Downstairs toilet overflowed so I pulled the tank and bowl and tried pouring water down the drain. It flowed quickly and without any apparent blockage. I put drain cleaner down and later flushed with a couple buckets of hot water per the instructions. Again all seemed fine. I went upstairs and flushed one toilet and it flowed fine but the second upstairs toilet caused the downstairs drain to rapidly overflow. Should I snake out the upstairs toilet or the downstairs toilet? Isn't my blockage downstream from the downstairs drain? How come it flows just fine? I don't understand.

Hi Deckster

Toilet drains can be funny like this! Because a lot of air mixes with the water as you pour buckets of water down the drain with the toilet removed there appears to be no issue, but when you reinstall the toilet the air mix is lessened and a partial blockage shows itself quite quickly! It could be a vent issue, but more than likely, you have a partially clogged drain.

Lift the toilet and snake the drain... sounds like a partial blockage in the drain line downstream of the downstairs toilet.

Mark