Leak somewhere, hot & cold mixed up somehow - mystery?

[Bipolar Express]

This is a weird one folks -
I had a new bathroom built. I can now hear water flow in the hot water lines throughout the house. I know I have a problem and I have narrowed it down, but here's the question:

The new tub has a single handle mixer valve. If I turn off the cold water supply to the water heater, the tub will flow cold water normally, but not hot (as it should be, right?)

However, if I turn off the hot water supply coming out of the hot water heater, the cold water flow at the tub is just a trickle (hot is off, as it should be).

No other fixtures in the house are affected. Turning off the hot either before or after the water heater shuts off hot and allows cold to flow normally.

The house is slab on grade, so I can't go under & look. I have tried to draw a schematic of how turning off the hot water supply at the heater can affect the cold flow, but for the life of me, it's got me stumped! I'm about ready to think I'm demon possessed & call for an exorcist!

Any thoughts?

[jadnashua]

An anti-scald tub/shower valve will do this...it is normal for it to shut down when you don't have equal pressure in both lines. This is called a pressure balance valve, and is the most common anti-scald method used. As to why you can hear what appears to be water flowing, that's another issue. If, when you run cold at a faucet, it is mixed with hot, or your hot is tepid, then you may have a defective cartridge in a single handle faucet somewhere that is allowing the hot and cold to mix. Solution is replacing the worn out or defective cartridge.

[hj]

I am not sure what you mean by "shut off the cold supply to the heater" and "shut off the hot water out of the heater", because either of them should do exactly the same thing, stop the hot water flow and NOTHING else. If EITHER the hot or cold is turned off you should not get ANY flow from the faucet, period. There is nothing "normal" about having cold flow when the hot water is turned off. In fact is it is very ABNORMAL and indicates a malfunction somewhere. Now the question is, "is this the new bathroom", or is it the existing one and the new one is still under construction?

[Bipolar Express]

The new bathroom uses what was the washer connection to feed the hot & cold. Something is wrong. I know there is a hot water leak because I can hear it -- I have an ultrasonic detector on the way to try and isolate it. Meanwhile, I do understand what you are saying, that either valve SHOULD do the same thing, but they don't. Shut off the cold input to the heater and you get a normal cold flow from the faucet. Shut off the hot out of the heater and you get a trickle. To me this means there is a problem in the hot side. I just don't know what it is.

[hj]

quote; Shut off the cold input to the heater and you get a normal cold flow from the faucet. Shut off the hot out of the heater and you get a trickle. To me this means there is a problem in the hot side. I just don't know what it is.

Are you reading what you wrote? UNLESS there is a cold water tap off between the heater and its cold water shutoff valve, it is IMPOSSIBLE for there to be ANY difference between closing the water in and the water out at the heater. In fact, having a valve on the hot water out side is so redundant, that it is almost NEVER done. You may have a problem, but it is not the water heater. Your sound, usually indicates an underslab hot water leak, and if so, I hope it is not under your newly completed bathroom. If it were I, I would be calling a leak locating specialist, not a plumber with an ultrasound detector, to pinpoint the location of the leak.

[gjunkie]

I'm a newb, but could these symptoms be explained by a hot water recirculation system (especially one that returns on the cold water line)?

[jadnashua]

Hot water recirculation or possibly a tempering valve could give you weird results. A properly functioning recirculation system generally has a check valve in it, so that might not happen. A tempering valve normally wouldn't be putting cold into the hot line since the hot would not be warm enough to allow it (no tempering needed!), but if that valve was defective, it could allow cold into the line.

[Bipolar Express]

Thank you jadnashua. You hit it on the head -- new shower valve is a pressure balance valve and operates exactly as you stated. As a charter member of the over-the-hill gang this was a new twist for me. I did find the leak by using an ultrasonic stethoscope I snagged from e-bay. The leak is under the concrete slab floor behind the bathtub next to an outdoor brick wall. I used PEX and went over the ceiling, abandoning this section of pipe. The weather is warm now, so there shouldn't be a problem. When we continue with our renovation, I'll make sure the PEX is under the insulation in the attic so it won't be prone to freezing. I was going to put ninety degree elbows in, but the PEX is flexible, so I don't have any joints above the ceiling. I really hate running overhead, but I just can't see ripping out a new tub and tile surround and cutting the floor.

This taught me a lot -- if I ever have to cut a floor and run new plumbing again, I will make sure the plumber does a pressure test of the entire system before anything is closed up.

Thank you jadnashua and Terry Love!

[hj]

quote; if I ever have to cut a floor and run new plumbing again, I will make sure the plumber does a pressure test of the entire system before anything is closed up.

YOU making sure should not be necessary, because any reputable plumber will do the test as part of his installation, AND a reputable one would NOT have any connections under the floor which could leak. So, unless this is a remodeled bathroom, rather than a NEW one, you should not have had the problem in the first place.