We have a serious problem with our drains when the wind blows from the west or north, no problem from the south or east
The house has two air admittance vents, (internal), one just behind the toilet cystine upstairs the other at the end of the drain system downstairs. Both vents are on the north side of the house.
When the wind blows from the north or west it seems to be blowing up the main drain and also in through the upstairs vent (It is in the coombes, internal) and trapping the rotten smell in the drains, which is then working its way into the rooms immediately above the drains.
After hours of checking drains etc the only way to get the smell to shift is to take the cap off the downstairs vent (end of drain system next to toilet cystine), and let the smell out into the shower room. This is better than having it coming up in the kids bedroom.
However when I open the shower window to let the smell out it tends to hinder the process as the wind coming in once again holds back the gas.
I should say we live on a very small island and the winds can be very strong.
Does anyone have any suggestions as to how I can draw the gas out of the drains so it vents outside without the wind forcing it back down the drain. Would an external vent at the end of the system with an opening that swings away from the wind possibly help?

We have a serious problem with our drains when the wind blows from the west or north, no problem from the south or east
The house has two air admittance vents, (internal), one just behind the toilet cystine upstairs the the other at the end of the drain system downstairs. Both vents are on the north side of the house.
When the wind blows from the north or west it seems to be blowing up the main drain and also in through the upstairs vent (It is in the coombes, internal) and trapping the rotten smell in the drains, which is then working its way into the rooms immediately above the drains.
After hours of checking drains etc the only way to get the smell to shift is to take the cap off the downstairs vent (end of drain system next to toilet cystine), and let the smell out into the shower room. This is better than having it coming up in the kids bedroom.
However when I open the shower window to let the smell out it tends to hinder the process as the wind coming in once again holds back the gas.
I should say we live on a very small island and the winds can be very strong.
Does anyone have any suggestions as to how I can draw the gas out of the drains so it vents outside without the wind forcing it back down the drain. Would an external vent at the end of the system with an opening that swings away from the wind possibly help?
I've never seen it done. But the weather vane did come to mind. I wander if your neighbors have the same issue and if any of them would know the solution.

The wind should not affect your system like this unless the AAV is failing. Try wrapping it with sveral layer of Saran wrap and rubber bands to see if the odor goes away. If it does, buy new AAVs.

Okay will try any thing

Let you know

If your AAV is bad the Saran wrap will give you some results in the first hour.

I have just tried sealing off both air vents, but that has resulted in the smell returning. It works with the first vent sealed but the end vent open, but once both vents are sealed the smell becomes locked in, and is forced up into the kids bedroom again.
Although the main drain is fully enclosed there is still too much through draught which is forcing the smell into the drains below the house and it seems by sealing the vents it is stopping the smell escaping out.
I think somehow I need to get the vent to vent out the house. The problem lies in getting the wind to draw the gas out the vent, rather than having the wind forced it back down the drain system

Do you have a vent stack that comes through your roof line? Which side of the house is it located, windward?

Both vents are internal, the theory being that the air is drawn in but nothing comes out... I believe! The vents don't actually let the smell out, (unless you remove the trap valve, which I have done to get the smell out the drain). Somehow the smell is escaping out the drain system under the house. The drains themselves are buried within the concrete floor slab.
Other than that are are no external vents.

OK, by process of elimination I have determined your house has a ghost who eats beans and hardboiled eggs. It's the only thing I can think of. No vents to the roof, both AAVs capped with Saran wrap and you still have the sewer gas. Toilet wax ring or a cracked drain pipe is about all that is left. Have you verified that all sink, tubs and shower traps are holding water.