Is manifold plumbing better than branching off?

[Bodgitnscarper]

Good morning. I’m looking for advice regarding plumbing the hot and cold around the house having built an extension and moved the kitchen and bathroom. There is an additional utility room and downstairs loo also. I’ve noticed that the Americans are big on manifolds and wondered if that would be a better way to run the supplies to showers etc? Or would it make little difference with regard to pressures/flows. What I’m trying to avoid is the showers being affected by taps being turned on etc, or at least try and reduce the effects of this. Any advice on where I can learn the difference would be appreciated thanks.

 

[bernardgreen]

What I’m trying to avoid is the showers being affected by taps being turned on etc,

To achieve that you need to minimise the amount of pipe that is shared between each of the showers, taps etc. When a tap is opened the flow to that tap will create a pressure drop along the length of shared pipe. This pressure drop will be added to the pressure drop affecting other taps or showers.

Manifolds at the source of the hot water achieve this minimisation but at the cost of more pipes to purchase and install. There will be a pressure drop created by flow along the pipe from source (manifold ) to tap but this will not affect the flows to other taps or showers. The manifold and the pipe feeding it are shared pipes.

 

[Bodgitnscarper]

What I’m trying to avoid is the showers being affected by taps being turned on etc,

To achieve that you need to minimise the amount of pipe that is shared between each of the showers, taps etc. When a tap is opened the flow to that tap will create a pressure drop along the length of shared pipe. This pressure drop will be added to the pressure drop affecting other taps or showers.

Manifolds at the source of the hot water achieve this minimisation but at the cost of more pipes to purchase and install. There will be a pressure drop created by flow along the pipe from source (manifold ) to tap but this will not affect the flows to other taps or showers. The manifold and the pipe feeding it are shared pipes.

Hi Bernard, thanks for taking the time to reply. I think I understand what you wrote. So the pressure along a branch pipe which is teed off to a number of appliances will reduce for each appliance in use, therefore the last appliance at the end of the run will have the least pressure? Is the flow directly related to the pressure and how does the above scenario differ from having a manifold with say 3 different appliances in use at the same time? Would it be equal pressures available with a third of the flow to each?

 

[bernardgreen]

Is the flow directly related to the pressure

In simple terms Yes. Flow = Pressure difference / Resistance ( to flow ) Pressure difference from end to end of the pipe.

Resistance = length of pipe X k/diameter

( a smaller pipe means greater resistance but halfing the diameter increases the resistance by more than 4 times and factor k depends on the diameter )

Effective Pressure = Static Pressure at Source - Pressure drop due to flow. ( if there is no flow then pressure at source and presure at closed tap are the same )

The flow from a fully open tap will be determined by Effective Pressure.

 

[terryplumb]

What is the source of your domestic hot water ,combi boiler ,unvented cylinder ,vented cylinder etc?
Are all your cold outlets ,taps/WC/bath etc directly from mains ?

 

[Bodgitnscarper]

Thanks again. Terry the mains water feeds a new combi boiler. The hot and cold come out below and above the boiler in 15mm.

 

[Bodgitnscarper]

The boiler is in the corner of the house with the kitchen sink position approx 3m away. The en-suite is directly above the boiler, the shower being approx 2m away and the main house bathroom is adjacent to the en suite. I was just trying to figure out if I should take a hot and cold in each direction and branch off for the basin(s) and bath or feed them all individually which would require a sort of manifold?

 

[terryplumb]

unfortunately you will suffer from reduced flow when more than one outlet is in use . Its highly unlikely that two showers will run simultaneously ,(or just one shower and another outlet simultaneously ) with satisfactory results . Bear in mind that all hot and cold outlets are essentially fed from the incoming mains ,so the more outlets that are being used at the same time results in reduction of flow to them all.

 

[John D v2.0]

The annoying thing about a manifold is having a shower will fill up all the pipework with hot water, but if you then use the basin you'll have to run all the hot through all over again.

 

[Bodgitnscarper]

unfortunately you will suffer from reduced flow when more than one outlet is in use . Its highly unlikely that two showers will run simultaneously ,(or just one shower and another outlet simultaneously ) with satisfactory results . Bear in mind that all hot and cold outlets are essentially fed from the incoming mains ,so the more outlets that are being used at the same time results in reduction of flow to them all.

Thanks. So I guess I was conned when I was told I would get better flow with the biggest boiler?

 

[terryplumb]

Thanks. So I guess I was conned when I was told I would get better flow with the biggest boiler?

You will get more flow from a " bigger" ,more powerful boiler ,that's quite so.
However that flow ,let's say 14 litres per min for example , that the boiler can deliver ,when routed through two outlets at the same time can not give 14 ltrs per min at both outlets. But it would be shared between them ,not necessarily equally.

 

[Bodgitnscarper]

Ok thanks to all for your replies. Would there be any advantage whatsoever then, in running the supply to the shower separately or will I just have to ban everyone from running the taps whilst I’m in the shower?

 

[terryplumb]

You have one pipe ,bringing domestic hot water from the combi boiler which is invariably 15 mm diameter. Extending this to all your hot taps / showers etc with the minimum amount of elbows and the shortest route possible would be your objective. You can't really run a " separate " supply to a shower as it would simply be another branch from the boiler ,unless I misunderstand your description.

 

[oldbutnotdead]

An addendum to this- if you're using plastic pipe then manifold is a much better solution- the plastic tees, elbows etc are bulky and expensive and really need to be easily accessible so they can be fixed when they leak. Using a manifold should mean you have a connector at the manifold and another one at the appliance and that's it- no intermediate joints to maintain access to. Of course you'll be using lots of pipe......