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    • 22 February 2013 at 8:45 am #31782
      Geoff Stow

        Bit of a practical question. Someone has been told that if you want to have oversized rads for a heat pump installation you would need to run the system with 22mm pipes. Any comments. Also does anyone have experience with Skirting Board radiators with a heat pump system. Ta.

      • 2 April 2013 at 9:42 am #38842
        Alan Clarke
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          Geoff
          I don't think the pipe size issue is directly related to over-sizing of radiators, but it is a feature of heat pump installation. Firstly heat pump manufacturer's specify fairly high minimum flow rates, which leads to running at lower delta T than we're used to for gas boilers.
          The impact of DT on efficiency is reversed compared with gas boilers – heat pump COP is determined by the highest temperature in the system, compared with boilers where low return temperatures are best. Heat pump COP is maximised by high heat transfer within the unit (hence high flow rates) and a heating flow temperature close to the average emitter temperature – again helped with high flow rates. On the other hand in a low energy house you are often sizing a heat pump system for a maximum output of 3 or 4 kW, so absolute flow rate is still reasonably low.
          Alan

        • 4 June 2013 at 1:05 pm #38843
          Anonymous

            Alan is correct, if there is only a small deltaT (and it will probably only be in the region of 5-10 degrees, and obviously at a lower overall temperature than for a boiler installtion) there is only so much heat that can be delivered through a small diameter pipe at reasonable flow rates.

            Vaillant offer a quick and easy guide to sizing your pipes, I will see if I can find it for you some time soon. Of course it doesnt mean you necessarily need to change your pipe sizes to 22mm, just that you need to take it into account when thinking about the design and adjusting your expectations of how it will perform. It may be that 15mm might be “acceptable”, but you could definitely expect better performance from larger pipes

          • 6 June 2013 at 9:22 am #38844
            Anonymous

              The Vaillant guide suggests the following

              For 15mm pipe (includes allowance for bends and fittings)
              DeltaT of 5 degrees can carry 3kW
              DeltaT of 7 degrees can carry 4kW
              DeltaT of 11 degrees can carry 6kW
              DeltaT of 15 degrees can carry 8kW

              For 22mm pipe (includes allowance for bends and fittings)
              DeltaT of 5 degrees can carry 6kW
              DeltaT of 7 degrees can carry 8kW
              DeltaT of 11 degrees can carry 13kW
              DeltaT of 15 degrees can carry 17kW

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