When I leave town I cut the water off to the house and turn the water heater down. When I return there is air in the water supply lines. Why and how do I prevent this?
Try leaving the kitchen taps on/open (after turning off the water) until the water runs out, hopefully pushing out any air in the pipes. Then turn the taps off (of course :0).
if you have air in teh lines after you cut off teh water supply for a couple days, then you have a leaking faucet somewhere. maybe even a couple of them. check them all for drips.
also, do you remember to kill teh WH when you do it? could be a drip there thru the T&P valve and this allows air intrusion when there is no water pressure to keep it out.
You have to understand the physical properties of water on this one and while it is technically true that you have a leak, I doubt very seriously if it is a leak that requires any action on your part.
Water reaches maximum density (8.34lbs/gallon)at 39degF and if heated or cooled from that temperature it expands.
When heated from maximum density temp of 39degF water undergoes a fairly linear expansion up to about 10% by volume at the boiling temperature and when it flashes to steam it immediately expands another 1628 times by volume (1sq.in of water produces 1sq.ft of steam).
The actual boiling temperature and rate of expansion is affected by the working pressure however I don't have the pressure temperature tables immediately at hand, and understanding that the working pressure can vary considerably from one location to another, for the sake of illustration allow us to use numbers that we are all familiar with. Water expands approximately 10% by volume from maximum density (39degF) to boiling temperature, which is 212degF at Standard Atmospheric Pressure) for a total differential of 173degF.
Per code, we set our water heaters to 125degF therefore the temperature differential between cold water coming in at maximum density, again 39degF to the set point of 125 = 86degF.
From this we can see that the differential between maximum density and the working temperature of our hot water system is approximately 50% of the total differential between maximum density and maximum expansion, therefore if we have a maximum expansion of 10% by volume, we could then say that the expansion in our hot water system is approximately 1/2 the maximum or 5% by volume.
No let us put this all in perspective. If you have a 50gallon water heater and you fill it from a cold start with water at 39degF as the water heats up to the 125degF calling temperature the water will expand to approximately 55gal by volume. Under normal circumstances as the water expands the additional volume is compensated by water backflowing through the supply line to the municipal main or the well pressure tank. (This also explains why we are required to install an expansion tank near your water heater whenever we install a pressure reducing valve or a check valve on your water main.)
Now in relation to the question at hand, how does air get in the lines? Under normal circumstances if we turn the water heater off the expanded hot water will slowly cool to room temperature thus the expanded hot water will contract. As the water contracts additional water will come in the supply line to maintain the total volume in the hot water system however, the post states that they turn the water heater off and close the house main water valve. Under these circumstances as the expanded hot water cools the supply has been closed off therefore as the hot water contracts it leaves a void in the pipes.
We previously concluded that the amount of expansion was approximately 5gal by volume. A 1/2" pipe contains 1 gallon for every 98 linear feet of pipe and a 3/4" pipe contains 1 gallon for every 43.4 feet of pipe thus in all likelyhood 5gal by volume is greater than the total volume of all your hot water pipes in the structure. Now given that the water will contract and flow back to the water heater, the lines are then left with a vacuum and nature abhors a vacuum.
Under these circumstances air will enter the pipes through microscopic pores or crevices or through faucet & valve mating surfaces which are no doubt to small to even pass a molecule of water.
In this case it is enough to just understand what is happening and there is no further action required.