked to I'd only fit something like Pegler PB300 or Pegler PB300T which can take the pressure and temperature. If fitting, fit them either side, so you are talking around the £35 - £40 mark for something which might never be used.
    3. Make sure you check pipe sizes before buying motorised valves and isolation valves. Probably 22 mm, but could be 28 mm.
     
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  6. FiremanT

    FiremanT

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    Why would you want to go to the time and expense. TBF, I always fitted S plans, but I see no benefit in changing once installed. Although, TBH, more RGI's understand S than Y
     
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  8. grommett

    grommett

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    That is my main question. What is the benefit of S over Y and is it worth converting if I have to replace the mid position valve anyway.
     
  9. stem

    stem

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    In practice as far as control of the heating system is concerned, S and Y plans work exactly the same. Both simply direct the water heated by the boiler to the central heating, the domestic hot water, or both, as dictated by the controls. But there are a few differences.

    1. With S-Plan, when the heating and the hot water are satisfied, both motorised valves are closed. If the boiler has a pump overrun then an additional by-pass is always required to allow water to flow during the overrun period. The Y-Plan's 3-Port valve on the other hand will always have one open port open to allow flow during the overrun. [Some Y-Plans may still have auto by-pass valves fitted to regulate the flow through the system to maintain flow when the TRV's are closing]

    2. With a 3-Port valve when the heating and hot water are both satisfied, and the boiler goes off, the valve remains in its last used position. So if the central heating goes off last in the evening, the motorised valve will remain powered to keep it held in the heating only position all night. This consumes electricity (only a few watts) and shortens the life of the valve motor when compared to 2-Port valves which are only energised when the heating or hot water are actually running.

    3. Y-Plans need an additional control wire to S-Plan systems (hot water satisfied / not required) to position the 3-Port valve correctly.

    4. A Y-Plan is only suitable for control of a single heating zone and hot water. It is not possible to use them where there are two or more heating zones and hot water, because one of the ports always remains open and would allow water to flow somewhere that didn't need it when the second zone was calling for heat. As building regulations now require new homes to have separate heating zones, only S-Plan's will be suitable as they fully close off each circuit when not required.
     
  10. fixitflav

    fixitflav

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    I have a Honeywell 3-port, and with that it's easy enough to remove the head, then the internals (isolate the water first!) and you might be able to free the mechanism up. If so, problem solved.
    If not, it's much simpler to just replace the 3-port than convert to S-plan. But you don't have to go for a mid-position valve, why not fit an either/or valve, ie System W, with HW preference? I've never seen much benefit of Y-plan over W-plan. You might at some point want to install increasingly popular weather compensation, and that needs an either/or valve. With weather compensation the boiler flow temperature can be reduced too low to heat the cylinder, so has to be bypassed when HW calling, reverting to normal control-stat temperature and HW only operation. Cheaper valve and you'll have some cores to spare!
     
    Last edited: 4 Feb 2019
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  11. icanfixitmyself

    icanfixitmyself

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    Hi Grommett
    I know it's an old post, but I'm considering a Y to S plan conversion.
    How did it all go?
    Regards
     
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