Dometic Hot Water Heater leaky pressure release valve

[Jason Worman]

So I came home and noticed water dripping from the external hot water heater door. I opened the door and the brass pressure relief valve was dripping steadily. Is this normal? Just had it installed about a week ago. Thanks

[Gerald Farris]

Jason,
No, a leaking relief valve is not normal. It is either a bad relief valve or an over-temp water heater.

Gerald

[Bill Sprague]

It is really a risky business to disagree with Gerald.  I should know better!  But....

We've recently been enjoying a PleasureWay Pursuit with a traditional propane tank water heater.   The pressure valve leaked and I replaced it with a new one.  It still leaked. 

Why?  It took some reading.  The design is meant to leave an air pocket in the top of the tank.  When you first fill it, it can't fill all the way with water because of where the hot outlet is.  After a week or so, the air in that pocket is absorbed into the tanks water.  The air bubble gradually disappears.  Without the air bubble, the water can't expand when it is heated so the pressure valve has to open a crack and dribble out some water.

The "fix" is to drain some water out through the (awkward) drain plug to recreate the air bubble.  If that doesn't work, you may then consider you have a defective relief valve. 

(I will never, ever even dream of disagreeing with Gerald if it is about engines, transmissions, wheels, brakes, etc., etc.!) 

[Dennis Belfils]

Yes, Bill is quite correct regarding the air pocket in the water heater. When water is heated it expands & without that air, the pressure builds until the relief valve does it's job. On other rigs we have owned, I've installed an accumulator, preferably
 near the pressure side of the pump. It consists of a pressurized air bladder inside a small tank & gives the expanding water a cushion to push against. Look into it. Hope this helps

[Gerald Farris]

Thank you Bill,
I stand corrected. I never thought of the expansion issue because the only coach that I ever owner with a propane water heater was my 93 Patriot and it had an expansion tank in the water system, therefore the pressure relief valve never leaked.

Gerald

 

[David T. Richelderfer]

I thought I learned about water in college chemistry.  Apparently, there was more to learn.  Water is at its most dense at 3.98 degrees Celsius (C).  0 (zero) degrees C is freezing and 100 degrees C is boiling.  If water gets either warmer or colder from 3.98 degrees C, then it becomes less dense (less dense means it expands).  I found this online:

"4 degrees C turns out to be the temperature at which liquid water has the highest density. If you heat it or cool it, it will expand. The expansion of water when you cool it to lower temperatures is unusual, since most liquids contract when they're cooled."

"As a result, ice has a lower density than water. Hence, ice floats on water. Water has a maximum density at 3.98 degrees Celsius because on heating ice, the hydrogen bonds break progressively and the molecules start packing in more closely in the vacant spaces."

So, Bill, your memory has served you well here.  But if your water heater had an expansion device of sufficient volume, then your pressure valve probably would not be leaking.