Cameron wrote:
Hi there. I just moved into a new home and got hit with an incredibly
enormous water bill. I checked my water meter and could see the little
dial moving - not all the time but every 10 seconds or so it would
move forward. Sometimes it moves backwards but never enough to make up
for the forward movement. I have made sure all water is off in the
house and have done visual checks for leaks as well as a dye test in
the toilets. I called a plumber over and he said this motion is caused
by the expansion tank on my water heater. Low and behold when he
turned off the expansion tank the motion stopped completely. He told
me that this is normal for expansion tanks to cause this motion -
however he said the movement forward and back should be equal. I have
not noticed this. Can anyone verify if their expansion tank causes the
same forward motion on the meter?

thanks so much.
Something doesn't sound right. The expansion tank is there to allow the
hot water in the heating system to remain full without any air in the
lines. In order to do this the expansion tank has a bladder that is
charged with air(except very old tanks which do not have a bladder). As
the hot water expands it displaces this bladder and the pressure in the
heating system remains relatively unchanged. When the tank looses its
charge and the furnace turns on to heat the water, the water will
expand, have no where to go, and will leak out of the
temperature/pressure relief valve. In this case you would be seeing a
leak from this T&P valve and the plumber should have recommended
replacing the expansion tank.
Why he didn't recommend this, and why you didn't see a leaking T&P
valve, just doesn't add up.
In addition to that, I doubt if the meter is sensitive enough to show
this relative small rate of usage.
Here is a test you can try for yourself.
Turn off the same valve that the plumber turned off. It would be the
fill valve to the furnace and is usually located just before a pressure
regulator.
Verify the meter has stopped moving.
Incrementally open a sink faucet until the meter moves at the rate you
have been seeing before.
What is the flow rate? Measure it by timing how long it takes to fill a
particular volume container.
I'm guessing it will be at least 1/3 gallon per minute.
My guess would be like the others have mentioned, leaking toilet,
leaking outside faucet, or a leaking underground feed to a auxiliary
structure like a pool house.