Crossflow Prevention in Soil Pipe

[tcarnell]

Hi,

I am what I guess you'd call a competent DIYer and I'm planning to replace the old cast iron soil pipe on the outside of the house with a new plastic one, but I plan to simplify the pipework by joining the bath and basin wastes into the stack rather than have them as at present; separate wastes into a hopper and from there down to the drain.

My question is this: when reading Building Regs part H, it talks of crossflow prevention, but the soil branch fitting I'm planning to use to join the WC soil branch to the stack is also where I was planning to join the bath and basin wastes.
I hope the following image helps to clarify things:

Is it still valid to join the bath/basin wastes at the junction, and if so is there any special pipework I would need to fit to them to prevent crossflow?

If I need to observe the offsets it speaks of in the Regs, one wonders why you can buy a branch with connections on it?

Many thanks in advance for any help.
Tony.

 

[migoplumber]

the t piece pictured is designed to be installed in your circumstance. (and comply with building regs)

 

[Phil]

Don't forget your deep seal traps if connected to soil stack.

Deep seal traps are not needed when discharging into a hopper as there is an air gap which will break the syphon effect when the toilet is flushed.

 

[Bernie2]

Good point this one. I asked the same question 35 or more years ago.

The answer was, if you look at the branch the waste pipe bosses are actually above the center line of the branch discharge point and so it complies with the regs.

In practice what we actually did, was install a bos pipe over the branch to pick up the washbasin waste and a bos pipe below the branch to pick up the bath waste.

The reason being, is that the bos on the branch, we intended using for the bath waste, was in practice to high for the bath waste pipe and would have next to no fall on it.

We did not like the idea of hot water from the basin causing to much expansion on the cold water wc branch.

We never used the side branch bos because of the amount of bends it would take to get into it and of course what it looked like.

We also used collar clips as anchor points on the branch and both bosses with pipe insert depths into collars checked for proper expansion gaps. And preferably all push fit pipes joined with the correct silicon grease used.

If solvent weld then expansion couplers fitted where required.

Once again it was doing it as the manufacturers wanted in case of anything going wrong, you could get them out to an installation, installed according to their standards.

And when you consider you may have hundreds of installations all the same on a housing site, the cost implications if a fault was found where great.

So if you had installed it as per manufacturers instructions you could pass the cost of repair along.

 

[tcarnell]

Thanks for everyone's help and advice, it's given me food for thought.

Don't forget your deep seal traps if connected to soil stack.

Would that problem be eliminated by using a seal resealing trap like that shown below, and if so would I need to fit one to both the basin and bath, or would one on the furthest item (the basin) be good enough?

I'm now beginning to wonder whether to join the bath and basin wastes, increasing the pipe to a 50mm from the point they join, and then taking just that one waste out and into the stack. It would lead to a neater job outside... what do you think?

Thanks again for continuing help.

Tony.

 

[justlead1]

HI The idea behind the single stack system was to install wastes and soil in such a way that the pipe never became full (allowing air to pass over the water when flowing away from appliance) and was not commoned with other waste pipes (Bath, basin etc) The bosses found on 110 mm soil fitting are 92.5 degrees if this fall is maintained from soil to trap, syphonage will not take place. But alas this is no longer the case, there are many products on the market for all areas of plumbing and heating. That will cure the problem however in my experience many are a short term fix. As mechanical solutions to problems need maintenance. And maintaining plumbing and heating systems on domestic properties is an area that is neglected when compared with car servicing etc. Good Luck

 

[tcarnell]

That will cure the problem however in my experience many are a short term fix. As mechanical solutions to problems need maintenance.

Actually I've just read elsewhere "resealing traps can be noisy and do not completely eliminate water seal loss", so it's back to the drawing board...

 

[Plouasne]

After having looked at the technical drawings of a hep2o waterless trap, I would consider them to be a massive retrograde step back in time, to the detriment of public health

Reasons::

The waterless trap relies on a plastic sleeve to open and let the water past, and closing when the water has gone past the sleeve
Water carrying bits of, soap scum, hair, scale, etc, could deposit some of this on the lips of the sleeve and thereby stopping the absolute closing of the sleeve, and so allow drain smells to percolate into the property, also if the trap is fitted away from the appliance, what is to stop somebody from damaging the sleeve if they try to clean a blockage with a wire, or rods etc

 

[david williams]

this trap is another atempt to lower plumbing standerds