Advice on my pipe size.

[obmitty]

Hi, I have recently run 15mm copper pipe from my kitchen to bathroom before we get the new boiler installed.

The boiler is about 10 meters from the bathroom, and on the lower floor.

Will my combi be able to supply hot water to the bath at a reasonale rate will I need 22mm pipe?

We are using an electric shower from the mains.

Cheers!

Tim

 

[Bahco]

The gas guys will tell you that min pipe size will be 22 mm and could even be 28 mm depending on the distance and resistance.

Don't curse as you remove all the 15 mm pipe you have made such a good job of

 

[clf-gas]

15mm is ok on a combi, though the bath tap may be 3/4, throttle it down here to balance through boiler.

 

[Bahco]

Hi, I have recently run 15mm copper pipe from my kitchen to bathroom before we get the new boiler installed.

Was the pipe you have laid referring to the gas supply to the boiler or to the hot or cold distribution pipes

 

[tryitandsee]

I assume you are talking about water pipes.
Be prepared, it will take a long time for the hot water to get to the bath and you will run/waste water until it gets hot enough to put the plug in. Most boilers are pre heat, that does help, but 10 metres is quite a distance.


Make sure you lag the hot water feed pipe really well as the heat loss will effect bath temperature.

Sizing of pipes I will leave to others.

 

[obmitty]

surely water waste would be less, as there is less volume of water losing heat in the pipe work in thinner pipes?

 

[stevesey]

surely water waste would be less, as there is less volume of water losing heat in the pipe work in thinner pipes?

Smaller pipe = greater surface area to cross section ratio, so greater heat loss at the same flow rate

 

[obmitty]

Sure, a greater surface area to volume ratio increases heat loss, but the length of time you wait, and the amount of water you need to heat assuming flow rate is constant must be more in larger pipe systems no?

This is assuming you are starting from a cold system, with no heat in the pipes when you turn the tap on.

 

[tryitandsee]

Interesting. I see what you mean. If you had a needle size pipe, water would reach the end quickly and hot. But this would take for ever to provide the quantity for purpose, ie full bath. There must be a size that can be formulated to give the best compromise.

 

[Agile]

I dont think you need to be posting this question because you already know that the pipe should be in 15 mm.

What you may not know is that you should insulate the hot supply pipes to every outlet.

I hope you know that the relevant facets of the boiler installation should be done by a CORGI who should notify the installation to CORGI who in turn will advise Building Control.

Tony

 

[obmitty]

I think the important factor goes back to my original post: What is the flow rate allowed by the taps (the bottleneck in the system).

If it is very low, you wait ages and you waste water with large pipes.

You wait less time and water at the same flow rate with smaller pipes.


However, if the tap is not much of a bottle neck, and the limiting factor is pipe size, you wait ages because the pipes are too small.

I've decided to wait before drilling holes in my bath until the central heating is intalled, then try the flow rate through the basin taps I want to use on my bath. If its ok, I will drill the holes!

 

[stevesey]

Actually I by flow I was thinking in terms of velocity - i.e. a needle side pipe with water flowing at 1 metre/sec will provide colder water than a 22mm pipe flowing at 1 metre/sec (but of course the 22mm pipe would fill the bath much quicker).

Of course with the 22mm pipe you have a lot more cold water to flush out before the hot arrives.

The comprimise is to size the pipe to match the flow rate your source (combi boiler) can provide - too small a pipe and the resistance will limit the flow and the bath will take ages to fill, too larger a pipe and and the velocity of the water will be so slow that it loses to much heat on the way (and take longer to flush out the cold when first turned on).

All very academic - what do the instructions for the new boiler say?

 

[Agile]

All of that is totally irrelevant!

Assuming as implied its a combi boiler the flow rate through that will be restricted towards what it can heat. That will be 9-13 li/min depending on boiler power.

The mains water will presumably have checked before choosing a combi and will be over 1 Bar and able to sustain a dynamic flow of at least 12 li/min at a residual pressure of 0.5 Bar.

A 15 mm pipe will be totally adequate for a combi flow rate and few taps will add much additional resistance.

Tony

 

[stevesey]

All of that is totally irrelevant!

I did say it was all very academic.

 

[obmitty]

But surely length of pipe, number of turns, diameter of tubing, height above the water main, diameter of tap connectors and tap itself and all these factors could collude to decrease my flow rate below what the boiler can heat?