Hot Water Cylinder Discharge On Soil Stack (Passive House)

[tuftythesquirrel]

Hi, I know that this issue was partially discussed some time ago but I have an question regarding the discharge pipe from the hot water cylinder temperature and pressure relief valve from the fitted by the tank manufacturer and where it is terminated. Particularly in a passive house, where a fully ventilated soil stack would appear to be undesirable.

 

Our general layout of the house soil pipes, we have a main pipe that traverses the length of the house, then 5 off 110mm spurs that go to each point of use e.g. bathrooms, bedrooms, kitchen etc. At the end of each spur there is an air admittance valve to prevent siphoning. So no soil stack as such.

 

Note my references below apply to the Welsh versions of the building regulations. Also before anyone points out that I need a G3 qualification to do this, my arrangement with my heating engineer is that I will do all the donkey work e.g. fitting the supporting unistrut, tanks, pipework and he will do all the clever stuff, gas work, testing and commissioning to sign it off.

 

Normally, the discharge would go via a 15mm copper pipe into a waterless tundish, then via a 22mm copper pipe to the outside world. So not great for a passive house. There is a section and diagram in the Building Regs. Approved Document (G3.50), describing the general D1/D2 arrangement and pipe lengths etc. However, it seems to be the trend to use a dry tundish that has a 32mm outlet that allows the fitting of an appropriate plastic waste pipe (as made by Hotun/McAlpine etc.). This can also be terminated on the soil stack, via a separate pipe, again permitted in the building regulations in G3.60.

 

My question is, am I OK to terminate my 32/40mm soil pipe onto the 110mm soil stack, when it is not fully ventilated and only has the AAV’s.

 

I always try to read and understand the regulations for each aspect of the build and that is why I am asking. I read the regulations and though I was OK. However, I then read the Part G FAQ’s (link below) in relation to G3.60 which says that the stack must be fully ventilated. If I hadn’t read it I would be blissfully ignorant.

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/509352/160321_Part_G_FAQ.pdf

 

I also read a few other articles, the first below by the CIBSE and the second from the NHBC standards. One mentions the fully vented requirement, the other doesn’t.

https://www.cibsejournal.com/technical/guidance-discharge-from-storage-cylinders-into-pipework-systems/

https://www.nhbc.co.uk/binaries/content/assets/nhbc/tech-zone/nhbc-standards/tech-guidance/8.1/discharge-pipes-from-unvented-hot-water-storage-systems-.pdf

 

Our BCO’s are fairly laid back and don’t seem to want me to jump through too many hoops, so I might be lucky. By the way my heating engineer says it conforms to the regulations. I just don't see where the fully ventilated stack requirement is defined.

 

Any assistance would be appreciated.

Edited November 25, 2022 by tuftythesquirrel
Typo

[JohnMo]

Your reference docs state,

 

"smaller hot water systems, BRE Information Paper 8/07 indicates that discharges can be made to PVCu stacks, provided that: relief discharge is from domestic unvented hot water storage systems only – not combi 
boilers or sealed system boilers. 
storage volumes do not exceed about 210 litres. stacks are fully ventilated (ie. no stack cap or air admittance valve). 
pipework complies with BS EN 1329-1:2000 or BS 4514:2001.

 

It clearly states stacks are to fully ventilated, no cap or aav.  So surely you don't comply.

 

Assume you are below 210L also.

[Iceverge]

What is the reason for it needing to be fully ventilated? 

 

I can't see any practical benefit. 

[MJNewton]

8 minutes ago, Iceverge said:

What is the reason for it needing to be fully ventilated? 

 

I can't see any practical benefit. 

Might it be to avoid any impediment to pressure relief that might otherwise occur from, say, a blocked drain?

Edited November 25, 2022 by MJNewton

[ProDave]

11 hours ago, MJNewton said:

Might it be to avoid any impediment to pressure relief that might otherwise occur from, say, a blocked drain?

More likely a pressure relief from an UVC could let off a lot of steam (literally) and it NEEDS an open vented stack for that to escape.  A waterless trap won't allow it to vent.

 

A 22mm copper pipe exiting the building seems the lesser of 2 evils compared to a vented 100mm drain stack.

[tuftythesquirrel]

Thanks for the replies.

I don’t know the absolute underlying reason behind the “fully vented” requirement, I just trying to understand why it isn’t explicitly stated in the building regulations approved document G. The G3 section is fairly easy to understand but doesn’t seem to mention it.

 

To find the requirement, I need to leave approved document G, look at the FAQ’s for document G, find the section that references the BRE paper and go to the BRE site and pay £9 for their Information Paper IP8/07 (link below) last updated in Jul 2007. This was published some 10 years before the last update of approved document G in 2017/18 according to the website information. To me this seems very convoluted and would be quite difficult to enforce.

 

https://www.brebookshop.com/details.jsp?id=287207

 

Yes it is a reference document that requires it, but I’ve also got other reference documents that don’t.

 

I was just wondering if anyone had practical experience of this aspect and what building control would accept.

 

[JohnMo]

Why don't you talk with the person signing your install of.  If he's not happy no sign off, if is happy you get a cert.

 

If he signs and something goes bang and someone get injured, it's him that ends up in court.

 

Pressurised hot water can easily expand into steam when you drop the pressure quickly. Hence all the rules and regulations and a need to be qualified to install.

 

If you don't know what or why you are doing it, pay the money and get someone qualified to do it.  There's some jobs you do yourself others you you give to someone else, this is one of those jobs.

 

If you think a 20mm hole is the end of the world, go invented or thermal store.

[Iceverge]

3 hours ago, ProDave said:

More likely a pressure relief from an UVC could let off a lot of steam (literally)

 

Would this not already have been taken care of by a tundish? 

[ProDave]

2 minutes ago, Iceverge said:

 

Would this not already have been taken care of by a tundish? 

Possibly but I am sure before I have seen if you use a waterless trap you don't have a tundish.  Anyone more knowledgeable care to comment? 

[JohnMo]

If you discharge in to a slightly  pressured or blocked, stack blanked at either end with aav, that could have been installed years ago, if you have an event, the flow will take the easiest route which may be out the open part of the tundish.  ie not safe.

 

The pressure relief operates on an assumed differential pressure, any blockage, or increased pressure on the unpressurized side, will increase the lift pressure on the valve, so will open later/ at a higher than designed pressure.  The worst case is, it doesn't open at all and your cylinder explodes.  Sounds a bit dramatic, yes, but the only purpose of the pressure relief is to protect against a run away heat source that hasn't been tripped by the temperature switch.

[tuftythesquirrel]

33 minutes ago, ProDave said:

Possibly but I am sure before I have seen if you use a waterless trap you don't have a tundish.  Anyone more knowledgeable care to comment? 

I don't think that is the case. The reason why it must be waterless is that water or steam very rarely enters the trap, so any water in a normal trap would evaporate. The tundish is a waterless or mechanical trap. There is a seal a the base of the trap held in place by a spring. The weight of a few grams of water in the bowl of the tundish is enough to push the spring back and allow the water to pass the seal after which it returns. 

[Iceverge]

I'm enjoying this iterative process.

 

@tuftythesquirrel is there an option to have the UVC on an external wall and run the discharge pipe directly through the wall as per red line. 

 

 

 

 

Alternatively have the discharge to an external vented stack. Wall as per red line. 

 

 

 

29 minutes ago, JohnMo said:

if you have an event, the flow will take the easiest route which may be out the open part of the tundish.  ie not safe.

 

Would the HepVo not prevent any backflow just forcing any steam to bubble out the sewer?

 

 

[JohnMo]

6 minutes ago, Iceverge said:

Would the HepVo not prevent any backflow just forcing any steam to bubble out the sewer?

 

Not sure it would, the HepVo is downstream of the tundish.

[MJNewton]

2 hours ago, ProDave said:

Possibly but I am sure before I have seen if you use a waterless trap you don't have a tundish. 

 

They serve different purposes - the trap prevents smells from the downstream drain/sewer and the tundish provides visibility of activation - particularly unintended passing, dripping etc that would otherwise go unnoticed. 

[Gone West]

I looked at the Hotun dry tundish when plumbing my last build.

 

https://hotun.co.uk/

 

 

Edited November 26, 2022 by Gone West

[Hilldes]

My UVC's pressure/temperature relief discharges to an internal soil stack via a HepVo. This soil stack has an AAV at the top - is is not vented to atmosphere.

 

This was approved by Building Control (England) because we have an outbuilding that has a WC and the stack there is vented to atmosphere. Because of this, all the soil stacks inside the house can have AAVs fitted and none vented to atmosphere. Jeremey Harris explained it on here.

 

Not sure though if I'm missing the point on Passive House - soil stacks internally are not a good thing?

 

P.S. the plumber refers to the HepVo as a "fanny trap". You have to look into it to realise why 😯.

[Iceverge]

That's the exact situation we're in. 

 

Soil stack vent to atmosphere by the garage . AAV Internally. UVC to Tunsish to HEPVO to soil stack. 

 

[tuftythesquirrel]

Hi, thanks for the responses @Hilldes and @Iceverge, you both seem to have arrived at the same solution, which to me is good thing. I've had a look at the HepVo and I think it's just a different brand of waterless trap to satisfy the regs. I picked the Hotun product because they make an audible alarm that can be fitted to the bowl to detect when the T&PR valve operates.

 

As suggested I read a few more posts (below) I relating to this topic and realised I’ve been concentrating on the section of the building regulations i.e. G rather than H.

 

https://forum.buildhub.org.uk/topic/6727-soil-stack-passive-house/#comment-113214
https://forum.buildhub.org.uk/topic/4459-air-admittance-valve-soil-vent-pipe/#comment-71107
https://forum.buildhub.org.uk/topic/8829-soil-vent-pipe-tata-roof/#comment-150735
https://forum.buildhub.org.uk/topic/3975-tv-aerial-in-closed-panel-timber-frame/#comment-63244

 

Particularly helpful was the input by @Jeremy Harris. It started to make sense when he said you can split up the under and over pressure functions of the traditional soil stack.

 

So what I have learnt from this thread is….

1.       Internal soil stacks are a bad thing if you want to conserve energy.

2.       I can just have an air admittance valve internally, at the end of each of my soil pipe runs to satisfy the under pressure function.

3.       Externally I need a low level vent to satisfy the over pressure function.

 

This sure beats punching a hole through the 300mm solid concrete wall and the 200mm of EPS insulation.

 

I’m still not crystal clear regarding the position and height of the external vent, since there are no outside toilets etc. but if anyone could suggest anything I would be grateful. Pipework drawing attached.

 

TIA.

 

Church Hall, Narberth - 4th Feb 2020 - DWG-Floor Plan - Substructure Drawing Model.pdf

Edited November 27, 2022 by tuftythesquirrel
Typo

[PeterW]

On 26/11/2022 at 10:14, ProDave said:

More likely a pressure relief from an UVC could let off a lot of steam (literally) and it NEEDS an open vented stack for that to escape.  A waterless trap won't allow it to vent.

not really - they are combined pressure and temperature and go at either 95°C or 3.2bar. Even at that point, the tank starts to rapidly cool as mains cold comes into the tank - if you have steam then it will exit via the tundish vents anyway. 
 

I’ve seen plenty of internal stubs with AAVs signed off with D2 connections on them and no-one has picked up on it. 
 

@tuftythesquirrel if your plumber is signing off the G3 it’s his cert anyway - BCO will just ask you for the book as proof.