UPDATE: Daughter bought another detector for safety sake and put it right in the furnace room near the heater. It has a digital readout and it is at zero. The flue was cheked and is clear. However, last September they had some one check the house for heat loss areas and some extra insulation etc. At that time they discovered a partial block (collapsed cap) and replaced it. She doesn't know when the plastic rings melted and right now we think it was when the flue was partially blocked. I am going there today (200 miles away) and will do further checks around the return air , the match test etc. Thanks for the help.