15mm or 22mm for cold mains?

[eveares]

Just a quick question, Just having a extension done with a new kitchen and two new en-suites.

For our hot water we have a new modern combi boiler, what as usual takes 15mm cold water in and 15mm hot water out.


The plummer has used 15mm pipe for all the hot and the cold water pipes, is this normal?

Would it not be best to use 22mm for all the cold water pipes, or at least until the pipe splits for the hot and the cold water supply's/circuits?

Just a bit worried the flow rate of the hot water being affected while the washing machine is on or the toilet cistern filling up for example.

Advice much appreciated.

 

[RickMoore]

There are sizing calcs that could/should be done. Yes generally a 22mm pipe will provide a higher flow rate, though it can be undesirable in some circumstances as it takes longer to draw off water that's been stood in the pipes.
In my own house I plumbed a 22mm cold feed up to the loft which feeds the UVHW cylinder and t's into 15mm to feed the bathroom. The Hot pipework is in 22mm off the cylinder and that T's into 15mm to feed the bathroom and kitchen. If I were looking for best flow rate I would plumb in 22mm for the most part, but it would take an age to get hot water through to the kitchen. Dumping one size or the other in just because ...... really isn't good enough, it should be sized properly and IMHO the pro's and cons discussed with the customer.

 

[Whitespirit66]

Does the combi only service the extension, or whole house?

You can put a 12" pipe in if you want, but a combi can only supply maybe 11 l.p.m. You won't squeeze out any more, unlike gravity fed supplies from a hw cylinder.

What's the boiler?

 

[eveares]

The Combi serves whole house, and we have an existing bathroom as well.

In total[//b]

1 bathroom

2 en-suites

1 outside tap

1 kitchen with dishwasher and washing machine

 

[Whitespirit66]

The Combi serves whole house, and we have an existing bathroom as well.

In total[//b]

1 bathroom

2 en-suites

1 outside tap

1 kitchen with dishwasher and washing machine

What size is the boiler? It's definitely a combi?

Unless it's huge or has some hw storage like some Glow Worms, you may be disappointed.

 

[eveares]

The hot water is and will be 15mm all over, just don't want the existing problem of the shower pressure going low when the toilet is flushed or similar.

 

[Whitespirit66]

What boiler is it?

 

[eveares]

The combi is a Vailiant ecoTec plus 937 - a storage combination boiler

Just over 15lpm at 65c, or 20lpm 42c blended water with boilers two 7.5 liters storage tanks

The main bathroom will not be used - only 2 of us living here.

 

[Whitespirit66]

The combi is a Vailiant ecoTech plus 937 - a storage combination boiler

Just over 15lpm at 65c, or 20lpm 42c blended water with boilers storage tanks

The main bathroom will not be used - only 2 of us living here.

That's OK. Not just a normal combi - has some stored hw.

Borderline re. 15/22mm, but I would almost certainly be running 22mm to new extension before splitting to 15mm for each bathroom/kitchen.

 

[eveares]

So would be best run 22mm to new extension and then split to hot and cold with 15mm hot / 15 or 22mm cold.

 

[RickMoore]

Given your description that would be my suggestion.

 

[eveares]

Thankyou you two, much appreciated.

 

[Agile]

I know its not very detailed whats been discussed but I would be treating it differently.

For a start all cold in 15 mm, not in a long daisychain but to a fair degress in a star for important uses like showers. That minimises pressure differences.

I would supply the 937 with 22 mm cold supply. The output to important uses would be in a star and all in 15 mm.

All outlets would be limited ( ideally with flow limiters---but I use cheaper iso valves ) to what they need, like WCs to 3 li/min, basins to 4 li/min etc.

Tony

 

[eveares]

The boiler only has 15mm inlet, the 22mm to the boiler would be limited by the 15mm inlet would it not?

 

[Dan Robinson]

No, but having oversized pipe work after the heat source is also wasteful.