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Recently I installed a new kitchen in a flat in a very old house (1830s) in a neighbourhood known for sagging pipes.

The old pipes up to the soil stack were cleaned by a pro, after the kitchen was installed. Naturally the sink got blocked (only draining very slowly). Plungers were applied and as last resort some chemicals. Nothing worked.

After doing dishes by hand for a while (2 weeks), rather than the dishwasher, the sink is magically free again. Is it possible that this is just delayed reaction to the chemical drain cleaner, or was it the increased flow from the manual washing that did the trick? I have never seen a sink unblock itself before.

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Happens here fairly frequently. Stationary tub and washing machine share a drain line. Tub isn't used much. Side flow from the washing machine refills the tub trap with dirty water. Eventually it gets enough crud that the drain slows. Use the sink, and the first 3-4 inches of water drain slowly, then it flushes the crud out.

In your case, a slow response to the chemicals seems probable.

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One possibility is that by filling the tub basin, the height of the water provided extra pressure to move the clog. This can happen with slow draining: there's a constant stream of pressurized water flowing through the clog which starts to break it down.

Plunging a sink is tricky because you have to have an airtight block on the overflow. It's probably your best method here if you can work that issue out. You should avoid putting harsh chemicals down old pipes especially if they don't have a good pitch on them. I once cleaned out a waste pipe that was not pitched properly to find that the accumulated grease was actually what was making it watertight. It was eroded by drain opening chemicals. You could end up with a major plumbing problem if you keep using them.

You might want to consider an enzymatic treatment to break down grease. Also try to avoid putting grease down the drain the first place. I pour things like bacon fat into a jar and let it solidify and then put it in the garbage. When you have to wash grease down the drain, use cold water, not hot. It's a little counter-intuitive but the hot water will cool as it goes down the drain and the grease will coagulate. It will not only clog things up but attract rats as well.

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You said the plumber cleaned the pipes after the kitchen was installed. So some of the crud from the cleaning probably built up around and in the smaller kitchen drain. After a fe washes, the built up crud broke free and into the larger drain where more water would be traveling and washed it out. the chemicals could have had an affect on it too. you're one of the lucky ones... Enjoy your new kitchen.

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