Electric heater spurts air

Hi,
My friends have an electric water heater about 10 yrs old. For the past few months when they open a hot faucet after not having used it for half an hour air spurts out with the water which is a scald issue. It's as though air is getting into the tank but we can't figure out how. Any ideas? I thought maybe a bad top thermostat, the water is very hot neither thermostat is turned up very high. I turned them down a bit the other day to see if it made a difference but haven't heard from them whether it did or not.

Are they on city water of well? If on well water they may have a problem with their AVC.

It could be that the plumbing is such that air takes a natural path up the supply line to the HWT.

If it is super hot and you have the thermostats turned down, you may have a bad element, or a bad thermostat...either one can cause the tank to overheat. If the T&P valve is blocked or defective, the whole thing could literally explode taking most of the house with it. If it wasn't super hot, there are other things that could introduce gas into the tank, like from a defective well pump, a reaction between the anode and the water, and probably other things.

If you test the T&P valve and it can easily open and drain a little, it's still safe other than the excessive temp. If you can't open the T&P and drain a little water out, immediately shut off the power to the tank. Note though, that on a 10-year old tank, opening the T&P valve, if it has some mineral deposits, or crud that gets into the seals, it may not close properly and leak a bit until changed. The T&P valve is your safety relief valve - it IS important that it can open and drain. When water goes to steam, it becomes about 1700x bigger...if the pressure can't be released, it makes a very spectacular steam explosion!

The other thing to keep in mind is that oxygen comes out of solution in the water heater.

Fish can breath under the ice in a frozen river or lake because there's plenty of oxygen dissolved in the cold water. Algae in the water convert sunlight and CO2 into energy and oxygen, and the oxygen dissolves in the water. When you heat that water, the oxygen comes out of solution. Just run your cold water faucet until the water runs cold, fill a pot with that cold water and put it on the stove to boil. You'll see little bubbles of oxygen forming on the bottom (and sides ?) of the pot as the water warms up and well before the water is hot enough to boil.

Same thing happens in a water heater.