Septic smell inside house, smoke test worthwhile?

[smellypot]

We were in our house for 10 years with no problems. We have the septic tanked emptied every 2-3 years. The 3rd time we had the septic tank emptied there was a strong stench of septic in the house immediately afterwards. The septic service said they did everything right, gave us a bottle of chemicals to pour down the toilet, and wiped their hands of the problem.

After a few days the smell dissipated to the point where it is now only noticable after doing 2 or 3 loads of laundry or taking 3 or 4 showers.

There is no gurgling in any drain, and no empty traps. The smell is not in the attic or in the basement. It is only in the laundry room and the adjacent den which has no plumbing. The laundry room is below the upstairs bathroom.

It now also smells of septic outside after heavy water usage. I suspected the small vent which was low to the ground had a disproportionate amount of smelly air flow due to the larger vent on the high roof being blocked.

We've flushed all the vents from the roof with a hose and replaced the toilet seals. When the vent was flushed from the roof the smell became very strong inside the house. I think the back pressure of gas from the septic tank is making its way back into the house and leaking out between the upper floor and lower ceiling before it can escape through the roof vent.

The problems are how to detect the leak and how to fix it?

A plumber gave a a quote to do a smoke test for $800. Is this worthwhile? If the smoke test results in smoke coming through the walls or light fixtures in the ceiling, which is my expectation, then will it be a waste of time?

Can anybody offer advice causes or troubleshooting that doesn;t require tearing out walls?

Is this kind of thing covered by homeowners insurance?

TIA

[Redwood]

A smoke test is a very good way of finding a leak in a DWV system.
If the smoke come out of a wall or, light fixture you know where to start hacking into a wall or ceiling...
Have lots of people looking for the smoke.
The first places it shows up are where you want to know, not just a room full of smoke...

Before doing a smoke test I would look at all obvious possibilities.
Dry traps, toilets that move, or, rock, inspect pipes in open locations for cracks and holes...
You also can do a pepermint test much cheaper with a few friends to see if it is indeed a DWV system leak.

Get a bottle of peppermint oil from the local druggist, or, health food store... (Note: Large chain phamacies are kinda useless) get a bunch of people inside to sniff and try to locate the first whiffs of the peppermint. Have one person outside mix the peppermint oil into a gallon of hot water and pour it in the roof vent(s) then stay outside so he doesn't carry the smell inside and contaminate the test.

If you get the peppermint smell inside you know you have a DWV system leak and hopefully you can locate it. If you can't you know the visual smoke test will yield results.

If there is no smell of the peppermint I would look for other sources of the smell than the DWV system. A smoke test would probably be a waste of money.