Plumbing cognoscenti--Cold water backflow to hot?

Short version: My wife takes a shower and has to wait for the hot water to make its way from the heater to the tub. I take one about an hour after her and have to wait almost as long for hot water to make its way out of the faucet. Why? It seems the hot water should still be there, maybe a little bit cooler but not all the way down to the temperature of the cold water pipe.

I noticed something that perhaps explains some of it when I had to replace my water heater. As it decided to start leaking at night, after the stores were closed, I shut off the water, drained the heater, and removed it. Wanting some water in the house, I added a valve to the cold water inlet (it T’s to the heater and to the house). With the valve shut but the house water on, within a few minutes water was flowing back out of the hot water pipe (that normally feeds INTO the house).

My guess is that this is water making it past the seals of any single-handle faucet where the hot and cold pipes go through a blending valve. City pressure on the cold side, nothing on the hot side (as I’d removed the bad water heater and kicked it several times) and the cold water flows backward in the hot water pipes.

But when the water heater is in the loop, the pressure on the hot side should equal the pressure on the cold side, shouldn’t it? Why does the hot water at the shower turn cold so quickly? The shower is the farthest fixture from the water heater so has the longest time to get hot water already.

Would it help to pull apart the faucet cartridge and replace its washers/O-rings? Replace the whole faucet assembly? I haven’t opened up a wall in a few months so I suppose I’m about due to do so.

BTW, I don’t have a water leak–or at least not one that would account for this. I’ve monitored the meter to the house and not seen it move for hours and hours on end.

That’s the case. But keep in mind that if you had actually turned on any of those faucets (or turned on the hot water at the same time as cold water at a two handled faucet) water would have come dumping out of that unvalved pipe.

It could be any valve, but that sounds like the culprit. If you have access to the backside of it, the easiest way would be to put your hand on the pipes and see if you can feel the hot pipe getting cold when you hot water else where in the house.
Another thing. Do you have other faucets that seem to start hot and then get cold after a minute or so?

Lastly, it could be the mixing valve, but it could also be the anti-scald thing that sits behind the valve. Part of it is made up of a rubber diaphragm that moves back and forth. After a while the diaphragm can fall apart and effectively connects the hot and cold side of your water system.

I didn’t know about the anti-scald thing but I’d almost bet that it has long since deteriorated and may not be there any more. Is this something that can be replaced, like as part of the cartridge? I don’t have access to the pipes without serious drywall removal–the shower piping is in a wall that’s pretty much out in the open.

Would there be that much heat loss, that fast? I was originally going to ask “Even if the pipes were tied together, shouldn’t the pressure still be the same on both sides” but on second thought I imagine actually mixing hot and cold would cause entropy to occur as quickly as possible, right?

It’s not that the hot pipe to your shower is losing heat, it’s that it’s being backfed with cold water when you turn on a hot tap somewhere else in the house. (If this is, in fact, the case).

Also, don’t forget, pressure isn’t the same as volume. The pressure is going to be the same everywhere, but the volume is dependent on the size of the leak. IOW, if your house pressure is 60PSI and you turn on a spigot so it’s just barely dripping, it might take an hour to fill a gallon bucket, but if you connect a pressure gauge to it, it’ll still read 60PSI.

Some day, when it’s been hours since any of the hot water has been used, try running just the hot water in your bathroom sink and see how hot it gets. A textbook example of a bad mixing valve (or bad anti scald thing [I really can’t think of the name of it]) is that you’ll get lukewarm water from other faucets as they get some hot water from the basement and some cold water that’s being pulled through the bad mixing valve.

As far as replacing it. It’ll come out when you pull the entire valve assembly out. It’s attached to the back. I don’t think they can be replaced as a part, but I could be wrong. I few months ago I replaced my parent’s (Kohler) and we had to replace the entire assemble.

ETA. I meant balancing valve, not mixing valve :smack:
The balancing valve is what makes sure your shower temperature stays the same when someone else turns on the water elsewhere in the house.

This is a pressure balancing valve on the left.

Here’s another type of balancing valve. You can see there’s a piece of rubber that allows a disc to move back and forth (easier to see on the bottom one). If the pressure drops on one side it pulls the diaphragm to that side which closes off the other side to equalize the pressure. After enough cycles the rubber degrades and disappears.

An hour does not strike me as a particularly short time for the water to be back close to room temperature, unless the pipes are insulated for most of the run. Your typical 3/4 inch pipe has a pretty high surface to volume ratio. I would be somewhat surprised to find the water even somewhat warm after an hour at my house. When I give my daughter a bath it cools noticeably in the 10-15 minutes between when I fill the tub and when I rinse her hair.

This.

This makes sense. Add to it that the pipes are most likely 1/2 inch so there’s considerably less mass to hold onto that heat with (over a 3/4 inch pipe). If it was summer and the pipes were running next to an AC duct that would make the problem even more apparent.

Also, WRT to water coming out of the open hot water pipe in the basement. That could have just been water draining out of the system.

But like I said above, there are some troubleshooting methods you can do to see if you have a bad mixing valve or pressure balancing valve without tearing everything out.

I agree an hour is a lot of time.

There is a test you can do to seeif you have a mixing valve leaking.
Close the inlet to the water heater. Open a hot water faucet to bleed of the pressure. Remove the water heater outlet. Cap the line going to the house with a fitting with a pressure gage. Shut off any and all faucets in the house. If the pressure increases near to supply pressure then cold water is bleeding back into the hot water system.
If cold water is bleeding back to the hot water,
At or under each sink should be access to the shut off valves for the faucets. Shut off each sink. Bleed the pressure off, see if rebuilds. If it does not rebuild repair the sink you have shut off. If it does rebuild go to second sink. Continue until problem has been found or all are shut off. After you have located and repaired problem area retest. Then turn one sink at a time checking the pressure.

I’m still going to go with “no leaks” because: (1) I can see the water meter doesn’t move at all over many hours. If there was a leak big enough to contribute to this problem, I would expect to see the meter show some water usage. For whatever reason, my city supplies water meters that read accurately to a gallon. (2) I really, really, really don’t want there to be a leak.

As to water flowing back out the hot water pipe due to the system draining, I apologize in not having stated that this is a single-story structure. With the heater out, the water was actually coming out of a pipe that comes out of the wall about 5 feet above the ground.

Thank you all for the ideas and data. I will work on the system this weekend and see what I can determine. I have to say that in fewer that a dozen posts you guys gave me more, and better, information than I got from several Google searches.

You can also use the isolation valves near a fixture to isolate the hot water pipe from any cold water than may be leaking past a worn valve. If you find that the water stops leaking when you isolate a particular fixture, you have found where your trouble is at.