22mm or15mm?

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.

i understood what you were saying dannypipe.

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 .

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

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