Every year I shutoff the taps in the basement leading out to the outdoor taps.
I was cleaning the furnace one day a couple of weeks ago and noticed that the relief valve on our hot water heater was leaking. We had the water heater rental firm out to the house 5 times to diagnose the leak and replace various parts during each visit.
On their last visit they decided to replace the water heater entirely. Still leaks from the relief valve everytime we use hot water, maybe a cup or so.
Through trial and error I figured out that if I don’t shutoff one particular shutoff faucet in our basement to drain the line leading outdoor, the water heater does not leak. If I shut the water off, the relieve valve on the hot water heater leaks. So an increase in pressure somewhere I suppose, but unclear as to why this is happening.
This may have been happening every year but I just never noticed it since I rarely go into our furnace room.