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Discuss Distance from main supply pipe or bolier most importnat for balancing in the Central Heating Forum area at

  1. Paul Taylor

    Paul Taylor New Member

    Im going to balance my radiators because some are struggling, but I would like to understand the calc better.
    I have a central 22m supply and return pipe running from far left of house to far right of house through first floor joists, Boiler is at far left, pump is near boiler on supply pipe. I have an openvented system

    All radiators are connected via 15mm copper pipe, nearly all have own loop. Some our on first floor, some drop down to ground floor, and some go up to second floor.

    (radiators are all newly replaced, so there is little sludge in the system)

    Qu 1. For working out resistance is it total distance from boiler to radiator supply valve or just distance of radiator 15mm (small pipe to main 22m pipe (large pipe) that is more important, is there a basic calculation

    I wonder this because one radiator that is at the far right end of the main supply and return pipe happens to be one of the fastest to heat up, but it also has one of the shortest 15mm loop

    Qu2. What is the effect if the supply pipe has to go down (to ground floor) and then return back up to first floor.

    Qu3. What is the effect if the supply pipe has to go down (to ground floor) and then return back up to first floor.

    Qu4. If a piped is teed to provide two radiators does that effectively half supply ?

    Qu5. I could easily move where the pipe is teed so both of these radiators mostly have their own 15mm loop, however to take out the tee completely would be difficult. Is there anything to gain by reducing the amount of shred 15mm pipe if I cannot elimiate it completely.
  2. tolly

    tolly Plumber GSR

    You dont need to go to all that trouble to balance a heating system. What are you going to do with these calculations anyway?
  3. Paul Taylor

    Paul Taylor New Member

    Yes I know, but Im trying to understand how central heating system works, so if you know the answer to any please humour me and give me an answer.
  4. Ric2013

    Ric2013 Plumber Top Contributor!!

    1 The actual calculation is very longwinded and I tend to use a spreadsheet. (I suspect that plumbers who do a lot of new systems just work by experience and rules of thumb). You'd factor in the heat required by the radiator and the heat lost from the pipe serving the radiator, guessing the pipe diameter at this stage. Then you'd work out the resistance in the circuit at this flow rate using a long table of figures to multiply by the pipe run length and see if it is accepable or if you need to change a pipe diameter. Any shared sections need to be calculated separately as they will have to carry the flow rate of everything they serve at the same time, and resistance increases with flow.
    There is also a maximum recommended pressure drop per meter that any section of pipe should not exceed. Beyond this, the risk is, presumably, cavitation, but I'm not actually sure what it's all about, nor is my teacher at college.
    To answer your specific question, it would be the resistance over the entire length of 22 (split into sections with separate resistance calculations for each section), plus the length of 15, and, of course, the resistance across the boiler, and the resistances of each fitting, valve, and bend, factored in as pipe-equivalent lengths.As to why a radiator warms up most quickly than its peers, this is actually only an indication of balancing. An ideally balanced system would require the rooms to be at designed temperatures and the boiler to be providing the exact temperature of flow required, while each room's heat loss is precisely that emitted by each radiator and associated pipework. Once the radiators were warmed through, they would each receive the exact flow required to reduce the temperature by xx degrees from the flow to the cold side.
    You'll probably find that whoever installed the system realised that the radiator you have at the far end of the 22mm run had the greatest resistance to flow and so left its lockshield fully open and over-throttled the others and so is slightly out of balance?
    However, to take two extreme examples, a heavy cast-iron column radiator in a perfectly-balanced system might still take longer to heat up from cold than a modern low-water content and narrow convector fins steel radiator, but this is merely due to the lower heat output:water content ratio of a column radiator and, once warmed, they might both be running at a 15°C temperature drop. So a radiator heating up quicker than other is not necessarily out of balance.

    2 This is the same as question 3, isn't it?

    3 I would imagine the potential energy lost by the water flowing down to a radiator would more-or-less balance that gained by the water flowing up. In theory, the water coming up is less dense than the water going down, so you are fighting the natural circulation, but standard calculation procedure for central heating design ignores this factor as it is insignificant. You'd only really care about the pipe length.

    4 + 5 Nothing wrong with a shared 15mm pipe in principle. Depends on the length of run and the radiator output. If the resistance to flow in the pipework is such that one radiator on a 15mm circuit gets only the required flow with no restriction required by the lockshield radiator valve, then adding a second radiator to this run (creating a shared 15mm section) would mean you'd need a more powerful pump (waste of electricity); if the two radiators both need their lockshields closed to some degree, then it is not a problem as less resistance in the run would just mean you'd need to compensate by adding more resistance by shutting down the lockshield further. Basically, if you are trying to improve flow so that you can run a smaller pump or set the pump speed down, then you need to concentrate on improving the pipework that serves the so-called index radiator: restrictions on other branches don't really matter.
  5. Paul Taylor

    Paul Taylor New Member

    Thankyou thats helpful information, but question 2 was meant to read
    Qu2. What is the effect if the supply pipe has to go up (to attic floor) and then return back down to ground floor. it is the two radiators that on this floor are hardly working and our teed, so that is why Im wondering if I should double up the pipe work here since everything else works fine. Im assuming that because water has to travel vertically up here there is more resistance to flow, where the supply pipe connects to these radiators it is warm but not as hot as the others.

    Im not sure what you mean about index radiator ?
  6. Ric2013

    Ric2013 Plumber Top Contributor!!

    Index radiator - the radiator that has the greatest pipework resistance to flow so that, if all valves are fully open (i.e. before any attempt to balance the system is made) is has the greatest temperature drop of all the radiators. In balancing the system, this is the radiator that will have its lockshield valve left fully open.

    I'll get back later re. Q2.
  7. Paul Taylor

    Paul Taylor New Member

    Well I think based on the fact that the supply pipe is not that warm and they are are the only radiators, that the loft rads are the index rads. The total pipe length may not be the longest, but since they share pipe work and these are the only rads where pipe work has to go significantly higher then boiler I'm assuming these have highest resistance. This is why I ask if I can essentially move the t-joint so its nearer the 22mm pipe so there is only 1 metre of shared pipe would that probably improve things.
  8. Ric2013

    Ric2013 Plumber Top Contributor!!

    Unless there is a partial airlock, or sludge preventing flow, then your reasoning seems to be about right. I don't think the height has anything to do with it.
    Before you do anything else, make sure the valves on the problematic radiators are all four fully open, otherwise it may be a simple case of very poor balancing. Personally, I would try to extend the 22mm pipework as close to the radiators as possible so the shared 15mm section is short or eliminated, and tee off closer to the radiators rather than extend the individual 15mms towards the 22, but depends on what is accessible and practical. If you have to leave a shared section of 15mm, just keep it as short and as straight as you can in the circumstances.
    How large are these radiators, and are they single or double, finned or unfinned?
  9. Paul Taylor

    Paul Taylor New Member

    All four valves are fully open, and the radiators are brand new so sludge is not an issue. They are quite large in terms of water volume in that they are column radiators (2 column) measuring 800 x 600. Maybe there is an airlock, but I have tried fully closing supply valve, open return and bleeding, and then fully closing return, open supply and it has had no effect.

    I'm surprised you say height makes no difference I understand there was less natural pressure to help get the water up when it is nearer to the f&e tank, but I must confess I don't really understand the pressure thing, and interplay with the pump.

    Using 22mm to upstairs rather than two set of 15 mm pipes is a good idea I hadnt considered, but it will be problematic to connect the 22mm direct to the central 22m pipe.
  10. Ric2013

    Ric2013 Plumber Top Contributor!!

    If the top of the radiators aren't lower than the water level in the F&E, then there will be a problem in that it is unlikely that they will stay full of water.

    Assuming the above isn't your problem, at an absolute guess of 800W nominal output for each radiator, I suppose you may be pushing the boundaries for one 15mm pipe, particularly if the water also has to run along a long length of 22 first as well.

    Before you do all this work, have you tried balancing first?

    Croppie's Balancing Thread.

    The other thing I would try is turn off every radiator you can except these two and run the heating and see if these two heat up in this case. If they do, you may find you've just cleared a partial airlock, or, at least, there's a vague indication the system may work well if balanced properly.
  11. Paul Taylor

    Paul Taylor New Member

    Top of the radiator is 1.5m below the bottom of the f & e.

    Hi, yes I have balanced the system so all the other radiators seem to have about the same temperature differential, but it didnt help. I then tried turning off all other radators and then I had the issue of hot water pouring out of hot pipe f & e. I guess this indicates an air lock meaning it couldn't get water around the radiators so found an alternative route, I tried bleeding some water of these radiators (first with supply valve on, return off then vice versa) but could shift it.
  12. Ric2013

    Ric2013 Plumber Top Contributor!!

    Then your basic system design is flawed. This shouldn't really happen if the pump, vent, and cold feed are properly located, even if all the radiators were off. Look up 'pumping over'. If it were an air lock, the pump would usually clear it in this instance, and via the vent would not be a possible alternative route.

    You can try the pipework modifications you have suggested, but I'm really starting to think you need a plumber on site to have a look at what is going on. An old system that has been modified on several occasions?
  13. Paul Taylor

    Paul Taylor New Member

    I have had a plumber doing some work, but he not sure what the issue is, he originally blamed it on height and size of radiators, and left me in the lurch a bit. I'm not going to do any of the work myself, just trying to be prepared when I do speak to this/or other plumber so I know the options. Why would via the vent not be a viable route, the vent pipe comes of the supply pipe is that not correct ?
  14. Ric2013

    Ric2013 Plumber Top Contributor!!

    I'm sorry to hear that. Sadly problem solving is one of those jobs that can take several attempts and a lot of head scratching, particularly when the system design is unknown and I do know plumbers who no longer take on such jobs on the grounds that the stress isn't justified.

    To answer your question...

    Best practice on modern systems is to have the following in this order:
    1 Flow pipe from boiler
    2 Vent pipe
    3 Cold feed from cistern
    4 Pump (water flow away from above items and pushing into zone valves, rads, etc)

    So turning off all the rads would not result in pumping over.
  15. Paul Taylor

    Paul Taylor New Member

    In my system the supply pipe leaves the boiler, then there is the pump, the supply pipe continues, a few radiators drop of it, hot vent pipe is of this pipe as well I think, and then think cold feed goes into return pump. So does'nt sound quite the same as yours although I don't quite get it are you saying ideally that between the boiler and the pump is the vent pipe coming of the supply or are you saying the vent pipe and old water pipe should be separate pipes going directly into the boiler, not directly attached to supply/return at all.
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