I would have to agree 18" is not deep enough but additionaly a run of
30 feet is a bad thing.

Even if you were doing this the other way around and heating the garage
hot water from a heater that is inside the house it would still be very
wrong.

the frost line in md is 36"

i am sure this has probably worked for a long time but you are probably
losing a lot of heat.

I would immediatly find a cost sheet that shows price averages for your
area using your type of system and compair the difference.

Call the gas company and they will help with information on average
heating costs and may even send someone out to look for free.

I strongly suggest that you think hard about getting the water heater
into the house this season.
At worst maybe you can cap the ho****er line comming into the house and
attach the ho****er heater at that point.

but call the gas company they may even give you a rebate coupon to buy
new stuff if not ask if the state will.

best wishes