Cold water, then hot water, then cold again on all cold side faucets

[Michael Civisca]

A friend of mine has an ongoing problem with all the cold water taps in their home.  When the cold lines sit for a while and then turned on, the cold runs for about 30 seconds.  Then the water heats to full hot for about a minute.  The water then cools to cold.  This will happen on all faucets. 

Talking to my friends on the phone (until I can get over there), the husband "apparently" confirmed to me there is no expansion tank on either water tank (they have two water tanks).  I also confirmed they have some single-lever faucets in the house.  The house is about 5000 sq feet.  I am thinking of checking the single lever faucets for cross connections from failed seals, but wondering if that would cause this problem at all faucets in the house.  Anyone ever come across this before?

[Michael Merino]

I do not think cross connections would be occurring, I think you would see leaking. Are there circulating pumps installed? They may be failing. How old are the water heaters?

[Cameron Anderson]

At this point I don't think you have enough details.  Here's some of the questions I would have for your friend:

1. Does "ongoing problem" mean it has always happened since they bought the house?  Or was there a approximate start date for the issue.
2. Are all the times you mentioned actually timed?  Tap on, then exactly 30 seconds of cold, then exactly 60 seconds of full hot?
3. Was the "full hot" a measured temp, or was it relative to the cold water he felt for the first 30 seconds?  All the times and temps sound very approximate and that in itself may be important.
4. Where is the home located?  In a cold climate where a furnace or boiler is running?

If it is in a cold climate, has he checked to see if he has water pipes running through or on top of ducts or boiler pipes?  I would look for some heat source that is transferring heat into the main cold supply line.  I have seen houses with similar setups run cold, then warm(hot may be relative), then cold again just because one of the cold lines is getting heated while the water sits.  Rule out the simple, then branch out into cross connections and all the other possibilities. 

[Dominic Maricic]

Any chance this is a tankless water heater?

[Michael Civisca]

Thank you for the quick replies gentlemen.  The questions you listed are what I will be checking when I get to their house.  I will repost once I gather the data.  BTW, this apparently has been happening since they purchased the home about three years ago.  I am interested to see if a pipe manifold is hovering over a heat source since it is happening at every faucet.  Chat soon.

[Michael Civisca]

No.  This house has two tanks, which I will get the data from.

[David A. Andersen]

I had one of these once where there was no expansion tank. The main cold water entered perpendicularly (horizontally) to the cold water inlet pipe at the water heater with a tee fitting.

After running the hot and cold water during testing, the hot water heater would recover and hot water would back up vertically into the cold water supply for the rest of the house.

When you turn on the cold water valve, cold water from the main would come in behind the hot water and push it to the fixture.

This house had PEX water supply piping throughout the house and solid copper from the street.

[Cameron Anderson]

After running the hot and cold water during testing, the hot water heater would recover and hot water would back up vertically into the cold water supply for the rest of the house.

When you turn on the cold water valve, cold water from the main would come in behind the hot water and push it to the fixture.

Did you suspect the PEX was acting like an expansion tank or did the water heater just raise the pressure enough to push the water back up the cold inlet?  And would a backflow preventer correct that?

[David A. Andersen]

A backflow  preventer (or pressure regulator) will make things worse.

Yes, I suspect that the PEX allowed more water to enter the cold side piping.

This was not that big of a problem for the client so nothing was done about it so I don't know the final result of the condition.

[Cameron Anderson]

A backflow  preventer (or pressure regulator) will make things worse.

How so? I'm not following.   

I can see a pressure regulator wouldn't accomplishing anything, but wouldn't a check valve of some type on the cold water inlet to the water heater keep the hot water out of the cold lines?

I wonder what the flexible properties of PEX actually are anyway.

[David A. Andersen]

I was thinking that you meant a check valve at the water supply inlet into the house (were they normally would be located).

It would be cheaper and less troublesome to put in an expansion tank then mess with check valves.

One of the problems that likely is occurring is that when the cold water valve is opened, a drop in water pressure occurs throughout the cold water system in the house.

The hot water system is under higher pressure than the pressure reducing valve is set at. This forces the pressure reducing valve to close even harder on the seat preventing cold water flow from the street.

The first water that heads for the open water valve will be from the water heater. This occurs until the pressure within the water heater equalizes with the setting of the pressure reducing valve. Then cold water will begin to flow.

The amount of volume that enters the cold water pipe has to do with the mass of the water in the storage tanks of the water heaters.

A significant amount of hot water will leave directly from the hot water heater into the cold water supply piping until the pressure equalizes and water can begin to flow through the pressure reducing valve.

Coldwater - hot water - Coldwater …
Does this make sense?


The closer the cold water supply is to the top of the hot water heater, the more hot water is trapped in the cold water pipe between that tee fitting and the open cold water valve.

If the cold water supply is far enough away from the water heater, even though hot water still flows backwards up the Coldwater supply pipe, it never makes it to the streets supply water connection  tee to be noticeable.

[Michael Civisca]

I am hoping to make it over to the house on Friday to start checking out the possibilities.  Very good thoughts guys.  I'm looking forward to solving this.

[Cameron Anderson]

It would be cheaper and less troublesome to put in an expansion tank then mess with check valves.

A significant amount of hot water will leave directly from the hot water heater into the cold water supply piping until the pressure equalizes and water can begin to flow through the pressure reducing valve.

Coldwater - hot water - Coldwater …
Does this make sense?

Yep, that's a good description of the possible cause of the problem.  If that is the case, Mike's friend must be exaggerating the amount of hot water coming out.  I can't see 2.4 gallons of hot water backflowing into the pipes just when the flow of water is shut off, and Mike said it happens when the water sits for a while.  But...   as soon as I say 60 seconds of hot water out the cold side doesn't seem possible, it will be.  

[David A. Andersen]

I hear ya!

[Jeffrey R. Jonas]

Is there a "tempering valve" installed that possibly became defective, or is installed improperly?

[Michael Civisca]

Hello Guys:  I have been cancelled to visit my buddy's house twice since I last posted.  However, one point I wanted to tell you was, I had my friend lower the temps on both water heaters to the "normal" setting, and bleed the lines at each faucet in the house before going to bed.  His opinion was that helped a lot, but upon talking with his wife, she said there was no difference.  So I continue to wait until their schedule bonds with my schedule and I can get over to their home for a look.  Thank you again for all of your comments thus far.

[Cameron Anderson]

I'd put my money on the wife. They're the temp experts. 

[Richard Overton]

Just another possible cause I have run into out here in California, mostly in multi family. You get the same described symptom, cold, then hot, then back to cold. In the cases I have been on, it has been because of the disk in the pressure/temperature compensating shower valves get stuck. The plumber fixed it by removing the disk and shacking it until you could hear the mechanism rattling inside. When they get a bit of corrosion or a piece of grit- they act just like a slow leak between the hot and cold side.

Additionally, the problem might show up in one unit but be caused by a shower valve in a different unit.