What size of waste pipe do I need for the various safety systems around our expected heat cylinder (250l)

[MikeSharp01]

I need to install the waste run in our plant area and was wondering if there was a specification for the size of the waste pipe needed to handle outputs from the tundish(s) etc around the tank. It will be an MCS install so I expect they have a rule for that but I cannot find it, it's probably buried in one of the standards or hiding in plane sight. 

 

Gemini says 22mm but that feels too small to me:

"For an unvented domestic hot water cylinder safety system, the waste pipe (also called the discharge pipe) should typically be 22mm in diameter, with the section leading to the tundish potentially being a smaller size like 15mm, but always ensuring the pipe from the tundish is at least the same size as the tundish outlet itself; always consult local building regulations and consult a qualified plumber for specific installation details. 

 

Key points about unvented cylinder waste pipes:

• Pipe size:

  22mm copper pipe is usually the standard size for the discharge pipe. 

   
• Tundish connection:

  The pipe leading to the tundish may be smaller (like 15mm), but the pipe leaving the tundish should be at least 22mm. 

   
• Fall and drainage:

  The discharge pipe should have a continuous fall and be routed to a safe discharge point outside the building. 

   
• Professional installation:

  Due to safety concerns, always have an unvented cylinder installed by a qualified plumber familiar with building regulations. "

   

   

 

[JohnMo]

G3 regs cover it all. 

[Nickfromwales]

Seems odd that the above bulletpoints don’t use standard terminology? 
 

Pipework for tundish connection are referred to as D1 and D2, D1 from PRV to tundish, and D2 from

tundish to ground outside or internal waterless trap etc.

 

@MikeSharp01

Do you have a 110mm foul drain in the plant?
 

I usually come off this with a reducer to 40 or 32mm waste pipe, (I don’t think any modern waste pipe cannot take high temp water these days, but that’s the only thing to check / catch you out), and then terminate the waste pipe work with a waterless trap (preferably mounted vertically). 
 

Into that goes the D2 from the tundish, (T&PRV from the UVC), and for belt & braces I would install a second waterless trap for the other D1 discharge; the 6bar PRV from the UVC control group and PRedV. 
 

Most fitters will tee these two 15mm D1’s together into one 15mm D1 and then go into one tundish, but that requires the D1 and D2 installation recommendations to be followed to the letter. For eg, if you don’t have min 300mm of vertical D1 arriving downwards at the inlet of the tundish then the water will still be trying to ‘turn’ and not just fire straight down, ergo some of the water will splash out of the tundish; this is only real-world problematic where the T&PRV or control group PRV have opened fully, vs are just dribbling, but I test for worst case because you can bet your arse that if you don’t it’ll come back and bite you.

 

*If the waterless trap has at least 300mm of vertical pipe dropping out of it, before it turns horizontal, and the exiting waste pipe is low to the ground, AND you have at least 300mm of pipe (more is better) between the sum of the 2x 15mm D1’s  teed together before that combined discharge arrives at the inlet of the tundish, then you can go with one waterless trap. Depends on the space / pipework / feasibility etc but usually doable, but this is often decided by the height of the control group (whether these can be teed together as high as possible and still have a min 300mm fall before connecting to the tundish.

 

FWIW, if I am struggling and have to tee the 2x 15mm D1 PRV’s together then I marry them with a 22x15x15 tee and then have a 22mm pipe going as the ‘combined’ D1 into a 22mm(in) x 28mm(out) tundish (instead of of the usual factory-supplied 15mm x 22mm unit (which then gets discarded)).

 

 

If you get a bit of 28mm copper out of the bottom of the tundish and have a 32mm compression waterless traps, that will tighten down onto 28mm copper.

 

*If not, fit 2x tundish and 2x waterless traps. It’s not much more money or time tbh.

 

These are the kind of things that get lip service only btw, until there’s a problem and then whilst functioning you then realise these aren’t capable of getting the discharging water out without causing a bit of localised splashing / flooding.

 

The above pic is from Hotuns site, but I really doubt the longevity of their waterless tundish offerings which have the non-return arrangement. Note that the above pic shows the tundish as the only air break stopping stench from the stack getting into the house.  
 

I’ve used these once and was not overly impressed tbh, as the guts and spring / plunger just seem ‘feeble’, so won’t use again. Tundish and proper waterless trap every time for me!

[JohnMo]

1 hour ago, Nickfromwales said:

the above bulletpoints don’t use standard terminology

Sounds like AI generated...

[MikeSharp01]

6 hours ago, MikeSharp01 said:

Gemini says 22mm but that feels too small to me:

 

yes I did say it was AI

[MikeSharp01]

2 hours ago, Nickfromwales said:

Do you have a 110mm foul drain in the plant?

Yes its in the corner about 2m from the Un-vented Cylinder (UVC).

 

I was proposing to run 32mm plastic round to that. Is a waterless trap better than a water filled one in this application.

[Nickfromwales]

1 hour ago, MikeSharp01 said:

yes I did say it was AI

I call it “aye,aye”. None of you lot seen the film iRobot? The end is nigh. 

[Nickfromwales]

1 hour ago, MikeSharp01 said:

Yes its in the corner about 2m from the Un-vented Cylinder (UVC).

 

I was proposing to run 32mm plastic round to that. Is a waterless trap better than a water filled one in this application.

It’s the only trap you can use for discharge as water isn’t ever routinely flowing through it; ergo a wet trap would run dry (evaporation) and fill the room with stench.

 

 

 

 

 

 

note: This post was not generated by AI, I used WI instead 🏴󠁧󠁢󠁷󠁬󠁳󠁿. 

[Nickfromwales]

Why are we using arse-tificial non intelligence people?

 

It just surmises general crap on da web and dishes it up for you, utterly random and completely unreliable / non-robust.