"Bettina" wrote in message
news:Eycqf.1288$0u5.665@trnddc01...
We live in a six-unit apartment building, and all units are served by a
single water heater.

Last month we had a sudden increase -- about 25 percent -- in gas usage.
Emphasize, USAGE, not price -- which also increased.

The water heater is around 10 years old. Do such heaters lose efficiency
with age?

There is no other gas appliance on this meter.

Any guidance or suggestions welcome.
Certainly they become less efficient as scale builds up inside but not
suddenly. I would look for a leak somewhere in the hot water line. Perhaps
one of the apartments has a dripping faucet or a hot water line may have
broken if it runs under the concrete slab where it wouldn't be immediately
noticeable. Have all apartments turn all water off and then check the water
meter. If it is still turning, turn off the water to the water heater and
see if the meter still turns. I had a hot water line break under the
foundation once and in addition to a big jump in electricity usage (electric
water heater), I also had about a 30,000 gal. jump in water usage for that
month. Of course, there's always the possibility that one of the tenants
has started offering hot showers to the homeless. Or even, maybe your
utility company simply estimated last months bill instead of actually
reading the meter.

Tom G.