Last night we smelled something burning coming through the heater ducts. I shut down the furnace and water heater, since I couldn't identify where it was coming from, and shut down the gas to both. Figured it was already past midnight, throw an extra blanket on each of the kids and deal with it in the morning.

Today, sent the kids to their grandparents while I fired the furnace back up. No smell anywhere. Hmm. Fired the water heater up and no smell. Checked the exhaust and it was drawing just fine. Hmm. Called the guy who does our heating stuff to come check it out.

He re-did what I did: Shut everything down, turned on one then the other. When the water tank fired up, the flame shot out the access port a little bit. It's got a metal shield that goes over it, so I had never worried about it. (It's done that before when I had to relight the pilot light after a blew out.)

He showed me the faint smudge of smoke above the T&P valve, which means every time the burner was lighting it was singing the insulation and venting through the opening for the T&P valve. Which is far enough away from the exhaust to not get drawn up, but right next to the inlet for the furnace. So if the water tank lights up just before the furnace kicks on, it draws in the burnt exhaust smell.

So even if the tank itself is still good, the regulator on the burner is shot. And it's 15 years old. And the gas line leading up to it is no longer up to code. I ain't working on a gas line. He's downstairs right now putting in a high-efficiency Rheem for $600. Which is about what I could buy the tank for retail.

I looked at the tankless, and best estimate is it would pay for itself in 15 years. I don't plan on still being here by then.