Hot water in the cold water lines?

[Adventure.AS]

Originally Posted by avionstream

Maybe the toilet is tapped into the hot water? Can you follow the line?

I think the OP was referring the lavatory tap in the toilet, not the toilet itself.

[SSquared]

We sometimes experience hot water coming from the cold side of the bathroom sink. We have a 2013 25FB Flying Cloud. It only happens when we are using the onboard fresh water and pump.

I think this is a result of the air bubble that is normally present in the Atwood hot water tank. I'll try to explain my theory.

When you turn on the pump, it raises the pressure to about 50 psi. The water pressure in the water heater compresses the air bubble. When you turn the pump off, the pressure is held. If you open a hot water faucet, the water flows, and the pressure in the air bubble supports the flow, pushing water out of the tank and into the hot water pipe. As water flows, the bubble expands and the pressure drops. After a few seconds, the pressure is low enough the pump comes on, moving water and raising pressure. When you turn off the faucet, the system again stabilizes at 50 psi, and the bubble is compressed again.

Now consider what happens when you turn on a cold faucet. You have probably noticed that water starts flowing instantly, and the pump waits a few seconds to turn on. Again, the system pressure is 50 psi, and the air bubble is compressed by the pressure. In my trailer, I don't think there is a check valve where the cold water enters the water heater. So when you turn on the cold, the bubble pushes water out of the water heater into the cold supply line--flowing backwards. From there, it heads toward the open faucet.

As the water flows, the pressure drops. When the pump comes on, it overpowers the (reduced) pressure from the bubble, and pushes cold water toward both the open faucet and the water heater.

I believe an expansion tank would reduce or eliminate this behavior.

A check valve would also fix it. Note that a check valve would eliminate the "benefit" of the air bubble--when you run cold water, the pressure would drop instantly and the pump would kick in right away. I wonder what would happen when you are taking a shower--the hot water pressure would be moderated by the bubble but the cold would not be.....