water pressure and pipe diameter.....

[Dazzathedrummer]

Hi,

I'm replacing a bathroom at the moment, im not a pro.

I've recently replaced the taps on the existing bath with a mixer shower tap. The pipes that run around the room are 15mm cold and 22mm hot.

When I put the new mixer in I took a spur off of the pipes using 300mm x 15mm flexible connectors - so the hot steps down from 22mm to 15mm a foot before it gets to the tap.

The old hot tap had 22mm all the way up to about an inch before the tap where it was then reduced to fit the tap.

The old tap hot tap had really good pressure - the new tap has really poor pressure.......I'm just wondering what the science is behind this. It's not really an issue as the whole lot will be coming out and will be replaced......but obviously, I'd like to get the pressure right on the new bath.

 

[diyAlex]

Is it really a change in pressure that you're seeing? Or a change in the flow rate?

Pressure and flow are two different variables - that took me a while to get my head around, and I did an engineering degree back in the '90s !!

Also, are you on a combi boiler or have you got a hot water tank?

 

[Dazzathedrummer]

......I guess it must be flow rate.....there's less water coming out with the tap full open.

No combi - hot water tank.

 

[denso13]

Some taps are only suitable for higher pressure supplies which you probably won't have if you don't have a combi.

 

[diyAlex]

Yes, as Denso mentions, if you're on a combi then you're pressure will be that of the mains coming off the street - usually around 3 bar, but must vary a lot by time and location. With a combi, your flow rate will be dictated by the boiler - basically the boiler lets the water flow through the heat exchanger at a speed that will impart the necessary temperature change. So you end up with a slow flow rate at a high pressure.

On your hot water tank, your boiler is topping up the tank but your pressure is a function of gravity - i.e. the greater the vertical drop ("head") from the outlet on the hot water tank to the tap, the more pressure. Since you're working in the bathroom, I'm guessing there's not much vertical drop. When compared to the mains pressure, 1 bar = 10 metres of head, so you can see that the pressure from your hot water tank is going to be a lot less than that of a combi.

If you've got the tank in the airing cupboard on the same level as the bathroom, then this has a pressure too - but in a way that uses the pressure of the incoming water that is topping it up to push the hot water out of it - I can't quite remember the physics behind that one at the moment!

Regarding the 22mm stepping down to 15mm, and where this is done, in your original question - this I'm pretty sure if to do with friction/efficiencies in the system ... the idea here being the more that hot water is conveyed in larger diameter pipes, the less surface friction compared to cross sectional area = the less heat loss. But you've got to balance that against the fact that larger diameter pipes will need more "old" water flushed out before your hot water arrives. I think this is why basin taps tend to have 15mm pipes leading to them (people don't want to wait long whilst washing their hands) and bath taps tend to have 22mm feeds for "most of the way" in (people don't mind so much if the bath's hot tap starts off cool for a while).

 

[roberta1000]

15mm flexible connectors often have quite a small bore at least at the ends and are a good deal more restrictive than 15mm copper.

 

[muggles]

using 300mm x 15mm flexible connector

This is your problem. The flexis might be nominally 15mm but the bore will be the same as a 10mm pipe, possibly even less. Strip them out and replace with proper pipework and your problems should be solved