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I am replacing my bathroom suite..(that's a glamorous word for what's in there at the moment!) and the whole thing has been done with 15mm copper pipe including plumbing to bath. The water is supplied by mains and combi boiler (Worcester24CDi). The bathroom is on the first floor but only about 3m as the crow flies from the boiler cos the house is so small. I'm putting in a new bath with taps in wall and thermo shower, all concealed pipework. After the sink (15mm) should I change to 22 mm for bath and shower and what implications does this have for the pressure? I don't want to fit a power shower but equally I don't want my new shower to feel like a baby spitting on my head. In my brain it feels like smaller pipe=greater pressure (thumb over the hose pipe theory) but apparently its not so. Please explain. Ta

The cold mains supplying the boiler will be 15mm, so there's no point in having 22mm from the boiler to the bath.

The pressure is what it is, you cannot increase it. You can alter the flow rate, however you cannot get a greater flow rate than what your mains cold water is at.

Basically, 15mm pipe for hot and colds. Poor mains cold water pressure/flow rate - combi boiler not much use.


htg

Ta v. much for prompt reply...
I know that pipes to boiler and up to bathroom are all 15mm..and was all set to use 15mm thru out job...obviously with adapts to fit bath taps etc... but when ordering all my stuff in plumbers merchants..(.v. proud cos had worked it all out and felt very prof. ) large plumbing man next to me said.. you don't want to use 15mm for your bath .. you want to use 22mm..etc etc
I didn't understand why and he didn't seem to be able to explain in terms I could understand but said it was to do with pressure. Then had to go off and read DIY books, plumbing manuals etc and got thoroughly confused and ground to a halt on my great project.
Then found here..just want to find out theory behind this..Why use different pipe diameters./. why are bath taps 22mm ./.is it a hangover from hot water tanks? Am going to use 15mm but didn't want to make an unfortunate error.
.. yours plumbingly obssessed Ms M

It is to do with tank/cylinders, if you used 15mm from a tank or cylinder the bath would take too long to fill, so a greater flow rate is produced by using 22mm pipe, with mains pressure you cannot increase the pressure and is normally a good flow rate.

People confuse pressure and flow rate, the pressure is whatever it enters the house at, if you want to increase that, you have to store the water and then pump it. The flow rate is the amount of water - litres per minute.

When choosing a combi, you want to best l/m possible to ensure good showers and quicker at filling the bath.

If you have 15mm feeding the boiler, then 22mm draw off for the hot taps, all that 22mm pipe and the water in it (cold) has to be drawn off before you get hot water.

Also 15mm with a flow rate of 8 l/m, putting in a 22mm or 28mm pipe, will still be 8 l/m it's not going to increase.


htg

Thank you so much for replying....
You know when you can't rest until you properly understand a thing and now I do.
Took out old pipe work, fitted service valves and all ready to go on new pipes tomorrow so job's back on track. Thanks again.

Ms M

When I installed my shower the instructions talked about the bends in pipework reducing the available head and 15mm reduced it by more than 22mm (albeit from a tank rather than mains). Would the same not be applicable for mains pipework (reducing maintained pressure rather than static)? Or is mains pressure so high the effect of bends can be disregarded?

From a tank it would have to be 22mm.

htg

Thanks, the reason I ask is that the person who called himself a plumber employed by my builder redid my heating and water with an unvented hot water system in the loft. The mains supply to the loft is 15mm and the output from the presurised tank is 22mm back as far as the bath with a 15mm branch for other hot water tanks. He routed the pipe around 3 sides of the loft so I plan to re-do it eventually anyway and it sounds like I would get the same flow rate from 15mm as I do from the 22mm.

the central heating pipework in the loft is also 22mm and I guess this would be best left as 22mm.

Thanks again.

Different needs apply to heating pipes. Pressure within a system is not the deciding factor when choosing the diameter of pipework but rather the flow rates. It is accepted if the feed leads to three or more radiators or if the distance is excessive then at least 22mm diameter pipe should be used.

If the feed only leads to one or two radiators then 15mm is sufficient.

Ms Marple

Just a couple of other points to add to Htg's, If you have 22mm pipe you don't have to change it, it just gives an extra choice. And to explain about 3/4 inch taps, although the 15mm pipework will not significantly alter the flow rate in a pressurised system, 1/2 inch taps could very well do. So to ensure a full flow rate, 3/4" taps are still used.

Thanks to everyone for your clear explanations.
Am in danger of spending my whole life on DIY Doctor..imformation is so addictive

Ta again

Ms M

Hi I am just having installed a pressurized hot water cyclinder using a system boiler.My installer wants to use 15mm pipe to feed two baths after an initial run of say 1meter from the tank of 22mm .i have a feed straight from the water mains nearby that then goes into 22 mm to feed the cylinder why cant we carry that on through to the baths in 22mm .i am thinking that if both baths were filling at the same time it will be slow or at least slower than 22mm.I have seen previous posts but that was assuming using a combi boiler with a 15mm feed , i have a 22mm feed to tank .Many thanks

Hello, I'm having similar questions... there is a mix of 15mm and 22mm in the pipe work for my shower. I have to replace the shower pump, and the instructions say all the pipes have to be 22mm. Is there a particular reason for this or is 15mm for the cold water mains ok?

It is extremely unusual to see showers that require different pipe sizes and non of them are pumped.

You mention the shower cold is connected to the cold mains. Are you sure this is the case? Pumps should never be connected to the house cold supply. The feed should come from the header tank.

22mm pipes were generally used only on old gravity fed showers and are not necessary on pumped systems. Just why your pump instructions suggest this is important is unclear.

If your shower has worked in the past,I wouldn't consider changing the supply diameter. However, check where the cold feed comes from to ensure it is correct.

thanks for the advice!! you're right, cold water is coming from header tank... it has worked in the past, it's just that there was no thermostatic mixer valve for the hot water and we suspect the water got too hot and ruined the pump. got a bit worried when we saw the instructions for the new pump talking about different size pipes to the existing pipe work!

Dear forum,

This is my first post on this site because for the first time ever I haven't been able to find the exact answer to my queries. so here goes.

I am installing a new bathroom and adding a shower as their is currently just a bath. I have a Combi boiler (Woschester bosch junior type). from here the hot water feeds through the kithcen in 15 mm pipe untill it reaches the place where the old water heater was (which I had replaced and moved the boiler to a more convienient location in the kitchen) at which point the 15 mm pipe conects to an old 22 mm pipe. from here to the bathromm 22 mm pipe continues. the cold water feeds all the way to the bathroom via a 22 mm pipe. both pipes are now on mains pressure. my question is will the change in hot water pipe diameter from 15 to 22 mm effect the performance of my new shower and would I be wise to use a 22mm pipe al the way. shower is a mixer with deluge head and spearate hand held and can have both opperating simultaniously. I have been considering attaching a 22mm pipe to the boilers hot water outlet and re routing it directly to the bathroom and T-ing off a 15 mm pipe to the kitchen?

Thanks in advance for any advice.

Daws