Old plumbing (galvanised steel?) flow question

[BenjyK]

The rising main in my downstairs bathroom looks like galvanised steel- 27mm by my measurements, which eventually joins to 15mm copper in the kitchen. I'm looking to tee off somewhere to feed an unvented cylinder, so need the best flow rate possible.

My question is, would the flow rate be better from the wider steel pipe? Or will it be much narrower inside the pipe, so little gained?

You see, the snag I have is that I have no way of isolating the supply before it joins the 15mm copper. I only get 12 l/pm flow from the 15mm copper pipe, so not enough for the unvented cylinder. My hope is that teeing off from the 27mm galv steel? rising main gives me the 20 l/pm I need. Otherwise I guess I'll be looking at installing an accumulator and extra ££.

Advice would be much appreciated.

 

[Gasguru]

Your priority is surely to sort out a way to isolate the pipework...

 

[BenjyK]

Your priority is surely to sort out a way to isolate the pipework...

I wish I could! There’s a service tunnel below us, I suspect that’s where it’s isolated from. But we can’t gain access without the management company, and they’re not doing visits until who knows when because of COVID.

 

[Gasguru]

Stuff the management company...the insurance policy might be void.
Is there a residents association?

 

[oldbuffer]

Please note than none of the following makes any allowance for corrosion or lime-scale build up on the pipes.
1. Steel pipe with an OD of 27 mm is almost certainly 3/4" bore.
2. If so it will have, surprisingly, an internal diameter of 0.75" or about 19 mm.
3. The internal diameter of 15 mm copper pipe is 13.6 mm
4. The cross sectional area of a 19 mm pipe is almost double that of 13.6 mm pipe.
5. For a given pressure not resulting in turbulent flow, you'll get almost twice as much water through the steel pipe as through the copper.

You can still measure the pressure via the copper pipe, that won't change, and you will need to measure both static and dynamic (with a mains cold tap running) pressures. Anything less the 2 bar and an unvented cylinder is iffy.

You could consider freezing the incoming main, but you may struggle to find a plumber with the tools and experience to cut the pipe, thread both ends and insert the Tee.

 

[Nige F]

, but you may struggle to find a plumber with the tools and experience to cut the pipe, thread both ends and insert the Tee.

I think you WILL struggle - and then they may consider the age of the pipe to be too old to stand dies turning on it . And they may not be happy as a " management company" is lurking - Just my thoughts on it, because I could do it.....but would not be keen because of what I mention

 

[ianmcd]

Every day is a school day, I have never seen Galv used for a water supply

 

[BenjyK]

Thanks @oldbuffer that’s a really helpful reply. Gives me the confidence that it’s at least worth exploring further, that it’s not a complete no-go. Sounds like it may just get me the flow rate I’m looking for.

Thanks @Nige F for highlighting those caveats.