Acceptable length for dead legs

Discussion in 'Plumbers' Talk' started by ajmoore, Feb 8, 2004.

  1. ajmoore

    ajmoore New Member

    Hello everyone this is my first post as i am new to this. I am about to install a Worcester oil combi boiler(heatslave i think) into my house. The problem i have is that the house(old barn) is narrow but long. The kitchen is at one end and the bath room at the other. I plan to site the boiler as close to the middle as possible but this still leaves a dead leg of approx 11m to the bathroom. The combi has got a 22mm hot water out pipe, i was going to put 22mm in to where it Ts to the kitchen and bathroom then run in 15 to the bathroom and kithchen. Will this be ok? The output of the boiler is 22l min, it is the biggest they do.

    Any help is much appreciated as i dont want to wait hours to get hot water.


  2. bilco

    bilco New Member

    That sounds ok...
    There will be some time lag before hot water flows and so not too economical for short draw offs.
  3. hillheader

    hillheader New Member

    I had a similar problem (old long barn), but a longer cold leg. In order to avoid the time lag for the hot water to arrive I installed a return loop back to the HW tank cold from very close to the HW taps. This is quite common. The return loop is 15mm and has a small pump and check v/v. The pump is turned on when the light to the bathroom goes on. If you turn on the light and then the tap, you get cold water, but if you relieve yourself first, the water is hot by the time you wash your hands! You can also put a timer on the pump instead.


    I have written out below the recommended maximum length of dead leg for the various diameters of pipe you may use.

    The design guide states that the length of hot water draw off pipework to taps and other outlets should be kept ot a minimum to avoid the undue waste of water. Where there
    is more than one diameter of pipe on a dead leg the equivalent length and size should be estimated. When a dead leg exceeds the recommended maximum secondary circulation or trace heating should be installed, controlled by a time switch and incorporating a motorised valve to stop gravity circulation.

    For 10&12mm max length 20m, 15&22-12m, 28mm-8m and 35mm and above 3m
    I hope this information helps you though secondary cirulation loops on combi boilers don't work and if you have more than one bathroom or a large family in the house then you may well find the combi boiler won't cut the mustard when the demand for hot water is high. Also remember that unless you have a fairly large buffer tank to store some water at 80* then you boiler can normally only lift the temperature by 35* above ambient.
    If there is a protracted cold spell this may mean you hot water has trouble getting above 40* so please m,ake sure that what you put in will do what you are wanting.

    Kind Regards,

    Master Plumber
  5. Tangoman

    Tangoman Active Member

    Just a kind of obvious point. You are going to run a 22mm pipe to the bathroom and kitchen and then change it to 15mm. So the overall flow rate at each end will be governed by the final pipe diameter here, i.e 15mm.

    The 22mm section, will however contain more water than a 15mm pipe would, so it would take longer to exhaust this cold water through the HW taps at either end before the newly heated water arrived.
  6. Bahco

    Bahco Guest

    I agree with Tangoman.
    There is no point having a 22mm feed pipe to your hot taps given the poor flow rate from a combi.
    I would think the best solution would be to run the whole hot supply in 15mm and insulate the whole pipe run.
    Still not ideal but there is not much else you can do unless you can stretch to fitting an instantaneous heater in the kitchen and the combi boiler close to the bathroom.
    This would solve your predicament and may be worth the extra expense
  7. bilco

    bilco New Member


    Why are you trying to confuse the poor chap?...He's already said he's running 15mm to bath and kitchen...His situation is not ideal, but it's not too uncommon either.

Share This Page