10mm hot and cold pipe runs vs 15mm

[Nickfromwales]

10mm Pipe For Hot Water Taps? (Content copied from EB)

 

Started by Eagerbeaver , 24 Jan 2016 07:56 AM

 

10 replies to this topic

Eagerbeaver

Posted 24 January 2016 - 07:56 AM

All the water pipe installs I've seen use 15mm pipe but the flow capacity of 15mm is far more than is needed for most hand basins and sinks. 

The down side of using 15mm for hot water is that you end up with a lot more cold water to shift in the pipe before it runs hot. 

Is there any problem with using 10mm for hand basins and sinks for the hot water?

 

tonyshouse

Posted 24 January 2016 - 09:13 AM

For very long runs pressure drop could result in too slow flow, avoiding using elbows will help

Would be fine with combi or pressurised system or three storey situation.

 

Nickfromwales

Posted 24 January 2016 - 10:21 AM

What lengths are the runs? Also, are we talking about a radial plumbing setup ( critical if considering using 10mm runs ). 
10mm is perfect for wash hand basins, but kitchen and utility would probably be better with 15mm for short, higher volume draw offs where 10mm may struggle. 

If there's not a good / very good cold mains pressure / flow rate at the incoming supply then excessively long runs may suffer, 
Regards, Nick 

 

ProDave

Posted 24 January 2016 - 10:38 AM

I'm considering this.

In my new house, the kitchen will have the longest run, about 6 metres. I'm planning to try a 10mm pipe for the kitchen hot water tap to minimise the volume of cold that has to be drawn off before the hot arrives. We have good water pressure and food flow at the source so I think it's worth a try. 

jsharris

 

Posted 24 January 2016 - 10:38 AM

It's commonplace on the continent to see narrow pipe runs to basin taps, not sure if it's usually 10mm or 12mm but it looks a lot smaller than our normal pipe of 15mm. With a decent head 10mm pipe will flow a fair bit of water, more than is usually needed for wash basins or even sink taps, dishwashers etc. For guidance, a 10m long length of 10mm OD copper pipe causes a pressure drop of about 1.7 bar at a flow rate of 10 litres/minute. Sink taps rarely flow at more than 6 litres/minute, 10 litres /minute is about an average mains pressure shower flow rate.

What this means is that if you have a head at the start of the pipe of at least 1.7 bar then you can get a flow rate of 10 litres/minute from the open end of a 10 metre length of 10mm OD copper pipe. We have an available head of around 4.5 bar, so I could have used 10mm pipe pretty much everywhere and still had good enough flow rates.

The downsides with using small bore pipe is that you pretty much have to use a radial plumbing arrangement, with the pipe from each outlet fed back to a common manifold, you can't really run two or more outlets from a single pipe, as they will interfere with each other to a greater extent than with larger bore pipe with a lower flow resistance. Another downside is that if you try and flow 10 litres/minute through a 10mm OD pipe then it will make a noise, especially if you have a fairly high head of pressure at the manifold. The noise is acceptable at up to around 6 litres/minute, but any more and you need to increase the pipe diameter to quieten it down.

For comparison, the pressure drop across 10m of 15mm OD copper pipe is a great deal lower, at around 0.2 bar for 10 litres/minute flow rate.

Plastic pipe flow rates are lower, or the pressure drop higher for a given flow rate, because they usually have a smaller bore for the same OD as copper pipes.

cjard

Posted 24 January 2016 - 10:46 AM

I looked a using a range of different pipe sizes but in the end just used 15 mm throughout for the following reasons

I'm using plastic, and the bore is a little smaller than copper 15ml so there's a little less volume of wasted water

A metre of 15mm plastic holds about 70ml more water than a metre of 10mm, given my longest pipe run from manifold to sink is 5 metres, that's less than a teacup. The longest shower run is 10m but, for the duration a shower runs, the excess is a drop in the ocean

Plastic doesn't suck the heat out of the water the same

The manifolds are 15mm, and I'd have to adapt down to 10. Plastic fittings at pretty bulky and ugly at the best of times and it adds cost and points of failure

Flow capacity isn't the be all and end all. If you rush water through a pipe you can create areas of low pressure where it will boil temporarily and make a horrendous noise. A bigger pipe flows more slowly isn't as prone to this cavitation 

 

cjard

Posted 24 January 2016 - 10:53 AM

ProDave, on 24 January 2016 - 10:38 AM, said:

I'm considering this.

In my new house, the kitchen will have the longest run, about 6 metres. I'm planning to try a 10mm pipe for the kitchen hot water tap to minimise the volume of cold that has to be drawn off before the hot arrives. We have good water pressure and food flow at the source so I think it's worth a try.

Consider an instantaneous heater that will use a little electricity to heat the initial water and then dial back on its output as he hotter water starts arriving. Saves water, maybe saves the planet if you have renewable electricity. Such instant heaters can be good for occasional use households as it beats having a large standing volume of hot water losing heat constantly

jsharr

 

Posted 24 January 2016 - 10:57 AM

cjard, on 24 January 2016 - 10:46 AM, said:

I looked a using a range of different pipe sizes but in the end just used 15 mm throughout for the following reasons



I'm using plastic, and the bore is a little smaller than copper 15ml so there's a little less volume of wasted water

A metre of 15mm plastic holds about 70ml more water than a metre of 10mm, given my longest pipe run from manifold to sink is 5 metres, that's less than a teacup. The longest shower run is 10m but, for the duration a shower runs, the excess is a drop in the ocean

Plastic doesn't suck the heat out of the water the same

The manifolds are 15mm, and I'd have to adapt down to 10. Plastic fittings at pretty bulky and ugly at the best of times and it adds cost and points of failure

Flow capacity isn't the be all and end all. If you rush water through a pipe you can create areas of low pressure where it will boil temporarily and make a horrendous noise. A bigger pipe flows more slowly isn't as prone to this cavitation

I did the same, used 15mm plastic everywhere, for much the same reasons. I did initially try running 10mm copper, but it was far too difficult for me to pull through posijoists on my own and I scrapped two lengths and then gave up and switched to 15mm plastic. Plastic is a hell of a lot cheaper and one roll did the whole house.

I'd definitely NOT use 10mm plastic, as the bore is a lot smaller than 10mm copper, and restricted further by the inserts.

Nickfromwales

 

Posted 24 January 2016 - 01:10 PM

I've ( on a previous job ) used 10mm Hepworth with the slimline stainless steel inserts and the flow rates were more than adequate. Longest run was prob about 13-14m. 

I used 10mm to each WC purposely to reduce water velocity and stop the WC from over monopolising on the available water pressure ( quite a significant short turn effect on showering when not reinforced with an accumulator ) which gave the desired results, and also reduced the noise of the cisterns filling too.

• The basin hot and colds were all in 10mm and performed well.
• Manifold 15mm outlet with a 15x10mm reducer. 
• Hepworth 10mm direct to outlet with zero joints. 
• 10mm exposed behind basin ( out of sight ) terminating into fitting reducers ( female each side ) and then a piece of 15mm copper made off to a male 15mm x 1/2" fitting which the tap connector fits into. 

Everything else in 15mm, for simplicity ( one size fits all ) more than anything 
Regards, Nick.

 

ProDave

 

Posted 24 January 2016 - 05:49 PM

Sounds like in my case 10mm to the kitchen sink will be okay. It's the only pipe run in this direction and the longest run from the hw tank. All the rest (bathrooms and utility) are the opposite direction and will be more conventional.

the bulk of the run will run parallel to one of the posi joists so I will try and run it in a coil of 10mm copper.

 

VIPMan

Posted 24 January 2016 - 06:20 PM

I've used a 15mm manifold system with 10mm copper to basins. 15mm plastic to bath and shower. Maximum runs around 17metre. Still a work in progress but flows seem reasonable with around 4.5 bar mains pressure. 

 

 

 

Edited July 8, 2016 by TerryE
Reformatted as discussed