I didn't want to derail the other thread about hot water coming from the cold-water pipe.

I'm having this problem in my small co-op apartment building, but only in the last couple of months, and we're trying to find the non-expensive solutions before we pay the plumber. (we're self-managed)

I only yesterday heard that a single-handle faucet's cartridge can fail and create this problem, and I have two of them (shower and sink) in the bathroom where I'm experiencing the problem.

It only happens in that bathroom--all three fixtures, though. (when I flush, the water in the toilet it warm. No one else in my line of plumbing has indicated that they experience this. But, I'm also the only one up late, and i'm the first one up in the morning.

Yesterday, I thought I'd figured it out; it was the cartridge in one of my single-handles. Maybe the shower, because it's oldest.

BUT . . . last night, at 11:30, the water coming out of the cold pipes (shower & sink) was cool. Even when I let it run a little, it was still cool.

And then this morning, I tested the water at both faucets as soon as I went into the bathroom; it was cool. But when I flushed the toilet, the water I washed my hands with was very warm. And it was warm in the shower as well.

If the shower hadn't been used for several hours (like, 15 hours), wouldn't the water closest to the tap be the hottest?

Ditto the sink--if that's where the problem is, wouldn't I get hot water out of the tap immediately?

If I can't get hot water until after I've removed a bunch of cold (by flushing), then wouldn't that indicate that the hot water is entering the stream somewhere below my "branch" of pipes?

Is there any other symptom that might help me figure out where to look for the origin of the crossover?