drain valve

[scottygio]

1)I want to put a drain valve immediately above my stoptap. The pipe is 22mm copper but screwfix don't sell 22mm drainvalves only 15mm. Why is that?
2)Why do baths use 22mm pipes when if you tracee the pipes back they are reduced to 15mm? I have a combination boiler.
3) How serious is siphoning on a single stack system? I'm going to use a compact shower trap for the shower and a shallow bath trap for the bath(both 40mm). My understanding, and if i'm wrong please tell me, is that on a single stack system as water drains away from the stack it can syphon the water left in the traps. How is the best way to avoid this?

thanks in advance

scott

 

[ChrisR]

1)I want to put a drain valve immediately above my stoptap.

Good for you, that's what the Water Regs ask for!

The pipe is 22mm copper but screwfix don't sell 22mm drainvalves only 15mm. Why is that?

Partly because that's Screwfix. But any slightly unusual plumbing fitting is disproportionately expensive - there are many ways to skin this particular cat. A reducing compression tee (15mm branch) with a standard 15mm drain cock in the branch, would be a cheap option, where the drain cock could be changed later if required.

2)Why do baths use 22mm pipes when if you trace the pipes back they are reduced to 15mm? I have a combination boiler.

Your system possibly has been changed to a combi system. Gravity bath supplies are 22mm, and the most common bath tap connector goes to 22mm.

3) How serious is siphoning on a single stack system?

It can be overwhelmingly smelly. You can use an Air Admittance Valve (32, 40, 110 mm or built into a bath or basin trap) at a suitable point to let air in if a negative pressure develops in the pipe.

I expect you're intending to use the most common shallow shower trap which has a "seal" of only 18mm, so gets sucked out easlily. It also does NOT comply with Building Regulations , which demand a 50mm seal. These are available, but obviously a bit deeper. McAlpine make them, for eg.

 

[Softus]

I expect you're intending to use the most common shallow shower trap which has a "seal" of only 18mm, so gets sucked out easlily. It also does NOT comply with Building Regulations , which demand a 50mm seal. These are available, but obviously a bit deeper. McAlpine make them, for eg.

Whilst this is your usual excellent advice ChrisR, is the 50mm trap actually mandatory? I can find it, as being recommended, in the practical guidance document (Approved Documet H), but the BRs (SI 2000:2531) simply say that the installation "shall be adequate".