22mm vs 15mm pipework?

Discussion in 'Plumbers' Talk' started by TeamRocket, Dec 6, 2023.

  1. TeamRocket

    TeamRocket New Member

    Hi All,
    Looking for some advice regarding pipework sizing for an upcoming renovation of a 1880s 4 bed property.

    Entire house is going to be re-plumbed in plastic rather than copper, gas combi boiler will be fitted and the old lead mains supply replaced with 25mm MDPE.

    The mains water enters at the front of the property before running approximately 10-12m to the rear where the kitchen on the ground floor and 2 bathrooms on the first floor (both will have a bath and shower) are located. The boiler is also positioned to the front of the property where gas enters.

    1. Should the cold and hot water feeds to the rear of the property be run in 22mm or 15mm? Given there are baths I would have thought that 22mm would be preferable for quicker filling?
    Can a combi boiler deal with 22mm for the hot water?

    2. Would the showers pressure be ok being teed off the 22mm in 15mm?

    3. For the central heating, what size pipe would be recommended for the flow and return?
    Downstairs there will be 8 radiators and upstairs 7 radiators. Previously we’ve had 22mm flow/return with 15mm rad tees, but that was within a warmer property.

    Thanks in advance.
  2. terrymac

    terrymac Screwfix Select

    Combi boiler domestic hot water outlets 15mm ,so no point in running 22mm, cold mains to bathroom in 15mm too.
    Central heating main flow and return in 22mm. Branch to rads in 15mm.
    And copper pipework would be my preferred install.
    arrow, WillyEckerslike and Schooldays like this.
  3. andy48

    andy48 Screwfix Select

    1. As terrymac.
    2. Have you done the necessary measurements to make sure you have adequate pressure and flow to run a combi. Pressure will be the same after moving to 25 mm. Flow may be better.
    3. Older properties have higher ceilings. You could well lose almost 0.3 bar of pressure per storey.
    arrow likes this.
  4. The Happy Builder

    The Happy Builder Screwfix Select

    Use 22 mm pipe up to the tee where the pipes split to two 15 mm pipes to supply the hot water via the pipe to the boiler and the cold water pipework, possibly even further on the cold pipework.
  5. Jimbo

    Jimbo Screwfix Select

    What is the heat loss? If it’s under about 12kW, personally I would design the system to work with a heat pump, even if you don’t fit one now, which will likely mean at least some sections in 28mm (flow is everything), as said 15mm tails to the rads.
    WH55 likes this.
  6. arrow

    arrow Screwfix Select

    As with terry and andy 22mm run with 15mm tee off to the rads, all done in copper which is the better job.
  7. TeamRocket

    TeamRocket New Member

    Thank you really helpful! Would it help the flow rate to run the cold water feed to the rear of the property in 22mm before teeing off in 15mm for each room?

    If combis come with only 15mm hot water outlet will this result in very slow filling of baths?
  8. terrymac

    terrymac Screwfix Select

    The flow rate from combi boilers varies / is dependant on the power of the boiler. For example,if your boiler delivers 12 litres per minute of hot water at X temperature, then it would take 10 mins to put 120 litres into a bath. If there are any restrictive valves or hoses on the supply to the bath taps , they would restrict that flow to a lesser amount of litres.
    You could run 22mm cold pipe before dropping to 15mm to taps ,but the resulting difference would be minimal
  9. TeamRocket

    TeamRocket New Member

    Sadly a heatpump isn’t going to work here, we’ve had a few specialist over they’ve all advised not to currently install one.
    Heat loss is currently just over 17kW, but the property will have the lofts and suspended timber floors insulated. There are 2 extensions to be added, but these will be modern construction so probably will end back up around 17-20kW I’d guess.
  10. TeamRocket

    TeamRocket New Member

    This is great to know thank you!

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