air in the lines

I went on vacation for a week and shut the water main off before I left.
No faucets were opened or toilets flushed after the main was shut off.
When I came back home, I turned the main back on and noticed a slight in-flow of water. Later during use, the faucets expelled air AS IF I shut the water off and opened a faucet or flushed a toilet.
I've checked every faucet and toilet and have verified there are no leaks.
My question is where did the air in the lines come from?

Turning the faucet from hot to cold didn't tell me which line had the air.
The only thought I had is that I turned down the hot water heater and that the contraction of the previously expanded water created an air pocket in the water heater. However, this brings up the question of how did the air get into the water heater (assuming this theory holds water .. haha)?

That's probably what it is.

H2O is strange. It shrinks as you cool it down to +4 deg. Celsius. As you cool it below that, it actually expands as it gets colder (below +4 deg. Celsius), expands as it freezes, and then continues to expand as you cool the ice. This is why ice floats.

Also, water is virtually immune to changes in pressure. (which is why the stuff is nearly incompressible), but it actually changes it's volume pretty substantially with temperature changes. Take a look at this chart:

H2O - Thermal Expansion Coefficient

The coefficient of thermal expansion, K, changes with temperature. At 50 deg. C, or 122 deg. F, or the approximate temperature of the water in the heater when you turned it down, the thermal expansion of water is 0.00045365 per degree Celsius, or about 0.045 percent per degree C. At room temperature of about 25 deg. C or 77 deg. F, the thermal expansion of water is 0.0002569 per deg. C. or about 0.026 percent per deg. C.

So, if we take the average thermal expansion of about 0.035 percent per degree Celsius and multiply it by the temperature drop of 25 deg. C, we get 0.875 percent. Assuming a 50 gallon hot water heater, that's 0.44 gallons change in volume, or about 7 cups or 1.66 liters.

Water is nearly incompressible, so even that small change (7 cups change in volume in 50 gallons) would have resulted in a significant pressure drop IF (big if there) your water supply system was totally rigid. However, much of the volume change in the water would have been compensated for by the deflation of the water heater tank.

Still the fact that you're saying that "air" came out of your faucets means that the pressure drop was sufficient that dissolved oxygen came out of solution from the water in your hot water tank and your water supply lines. Air is 78 percent nitrogen gas, but the 21 percent that's oxygen is much more soluble in water than nitrogen. So, when you boil water, the bubbles that form on the bottom and sides of the pot are almost pure O2. So, most likely, the stuff that came out of your faucets wasn't "air", it was nearly pure O2.

I think you're right that what happened was that the hot water in the water heater cooled down during your vacation, and the resulting change in the water pressure allowed dissolved oxygen to come out of solution and form bubbles in your supply lines.

If you have a well system, your foot valve may be no good at the bottom of your pipeline in the well.
Other than that, I have not seen air in a system that was town water.
happy hunting.
happens here all the time. Water drains back down into the well when the pump is not on...and air comes rushing out of any one of my faucets.

A small drip in a lower level could allow air to be drawn in through a faucet at a higher elevation over time. Air can get through a much smaller opening than water can.

That's the only explanation I have since you said you shut off the main valve and we are assuming it works.

The footvalve theory is feasible also if you are on a well, but there would have to be a leak in the feedpipe to the house to let the air in, just like in the faucet theory. When these pipes are under pressure, there is no way air can get in.

Just for grins put a bit of food coloring in your toilet tanks overnight. See if any color ends up in the bowl. Flappers can have very small leaks. As for air entering those guys ^^ have suggestions, i don't have anything to add.
If you do have a leak here are some troubleshooting tips.

Episode #8 – In The Toilet!

Pic of the flapper

I don't have a well but, suspected a leak in the toilet flapper. So I put a hash mark in the toilet tank to inspect upon returning from vacation. This comparison indicated that I did not have a leak in the flapper and that is what prompted me to post my question as at that point, I was at a loss as to where the air had come from.
However, I had my water meter changed out a few weeks ago before going on vacation and within the 15 minutes that it took to change it out, I experienced the same in-flow of water when the main was turned back on. This in-flow was definitely less than the 7 cups that Nestor is describing. The DPW worker that changed the meter even commented that it shouldn't have in-flowed unless I have a leak somewhere and in his 25yrs. of experience it is usually attributed to a toilet valve leak. That is what why I marked the water level in both toilet tanks before I left. Unfortunately, this negates what Nestor & I thought would be the problem because I didn't turn off the water heater when I had my meter changed out so contraction of hot water wouldn't figure into the equation. I'm completely lost on this one unless it is the fill valve that allowed water in the lines to be exchanged with air and maintained the tank water level. I'll try the food coloring test tonight and post back my findings.
Thanks guys.

Gremlins.

or gnomes, maybe.

What about a leak at your dishwasher fill valve...maybe the valve is no good..or some debrie got in there...and lets water sneak by even when the unit is off., even a small amount.A couple of cups of water will not be noticed in the bottom of the unit.
just a thought.

I can see where the little tube going into the overflow tube in the toilet tank(or whatever it's called) could let in air if the toiler fill valve could leak air in. But the water would have to go somewhere below it to create the vacuum.

It could also easily be a tap on the external portion of the home or in the line leading to the home. neither of these would be noticed but can easily do what you describe

Redwood slide a tall stack of chips on handyguys post and grins anticipating the win...

I added the food coloring to both toilet tanks and let set overnight.
One toilet had two streaks that came out of the rim but, never dripped into the bowl.
The other toilet appeared to be somewhat blue but, not enough for me to tell if it was serious enough to constitue the amount of in-flow that I experienced when I turned the water back on after coming home from vacation.
I also suspect that the in-flow from the meter change was a result of removing the meter and refilling it with water.
Sometime in this coming week, I'll shut off the main again and leave it off for a couple of hours and see if I experience the in-flow again.
An additional point of interest is that I do have an RO system that has a drain tube that taps into a drain line that could possibly bleed down and account for some air in the system.