Duct Vents Bang After Heat Turns Off

[bostonman]

Not sure if this is an easy question, but this has baffled me for years.

At my parents house is a forced hot air heat system that goes through galvanized (?) aluminum duct vents that are shared with the central AC in the summer. Due to the age of the house, the ducts aren't insulated (nor can be due to how they are run), however, about 80% of them are not accessible due to them being in finished ceiling sections in the semi-finished basement.

Every year in the months of October and November, about 1-3min after the heat turns off, a bang can be heard that echoes throughout the system. The vents don't bang in the summer when the central AC is running and the banging stops routinely after November.

It obviously has to do with expansion/contraction, but I find it odd it only happens for about two-months out of the year. We've tried to narrow down the location, but seems it changes (or we just can't narrow it down).

Currently the outside temperature is around between 50-70 degrees F, and inside is set for maybe 70 degrees F. If it's due to expansion/contraction, I'd assume it would occur throughout the year; especially during the much colder months.

A air duct installer I chatted with once said it's probably the cooler air getting sucked back into the vent after the heat turns off and causes a bang when the two different air temperatures collide. While that seemed logical, again, this doesn't happen post November months.

Why would this happen only two-months out of the year?

[pcprogrammer]

This is indeed a strange phenomenon, and I can only guess.

Maybe the ducts are restrained by something that when contracting after the expansion that only gives way at some point creating the shock wave, and this restrain goes away when the house is warmed up in it's whole, so not happening after this warm up period of a couple of months.

And not happening on the cooling cycle since the duct does not expand beyond the restraining point.

No idea if it makes any sense at all.

[bostonman]

If I understand your thought correctly, we share a similar thought.

The wrench in the gears is that December and the following months outside is colder which would seem things would cool quicker when the heat turns off, so everything surrounding the vents would be colder and cause a greater delta.

Seems the bang changes location, but it's been tough to narrow down especially since most areas are covered by the ceiling in the semi-finished basement.

My thought was just to bang on the main vent run in the directions of the vents to each room. This may jolt the vents and possibly jolt them into a better spot.

[ataradov]

It may not be related to the temperature. I had banging noise in my system too, but it was immediately after air shut off. The reason was flexing of the duct under lower pressure of the intake. Turning on it was fine, but turning off would result in a loud bang. i propped it up in the "compressed" state and the issue went away.

I would look at all the accessible ducts and poke at them, you may find one that flexes.

[jpanhalt]

I suspect it's "oil canning."  Contraction with cold air may not produce it, but expansion with heat can.  The ductwork in my other home had very shallow breaks (bends) in a crisscross along long runs or large flat areas.  It was relatively silent.

[bostonman]

I suspect it's "oil canning."  Contraction with cold air may not produce it, but expansion with heat can.

Why only two-months out of the season?

I'll check for loose vents and stuff, but all the vent runs are tightly strapped into the rafters.

[jpanhalt]

Restrained expansion causes oil canning.  Once it's done, the severity decreases.  Cooling doesn't cause it.  In some homes, where heating is infrequent, it occurs every time the heat comes on.  In your area, once the heat comes on, it is pretty much on intermittently  the rest of the season.

[pcprogrammer]

If I understand your thought correctly, we share a similar thought.

I hope you do. 

The wrench in the gears is that December and the following months outside is colder which would seem things would cool quicker when the heat turns off, so everything surrounding the vents would be colder and cause a greater delta.

You stated that there is no insulation on the ducts, so the surroundings will also get warmer, even when the outside is cooler. The heating time will extend to keep the house warm and even with the bigger delta cooling may not be that much faster.

Seems the bang changes location, but it's been tough to narrow down especially since most areas are covered by the ceiling in the semi-finished basement.

And that is what makes things very difficult to diagnose. If it was always in the same spot you could consider making an access to that spot. But if you start poking holes everywhere your parents might disown you. 

My thought was just to bang on the main vent run in the directions of the vents to each room. This may jolt the vents and possibly jolt them into a better spot.

Who knows, maybe that will scare away the poltergeist. 

But like I wrote before, no idea if it makes any sense at all.

[richnormand]

Had this issue for several years and as you described it would appear for a few week/month only during the beginning of heating season.
Seemed dependent on outside temperature or if I used the smart thermostat (letting the house cool during work days).
If I let the furnace at constant temperature 24/7 it would not show up. Took a long time since all was OK most of the year.

The bang noise was quite loud and transmitted (via the heating ducts) to the whole house.
One day I was lucky while in the room and it popped! The heating ducts have a "lozenge"  diagonal pattern in the flat surfaces to make them rigid and stiff. This can be seen in the photo as the two diagonal lines on the surface.
The center is raised about 4 mm at the center. All the cold air returns and heated air distribution are like that.

Using very little hand pressure I found out the bang seemed dependent on temperature and air pressure inside as the center snapped in or out.

Solution in the photo is a metal strip screwed in the duct and a small wood spacer between to create a pressure. Adjusted the screws tightness to deform the panel so it does not pop back and forth anymore.
Been like that for over 10 years now.

And yes it was loud!
Hope this helps.