House trap before septic tank?

[Qwertyjjj]

I have a family member who has property in a country where code requirements are not exactly strict

The septic tank smells in the house.

There is a section of the plumbing for the main waste pipe that comes from the house and runs a70 ft (slight angle from horizontal) before running down 20 into the septic tank.

There is either a problem with the venting at the top of the house - the pipe does not extend beyond the roof but rather into a little roof cap or there is an open pipe in the house (in the concrete cavities) or some other problem but when the central ventilation/AC is turned on, septic tank smell comes out of the AC vents. So, somehow the smell is coming from the main cavity in the walls where all the pipe work is hidden and cannot be reached.

To try and improve this, they added a vent in the main waste pipe and knocked a hole in the wall of the house underneath one of the toilets!
I can only assume this was a way to increase the ventilation of the tank but it has made no difference to the smell.

Can you add a p trap in a long section of 4" pipe or will this cause back up problems?

 

[Reach4]

There is either a problem with the venting at the top of the house - the pipe does not extend beyond the roof but rather into a little roof cap or there is an open pipe in the house (in the concrete cavities) or some other problem but when the central ventilation/AC is turned on, septic tank smell comes out of the AC vents. So, somehow the smell is coming from the main cavity in the walls where all the pipe work is hidden and cannot be reached.

Can you extend the pipe at roof where the little cap is?

To look for a leak inside, there is a smoke test you can do. Check locally. Search terms could include "smoke candles" and smoke test sewer.

You would not want to put a trap between your house and the septic tank. I am not a plumber.

 

[Sponsor]

 

[Bluebinky]

Where is the the A/C condensate drain plumbed to? If into the sewer, it needs an air gap and a way to keep the trap from drying out...

 

[Qwertyjjj]

Can you extend the pipe at roof where the little cap is?

To look for a leak inside, there is a smoke test you can do. Check locally. Search terms could include "smoke candles" and smoke test sewer.

You would not want to put a trap between your house and the septic tank. I am not a plumber.

Where is the the A/C condensate drain plumbed to? If into the sewer, it needs an air gap and a way to keep the trap from drying out...

Trap on the condensate drain? Where would that be.
I suspect it's plumbed straight into the main waste stack. The lines go down through the roof cap in the same area but I cannot see after that where they go.

The AC is out on the roof so it might just drain onto the roof area and evaporate.
There are copper coloured lines that go down into the house... maybe 1/4" thick.

The smell also happens when turning on the bathroom extractor fan and when ac is off. All in the same cavity area.

 

[Bluebinky]

Trap on the condensate drain? Where would that be?

Hard to say, could be under a bathroom sink, or like you say just onto the roof. There needs to be one if it happens to drain into the sewer somewhere. Just an idea to investigate further...

 

[Qwertyjjj]

What's the reasoning that a house trap can't be used?

 

[Reach4]

What's the reasoning that a house trap can't be used?

Gases from the septic must and will go somewhere. Unless you put a vent pipe on the septic side of the whole-house trap, the gasses will burp through the whole house trap, and will work against the flow of sewage by keeping the septic tank pressurized to an extent.

 

[Qwertyjjj]

Gases from the septic must and will go somewhere. Unless you put a vent pipe on the septic side of the whole-house trap, the gasses will burp through the whole house trap, and will work against the flow of sewage by keeping the septic tank pressurized to an extent.

There is already a vent near there that I could use. I guess I'm more worried about whether a house trap will clog.
They seem to have been installed in many places though without problems.

 

[Reach4]

There is already a vent near there that I could use. I guess I'm more worried about whether a house trap will clog.
They seem to have been installed in many places though without problems.

In places that call/called for those, I would expect that the sewer is vented somehow on the street side of the whole house trap.

What is your reasoning that you wanted to consider the whole-house trap? I think it was to prevent septic tank gases from going to the house. The whole house trap would not do that, unless you could vent the septic tank side of the whole house trap.

Whole house traps would normally have cleanouts.

 

[Qwertyjjj]

In places that call/called for those, I would expect that the sewer is vented somehow on the street side of the whole house trap.

What is your reasoning that you wanted to consider the whole-house trap? I think it was to prevent septic tank gases from going to the house. The whole house trap would not do that, unless you could vent the septic tank side of the whole house trap.

Whole house traps would normally have cleanouts.

We can't find the source of the smell in the house. We have filled all traps. The ac condensate goes onto the roof. So, this is a last attempt before having a smoke test/camera to see where the issue is.
There is a vent on the septic side near where I could put the house trap.
I can only think there is a pipe open in the cavity or because the pipe is not extended beyond roof that somehow the air is getting blown back in and down into the cavity.

 

[Qwertyjjj]

I guess my main worry is do house traps clog often or are they fine?
Presumably as many of them were buried in the ground, they didn't get clogged much at all.

 

[bestseed]

I saw this problem once. Same symptoms , same incorrect ideas as to cause. It actually turned out that the septic gas was entering the basement via the perimeter drains outside the foundation and traveling into the concrete blocks of the basement walls then escaping the top of the blocks into the house. Many old houses have old plumbing where there can be odd things like gutters and septic connected to pipes that may allow sewer gas to be pulled into a house by the vacuum caused by heated air or ventilation fans drawing air from basement floor drains or as in this case outside perimeter drains. An extremely complete analysis of the problem probably involving some digging may be required. In your case it is obvious that sewer gas is escaping from DWV pipes or drains or it may be entering via another path such as backflow in a seemingly unconnected basement or foundation drain. Creating the symptom and use of a carefully calibrated nose to sniff out the origin of the odor then following it back to a source might save a lot of digging and expense.