Bump in the Night

This winter, we've been awakened every night by a house-rattling thump. It sounds as if someone dropped a 50 lb sack of flour on the floor. Every night between 2 and 6. What could it be?? 

The boys?: Kinda sounded like one of them falling out of bed, but with out the ensuing swearing and moaning. This theory went out the window after the went back to school and the bumps continued.

The dog?: Was it the dog falling out of bed? After a thump, I jumped up to check on him. Nope - he was still in bed, looking at me and wondering why the bump woke him up.

Planes?: We're a few blocks from the approach into MSP, but if it were from a plane, you would expect to here it more than once per 24 hours.

Ice?: Is it the ice cracking every night and shaking the ground? There are also things called cryoseisms that are frost-induced earthquakes. There was no mention of this in the news or at the University of Minnesota seismology site.

We set out a sound recorder every night in a different location to try to pinpoint the source of the sound.

It was the loudest in the living room next to the fireplace. Barely audible in the garage.

The recordings were all similar and they showed a surprise: there's an echo!! It shows up in every recording between 13.8 and 14.1 seconds after the main thump. What is the sound echoing off of?

Listening to the recordings, the thump sounds like a poof or whoosh; just like lighting a grill that has too much gas built up. Do we have something wrong with the gas fireplace or furnace?!!?

We turned off the gas to the fireplace with no effect; we still had bumps in the night.

Looking at the furnace, we found a few things: 

1. The flames on the burner were bright orange, not cool blue like they should be. This indicates that it's running rich: either too high gas pressure, or not enough air.

2. The fresh air intake hose into the furnace room was sorta crushed down and preventing full flow.

3. The exhaust damper was stuck open and was not turning off after each cycle.

4. There was some dusty residue on top of the hot water heater that had fallen out of the flue.

Hypothesis: The furnace is running rich, which produces lots of carbon monoxide (CO), which is flammable. There's a leak in the metal chimney flue pipe that is allowing the CO to accumulate in the chimney. The rich mixture would also cause soot buildup. At some point in the night, the cold air blocks up the flue and a buildup of hot exhaust finally torches off the CO and it shakes the house.

As soon as we put this together, we called the furnace company.

The gas pressure was OK - that meant we were not getting enough air to the furnace. 

The exhaust damper motor has a switch on it that had been switched to "manual". Switching it back to "auto" got it moving again.

The furnace guy could find no issues. No soot. Nothing. Now what? We called the chimney guy to check out the chimney. Is there a blockage?

Our exhaust flue was 30+ years old, so we had the chimney guy replace it. We were afraid that the explosions in the night would destroy the chimney.

Two days later we had a completely new furnace/hot water heater exhaust system with the right size pipe, the fresh air intake had been corrected, and we installed a louvre in the furnace room door. 

That night: BOOM! Still a house-shaking bump in the night!!! What the heck?

In a moment straight out of Christmas Vacation when Ellen Griswold realizes that Clark's lights aren't working because of the garage light switch, I unplugged the TV system for some reason. The subwoofer let out a beefy thump. ??? Seriously? Could it be the subwoofer?

Yes. It was the subwoofer all along. Every night the TV or cable box reboots itself and sends a massive pop to the subwoofer that converts it into a housequake that wakes every from a dead sleep. The smaller echo is when the TV comes back to life during the reboot process.

At least we know that our furnace is in tip top shape and should be running much more efficiently now!