Using all water pressure from 25mm water mains into 15mm domestic pipes.

[hetheboss]

I recently had my 15mm copper water mains into the house replaced.
I now have a 25mm pipe going into the 15mm pipe domestic pipe circuit.

The house pipes are all 15mm and the same pipes feed the combi boiler.

Therefore when I have a shower - for example - I'm getting 7.5mm of hot and 7.5mm of cold - if that makes sense.

If I put a seperate cold feed into the boiler coming off the 25mm pipe will that increase the pressure at the shower?

My thoughts are - I'll then get up to 12.5mm of hot and 12.5mm of cold coming out of the shower - an increase and the possibilty of being able to have two showers at the same time.

(obviously the shower won't spurt out more water than it's able to - but it will help if someone starts washing up - or goes to the loo)

Or is all that ******?

 

[terryplumb]

It's all *****. The combi will deliver X amount of litres per minute of hot water from its 15 mm outlet no matter if you fitted a 6 inch cold supply.
Pressure and flow rates are two completely different things.

 

[hetheboss]

I understand that - sort of - but if the pipe circuit is 15mm throughout the house - also feeding the combi - then surely a seperate feed off the 25mm pipe going directly to the combi will give me more water coming into the house and out of the taps?

Still ******?

 

[Lower]

If you run 25mm all the way to the combi, and increase the cold feeds to 25mm, you will likely get more flow. However, it will always be limited by the combi output and the incoming supply pressure and flow rates. The reality is that if the boiler is the limited factor, increasing the cold flow rate and pressure to a shower is irrelevant because you always need to balance the hot and cold top get a comfortable temperature.

 

[terryplumb]

Afraid so. Consider this... For arguments sake let's say you have 2 bar of pressure at your 25mm supply ,and that pipe can deliver 20 litres per minute. You then take that 25 mm directly to two taps ,so each tap is connected to a 25 mm branch. Open both taps fully. Would each tap deliver 20 litres per minute ???

 

[Hugh Jaleak]

Theoretically if you came off the 25mm with 22mm copper then split that into 2x15mm feeds it might improve matters. No harm in trying. Although as said, you may have to look at reducing the cold to the shower, as the flow from the boiler will be limited to what it can heat rather than what it can get.

 

[hetheboss]

OK - so because the new cold feed would have to go through the garage roof externally and into the wall that the boiler hangs - I'll shelve that idea then!

Cheers folks!