Plumbing

Okay, this is going to be a bit long. First, the setup. We have a two tank system, but the second is unpowered and connected to both the main tank and our geothermal system. There was a recirc pump installed prior to us buying the house, but it failed and was melting so it's unpowered and still in place. The tank is downstairs at one end of the house, and our issues are occuring upstairs at the opposite end. We are on a well, and had a fair amount of sediment in our water. We installed a three stage whole house filter, but some sediment likely remains in the tank. We haven't seen any of the previous issues associated with the sediment, so I don't believe any new sediment is entering the house.

This started several months ago, and is weird (for me, anyway). The master bath is the farthest from the tank. A few months ago, the water in the shower spontaneously (or so I thought at the time) would only get lukewarm. The other upstairs shower, much closer to the water heater, still had spicy hot water. I thought it was the mixing valve in our shower handle, so I replaced it and everything was fine. This was right after the master tub was used, which is seldom used at all.

Another month or so goes by (after the tub was used again, still no 2+2 for me), and now the farthest faucets only get lukewarm water. The closer bathroom is fine, the kitchen (midway between upstairs bathroom #1 and the master) also has lukewarm water. I did some googling, and decided a back flush of the hot water lines was needed. I capped the sink in the master, did the back flush, hot water was restored.

Another month or so elapsed (the tub was used again), the lukewarm water returned, and now I'm starting to get suspicious about this damn tub. I flushed the tank, did yet another back flush of the lines, and hit water was restored. Then my wife used the hot water in the tub to clean it today, and the lukewarm water returned. She did not used enough to empty the tank and draw up more sediment (if that's the cause), so I don't think it's a partial blockage. Tonight when I was using the kitchen sink, the hot water was measuring at 95 degrees. On a hunch, I went and opened the hot tap on one of the master sinks. The water in the kitchen instantly jumped to 115. I turned the tap back off and the kitchen water dropped back to lukewarm.

I'm pulling my hair out over here. Somehow the usage of the master tub is throwing the upstairs water into a tizzy. We can't avoid using the tub, as its where our kids take baths. I would live to not have to keep capping the master shower and back flushing, although that has worked 100% of the time.

Any ideas from you experts? Is this something to do with the tub valve, or some weird pressure imbalance caused by using the tub?

I appreciate you taking the time to read through all this noise.

Edit 1: My wife used the master sink just now after I posted this (again, farther downstream than the kitchen) and the sink in there was plenty hot like it should be. So now I'm even more confused. When this happened before, the master faucets were the lowest temp as they're the farthest.