hello, i have a 30 year old furnace ,oil hot air.i had repairman out, he changed the filter, nozzle, cleaned it,and checked it for smoke and burn eff.he said it was 80 % and ok to keep.it blows blue smoke out the chimney for about 5 seconds then it turns like a white color and then disappears.it seems to do this the worst after getting up in the morning and turning heat up to warm the house.anyone have idea what is causing this blue smoke at start-up.thanks