best piping layout to make the most of combi boilers?

[Tipper]

....22mm to a combi is pointless as the biggest can only deliver 16l/min (easy for a 15mm pipe to cover)

It's not actually pointless just that most plumbers don't bother to do it!

The point about a larger bore pipe to the combi is to reduce the pipework losses before the combi and thus get better pressure and hence flow after the combi. Similarly for the outlet pipework but having a large pipe takes longer for the hot water to get to the outlets and wastes a lot of heat so it's not done.

The other thing worth thinking about is keeping the inlet and outlet pipework as simple as possible and as 'straight' as possible because every bend adds to the losses in the pipework by a large margin and reduces the flow to the outlets. Swept bends are much better than elbows and in my opinion Speedfit fittings in particular with the sleeves that go with them are far too restricting.

IT IS POINTLESS

a combi will only provide say 11 litres/min @ a 35* rise so you try putting 20Litres/min through it and see what happens.

If you have ever installed a combi on a old vented system the cust will nealy always find the bath tap runs cold if fully opened

Guessing you aint a plumber just a google warrior

But you're missing my point!

Yes 15mm pipe is capable of more than most combis rated outlet performance but the longer and smaller and the more bends the inlet pipework has the worse the performance of the combi may be. Any restriction in the pipework before a combi will have an impact at its outlet due to the increased friction losses generated (Bernoulli's theorem iirc). This is particularly so if the inlet pressure/flow is right down on the minimum.
Each fitting, particularly elbows and tees and even more for those with restrictive pipe sleeves will increase the pressure losses and hence flow. A restrictive sleeve is like putting an orifice plate in a pipe which is used in pipework design to reduce pressure. It's called throttling.

Oh and you're right, I'm not a plumber, just a humble (retired) power station engineer. (Yeah, and before anyone says it , I am full of ****, but at least my knees are intact!)

 

[Bunnyman]

Yes but u know as much about plumbing as i do abt power stations. Best to keep ur comments to urself instead of misleading

 

[Tipper]

I suspect I know more than you think about plumbing after all there is rather a lot of plumbing in a power station! 100m high boilers would give most plumbers nightmares!

Just joshing, I know you all know more than me...that's fairly obvious judging by the content of many of the posts...well, by those who do know what they are talking about. I had you, Bunnyman, included in that group but judging by your last post I'm not so sure now.

 

[picasso]

If you have got to the stage where your worrying about pressure loss in your mains due to pipe sizing you shouldent be fitting a combi, maybe time to start thinking about upgrading the mains or booster sets.

 

[dreadnoughtheating]

No tee's needed, feed everything with hot water, helps keep the WC pan clean too.

rogue, the real bonus is no condensation on the outside of the WC cistern in the winter...



DH

 

[xelaris]

unvented cylinder for high water demand NOT two combis.

22mm to a combi is pointless as the biggest can only deliver 16l/min (easy for a 15mm pipe to cover)

Too much water through your Biasi will lead to luke warm water

that's what the manual says - 22mm recommended...

 

[xelaris]

Only if you don't want cold water in the house. You'll obviously have to tee off before the boiler. A combi is fed with a 15mm mains pipe and as a result you'll never have great flow which is why you wouldn't use one where you have more than one bathroom.
J

The minimum requirements for my Biasi is 22mm...
I guess with central heating on and a second toilet is when problems starts to appear - that's why I'm considering going UFH for heating and Combi for two separate toilets and 2 showers - maybe it's worth considering 2 combis (remain to be seen if the water flow is going to be adequate).

What are you talking about? Have you any idea?

I have a very fuzzy idea that's why I started a discussion in the first place (doh, this is what forums are for)...