Water leaks when hot water heater is on

[Michael Sichley]

My Montana 5th wheel has water leaks from the water pump and other water connections after my water heater has been on for awhile. It doesn't leak when the water heater is no on. What is causing this? I guess it is more water pressure in the lines due to heated water pressure expansion. Why is this feeding back through the cold water lines?

 

[twindman]

Could the thermostat on the heater be bad and the water heater is boiling the water and thereby causing a lot of pressure? I can't think of any other reason.

 

[ChuckS]

If you are actually getting hot water feeding backwards into the water pump then I would be looking at a stuck check valve on the output side of your water heater... Didnt see the year/make of your unit but it should have a Suburban SW series water heater... Typically the check valve on this brand WH is only onthe output side ( some also have on on the inlet side ) depending on how the winterize valves are setup...

Hot water feeds out the outlet side of the WH and never is run through the water pump.... so if you actually have hot water leaking at hose fittings on the water pump you need to be looking at the WH check valve.. IMO

Image of typical RV water system plumbing

 

[BB_TX]

Water does expand when heated snd would cause an increase in pressure. But the system should be designed to handle that “normal” increase.

Twindman has a good thought. If the water is actually boiling, it would cause a more significant increase in pressure as water turning to steam would cause a more significant expansion and resulting pressure increase.

Increased pressured inside the water heater would force that same pressure increase back thru the water heater cold water inlet to the cold water lines as well as thru the water heater outlet to the hot water lines.

 

[mlh]

I would check the temperature of the water. It shouldn’t be over 140 or 160 degrees. If you have a digital thermometer point it at the line out of the water heater and turn the water on then check the temperature. If you don’t have one Harbor Freight and other stores have them. They aren’t expensive, about $20 or so.
Lynwood

 

[Bourbon County]

If the water was boiling, that would mean that the thermostat is sticking, the high limit not opening and the T&P valve is not opening. The T&P valve is supposed open at 210 degrees. The likelihood of 2 or more components failing at or near the same time is highly unlikely.

I believe the check valve failure is a more likely approach.

 

[BB_TX]

The only way water can flow backwards from the water heater into the cold water lines is if the water heater pressure were higher than the pressure in the cold water lines. That pressure increase would soon cause the pressure in the cold water lines to stabilize at the same pressure as in the water heater itself. And it would likewise increase the same pressure to the hot water lines, assuming the WH outlet check valve was opening. But that pressure increase due to heating would be normal and expected and should not cause leaks.

An increase in pressure sufficient to cause leaks would be outside of normal pressure increases. The question is what is causing that excessive pressure. Or is the water leaking at weak water connections?

 

[BB_TX]

Another thought.

What is your fresh water supply pressure? If it is higher that normal, then it is possible that the pressure increase due to the water heater could be enough to push water thru weaker connections.

 

[firestation12]

There is a check valve built into the water pump. Its job is to prevent pressure from bleed back to the fresh water tank. Any plumbing system whether an RV or residential or commercial that has check valve to prevent back flow into the city system, is then required by code to have an expansion tank to allow heating water to expand. Water heaters contain a TP valve the works in conjunction with the expansion tank, but in the case of our RVs that feature is built into to the hot water tank. Note, that our TP valves are located an inch or two below the actual tank top. Is is by design to create an air space above the TP valve to allow expansion. Occasionally for reasons I can’t explain, the water level in the tank creeps above the space above the TP valve, and when that happens, the TP valve will begin to drip. White calcium buildup on the outside cover is a telltale sign. Often the TP Valve is replaced unnecessarily as being defective, when in reality it just needs to be burped. In replacing the TP valve, the air gap is restored and the owner mistake thinking the replacement fixed the problem. Instead, if the TP valve drain lever had been opened with the pump and city water turned off, and the lever left open till no more water came out of the TP valve, 99% of the time the problem is fixed. Any time the camper is on city water pressure and the fresh water tank fills and begins to overflow, it is because the pump check cave has failed. This problem can be fixed by either replacing the pump or much easier and cheaper by adding a check valve at the pump entrance with the direction of flow arrow pointing to the pump inlet.

 

[Oregon Coast]

I am responding to "other water connections after my water heater has been on for a while." Plumbing connections, can leak intermittently due to temp changes, which causes expansion, and contraction. This problem exists with every type of connection. Flare, threaded, brazed, rubber seals, and compression fittings. I have even found leaks in Pex and copper pipes where there were no fittings, just pinhole leaks. An example, when I bought my Montana, there were a lot of calcium deposits around the bottom of the water heater. The leak was at the top hot water exit fitting, and only leaked after hot water was in use for a while, like a shower. Once the fitting heated up, and expanded, it leaked. Easy cheap fix.

 

[firestation12]

The only way water can flow backwards from the water heater into the cold water lines is if the water heater pressure were higher than the pressure in the cold water lines. That pressure increase would soon cause the pressure in the cold water lines to stabilize at the same pressure as in the water heater itself. And it would likewise increase the same pressure to the hot water lines, assuming the WH outlet check valve was opening. But that pressure increase due to heating would be normal and expected and should not cause leaks.

An increase in pressure sufficient to cause leaks would be outside of normal pressure increases. The question is what is causing that excessive pressure. Or is the water leaking at weak water

The only way water can flow backwards from the water heater into the cold water lines is if the water heater pressure were higher than the pressure in the cold water lines. That pressure increase would soon cause the pressure in the cold water lines to stabilize at the same pressure as in the water heater itself. And it would likewise increase the same pressure to the hot water lines, assuming the WH outlet check valve was opening. But that pressure increase due to heating would be normal and expected and should not cause leaks.

An increase in pressure sufficient to cause leaks would be outside of normal pressure increases. The question is what is causing that excessive pressure. Or is the water leaking at weak

Since water is not compressible, and its volume increases with heating, the slightest expansion is sent throughout the plumbing system as you explained both hot and cold. The volume of hot water in its tank far exceeds that of the cold water lines which would make pressure in the entire system spike rapidly. Without an air pocket to absorb the expansion, and a backup TP valve to open, it won’t take very much volume increase to cause a failure somewhere. The water heater by design retains air at the top to allow expansion. When the air pocket becomes filled with water, the TP valve does its job and releases water. I would venture a guess that as little as a cup of expansion volume without some means to absorb the volume increase or release it, will cause something to fail. A hydraulic brake system works on the principle that a small amount of volume introduced by the master cylinder produces impressive braking power.