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Newbie question - Adding rad to S-plan close to boiler

Discussion in 'Plumbing and Central Heating' started by I-DIY, 14 Feb 2021.

  1. I-DIY

    I-DIY

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    Hello and thanks in advance for any help.

    Firstly, I have searched both here and more widely and I’m struggling to find a complete answer – apologies if this has been done to death. Hopefully a fairly simple question regarding adding a radiator to a pressurised S-plan system prior to the CH / HW split or on the return.

    We want to add a rad to our kitchen as the dimensions are 5 x 6 with a huge window and the ceiling height is over 4m. Currently this is ‘heated’ solely by a 1800*450mm vertical rad which I’d estimate at 1250-1500 watts. Unfortunately, the floor is a rather lovely engineered wood type which after some prying appears to be glued together. Thus teeing off the main CH circuit will be next to impossible. The boiler is in the kitchen close to where we’d like to put an extra rad. I’m trying to understand the feasibility of teeing from the main flow & return. Diagram and questions below:


    1. Current simplified system.

    2. Tee from flow and return. Issue as I understand it is the heat into the return causing the boiler to short cycle.

    3. Tee from flow only. Struggled to find much about this.

    4. Tee from return only. Struggled to find much about this.

    If 3 or 4 were viable I’d look to control the rad with an additional programmable thermostat so that it can be switched off when the hot water is being heated. Thanks for any help and apologies for the noddy questions.
     
  2. MJN

    MJN

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    #3 and 4 are just bonkers and wouldn't work at all (no flow). :)

    The issue with #2 is that the new radiator would heat up even when hot water (only) is called for. It shouldn't short cycle any more than any other radiator though as as the lockshield would be throttled to create an 11C/20C drop just like the others.

    You could do a variation on #2 and add a zone valve, wired in parrallel to the CH zone valve, in the flow so it only comes on with a CH demand. Could even do it with a smart TRV depending on what controls you've got / willing to get.
     
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  4. I-DIY

    I-DIY

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    Thanks MJN, appreciate the help. Poorly explained re the TRV - but that was what I had in mind. I currently have a very basic time and thermostat but intend to switch to a Drayton Wiser and then add smart TRVs as needed. Can you clarify why 3&4 wouldn't flow? Is it to do with the distance between the tees or is a temperature differential issue? In basic terms the flow eventually becomes the return anyway. Thanks
     
  5. MJN

    MJN

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    The reason the water flows through a radiator is down to the pressure differential that the pump creates between flow and return. If you connect both ends of a radiator to the same branch (eg the flow in #3 and the return in #4) then there'll be no pressure differential hence no flow.

    It's like connecting a bulb to a battery - connecting both sides to positive (or both sides to negative) results in no current flow therefore no light. You have to connect one side to positive (the 'flow') and one side to negative (the 'return').

    Does that make sense?
     
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  7. I-DIY

    I-DIY

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    Thanks that does make sense and appreciate the explanation. With a seperate pump, this seems simpler to understand in practice. With the pump being in a system boiler then as i understand it the friction in the flow forces the differential. So the key is to ensure the tees are sufficiently far apart to enable this - in practice this means one on the flow and the other on the return of the loop. Thanks for the bulb / battery analogy which is helpful. I clearly need to do some more reading up - personal intersest as much as anything.
     
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