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Discuss One Pipe System - Help Please in the Central Heating Forum area at

  1. j4jav

    j4jav New Member

    Hi All,

    I am so glad I found this forum whilst looking for advice on the net. I am not a plumber as such but I do know a thing or two about DIY.

    Our house is a 1937 built detached which had a central heating system installed in probably around the early 80's, originally oil boiler system. But now this has been converted to a newer condenser boiler, a Vaillant 938 to be precise.

    Our heating works very well and throws out a lot of heat, we have really old style long radiators (2.5m wide) but they do warm the house up really well. The system is on two zones, with an upstairs and downstairs circuit run off two individual Nest thermostats again work brilliant no issues at all.

    This weekend my DIY part of me has decided to install 2 new radiators into our second hallway and utility room. I have used the online BTU calculators and bought the following from Wickes in terms of best output and what can be fitted in the space we have: -

    Rad 1 - 600(H) x 800(W)
    Rad 2- 600(H) x 900(W)

    All these rads are the Type 21 slim double style ones. We have a family member work for Wickes so with a nice 20% staff discount they came out at a decent price. TRV's I already have new spares (Drayton) the fancy expensive ones, although we never control our heating with thermostats. They always run on open full, as they never have seemed to work right and now I probably know why. We have only been in the house for a year so still tinkering away with DIY projects and discovering new things - the house really is a world of wonders! Luckily we bought it off the chap who's father had the house built from scratch and he lived here with his father ever since a child and we have regular contact - so know a lot about the homes history.

    I am going to get a plumber in to do the job (pipework) and have already been quoted by 3 of them who all have their separate opinions with is why I am here to ask the professionals. I will be putting the rads onto the walls as I have a bit of OCD and want them to be fitted perfectly and straight. All plumbers have agreed to this and said would come and simply solder up the pipework.

    Before calling the plumbers in I went into our crawl-under cellar which houses all the pipework for the downstairs and I though it would be a simple matter of hooking the new rads up to the Flow and Return by tapping a 15mm into the 22mm main pipes that run right across the cellar of the house. But to my shock the house is on a one pipe system. This now explain why the hallway and kitchen radiator always take longer to heat up than the rest of the system, as they are the last on the run!

    Now the flow and return pipes for the main house heating system run from the garage where the boiler is and go through the utility, store room and second hallway and we would be tapping into the flow/return pipes from under the basement in order to hide the pipes as best as we can.

    I have attached 3 pictures, one of the system how it is currently and two further pictures of what the plumbers have proposed. But I am yet to find out which is the correct one that will work properly.

    We do not want to loose any heat from the house main radiators, and we just want these extra 2 rads to break the chill in the other 'outbuilding' areas . So my understanding from 'common sense' was that hooking these two new rads to the return pipe would be the best way forward as eventually they would get hot once the water has made its way around the system all hot and warm, just like the kitchen and hallway one do.

    But one thing I will note is that we changed the main hallway radiator for a new Type 22 radiator and although it gets very hot eventually, the bottom is always cold and I have tried everything to rectify this and only this weekend I have found out it's on the last leg of the return pipe on our one pipe system! I read somewhere that Type 21 and Type 22 are no good for one pipe systems, but not sure how true this is.

    The good things about the system is that if we turn off a rad at the valves it dos not **** off the system. As you will see in the pictures there is a main 22mm pipe what goes around the system and each radiator is spurred off this using a T 22x15x22 going into each radiator. So I was hoping to do the same but instead this would be on the Return flow very close to the boiler, as the return pipe and even flow pipe have about 4-5m stretch before they reach the main house circuit.

    However further into this I am unsure how it would affect the upstairs too as I cannot make a diagram of this as we have had new carpets down which I do not want to rip apart just yet. Another thing I would like to add is that the house has a total of 11 radiators (pretty large ones too) and out of them 11 rads, 2 down stairs ones are fitted with top entry and all the upstairs barring then towel radiator are also top entry for some strange reason.

    What are your thoughts on this? Oh and by the way the 3rd plumber has completely disregarded the job and said we would be better off with electric heaters which I am not going to do. Lastly changing over to a 2 pipe system is an option downstairs as it's easily doable in the basement, but not upstairs. Although I would like to keep this 1 pipe system and just add the extra rads to make life easy. Then when and if we do a big renovation we can change the whole piping system for the better one.

    Please also be easy on me it's only my first post! haha

    Thanks in advance!!!

    Screenshot 2019-01-07 at 23.16.10.png

    Screenshot 2019-01-07 at 23.16.21.png

    Screenshot 2019-01-07 at 23.16.32.png
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Middle pic but I think installing trvs are a bad option

    Also your going to require diverter tees or swept tees

    There's a chance you drain down and it won't work right again

    TBH I would be looking at a re pipe and if you disagreed I would be the same as the 3 plumber
    • Agree Agree x 3
  3. j4jav

    j4jav New Member

    Thanks for the quick reply.

    If we was to go down the re-pipe route, as the house is on two zones. Could we repipe just the downstairs to a 2-way pipe system and leave the upstairs as it is?

    Or would this cause problems with the boiler in regards to the return flow... as that is obviously the same pipe for both circuits.

    Or do I just risk it, don't install TRVs and go for the second pic option and hope for the best?? Surely if it does not work then I could just remove these 2 new rads from the system...

    It has been drained down before when the boiler was installed and fills up and bleeds perfectly fine, but that may not be the case with 2 new rads!
  4. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    You could but it will cause problems for the upstairs zone

    That's down to you sorry the best option is full flow valves (lockshield type) and swept tees and pulled bends but then the new rads might have too much internal resistance so the water takes the easy route eg next rad

    It's your money at the end of the day

    Are you going to be spending a lot of time there eg not moving etc
    • Optimistic Optimistic x 1
  5. j4jav

    j4jav New Member

    I understand what you mean... so it is a bit of a gamble really.

    My neighbour across the road advised his opinion of just using a load of spare plastic pipe he has and push fits from his recent new CH system install and simply trial these on the second picture proposal and see how it performs. If it works good enough then have the plumber come in and do the soldering, but my argument was that the flow might be affected with plastic/copper and it may not work in the end with the permanent copper solution lol

    If it all becomes a shambles, then we could remove the push fits and solder in a straight connector and forget all about it! The pipes are very easily accessible so it could be an option, would probably waste some inhibitor during the drain but again we have a bottle spare so...

    Or am I just wasting my time?
  6. chris watkins

    chris watkins Plumber

    Shaun does this post hold the record for the longest ? Almost lost the will to read on.
    OP are you 100% sure it is a one pipe ?
    Are the connection TBOE (rad cold at bottom mentioned).
    • Funny Funny x 1
  7. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    To be honest plastic would be worse than copper smaller id bore and more restrictive fittings
  8. j4jav

    j4jav New Member

    Yes 100% one pipe system, I literally sketched up that diagram whilst in the basement turning the heating on and off to check the flow pipe and double checked everything. That diagram is 100%.

    I wish is was a 2 pipe system haha, the rad which is always cold at the bottom is the Rad 6 on my diagram, all the rest heat up really hot on all four corners, and this one which is cold at the bottom is a new type 22 one which we replaced from a single one which started to leak.
  9. JCplumb

    JCplumb Plumber

    If you already have a badly performing rad, adding more rads to the circuit will give the water even less incentive to heat the bad one.
    • Agree Agree x 1
  10. Rob Foster

    Rob Foster Top Contributor!!

    In my own opinion there is nothing wrong with the one pipe system. Obviously a new system will be two pipe. A carefully balenced one pipe system is fine. I would flush out the whole system throughly check the rad valves and see where you are then. You might well beca lot richer. Rob Foster aka centralheatking
  11. j4jav

    j4jav New Member

    Thanks for the reply everyone.

    There is nothing wrong with the system in my opinion. The cold rad at the bottom is not bothering us as the hallway still gets nice and toasty.

    My question was adding a couple of radiators on the return flow and how it would affect the system.

    I think I’m going to give it a try and see what happens. After all they can always be removed and reverted back to standard.

    Also putting the new dads closer to the end of the circuit may just mean that take a little longer to get warm which again is fine as long as they do get warm.
  12. chris watkins

    chris watkins Plumber

    So suitable reassure that there will be problem, he soldiers on, in spite of plumbers best advice!!
    What a waste of all that reading :confused:

    It will not balance now, what do you think the chances are of getting the flow through the new rads is!
  13. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    • Useful Useful x 1
  14. j4jav

    j4jav New Member

    The system isn’t balanced at the moment. All valves/TRVs are fully open.

    I will report back and let you guys know how it pans out. I just don’t want it to be something I didn’t try and always look back and think what if I had tried scenario.
  15. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    You have too much time on your hands. Remember the swept tees and TRVs for one pipe. Plastic won’t work
    • Agree Agree x 1
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