Recently, our toilet has been occasionally filling with hot water instead of cold. This primarily happens when the toilet hasn’t been flushed in a few hours (such as first thing in the morning). As we continue to flush, the water goes from hot to warm to cold, and the flushes stay cold until the toilet isn’t used again for a few hours. We live in a condo building, and have been advised by the building’s plumbers that this is caused by faulty mixing valves which are located behind the shower faucet, which is causing the hot and cold water supplies to get “mixed up”. They did not examine anything, just simply stated that this is what is causing the problem and quoted around $1200 to replace the valves. However, if this was the case, wouldn’t the water be mixed up for other things in the bathroom, such as our shower, bathtub, sink, and washing machine (located in the bathroom), and wouldn’t this happen at all times, not just when the toilet hasn’t been used for a few hours? I am not sure if this is pertinent, but when the faucets in the sink and shower haven’t been used for a few hours we get only hot water from them (no cold) for a few seconds when the taps are initially turned on.. Any advice would be most appreciated as I would like to have some idea of what could be causing this before having a plumber in. Thanks!

The building plumber's correct. This is called a "cross connection" and if it's located in the shower mixing valve it should be corrected by changing out the innards of the valve which doesn't come with a $1200 price tag. You ask if the other fixtures in the branch would be affected. From what you've told me they already are. Good luck, tom

Thanks. Any guestimation as to cost so I have an idea?

I can't give ballpark figures on a job because of local labor rates and the material used, but I can tell you it should be well under 100$. Good luck,Tom