Yellow

Paul S_3

What would cause a yellow smoke when you take a smoke test?and what exactly is it?

toilburn

Air

Yellow smoke is usually caused by too much air

billtwocase

yellow

Or could also could be condensation.

Steamhead

Probably unburned fuel

too much excess air or the wrong nozzle pattern are two of the usual culprits. 



What boiler and burner are involved here? What nozzle are you using? What type of test equipment are you using?

Charlie from wmass

Just making a bet

I would say leak but I just happened to come across this this winter a couple of times.