Trying to sort out the design of my new CH and am tring to find out what the effect of pipe length is on flow and output, so i can work out what size pipes i will require to put in. I have calculated all the radiator sizes so this really is the last thing i need to work find out before I can start.
Thanks for your help.
Pete
If the flow and return are any more than 12 mtrs in length, then they have to be insulated 28 mm. Anything under that length can be insulated 22 mm. The insulation is the important part as it stops you losing the heat in the parts you don't want to.
Broadly, to avoid excessive noise, the flow rate should be kept at under 1.5m/sec.
The radiator outputs, which you need to correct for operating temperature, allow you to calculate the flow rate (mass x specific heat x temperature rise).
As you then group the radiators together into trunk pipes the figures are added and you then need to account for the total flow of those sections.
The Copper Development Association has a useful article for working it all out fairly easily.
If you wanted to be really scientific you can use the graphs of flow data, but it isn't necessary. Article P150 has a simple to use method to work everything out using tables of numbers. A little work with a calculator and you're done.
Typically you will find that you may need 28mm close to the boiler or for long runs, although usually 22mm is enough, then 15rmm for most other requirements being careful not to aggregate too much load onto
15mm lengths. If you want to then go for 8 or 10mm microbore you can use the same method to calculate that..andy
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I once extracted a formula to relate flow velocity, pipe resistance and bore:
R = y * (V ^ x)
where R = Resistance in inches of water per 100 foot pipe run V = Flow rate in pounds per hour x and y vary for different pipe sizes
Size(mm) x y 8 1.7650 1.225 *10^-2 10 1.7430 4.150 *10^-3 15 1.7180 5.836 *10^-4 22 1.7495 7.070 *10^-5 28 1.8913 6.648 *10^-6
(Table looks better in fixed point font)
For 8mm tube this is for 0.6mm wall thickness, for 10mm it is 0.7mm wall.
These are empirical formula that I derived from tables. If anyone has an analytic version or a single formula that takes bore into account I would like to have it.
From this a 22mm pipe has about 1/7th the resistance of a 15mm pipe of the same length and flow rate. A 28mm pipe has about 1/30th the resistance of 15mm.
-- Phil Addison The uk.d-i-y FAQ is at
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