# Flow loss through 15mm versus 22mm pipe

#### chrisgarlick

Hi List -

I need to get a run of pipe from where the (hot water, 22mm) supply emerges at ceiling height, down a wall, to meet the exit point which is a stub of 15mm copper pipe at floor level leading through the wall to the kitchen beyond.

The run, from ceiling to floor level, is about 3 metres. It will be easier to dig out a chase for 15mm copper down the (brick) wall, but for some reason I'm worried about loss of flow and feel I should be using 22mm as far as possible.

From hot water cylinder to where the 22mm pipe emerges in the ceiling, is only about 3m, then I need to go 3m down the wall, to meet this 15mm pipe which then runs through the wall and sub-floor, about 6m, to the kitchen sink etc.

So, bearing in mind that most of the run is already in 15mm, am I going to notice any significant reduction in flow at the taps, if I complete the link in 15mm, as opposed to in 22mm...?

Sorry for the length of this post, hope it makes sense !

All views gratefully received - thanks in advance.

Kind Regards.

My opinion, for what it's worth, use 15mm.

Using a flow rate of 18 l/min you'd lose 1.2 metres head in 3 m of 15mm, or 0.21m head in 22mm. Slight difference, but I'd forget it.

Thanks guys, this is helpful.

I suppose the more I think about this, the more I conclude that the flow rate should be constrained by the most restrictive part of the pipework end-to-end... that is, if there's already a 15mm section then I will not lose any flow by extending that section. Is that a valid theory...?

ChrisR - thanks for your comments - what's the formula you're working to there, please......?

chrisgarlick said:
I suppose the more I think about this, the more I conclude that the flow rate should be constrained by the most restrictive part of the pipework end-to-end... that is, if there's already a 15mm section then I will not lose any flow by extending that section. Is that a valid theory...?

Not quite. Any length of pipe with a flow rate will give a pressure drop, and the fatter the pipe, the lower the drop ( fourth power of the radius, ie double the radius increases the flow rate by 16!). A bigger effect is caused by joints and elbows, and of course, the tap at the end. However, 18l/min will probably give you the problem of water splashing off the bottom of the sink and up the walls, but this flow rate is unlikely with water from a vented system. If the flow rate is not so large, the pressure drop will be lower. Using plastic pipe will reduce the need for joints and bends, and will keep the heat in the pipe better than copper.

Standard 3/4" bath tap from cistern in the loft above gives about 20 l/min.

Box's equation might be what was asked for - -I used a chart.

ChrisR said:
Standard 3/4" bath tap from cistern in the loft above gives about 20 l/min.

Don't disagree with this, but out of a kitchen tap would be quite a surprise.

........................

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