Dear Dave,
Occasionally my water heater drips from the pressure relief valve. I’ve been told that this is caused by the loss of the air bubble at the top of the hot water tank. Is this true? What’s the best way to prevent this from happening? Thanks. —Dave, 2017 HOST Mammoth
Dear Dave,
The most common water heaters used in RVs are Suburban or Atwood, and both have a pressure relief valve. The valve on this unit has the red label with a spring-loaded handle and a yellow identification tag.
When water is heated inside the tank, it will naturally expand. Both Atwood and Suburban have an air gap designed in the tank to accommodate this.
Over time, the water level can increase and absorb this air gap. The pressure release valve is designed to relieve pressure somewhere between 125-150 psi. This is what you are experiencing with the occasional leak or drip. This does not mean that the relief valve is defective, rather that the system needs to be purged to re-form the air gap.
How to purge the system
This can be done with the following steps.
1. Turn off water heater.
2. Turn off incoming water supply.
3. Open the closest hot water faucet in the coach.
4. Pull handle of pressure-temperature relief valve straight out and allow water to flow until it stops.
5. Allow pressure-temperature relief valve to snap shut, turn on water supply and close faucet.
Could be sediment in tank
Another issue could be the buildup of sediment at the bottom of the tank. Most often you will be using hard water from a campground source which has not been treated or softened like residential water. The hard water will have calcium, lime, and other minerals that can cause a buildup of sediment at the bottom of the tank.
Just draining the tank during storage is not enough to remove the sediment as the drain plug is not at the bottom of the tank, rather it is an inch or more up on the side.
It is recommended to flush the tank for several minutes using a flush wand like this one available on Amazon.
I like this brand as the tube is secured with a screw-on fastener. Some models have a plastic tube that is just pushed on and can come off with pressure and go inside the tank.
Procedure for flushing the Atwood water heater
1. Turn off your main water supply. Drain you water heater tank. Reinstall drain plug. Remove the pressure-temperature relief valve. With a funnel, use 4 parts white vinegar to two parts water. (In a 6-gallon tank that would be 4 gallons vinegar to 2 gallons water).
2. Cycle the water heater, letting it run under normal operation 4-5 times. At no time do you remove the vinegar from the tank. Once this has been completed, remove the drain plug and drain the water heater.
3. After thoroughly draining the tank, to remove the sediment, flush the water heater. If you elect to use air pressure, it may be applied either through the inlet or outlet on the rear of the tank or applied through the pressure-temperature relief valve. Remove the pressure-temperature relief valve and insert your air pressure through the pressure-temperature relief valve coupling. In either case, with the drain valve open, the air pressure will force the remaining water out of the unit.
If air pressure is unavailable, your unit can be flushed with fresh water. Fresh water should be pumped into the tank either with the onboard pump or external water pressure. External pressure may be hosed into the unit either through the inlet or outlet found on the rear of the tank or the pressure-temperature relief valve coupling located on the front of the unit. Continue this flushing process for approximately five minutes, allowing ample time for the fresh water to agitate the stagnant water on the bottom of the tank and forcing the deposits through the drain opening.
4. Upon completion of the steps above, replace the drain plug and the pressure-temperature relief valve.
5. Refill tank with fresh water that contains no sulfur. The Atwood water heater is designed for use in a recreation vehicle. If you use your vehicle frequently or for long periods of time, flushing the water heater several times a year will prolong the life of the storage tank.
The pressure relief valve may need replacing
If you have replaced the air gap and flushed out the tank and the pressure relief valve still drips, it may have gotten weak and then should be replaced.
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Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
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That Atwood/Dometic flush procedure uses a lot of vinegar! From experience, do NOT let the vinegar drain to the ground, especially if grass is involved. It took two years of flushing, applying lime and reseeding to repair the dead spot. Put a large tub under the drain to catch the used vinegar.
If you do need to remove the pressure relief valve, you will very likely want this socket.
Thank you for the careful instructions, Dave! Have a great day and safe travels!
Under no circumstance should you put a plug into pressure relief valve. You would be sitting on a bomb