Hot Water Pressure Relief Valve

15 yrs old so chances are T&P Relief Valve has built up enough scale that it is NOT a matter of re-establishing the air pocket. Time to replace.

15 yrs old it is well stuck

Try 'tightening' first then try removing. Might have to work back/forth to get it to break free.

Remove the exhaust shield so you can get better access.
And if you have a propane torch slightly heat tank area around the valve...but stay away from valve...it is brass and will absorb heat quickly (making it harder to remove)

Use a breaker bar for extra leverage to remove old one.

15 yr old T&P is a 1/2" NPT-------Atwood changed to 3/4" NPT when they changed design (2004)

Before you replace the t&P valve try the following;
Your tank needs an air pocket at the top or else it may leak so,

Get a can of PB BLASTER and spray inside the drain opening then
turn off water supply and open a hot water faucet that is higher than the heater, now go out to the heater and open the T&P valve until water stop draining out
Spray inside the drain with PB BLASTER again you are okay if some gets into the heater, you are trying to spray the sealing surfaces which may be rubber, then close T&P valve
Now go inside and close the faucet and lastly turn on water and after the heater is filled start the heater. Do not open faucet to purge air

This may save you a lot of grief takes about 10 minutes

I discovered the hard way (after paying for a replacement to be installed).....

That when my water heater initially heats up, if you are on the pump and it is ON and all the faucets remain closed during the heat-up (about a half an hour)......the extra pressure of the heated water is enough to make even a properly operating relief valve leak a little bit.

You can prevent that by shutting off the pump until the water heats up and crack a faucet open.

And let's hope that the amount of muscle required to remove the valve won't also damage the tank.

Rambler wrote:
Okay thanks, I have already tried to turn but couldn't move?
The heater is about 15 years old and runs very good.
Didn't know if there was some secret method of getting out besides muscle.

Rambler

well 15 year old heater the T&P valve could pretty well attached to the tank it's self. May take a bit of muscle to break it loose.

As the other person said, Roughly as hard as hooking up a garden hose to the park water spigot.

TURN OFF WATER (or bypass the water heater) turn off heater, Remove drain plug and replace with a short tube so as to get water out past the edge of the RV. remove TPR valve (unscrew) go to Lowes, home Depot, Sears, Joe's hardware, most any plumbing supply, Hand to customer assistance clerk, get new one, pay, screw back in (Brass should not need tape or putty but you can if you like) remove drain tube, replace plug (See above comment applies to nylon as well)

Restore water, restore power.

Okay thanks, I have already tried to turn but couldn't move?
The heater is about 15 years old and runs very good.
Didn't know if there was some secret method of getting out besides muscle.

Rambler

Very simple...unscrew the old one, and replace...Use a couple of wraps of Teflon tape on the threads.

Home Depot, Lowe's.