Modeling of soil stacks

[Mark Bennett]

Hopefully the last question for a while.

Following a comment from Mark Siddall, I'm now wondering if I should be attempting to model the soil pipes in my planned PassivHaus. My current plan is to use HepVO self sealing waste traps for the upstairs bathrooms, which I believe means that I do not need an externally vented soil stack. On this premise, there will be no regular air flow through the soil stack unless there is a negative pressure event which will suck air out of the house.

However, there will still be soil pipes containing air at an uncontrolled temperature in several locations. Should I be modelling this also, or is it safe to ignore? I'm somewhat scared by the impact of the chimney, and hope that I won't have similar losses here.

Even if I don't need to model the losses, should I be looking at insulating the soil pipes?

Thanks to everyone for their advice and help so far.

[Mark Siddall]

Mark,
The best solution is to design out the problem. Why not prevent the soil stacks from rising through the building? You could just provide an external riser that links under ground to the foul drain. (This strategy was undertaken at the Denby Dale PassivHaus.)

Yes, certainly insulate any cold pipes - if for no other reason than to prevent condensation. Consider using Armaflex, make sure all joints are sealed/glued to prevent moisture getting to the pipes.

Cheers,
Mark

[Mark Bennett]

Hi Mark.

Mark,
The best solution is to design out the problem. Why not prevent the soil stacks from rising through the building? You could just provide an external riser that links under ground to the foul drain. (This strategy was undertaken at the Denby Dale PassivHaus.)

Yes I intend to design the largest part of the problem out with one of two solutions - either an external riser as you suggest, or by eliminating the need for a riser altogether by the use of HepVO type AAV. The latter is my preference to avoid an unsightly external riser

Whichever route I take, there will still be pipes from the drain/outlet of each piece of sanitaryware that will eventually connect to the foul drain. The air inside these pipes is uncontrolled and theoretically outside the thermal envelope, in the same way that a fire chimney is, albeit small in volume and pretty unlikely to be changing very regularly due to the traps/AAV.

Can I ignore these pipes to the foul drain in PHPP or do I need to model them somehow?

Even if I can ignore them should I be looking at insulating them?

Yes, certainly insulate any cold pipes - if for no other reason than to prevent condensation. Consider using Armaflex, make sure all joints are sealed/glued to prevent moisture getting to the pipes.

Would you expct the soil pipes to get cold enough to suffer from condensation? I'm pretty sure that in my house they don't suffer from condensation in winter even just after a shower when the air is very humid (no extraction unless I open the window).

Thanks,
Mark.

[Mark Siddall]

Arguably best practice in its purest sense would suggest that you consider the thermal bridging introduced - it in reality  may be a tad excessive for small unventilated lengths. The main thing is to insulate them so that they don't become problem in the first place!

I have seen condensation of soil pipes - and in an experiment (that was, I must confess, a little extreme) I've even seen it on an uninsulated SVP in a PassivHaus ... it was a gentle film that was hard to see at first. So, yes. Insulate them.

Mark

[Mark Bennett]

OK, so insulate but don't bother modeling them in PHPP. Sounds good to me.

Forgive the apparently stupid questions - I'm still struggling to understand the correct level of pedantry in the modeling, which has been surprising in some cases!

Thanks again for the help.

[Rob Jarvis]

Please forgive me for butting in but I just wondered if you had considered that if using an AAV, there should be adequate ventilation for it to function correctly.

Obviously in 'conventional' houses they would be located in a ventilated roof-space but I don't know how this would work in a PassivHaus.

I would be interested to know how this works.

Regards

Rob

[Mark Bennett]

If I go for the external ventilation stack then I don't think it is a problem, since the air traveling into the AAV/HepVO would be small, and purely caused by appliances inside the house.

I agree that if I'm trying to rely on no external stack, and just the AAV then there is some risk. I can't find anything in Part H that talks about expected air flows for the ventilation stack, just recommended sizes, so it's hard to estimate how this compares to the typical air flow through the MVHR/uncontrolled leakage.

Something to sort out with building control in the near future.

[Nick Grant]

For anyone coming to this thread it is worth mentioning that vent stack performs 2 functions. One is allowing the drains to vent to prevent the build up of gases and smells that could seep out of inspection covers and the other is to prevent traps self siphoning and so loosing their seal under certain conditions. AAVs can deal with the latter function but don't vent the drains.

At least that is what I have always thought but just skimming Approved Doc H I am left uncertain that actual vent is a requirement?

The vent is usually provided at the head of the drain and can be outside the building envelope. A vented soil stack inside the building would be a theoretical thermal bridge but like Mark, I have never seen condensation on one. The air would be a bit warmed by the sewer and if we were having this discussion in winter we could all go off and take some temperatures.

Uninsulated, 110mm PVC stack full height of building has a surface area of say 0.11m x pi x say 5m = 1.7m2
Uninsulated U value say 5W/m2 and assuming air in the stack is halfway between ambient and indoor we get anual heat loss of about:

1.7m2 x 5W/m2 x 0.5 (temp factor) x say 70 kKh = 300 kWh or 3kWh/m2.a for 100m2 house so significant in PH terms (if my assumptions are right).

However stack may be warmer than this (bathwater etc) and is likely to be a bit boxed in but these sums suggest its worth sorting by insulation or external stack.

[Rob Jarvis]

I've always assumed that a 'proper' stack was required to vent to the outside and that if a second stack was required, it could be located inside and fitted with an AAV. I did just this on my previous renovation and I do remember the building inspector specifically looking at the outside stack.

"In Part H, 13.3.7 Discharge stacks - AAVs should be located in areas which have adequate ventilation, accessible for maintenance, removable to give access for clearing blockages.
AAVs should not be used... where there is no open ventilation on a drainage system or through connected drains - other means to relieve positive pressures should be considered."
This is taken from Building Regulations - Explained & Illustrated

[Mark Siddall]

Nick,
I get something similar - but a little different. The reason for this is that I calculated the U-value of the SVP using a different RSE. The reason for this is that the inside of th SVP is shaded from the wind. I allowed for a 0.2m/s, resulting in an Rseeof 0.106 W/m2K stack effect (this figure may be to low...). As a consequence I get:

1.7m2 x 3.8 W/m2K x 0.5 (temp factor) x say 70 kKh = 231 kWh or 2.3kWh/m2.a for 100m2 house. This is still significant in a PH.

Mark

[Nick Grant]

My U value very approximate and I have no idea if air is whistling up the pipe due to stack effect or still. Temp assumption likely to be greater issue and I think we have determined that external stack good idea for the open vent. I'm not too worried about stacks with AAVs as air should not be circulating much.

Rob, I share your assumption and think the 'should not be used  where there is no open ventilation on a drainage system . . ' bit confirms it.

Nick

[Mark Bennett]

Thanks. So the conclusion seems to be that an external stack is necessary. Next step will be to sit down with the architects and work the detail out.

Thanks again to everyone for their advice.

[Nick Grant]

Desirable and simpler. If internal then if our assumptions about temperate of the air circulating through it from outside drain are correct then it would benefit from insulation but that would be tricky to do as need to avoid condensation risk as with MVHR ducts. If you have to do this I'd probably suggest Armaflex but would be difficult to do with clips, bends, floor and roof penetrations etc.

Outside good I think.