When we moved into our house the furnace had just been replaced. Once in a great while I would smell gas but never found the leak.

One day my wife smelled gas and immediately called the gas company. DTE sent someone out with a leak detector but he didn't find any leak either.

When I redid the laundry room I also fixed some long standing code problems, like a gas flex line to the hot water heater and 6 (!) flex lines end to end for the dryer. Everything was redone with black pipe and some lines were relocated so they wouldn't hit someone in the head.

I found the leak. It was the cap on the end of the drip leg to the furnace. It had been screwed on hand tight but never had seen a wrench. I haven't the foggiest why it would leak sometimes and not others. It only took 18 years but by gum and by golly, I found it!

For leak detection at work we used Cool Snoop which had an antifreeze for outdoor use in winter. At home I use Dawn dish soap spiked with a bit of glycerin to make it sticky. If you want to buy something the stuff they sell in the toy aisle for making super bubbles works well.

-Tom