Greetings this is my first post .
I'm having a problem with a Weil McLain furnace that is probably 50 years old. It was running fine till last week when I went to start it up. It has always made a lot of noise when running but this time it backfired loudly and shook the house . I also smelled some smoke . I turned off the system called a local plumber. Said the burners needed to be cleaned so he took them out and cleaned them with a brush and air hose. Turned the heat on again after one hour had an other loud bang that shook the house also there was smoke in the basement and the smoke detector went off . Lucky I was home to turn off the system. I'm sure it's time for a new system but does anybody have any ideas on what the problem is since the burners were cleaned?

Dangerous situation as you could easily crack the exchanger. Why a plumber? Go with an experienced HVAC person. Burners running too rich and then firing the excess gas will cause a backfire. Smoke is another excess rich mixture indication.

The heat exchanger may need to be brushed & vacuumed out.