Toilet intermittently won’t flush - strange seasonal venting issue maybe?

Question

Backstory: last winter we had an issue where Toilet #2 of our 2 toilets (the one furthest from septic tank) would intermittently not flush. The tank would drain completely and refill properly, the water in the bowl would churn a bit, but the water level in the bowl stayed fairly constant. The only way we could get this toilet to flush would be to flush the toilet closest to the septic tank (Toilet #1) a couple of times. Then the water level in the bowl of Toilet #2 would drop a bit and it would flush properly for a bit - hours or days - until it stopped, at which point we would have to repeat the process.

Sometimes (but not always) during the season this problem shows up, when flushing Toilet #2, Toilet #2 will do some extra “glugging” as the water goes down. Sometimes Toilet #1 will spontaneously lower its bowl water level and have some of this same “glugging” at the time Toilet #2 is flushed (without it having been flushed itself)

This was annoying, and got to the point this past spring when I was ready to call a plumber to have someone take a look. But around that same time in spring the problem went away. It’s been 7-8 months and we’ve had no issues with either toilet.

Now
Two days ago the same problem started again. Same exact situation. Toilet #2 only.

Is this a venting issue, or something else? And perhaps more importantly, why would this problem be seasonal?

Technical details:

• toilets are on the same floor
• ranch style home
• Septic System
• Western New York State
• Toilet #2 is less than 10 years old, wax ring less than 2 years old
• We’ve lived here just over 6 years
• This problem only started a year ago

Answer 1

The obvious reason for seasonal behavior in western NY would be leaves.

You've got a septic system, so there are two venting opportunities: airflow into the house via the vent stack to support flushing, and airflow out of the septic tank displaced by inflow from the flush.

In many cases, the septic venting is large and widely distributed -- for example, air flowing around the cover of a manhole, or through an inspection port. But if your manhole is covered in a thick layer of wet leaves, it's sealed. Normally, I would expect a septic vent problem to affect more than just a single branch, so I don't think this is the problem. (But it could be!)

You don't say how the system is vented. If possible, look at the roof to see where vent pipes come through. In a newer house, the vents may be merged together inside the house to reduce roof penetrations. In this case, you may need to check the attic to find how many vents you have, and where they run.

Horizontal runs in the vent pipes are vulnerable to accumulations of dirt, nuts and acorns, and leaves. If your #2 toilet is vented differently than your #1 toilet (because they are widely separated), this may be an issue. You may have a bird or animal nest, a buildup of leaves, a collection of sludge (air-born dust or dirt plus rainwater), a hoard of nuts, or some other "natural" accumulation of things that combine to block air flow. Depending on the construction of your house, you may be able to see or hear water flow from the vent through the drain, so running a hose up to the roof and pouring water into the vent (not a lot of water, please!) may help you check if the vent is blocked. Alternatively, if you have a 25 foot drain snake, that is probably enough on a one-story ranch house to reach from the vent down to the toilet - try using it as a probe to check if the vent is blocked.