Old house, bad plumbing

Wow. That would be more than a little rotten. A lot would depend on what is causing the back ups. I know that you put grease down the drain cold with cold water and not hot. If you use hot water t he grease is liquid but solidifies as it cools father down the pipe. Pouring hot water down a drain might help if hot water softens up the blockage. It might also do nothing at all. Bleach probably wouldn't be helpful. At least, it would be less helpful than liquid plumber. Same with vinegar. Hrm.. you say the backups happen in the kitchen? This means, when you have a blockage which is causing water to back up in the kitchen, you can still flush to toilet and it drains? If that is the case, then the problem area lies in the kitchen sink or in the washing machine. From your description, it sounds like the blockage happens after the washing machine. ( This would mean your problem isn't caused by roots growing into the exterior pluming. If that were the case, you couldn't flush the toilet while a backup is happening in the kitchen. This is good. ) For starters, perhaps you could be very careful not to use a garbage disposal in the kitchen. If nothing but water goes down the drain.. then it shouldn't cause blockages. The walls of the existing pipes might be caked and coated with this black nasty stuff that accumulates over time. The washing mashine's run off might be hot and forceful enough to break some loose which then floats down the pipe and creates a clog anew. Therefore, if you can get someone to replace the pipe between the washing machine and the toilets, I would expect a clean pipe to cause less problems. (Especially if no food goes down the kitchen sink.)