Flow Rates

[pcsparky]

I've tried to search previous topics and other internet info but have only served to get confused.

I am in the process of gutting my bathroom and moving certain items around. While I will get a plumber for some of the work I do intend to undertake some of it myself.

Somebody stated that using Pascal's Law the pressure in my system from my combi boiler will be equal throughout the length of pipe regardless of the diameter but that the flow rates will be different given different diameters. Yet Poiseuille's Law shows that there would be some pressure loss due to friction.

All I need to know is that I want to be able to fill my bath up at the fastest rate. Though harder to achieve, I could extend the 22mm pipe direct to the bath. Or I can use existing 15mm pipe that was for a sink but then (presumably) have to go back up to 22mm for the bath taps.

So will the latter option make much difference?

 

[Master of None]

Pascal's Law appies to static pressure. And the pressure in the pipeline will only be equal if the two points of measurement are at the same height or "head" and there is no flow. Never read Poiseuille's Law, but he is obviously talking about the dynamic pressure losses you are intrested in. However given the flow of mains pressure water available through 30m of 1/2" hose in a garden. I would suspect that is more than your combi can deliver anyway at mains pressure. In my opinion the minimal pressure losses using 15mm pipework for mains fed hws from a combi (around 11-18L/min) are outwheighed by the waste of heated water sitting in a long length of 22mm pipe after the tap is closed. And the extra time it takes for the hws to reach the tap in the first place. This is why "Maximum recomended dead leg volume for Energy Saving Trust Advanced Standard is 1.5 litres."
Equivelent to 10m for 15mm and 5m for 22mm.
Couple of links below.
PS I am only a DIYer.

http://www.environment-agency.gov.uk/homeandleisure/drought/38529.aspx

http://www.ukcopperboard.co.uk/literature/pdfs/Installation-Tips/Pipe-sizing-basic-principles.pdf

 

[kevnurse]

As you say, this is about maximizing flow. If possible, fit the 22mm pipes, thus making the boiler work at max rate when you fully open the hot water tap in the bath. It will fill quicker, but at a slightly lower temp, I think. Just like an electric shower...

 

[Master of None]

Yep, a quick cold bath is sometimes best.

 

[swbjackson]

On most domestic systems on mains pressure the pressure loss will be minimal.

Assuming 0.2 l/s flow rate for a mid range combi you would be looking at a water velocity of around 1.3 m/s and a head loss of around 0.02 bar per m. To lose a bar of pressure you would be looking at a 50m pipe run so I think you can discount head losses.

Of bigger consideration is wasted energy as master of None states. When you open the tap and the boiler fires it has to displace the cold water in the pipework before the hot reaches the bath tap (okay not 100% true as some mixing will occur but good enough for this example). As 22mm pipework holds just over twice the volume of the same length of 15mm pipework the wait for hot water will be doubled.

Mike

 

[pcsparky]

So to sum up, now that we have a combi boiler there is no real need for 22mm pipes anywhere from the boiler if I can utilize 15mm ones?

 

[bengasman]

So to sum up, now that we have a combi boiler there is no real need for 22mm pipes anywhere from the boiler if I can utilize 15mm ones?

correct

 

[mogget]

So to sum up, now that we have a combi boiler there is no real need for 22mm pipes anywhere from the boiler if I can utilize 15mm ones?

correct

I'd like to add a caveat to that - your mains pressure and flow have to be adequate. Mine is 1.5bar static on a shared supply, and even my kitchen tap runs slowly. As long as you have a more normal mains pressure/flow then you'll be fine with 15mm for the bath.

 

[AMEdinburgh]

I'm a DIYer, not a plumber but when I did an ensuite a long way from one of my combis (a Worcester 350) I was advised not to use 22mm pipe at all. It was suggested there would be little or no benefit in flow and the amount of cold "dead" water in the pipe would be tiresome.

The boiler DHW output is 15mm and I ran 15mm in well lagged end fed copper with swept bends and the flow was pretty close to the quoted spec of the boiler (14litres/minute springs to mind)

We have a large 1800 x 900 plastic airbath which takes 8 mins to fill which is satisfactory (as it coincides with the warm up of the additional electric hot air/water feed). I would be wary of a cast bath as the heat might be lost into the cast with the slow fill compared to a tank system but I'm sure the pros would advise you properly on this one.

Good luck with your project.