I have a slow drain, everything works great until it doesn't. Example: I wake in the morning and take my morning "break" the toilet runs great and will do so until I wash dishes after lunch. The sink drains very slowly and the toilet will no longer flush. If I plunge the toilet it comes up in the tub. It will all drain eventually, but takes a long time. Throughout the day, I can hear a "gulping" noise in the drains. I had a plumber come out and he checked and removed the roots that were clogging the main line. He said it was all clear, but I am still having issues. I don't want to call the plumber again unless I have absolutely no choice. Is there anything else it could be? FYI: it is an older house, no upstairs, one bathroom, I do not have a washer or dishwasher. We stopped flushing toilet paper and I don't dump food/grease down the drain when I wash the dishes.

I am in the process of replacing 2 sink valves in the downstairs bathroom, one cold and one hot. First I turned off the water at the main water shutoff, then I opened up some cold and hot water taps in several (not all) locations on both the first and second floors until no water was coming out. Then I went back to the bathroom sink valves (the ones I am replacing) and opened the hot water valve. I let it drain until empty, which took only a few seconds. The weird thing is that when I then opened the COLD water valve, a strong stream of water started coming out. At first it was cold, but then became very, very hot. I let it drain about 2 gallons without showing any signs of slowing down before I decided to give up. I assumed I was actually draining my hot water heater, and I have heard it can be damaged if drained while left on. I guess my question is, what is going on and what do I do? In order to replace the valves, the water obviously needs to be shut off, but what is going on with the cold water drain? Do I need to completely (and safely) drain the hot water heater first?