I had Safeway come out and do a cleaning but because they did it from the inside the others were still saying that the flue must still be blocked and cleaning from the inside did not catch it. I called Safeway about this and, at no additional cost, Safeway came out and recleaned the flue, including going onto the roof from the outside, and determined there was no blockage. VERY PROFESSIONAL, Highly Recommended. (P.S. I eventually found out that the water heater installer had improperly vented the heaters which fed into the same flue as my furnace.)