22mm or15mm?

[d4nnyboy]

Hi and thanks for any input....

I'm fitting my new corner bath and shower this Saturday. The hot tap on the bath that's currently there has a 22mm pipe approximatley 13" long that's T'd off from a 15mm feed.

Would I need a 22mm feed to the hot tap on the new bath or could I use 15mm? What does 13" of a 22mm feed do that a 15mm can't? Does it increase the flow rate? The pressure?..
I have a combi boiler supplying the hot water.

I was going to copy what's already on the older bath because it runs fine, I'm just generally curious to the properties of a 22mm feed to a hot bath tap?

 

[ecowarm]

ith a combi 15mm is all you have to supply the hot its just the bath tap is 3/4" and has to be reduced somewhere

 

[Dannypipe]

The 22mm was probably just fitted as the bath tap would have a 3/4 tap connector - which would as standard go to a 22mm pipe.

Baths should really have a 22mm supply all the way with a gravity system, and in my opinion even with an unvented system. With a combi, its a different matter as it depends on the flow rate. 22mm all the way will dramatically improve the flow rate, which when it comes to filling a bath matters. Most baths in the country (existing) are plumbed in 22mm, so I'm suprised that you say you are cautious of baths plumbed in 22mm.

However that is another subject.

In your case, the 22mm pipe is doing nothing, so do whatever you wish. Replace it to 15mm all the way.

 

[d4nnyboy]

The 22mm was probably just fitted as the bath tap would have a 3/4 tap connector - which would as standard go to a 22mm pipe.

Baths should really have a 22mm supply all the way with a gravity system, and in my opinion even with an unvented system. With a combi, its a different matter as it depends on the flow rate. 22mm all the way will dramatically improve the flow rate, which when it comes to filling a bath matters. Most baths in the country (existing) are plumbed in 22mm, so I'm suprised that you say you are cautious of baths plumbed in 22mm.

However that is another subject.

In your case, the 22mm pipe is doing nothing, so do whatever you wish. Replace it to 15mm all the way.

Curious bud, I'm just curious. Like I said I was going to copy what was on the old bath to my new corner bath. It makes no difference to the job really I genuinly want to understand the differences.

 

[dancinplumba]

The 22mm was probably just fitted as the bath tap would have a 3/4 tap connector - which would as standard go to a 22mm pipe.

Baths should really have a 22mm supply all the way with a gravity system, and in my opinion even with an unvented system. With a combi, its a different matter as it depends on the flow rate. 22mm all the way will dramatically improve the flow rate, which when it comes to filling a bath matters. Most baths in the country (existing) are plumbed in 22mm, so I'm suprised that you say you are cautious of baths plumbed in 22mm.

However that is another subject.

In your case, the 22mm pipe is doing nothing, so do whatever you wish. Replace it to 15mm all the way.

you can only get out wots going in with both systems
imo

you can use existing pipework if you want or change it with 3/4 by 15mm wont make any difference to flow rate

 

[JCplumb]

**First post alert** !!
If the run from your boiler/hot water tank was all in 22mm the water would have an easier time getting to your hot tap, the thinner the pipe - the harder it is for the water to get through it.
This is important with a gravity fed system because there is relatively little pressure pushing the water through the pipes.
Because you are on a combi, you have mains pressure in your hot water supply so 15mm is fine, you may see a little improvement if the supply was all 22mm but not much.
Your branch to your bath comes from a 15mm pipe so there's not really any reason for the 22mm pipe other than to make it easier to connect to the tap as bath tap connectors are usually made to connect to 22mm pipe.
Hope that explains it for you

 

[d4nnyboy]

**First post alert** !!
If the run from your boiler/hot water tank was all in 22mm the water would have an easier time getting to your hot tap, the thinner the pipe - the harder it is for the water to get through it.
This is important with a gravity fed system because there is relatively little pressure pushing the water through the pipes.
Because you are on a combi, you have mains pressure in your hot water supply so 15mm is fine, you may see a little improvement if the supply was all 22mm but not much.
Your branch to your bath comes from a 15mm pipe so there's not really any reason for the 22mm pipe other than to make it easier to connect to the tap as bath tap connectors are usually made to connect to 22mm pipe.
Hope that explains it for you

That's correct, the hot water feed from the boiler is all 15mm apart from the last 13" which is 22mm to the hot bath tap.

 

[Paulus]

**First post alert** !!
If the run from your boiler/hot water tank was all in 22mm the water would have an easier time getting to your hot tap, the thinner the pipe - the harder it is for the water to get through it.
This is important with a gravity fed system because there is relatively little pressure pushing the water through the pipes.
Because you are on a combi, you have mains pressure in your hot water supply so 15mm is fine, you may see a little improvement if the supply was all 22mm but not much.
Your branch to your bath comes from a 15mm pipe so there's not really any reason for the 22mm pipe other than to make it easier to connect to the tap as bath tap connectors are usually made to connect to 22mm pipe.
Hope that explains it for you

Spot on JCplumb, really good,clear explanation.

 

[d4nnyboy]

Spot on JCplumb, really good,clear explanation.

I don't think it could be worded out as simpler as that... It must be! because I understood it good post.

Thanks for everyones replies

 

[Dannypipe]

JC plumb has in effect said the same thing as me!! Maybe I just didn't word it well enough.

A 22mm supply is standard for a bath filler. (Hence why I asked why you are cautious of baths plumbed in 22mm).

The fact that this system is a combination boiler, with a 15mm hot feed to the bath changes the rules. The bit of 22mm pipe leading up to the bath taps was probably just put there as the tap connectors would have been 3/4 by 22mm (standard) the plumber has then reduced to 15mm.

You can buy 3/4 by 15mm tap connectors but they are not a 'standard' item.

1/2" by 15mm is standard. (basins)
3/4 by 22mm is standard. (baths)

A combi boiler will only provide hot water at a certain rate. It needs to raise the temp' of the water as it flows through the boiler, the quicker you run the water, the less heat you get in the water. (In laymens terms).

So putting a larger pipe (with a better flow rate) on the supply could well be a waste of time and money if the boiler isn't capable of heating the water enough at the point of use.

However when the water is coming from a stored vessel (unvented or vented cylinder) then you ideally want the water as quickly as you can get it. Which is why I suggest a 22mm supply to a bath as it will fill much quicker.

Sorry to bang on!!

 

[cr0ft]

My advice is if you have a combi-boiler plumb as much of the pipework to the bath as you can in 15mm. You won't benefit from installing 22mm pipework as the limiting factor will be the boiler hot water flow rate for sure, not the maximum flow down a 15mm pipe. The hot water outlet on combi boilers is in 15mm for this reason.

If you up all the pipework to 22mm you will just end up with 3x the amount of cold water to run through the taps before the hot water reaches it which on a combi-boiler will take a while.

 

[d4nnyboy]

My advice is if you have a combi-boiler plumb as much of the pipework to the bath as you can in 15mm. You won't benefit from installing 22mm pipework as the limiting factor will be the boiler hot water flow rate for sure, not the maximum flow down a 15mm pipe. The hot water outlet on combi boilers is in 15mm for this reason.

If you up all the pipework to 22mm you will just end up with 3x the amount of cold water to run through the taps before the hot water reaches it which on a combi-boiler will take a while.

I went the plumbers merchants after work to get all my parts. I'll be using 15mm pipe right up to the point a 3/4" flexipipe with a 1/2" reducer to connect to the bath taps, essentially doing it the way you described.

 

[AWheating]

3/4 x 15mm tap swivels are a standard fitting you can buy from any plumbing merchant and diy shop. Any 22mm will just waste water and is not needed. Even with unvented systems there is no need to run 22mm to the tap in most cases.

 

[Dannypipe]

I realise they are a standard fitting in as much as they are available from plumbers merchants!!

What I'm saying (and no one seems to grasp my point) is that a bath tap generally has a 22mm feed as standard as it requires a better flow rate than a basin.

I don't come across many combi boilers as I tend to work on large properties where a combi wouldn't cut it. (I don't live in a large house, but that's just my client base).

The correct by the book way to supply a bath that's being supplied by a gravity or in my opinion unvented cylinder is in 22mm.

I'm fitting a shower in a wetroom at the moment that feeds a very large head. The manufacturers stipulate 22mm feed all the way from an unvented cylinder as they want the flow rates.

As I have said, this is all off point, as the OP is talking about a combi boiler.

He asked why the bath taps had a few inches of 22mm pipe and then 15mm all the way back, and I stand by my original answer. Because the bath taps are 3/4 and you will find 3/4 by 22mm tap connectors more readilly than 3/4 by 15mm. I have both in my van, but the difference is the 3/4 by 15mm ones have been there for a few months and the others are new as I reguarly use them.

 

[Simon F]

You can buy 3/4 by 15mm tap connectors but they are not a 'standard' item.
1/2" by 15mm is standard. (basins)
3/4 by 22mm is standard. (baths)

3/4 x 15mm tap swivels are a standard fitting you can buy from any plumbing merchant and diy shop. Any 22mm will just waste water and is not needed. Even with unvented systems there is no need to run 22mm to the tap in most cases.

the 3/4" x 15mm is standard fit for the bath cold pipe in properties with mains feed to cold outlets.

 

[Dannypipe]

If you read my first post I said that as this was a combi he should:

"Replace it to 15mm all the way."

I've been trying to explain why the 22mm is there...and what service it can provide (better flow rates with the correct system).

22mm pipe was standard on gravity systems to a bath to speed up the filling time. This is probably why there was a few inches of 22mm pipe on the bath taps before it was reduced to 15mm (a hang over from the past).

I imagine most systems still in use are gravity. I know that virtually no new build is going to be gravity fed (domestics). I was trying to make a point, without much sucess I should add.

 

[kay-jay]

i understood what you were saying dannypipe.

 

[AWheating]

as regards unvented systems, it really depends on the size and flow rate of the cylinders cold supply and numer of fixtures and there use. So you need to design each system and pipesize to suit each situation.

Granted if its a large house with a decent supply and flow rate then 22mm would be more likely needed at a bath and more likely the bath to be larger than a standard 1700x700, the problem you get for med to small propertys is that the cold supply is normally less suited for high flow rates at multiple fixtures at the same time. So you run the taps on the bath at high flow rates and then other fixtures suffer poor flow while the bath is filling. So normally i would recommend 22mm hot and cold to near the bath with 15mm to the bath taps. If flow rates are really poor i would even consider 10mm to basins or even flow restrictors to help avoid poor flow rates at peek use times.

This is what i have always been told to do, and what was always designed by the design engineers at my old firm. Sometimes no matter how well you try and design correctly sized supplys it can go wrong. Customers change there mind and add extra's and i have even had a 6bed 5bath property where the cold supply fitted by the builder ended up being only 25mm with a flow rate of 20lpm so if you run the bath nothing else worked .

 

[glenno1]

The 22mm will only get get the volume that the 15mm will give it so imo i would change it to 15 especially as the 15 cannot fill the 22 demand when tap is open so you may get water hammer

 

[d4nnyboy]

What have I started! lol It's ok guys, I understand....

The job's done and all is well, 15mm works fine. The only thing wrong is that I'm 6ft 2 and the bath is 1500mm long!

So despite it being a very attractive corner bath with fancy (and very expensive!) taps. It sadly is very uncomfortable lol... but my heavily pregnant and hormonal girlfriend chose it, tried it and she approves so that's all that matters

To be honest I'm a shower man anyway