Our Empire GWT-50 two-sided natural gas heater has started creating soot on and around the ceiling of the unit. The ceiling is 8 foot and the unit is installed to specifications. We have had a service guy come out recently. He said pilot and the heater flame was burning correctly. He cycled the unit several times and found no problems based on his knowledge. He did not charge us for the site visit. The other night my brother had a “flash” from the heater starting up. It was described as a “whoosh with a yellow glow outside of the burner tubes”. This flash has happen two times now. The unit is very clean with no cobwebs or dust. We are at a loss on this problem. Ken