Are long 10mm pipes to kitchen sink OK?

[ringi]

I will have a kitchen sink that will be between 10m and 16m (depending on kitchen layout) from the main pressure hot water tank.

Normally the hot water would be run in 22mm pipes, branching out into 15mm pipes going to each outlet. However I wish the water to be quickly at a useful temperature.

So using a 10mm pipe to reduce the volume of cold water that needs to be run off looks like a good option, however I wish to be able to fill a sink in a reasonable time for doing the washing up.

Has anyone get experience of using manifolds with 10mm pipes going to sinks and basins? (With 15mm pipes going to showers.)

Another option for the kitchen, is to fit an electric under sink water heater, so removing the cost of heating water in the pipe that is never used, but having to use more expansive electricity for the kitchen hot water.

 

[Dan Robinson]

Mine is fine.

 

[OwainDIYer]

Another option is to fit a return pipe from the sink back to the cylinder and use a small circulating pump.

If all pipes are lagged, and the pump is only operated when required (a PIR near the sink?) then heat losses and running costs should be low.

 

[Dan Robinson]

Seriously?

 

[ringi]

Please can someone check my maths, I work it out that 10m of 15mm plastic pipe holds about 1l of water and 10m of 10mm pipes holds about .3l of water.

A sink tap gives a flow rate of about 6 l/m for both hot and cold.

So with a 10m long 15mm pipe the wait will be 10 seconds, but with a 10mm pipe the wait will be 3 seconds, so very little difference.

However this does not seem right!

My sink at home has about 10m of 15mm copper pipe to it from the tank and about 6l/m hot water flow rate. Yet it takes about 20 seconds before it starts to get warm, and then another 20 seconds before it is hot. (So about 4l of water before down the drain before it gets hot enough to be useful.)

So about half the time is due to heat loses into the copper of the pipe – this is clearly less of an issue with plastic pipes.

Am I thinking along the right lines….

 

[JBPHS]

To find the pipe volume per metre, first work out the pipe area (internally).

Using 10mm pipe with a wall thickness of 1mm:
Area (m2) = Pi x ((Pipe diameter (m) / 2) - pipe wall thickness (m))squared)
= 3.14 x (((0.01/2) - 0.001) x ((0.01/2) - 0.001)))
= 3.14 x ((0.005 - 0.001) x (0.005 - 0.001))
= 3.14 x (0.004 x 0.004)
= 3.14 x 0.000016
Area = 0.00005024m2

To work out the pipe volume per metre, remembering that there a 1,000 litres in a cubic metre:

Volume (m3) = Pipe Area (m2) x 1,000
= 0.00005024 x 1,000
Volume = 0.05024 litre/metre

Then simply multiply by pipe length:
Total volume (m3) = Volume per metre (l/m) x Pipe length (m)
= 0.05024 x 10
Total volume = 0.5024 litres.

Using the same calculations 15mm copper with a wall thickness of 1mm (copper is usually 0.9mm though) will yield a volume of approximately 1.3677 litres.

James.

 

[JBPHS]

So following on with a flow rate of 6 litres/minute:

To calculate how long it would take for the water at 0m to pass through 10mm pipe at 10m:

Duration (s) = ((60 / Flow rate (l/m) ) x Total pipe volume (litres) )
= ((60 / 6) x 0.5024)
= (10 x 0.5024)
Duration = 5.024 seconds

Again, using the same equation for 15mm pipe in my previous post would give a time of 13.677 seconds.

James.

 

[ringi]

Thanks James,


That is what I did, but I used a wall thickness of 1.65mm that I got from a Hep20 technical handbook. When I plug your wall thickens in to my spread sheet I get about the same results as you.

Do you know how to take account of the water loosing heat to the pipe? (It is clearly not a steady state, so normal methods using u-values will not work.)

 

[JBPHS]

Thanks James,

That is what I did, but I used a wall thickness of 1.65mm that I got from a Hep20 technical handbook. When I plug your wall thickens in to my spread sheet I get about the same results as you.

Do you know how to take account of the water loosing heat to the pipe? (It is clearly not a steady state, so normal methods using u-values will not work.)

No problem, obviously pipe thickness varies considerably between manufacturers. For your needs, the heat losses involved will be negligible. Don't forget to include the pipework from the DHW cylinder to the start of the 10mm.

James.

 

[OwainDIYer]

Seriously?

Seriously. Absolutely standard practice on commercial installations especially where water in pipes must be kept hot to prevent legionella.

 

[JBPHS]

Seriously?

Seriously. Absolutely standard practice on commercial installations especially where water in pipes must be kept hot to prevent legionella.

In that case shouldn't the secondary circulation pump be left on continuous, I.e. not operated by a PIR or a time clock?

James.

 

[ringi]

Another option is to fit a return pipe from the sink back to the cylinder and use a small circulating pump.

If all pipes are lagged, and the pump is only operated when required (a PIR near the sink?) then heat losses and running costs should be low.

This will require nearly double pipe work and lots of pumps for me, unless I put in a long 22mm (or 28mm) pipe run round the complete building, so everything can have short branches of it.

Controlling the pump will not be easy as the kitchen is also going to be used as a dining room, so more than a simple PIR is needed. If there was a nice “off the shelf” control system when the user pressed a button for hot water and a light indicated when the hot water was ready for the tap to be used, then it would be more of an option.

Without controls so the pump only runs when need, the heat lose will be far too high regardless of the lagging. (The dT is over 35c between the pipes and normal temperature of the areas they will go var.)

 

[ringi]

No problem, obviously pipe thickness varies considerably between manufacturers. For your needs, the heat losses involved will be negligible. Don't forget to include the pipework from the DHW cylinder to the start of the 10mm.

James.

The 22mm (or 28mm) pipe from the DHW cylinder to the start of the 10mm is clearly of great importance and is also the hardest to insulate. (Just 1m of 28mm has more water in it then 10m of 10mm!)

I think with copper pipe heat loses are important as the copper pipe itself must heat up taking heat out of the water. Otherwise my real life experiment would have had cold water coming out of the tap directly before the hot water; however it had about the same amount of warm water then cold water.

If using 10mm pipe truly only give the small improvement our calculation shows, I am questioning if it is worth it compared to 15mm pipe, as 15mm gives a lot better long term flexibility to connect additional outlets.

However I am getting convinced not to run 22mm (or 28mm) pipe as a backbone with all rooms coming off it, as using “home runs” does not use that much more pipe for my layout, gives quicker heat up times, and puts all connections in easy to access locations.

 

[OwainDIYer]

Seriously?

Seriously. Absolutely standard practice on commercial installations especially where water in pipes must be kept hot to prevent legionella.

In that case shouldn't the secondary circulation pump be left on continuous, I.e. not operated by a PIR or a time clock?

Yes, for legionella prevention.

 

[Dan Robinson]

Putting a secondary recirc onto 10m of 10mm plastic fed from an unvented cylinder is bonkers mate.

Don't wanna be rude... but...