One of 2 things is happening...
1). You or someone else reversed the lines (cold and hot run to the sink next to the toilet or shower... you tapped the wrong one).
2). OR if you are flushing “WHILE” a sink or shower is running the anti-scold mechanism in it is allowing the pressure from the lines to mix (hot goes into cold and then feeds back to the toilet.
IF you only get hot water in the toilet while a sink or shower is running then it’s #2.
I had a tenant “replumb” his bathroom in the basement... he was OK with the hot and cold tap’s reversing themselves but not so much the “ball sauna” that happened after a kid took a bath and he proceeded to use the toilet...
I.e. - when a toilet flushes that tube on the top of the filler value puts fresh water into the bowl to refill the trap and bottom of the toilet - that’s how the hot water ends up in the toilet vs. going through the tank and cooling...
Hehehehe.... that was a fun phone call... Hey can you help me out.....
