for your advice, suggestions, and concern. The furnace and water heater are in the garage...but in a little tiny room with doors across the front. Now, I'm quite certain that if someone left the car running in the garage, we'd all end up pushing daisies. However, this duplex is on a concrete slab, and I honestly don't know where else they would put it. When I moved in (late October) I smelled gas coming from a supply pipe in the back yard. Called the gas company and they came and repaired it immediately. The guy from the gas company had to re-light the pilots on the water heater and furnace, and he didn't say a word about them being in the garage. Nor did my mechanical/structural inspector or the furnace repairman that I had over last week. So I'm guessing, it's acceptable. Not desirable, but acceptable.
By the way...I'm in a suburb of Kansas City and yes, I do recall the homes exploding due to broken gas lines. We had a spot in our front yard where grass would never grow. I always thought I smelled gas when mowing around that spot, so finally called the gas company. Sure enough, there was a gas leak that had been there for quite some time. Fortunately they repaired it before any disaster struck.
Now, we are dealing with water main breaks throughout the city due to the ongoing below-freezing temps. Are you in Kansas City as well?