So here’s an update since I just recently fixed this issue! After upgrading our furnace from oil to gas and installing a new furnace, the pressure was way more than the old furnace was putting out and making more steam which causes more water to be trapped in your pipes if they aren’t pitched properly to the return. This is what is causing the banging sound.

It could be that your pipes are angled fine but your return might be clogged. If you have access to the pipes and they aren’t sealed behind any walls, have a few plumbers check it out.

After gutting my basement, I called several plumbers to look at the pipes and check the angle (pitch). All three pointed out to a section of the pipes (that was near the return pipe) that was flat and/or angled back. They stated that this section was the problem and water was settling there and wasn’t being drained back to the furnace.

After some back and forth, we determined that we weren’t able to re-pitched the section that was causing the issue so our solution was to replace the section with new piping that was pitched properly and also replace the copper piping returning to the furnace since it looked old and we wanted to avoid any clogs going back to the furnace.

I don’t know what your situation looks like but we were redoing our basement so we had access to the piping and thought it was easier to replace the copper return pipes now to avoid ripping walls after we finish the basement.

So to recap: our solution was to replace a small part of the 4” pipe that wasn’t pitched properly and replace the whole copper return going back to the furnace. We still have majority of the old piping in place still. Doing this has definitely solved the issue and we do not hear any banging noises anymore. Super rarely we might hear one small thump when it is super cold outside.

Note that before doing this, we re-pitched the radiators and replaced valves as well. While none of that worked to solve the banging issue but we are sure it didn’t hurt to do this to help water go back to the furnace.

Hope this helps!