Mystery Cross Connection caused by parallel connected water heaters

JUGHNE

Stopped at a school to just check the steam boiler water line.
Teacher said that they were getting scalding hot water out of the cold faucets.
Previously I noticed that the boiler fill CW line would deliver a little hot water before turning cold, didn't think much about that. But today it stayed hot. WC tanks were hot. Water cooler was overworking as it was fed with HW thru the CW line.
Eventually discovered that the basement electric water heater was in run-away mode and overheating.
The Gym electric WH had a HW connection to the HW of basement WH.
They were in effect connected in parallel with about 100' of 3/4" copper between them.
By closing the valve on the common HW line the problem was solved.

These had been connected this way for years with apparently little cross flow....no real problem.

But when the basement WH overheated it seemed that the hot from it would back flow thru the Gym WH and push hot water thru the cold water line. Apparently the super hot water developed enough pressure to overcome the cold pressure?? Just enough pressure difference to tip the back flow delivering the hot to the cold faucets.

I locked out the basement WH switch, closed the CW valve to it and re-opened the common HW valve between the 2 WH.
Now the Gym WH will feed the entire building. Only maybe 10 special Ed students and about 5 teachers. They believe they will move into a new building next year.

Just posting this for anyone to read and consider any cross connections of H&C that can't be blamed on the mop sinks. (the first think I checked BTY). Seemed to be a unique story to share.

EBEBRATT-Ed

I am not a plumber so this more of a question than a suggested fix. Would check valve on each water heaters cold water line fix this?? Could lead to an expansion tank job as well.

LOL

JUGHNE

Yes, I believe the check on each cold water line would stop the reverse. And yes would need an expansion tank for each IMO.

It was much cheaper to close the valve in the common HW line.
And so far the 1 WH delivers enough to keep most happy.