Article Contributed on: 10/13/2006 2:02:58 PM
A fellow blogger recently asked me:
Dr. Plumb, Why does our hot water tank have a copper pipe running to a drain in our basement floor? Why would water ever dribble from the tank?
Excellent question! The pipe you refer to is a drain for the "temperature and pressure" valve, commonly referred to as the T&P valve. Aptly named, this valve monitors the temperature and pressure inside the tank of a water heater. Without this valve your water heater can become a steam-powered bomb.
Most of us know that water heaters heat a tank of water to a certain temperature and automatically shuts down, storing the hot water for future use.
The control valve, located at the bottom of the heater (see picture), is responsible for this process. Let's just say the control valve quits working properly and fails to tell the burners to stop heating the water (usually around 125-130 degrees). It's not just the rise in temperature that's dangerous in this situation.
You need to remember that your water supply system is under pressure. When the temperature of water rises, so does its volume. As the volume rises, so does the pressure. Are you starting to get the picture? BOOM!!!!!!!!!!!
The danger involving temperature is just as bad. Once the temperature reaches 212 degrees, it is said to be "super-heated." Opening a faucet with super-heated water could be very dangerous. The moment the water is exposed to the atmosphere, it can turn instantly to steam, literally blowing up in your face.
I will admit this phenomenon didn't occur to me until my son and I were watching an episode of Mythbusters involving heating water in the microwave for coffee. I highly recommend the show by the way.
I digress.... Now that you're all thoroughly worried about your water heater blowing up, let's talk about the marveled T&P valve.
We know what it is and what it looks like (look at the pictures), let's briefly discuss what you don't see and what you should be checking.
The T&P probe (see picture) makes the valve open when temperature or pressure becomes dangerous, conveying the hot water to a nearby drain and allowing cold water to enter the tank. You should test this valve at least once a year by opening it up.
If the valve fails to close after testing (dribbling), you may want to change it. Remember, the T&P is a very important safety device. Don't let this one go because you don't want to fix it. Call a good plumber (Dr. Plumb) if you have doubts. It'll be money well spent.
We all seem to take our water heater for granted but don't ever scimp on safety.
Hopefully, you have a new perspective on how dangerous a water heater can be.
--Dr. Plumb
P.S.--Your T&P valve should be piped to a nearby drain. If there is not a drain in your mechanical room, a drain pump is recommended.