Interesting cross-connection in house

Thurman

Just to share. So, suddenly there is hot water coming from the cold water side of all faucets in an upstairs/downstairs duplex my daughter owns and is rental property. The hot water and home heat is via a boiler system. The boiler/heat company comes out and determines that there is an additional check valve needed at the boiler and install one. Problem still there. Another tech comes out from the heat/boiler company and determines that there is a cross-connection of some type within the unit and starts tracing pipes. He admits he is not a plumber (give him credit for that) but really gets into the problem. After about six hours he find the problem: underneath the bath sink upstairs (he had to cut a hole in the wall), and between the walls there is a thermal control valve which cross-connects the hot/cold lines. This valve has gone bad and is wide open letting hot/cold water mix at will. His theory: when the water gets heated it expands (right) and expands into the cold water lines. Therefore: hot water in the cold water faucets for approximately three minutes and then cold water. He takes the thermal valve out of the system, caps off the lines which went to it, and now all is working well. We both know and understand what a thermal valve is and how it works but why was it there, why wasn't there one downstairs, and why was it within a wall? Beats me, but the problem is fixed now, by the non-plumber at that. I gave him lots of credit for hanging in there on this one. UH- the bill will come later. Thanks, David

Grampa Bud

What you are refering to is called an auto recirculation line for providing instant hot water usually at the farthest point from your water heater. Since you already have removed the crossover valve you should kook for the circulating pump that is or should be near or in the hot line coming off of your heater and remove it and reconnect the two pipes back together. People objected to waiting 3-9 minutes to get hot water in their Master Bath or such and some entrapuneur came up with this idea for homes that were already completed rather than rip walls and floors to put in a hotwater return loop to the water heater.

Maintenance 6

There might be a check valve in a circulating loop in a domestic hot water line, to keep water from flowing from the return line when a faucet is opened, but it would not be tied to a cold water line. This sounds more like a small mixing valve that some people use on the supply to a commode. The idea is to mix some hot water with the cold so that when the commode refills, the water is tempered. It is to prevent the cold water from causing the toilet tank to sweat and drip. Sounds like this one has failed. I ran into a failed one that did basically the same thing.

Thurman

GrampBud, and Maintenance 6, Thank You for your suggestions. As I stated, I know nothing about these systems that us a boiler to heat water and homes. Grandpa-I understand what you have stated, but I also know there is only one pump (Grunfoss ?) on the boiler system and the tech explained to me that it was for moving water throughout the baseboard heaters as needed to heat the home. I'm sure there is not another pump. Maintenance 6- -This was indeed a mixing valve, the tech and I just happened to call it a "thermal mixing valve". I understand your explanation as to the toilet use. This vavle was not connected to a toilet in any manner, we did find the line supplying the toilet branching from the cold water line before the cold line reached the sink. I know the tech removed the valve he found from the system. There are not pipes connected to it at this time, all faucets have hot/cold water as they should. It still baffles me as to why someone would put this valve within an interior wall so that it could not be easily accessed, or be seen to know it was in the system. Thanks, David

Grampa Bud

Probably to curtail questions from the building inspecters when/if the building was sold.