I recently replaced my water heater. It now seeps occasionally from the pressure relief valve. I plan to add an expansion tank. Can I connect the expansion tank to the water heater by using a drain valve at the bottom of the water heater? Usually it is connected to the cold water inlet, but since all the water is at the same pressure, why not use the drain valve. I will probably use the cold water inlet later when I am not so busy.
Thanks, ken

Before you jury rig the tank have you checked your pressure and insured that it is excessive? Did you recycle the old pressure release valve or buy new? Is there a anit-back pressure device installed on your in coming line? Lets be sure the problem is identified before we go furher.

ballengerb1, thanks for the response.
The pressure spikes to 120 psi when the water is heating. Drops to 50 or so when the water is turned on. The pressure relief valve is on the new water heater. There is a pressure regulator where the water enters the house. ( I assume it has an internal check valve.)
Thanks, ken

Yep, it does sound like you are in the market for an expansion tank. Send this post to Speedball1, he is near the top of the members list. He's a pro. I'm not a fan og your idea about using the drain valve but maybe Speedball can give you more info. In the meantime I'd consider shutting the fuel off or greatly reducing the temp. 45 lbs is more norma, 120 could blow a toilet or lav supply line that is weak. Buddy of mine is facing $185,000 in repairs due to a blown toilet supply.

Your pressure spike may indicate your temperature is too high. I suggest an expansion valve rather than an expansion tank.