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

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.

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?

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.

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.

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.

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.