I have a State Select Heater model P65220RTGW, it has 2 elements. Couple of months ago it shut off, after pressing the reset button it started working again but even though it is heating the water (and I raised the water temp) it doesn't even fill a regular bath tub. It's like it's working halfway... It seems that the lower one is not working, (I think so because when I raised the temperature on this one the water stayed the same, but I can't be sure since I don't know how to test it)
However will it affect the amount of water heated? Can you give me advise? Thanks

Classic symtems of a bad lower element. You must understand how a hot water heater works. Water enters and is directed to the bottom by a plastic "dip tube" where it's heated by the lower element. It then raises to the top and the upper element kicks in and maintains the heat. When you make a draw the process starts all over again. With a bad lower element the top few gallons are being heated and when you draw those fer gallons off you get cold water because that's all that's left. Little clearer now?
Let's check that hard working lower element. First turn the heater off AT THE BREAKER BOX NOT THE WALL SWITCH. Disconnect one wire from the element and check across the poles with a resistance checker. You should get a reading on the lowest setting, if not replace it. Now as for changing one. With the electricity still off, turn off the cold water supply to the heater and open the nearest hot water faucet faucet to prevent air lock. Now attach a hose to the boiler drain located at the bottom of the tank and drain the tank. If your in a basemrnt drain into a lower floor drain. You already have the inspection p-lates off so after the tank drains we are ready to remove the faulty element. There are two types, flange and screw in. The flange type just take a socket wrench to remove. THe screw type has a large hex nut built in, if you don't have a wrench big enough they sell a element wrench at your local hardware store. Replace with the same type and the same wattage. I use gasket cement when I install the flang type and wrap the threads with teflon tape on the screw in. After you install the element, with the hot water faucet still open turn on the cold water supply. DO NOT TURN ON THE ELECTRICITY YET!! After, AND ONLY AFTER, you grt a full stream out of the hot water faucet can you shut it off and turn on the electricity. Put the inspection plates on, wait a 1/2 hour and jump into a hot shower. If the elements check good the next thing I would ncheck is the dip tube. Hope this helps and thank you for rating my reply, Tom

Thanks for your promp reply! I'll let you know as soon as I can fix it :)