excess air fluctuation importance

flamingo

How important is a fluctuating excess air reading? My oil company insists I need a new furnace solely due to an excess air reading of @30% with FAN OFF and @49% with FAN ON. They say the reading should stay the same with fan off or on. They have never recorded this reading during service calls, only the usual stack temp, draft, smoke, C02 and efficiency readings. My York furnace is 25 years old, but has no operating symptoms; no smell, soot, noise. Subsequently, two independent techs have found no indication of cracked heat exchanger (including visual/scope inspection), said they would use the furnace. Testo 327-1 used to diagnose. I dont doubt the readings, but do they indicate a dangerous condition? I dont want to be the woman who falls for a scare tactic. Who to believe? Many thanks for any advice or information.

Robert O'Brien

Excess air

Shouldn't change with fan on or off. If the HX is cracked,which at 25 years is pretty common,over fire draft should also change with fan operation.

chapchap70

Blower motor stronger than burner motor.

If there is a small crack in the heat exchanger, you may not smell it.  The burner motor comes on for a short time before the blower motor comes on otherwise cold air would get blown around the house.  Since the blower motor is stronger than the burner motor, the air in the ducts gets forced into the combustion chamber when both are running so you would not smell combustion gases.  This is why there is an increase of excess air in the combustion zone. 



There would more than likely have to be quite a bit of flue gases forced from the combustion zone into the ducts before the blower motor comes on for you to be able to smell it meaning there would be a rather large crack.  It would be prudent for you not to let it get to that point since the crack isn't going to get any smaller.



Excess air is not normally noted in old fashioned reports for annual

service.  My kit does not measure excess air.  I would have to lean on draft readings (higher draft in flue pipe when blower comes on) to determine if there is a cracked heat exchanger.  I would think that the excess air method can "see" smaller cracks.  A furnace heat exchanger is not usually 100 percent visible

for inspection unless it is removed from the furnace.

Aaron_in_Maine

It's cracked

Sorry triple posted

Aaron_in_Maine

It's cracked

At 25 years old I would change it its live out it's useful life. Do you still have the car you were driving 25 years ago?

Aaron_in_Maine

It's cracked

At 25 years old I would change it its live out it's useful life. Do you still have the car you were driving 25 years ago?