Unvented Cylinder D2 discharge pipe calculations

[ProDave]

I want to buy some pipe and install this tomorrow so I can get on with my flooring. I need to check some calculations.

 

I was going to play safe and pipe it in 28mm copper.  Until I realised I don't have a drill bit to drill a large enough a masonry hole for 28mm.

 

So I am looking for the exact "rules" for the D2 discharge pipe.  A quick gogle finds this, for NI http://www.buildingcontrol-ni.com/assets/pdf/P1994.pdf  I would have thought it was the same rules in all parts of the UK

 

From that, table 4.1 says if my D1 pipe is 15mm (which it certainly is on my present UVC so is reasonable to assume it will be on whatever one I buy for the new house) then I can use a 22mm D2 pipe up to 9 metres long. Each bend counts as 0.8 metre, and I can do it in 3 bends, so as long as my pipe run does not exceed 6.6  metres then 22mm will be fine.  I am confident I can do it with 6 metres of pipe.

 

Can anyone confirm this is so or correct me please?
 

[Jeremy Harris]

This any help? : http://wiki.diyfaq.org.uk/index.php?title=Unvented_DHW#Discharge_Pipe

 

Seems to be compliant with Part G as far as I can see, so 22mm should be OK in your case, I think.

[Nickfromwales]

How much vertical fall can you get from start of D2 to first invert ( to horizontal ), and then how much horizontal before the next drop Dave? 

[ProDave]

It will be pretty much straight down vertically 3 metres from the tundish (VERY slight set then opposite set in the pipe to kick it out from the wall very slightly so I am ignoring that) then 90 degree bend, 0.5 meter horizontal * , 90 degree bend 1.5 metre horizontal out through wall, 90 degree compression elbow (as I am not trying to solder there)

 

* All "horizontal" runs will be set to a slight fall. All bends apart from the last elbow formed in the Hilmor.

 

The final horizontal / slight fall section is just to ensure the pipe comes up through the floor in front of where the cylinder will go. I could shorten that if I had to and have the pipe rise up closer to the front wall and come up alongside, rather than in front of the cylinder.

 

[Nickfromwales]

22mm will be ample if there's no immediate invert. They're what cause the problems. ?

[ProDave]

Here you go:

 

The pipe starts in what will be the airing cupboard, 400mm above the floor. I expect the tundish to fit straight onto this.

 

Then it runs down inside an internal wall

Then under the floor and out through the wall just under the sole plate

Where it discharges into the French drain around the house with a short discharge pipe down into the stones so you can't put your finger under it. All under the overhang of the EWI

I have ran the hose into it and it runs free without backing up and discharges with a satisfying gurgle into the French drain.

 

5.5 metres of pipe, 3 bends @ 0.8M per bend = 7.9 metres. Even if you counted the very slight set to kick it out from the wall as another "bend" it would still just be okay at 8.7 metres.

 

 

 

[Nickfromwales]

Eeeep. 

Its 'supposed' to terminate 100mm above ground level so damp leaf litter etc can't surround it and freeze. 

I'll understand if you don't speak to me ever again. 

Regs should have clearly stated that the discharge should be clearly visible ( as it would be discharging potable water ) as it would be with the overflow from a CWS. 

I'll get my coat and hat. 

[ProDave]

I thought it said it MUST discharge below ground level so nobody could put a finger under any potentially scalding hot water running out of it.  it mentions discharging below the grid of a gulley, or onto a pavement IF a wire cage was fitted around it to prevent anyone touching it.

 

I could locally reduce the ground level there by lowering the height of stones in the French drain so the pipe is exiting 100mm above the "ground".  I could install a gulley with a grid with a hole in it if it would look the part better. What I cannot do is raise the point of exit any higher up the wall.

 

 

Re reading the document I linked in my original post it says "Downward discharge at low level but no less than 100mm above external surfaces and having a wire cage or suitable guard to prevent contact"

 

So if I shorten the pipe out of the bottom of the elbow to almost nothing, lower the level of stones so the pipe is 100mm above the stones, and fit a wiore cage it should comply?

 

 

I better not start putting the floor down yet...... 


 

[Nickfromwales]

If it's not too tight, just remove the elbow and solder on an M&F bend so you have your down turn with near zero length. 

It allows termination below the grid of a gulley because it's assumed that the water level will still always be lower. I personally don't like that option as there is, IMO, more chance of blockages by leaf litter / other causing freezing issues. The end of the pipe getting blocked is the number one concearn. 

[joe90]

Ooh errr, didn't know about having metal pipe all the way, this will be a problem ( expensive) for me as horizontal distance is nearer 8 meters.

[PeterW]

@Nickfromwales can confirm but I don't think you have to.

 

The HepVO can be used for a discharge pipe from memory so you can go into a stack, but I can't remember the exact rules. 

 

... and here's the link ! HepVO for discharge

Edited February 26, 2017 by PeterW
added the URL for the G3 link

[Jeremy Harris]

The key here is that if you want to use plastic fittings or pipe then you have to provide the evidence to BC that it will withstand the temperature OK.  If you use copper, in compliance with the approved document (Part G) then you don't have to prove anything, you can just follow the guidance and be sure you're OK.

 

In our case I used plastic waste pipes from the PRVs, and wrote a short justification based on the specification of the pipe used and the maximum possible temperature of the discharge (75 deg C from a Sunamp, but less than 15 litres, so it's debatable whether I needed to do this).  My other PRV  is a 1.5 bar one on a system with a capacity of around 70 litres and an absolute maximum temperature of 40 deg C (the highest the ASHP can run at).  In both cases the temperature was within the safe working range of all the fittings, and so was compliant with the regs, even though it's not a standard detail in Part G.

 

Worth remembering that the approved documents aren't the actual regulations, they are just some ways that the regulations may be complied with, but often aren't the only way of doing things within the regs themselves.

[PeterW]

12 minutes ago, JSHarris said:

 

The key here is that if you want to use plastic fittings or pipe then you have to provide the evidence to BC that it will withstand the temperature OK

 

 

Yep.. ! ABS, PVC-U and PVC-C all have different ratings (from Wavin Tech):

 

Pipes and fittings manufactured from PVC-U or ABS Osma Soil systems and ABS Solvent Weld Waste system – OsmaWeld:

• Suitable for use with intermittent discharges of water up to 90°C

Pipes and fittings manufactured from PVC-C or Polypropylene PVC-C Solvent Weld Waste system, PP Push-Fit Waste systems (to BS EN 1451-1 and BS EN 1566-1) and V-Joint Traps:

• Suitable for use with intermittent discharges of water up to 100°C

So for a 90c thermal/pressure relief you could use either but Polypropylene would give you a margin of safety.

[Nickfromwales]

1 hour ago, joe90 said:

Ooh errr, didn't know about having metal pipe all the way, this will be a problem ( expensive) for me as horizontal distance is nearer 8 meters.

A decision you don't have to make, as if you go through the house to run the D2 you won't get a choice as it'll HAVE to be in copper. That so expense ? ~£20 a length and 3 lengths  ? 

@ProDave, I've just realised something. You'll have to fit a hep trap doo-da or your D2 is going to make a lovely chimney buggering up your airtightness

[ProDave]

Here we go, take 2.

A very short pipe out of the elbow (too tight to solder in there with the overhanging EWI on a plastic support bracket). Ground level lowered so there's more than 100mm gap. All I need now is a wire cage to go round it. Any ideas where I can buy one?

 

I was originally thinking of the hepvo and into the stack, but it would have been a long run to get to the stack and very congested to get yet another pipe into it, so I changed to a copper D2 pipe but wanted it hidden, unlike the present house where it runs down the outside wall of the house.

 

Yes I know it's a compromise to the air tightness, but an almost unavoidable one.
 

[Nickfromwales]

One of these ?

Cant find a small basket, and the flue ones are much bigger ( way too deep ).

Maybe get some mesh from b&q and make one yourself? They sell small pieces btw, save you buying a big sheet.  

[Nickfromwales]

1 hour ago, ProDave said:

Yes I know it's a compromise to the air tightness, but an almost unavoidable one.

Get signed off and retrofit one    

[ProDave]

This cover should do the job http://www.ebay.co.uk/itm/Plastic-Coated-Overflow-Discharge-Terminal-Guard-Boiler-Relief-Outlet-Cover-Cage-/381591128473?hash=item58d89a8999:g:iSkAAOSwsN9XAktq

 

[ProDave]

On the subject of who follows the rules...... I found this home made "tundish" on a job today

 

the D2 pipe went down to the ground then along the floor with no discernible fall before exiting at garage floor level.  Part way along the discharge from the boiler expansion vessel joined it with no tundish.

 

Re my vent pipe being a chimney. I have long thought it should be possible to encase the tundish in something transparent and sealed to both inlet and outlet pipe. That would not impede the flow of water and it would still be visible, but would stop air leakage up the vent pipe.  Plastic bottle or similar perhaps?
 

[Nickfromwales]

A Hep waterless trap would do it, but one of the hotun traps is ready to go. Iirc this is from the guy who registered on EB where a lengthy volley ensued as to its suitability for retro fit ( where you couldn't guarantee that the soil and waste network could cope with high temp discharge over long periods e.g. an UVC losing its contents ). 

Your in copper to outside so it should tick all the boxes. 

On 26/02/2017 at 14:59, ProDave said:

This cover should do the job http://www.ebay.co.uk/itm/Plastic-Coated-Overflow-Discharge-Terminal-Guard-Boiler-Relief-Outlet-Cover-Cage-/381591128473?hash=item58d89a8999:g:iSkAAOSwsN9XAktq

 

I looked high and low. Just not for the right thing ?. Good find. ?

[ProDave]

That Hotun looks interesting.  Fit one now? or wait until after BC sign off and swap it?
 

[Jeremy Harris]

The Hotun looks exactly like the tundish fitted to the PRV on our boiling water tap.  I'll check later, as I'm sure it's the same thing.  From what I remember when fitting it it seemed pretty well made and ours hasn't given any trouble at all.

[Nickfromwales]

15 hours ago, ProDave said:

On the subject of who follows the rules...... I found this home made "tundish" on a job today

 

the D2 pipe went down to the ground then along the floor with no discernible fall before exiting at garage floor level.  Part way along the discharge from the boiler expansion vessel joined it with no tundish.

 

Re my vent pipe being a chimney. I have long thought it should be possible to encase the tundish in something transparent and sealed to both inlet and outlet pipe. That would not impede the flow of water and it would still be visible, but would stop air leakage up the vent pipe.  Plastic bottle or similar perhaps?
 

The pipe that rises before meeting the tee is my variation bit. Not quite a vertical fall on that bit of D1!

[ProDave]

That wire cage from ebay fits well.

Just need to screw it to the wall.

 

 

[Nickfromwales]

Patch the render and paint the 90 white first ? .