A plumbing conundrum

I’m simplifying a bit, but shouldn’t affect the analysis. This one has me stumped.

I have a blockage somewhere in my sewer pipe. (I’m going to deal with it tomorrow, but for the time being I don’t understand why it’s acting this way). The set up is:

right side of house: Kitchen sink and dishwasher

Left side of house: Bathroom, laundry and this is where the sewer exits the house, running underground towards the front.

The kitchen is a few steps down from the bathroom, so my sink and tub are at about the same level.

When I run my dishwasher, the water backs up in the kitchen sink a few inches, but not in the bathtub in the bathroom. When I run my clothes washer, the water backs up in the bathtub, but not the kitchen sink.

Why doesn’t it equalize and back up in both the kitchen and the bathtub? IANAP, but this makes me wonder if I have a venting problem in my pipes.

May have to do with how the pipes from the 2 sides of the house are run.

If you have like 2 main lines from the 2 sides of the house, and the clog is at the T or Y joint when they come together and head to the street, then each side will clog and back up independently and not mix with each other.

I am not a plumber, and i of course have no idea what the plumbing in your house looks like under the floor.
But that is my guess.

Does the dishwasher drain run through the sink trap? If it does the dishwasher outflow may back up into the sink in the smaller drain pipes but the larger drain connecting to the tub and laundry holds enough water to keep from backing up into the tub. If the tub is just a little lower than the sink the water backs up into the tub before it gets high enough to back up into the sink. All that assumes the clog allows some water to keep draining.

Right now the water has been sitting in the tub at the same level for about 5 hours. Nothing is draining.

But no water in the kitchen sink.

A few days ago, I ran the d/w and the kitchen sink did the same thing. Water sat for quite some time, but never backed up into the tub. I used some Liquid Plumber later in the day, and I thought that fixed things, but apparently not.

LP can loosen up a grease clog only to have it form up again at the next major bend in the pipes. Basically it just melts the grease enough to move it, then it hits cold water again and you’ve got a whole new problem.

My Grandfather always recommended a salt “Scrub” (not really) for clogs like this. Go to the store and get a few pounds of salt - whatever kind is cheapest. Fill your biggest spaghetti pot with water, boil it, and then add salt until it won’t take any more. Pour this down the problem drain.

These day I would also add a hefty dose of Dawn dishwashing liquid (It really does cut grease).

That should melt and break up the clog, getting it all the way to the sewer.

If that doesn’t work, you need to rout it.

Sounds like two clogs. Doesn’t sound like a vent problem. But plumbing acts weird.