Our Heating Bill Doubled and Every Window Tested Fine

Our January heating bill landed at $340. The month before? $180.

We hadn’t changed the thermostat. We hadn’t left windows open. The weather was cold, sure, but not twice as cold. Something in the house was bleeding heat, and we had no idea where.

I started with the obvious suspects. Every window got the hand test—palm pressed flat against the glass, then along the frame, feeling for that telltale cold spot or whisper of moving air. Nothing. I moved to the baseboards next, running a lit candle along the gap where they meet the floor in every room, watching for the flame to flicker or bend. It stayed straight and steady.

My husband checked the weatherstripping on both exterior doors. He ran his hand along every inch of the rubber seal, looking for cracks or compression damage. Totally intact. We even went down to the basement with a flashlight and inspected around the foundation where the rim joist meets the sill plate, because we’d read that’s a common culprit for air leaks in older homes. We could see the fiberglass batts stuffed between the floor joists. No obvious gaps, no daylight peeking through.

Still nothing.

The furnace was running constantly. We could hear it kicking on every twenty minutes, laboring to keep the house at 68°F. The blower would roar to life, hot air would flood the vents, and fifteen minutes later the cycle would start all over again. Somewhere, all that expensive heated air was escaping fast enough that the thermostat never caught up.

We just couldn’t figure out where.

I was starting to wonder if the furnace itself was failing—maybe the heat exchanger had cracked, or the ductwork in the crawlspace had come apart. Then my friend Sarah came over to drop off a casserole dish she’d borrowed. She walked into the upstairs hallway, glanced up at the ceiling, and asked a question we hadn’t even considered.

It wasn’t about the furnace at all.