July 2017

Heat Exchanger Inspection…

do it first

Hello again!

It was great to see so many industry family members and new additions our craft while attending a few of the many classes held at both the Eastern Energy Expo in Hershey, Pennsylvania, and the NEFI event in Massachusetts this year.

With that in mind, and because we’re in the heating system preventative maintenance time of year, here are share some tips on checking oil fired warm air furnace heat exchangers.  This check should be part of the furnace service procedure, and hopefully will prevent some of  the drama, and call backs that occur during that first cold night when most of our customers decide it’s time to raise the thermostat on the same evening.

Perform the following heat exchanger inspection before starting service on the burner. This way, if a unit fails the test, it’s taken out of service, (recommended) or follow your company policy. Valuable time is not wasted on servicing the burner, vacuuming the unit, replacing filters, oiling motors, inspecting belts, motor mounts, cleanliness of blower wheel, only to shut it down and becomes the cleanest new space taker in the scrap metal dumpster.

Just to add, if the unit is being used for air conditioning, then just disable the burner when taking it out of service.  It will be much easier to sell the customer a new unit if they stay comfortable.

 Draft test.

De-energize the unit. Furnaces switch off.  Remove one lead from the primary control. If the unit serves dual purpose, as it has an evaporator coil and is used for air conditioning, just removing the thermostat from its subbase.

Remove the flue pipe. This is done to achieve a more accurate result.

Insert a draft gauge into the over fire inspection port. It should be zeroed.  Because the flue pipe is now removed, there will not be any draft, or wind interference in the reading.

Turn on the blower while watching the draft gauge.

No change; Great!  If all looks good, no complaints of odors from the customer, signs of discoloration around the heating vents, and no signs of water leaks from restricted or clogged condensate drains on units that have air conditioning coils, then it’s ok to proceed with the rest of the seasonal maintenance.


If there was a change in draft readings, or the customer happened to mention a complaint of odors, another test needs to be performed, preferably with the heat exchanger warmed up and expanded.

Making sure the furnace cleanouts are confirmed sealed, and in good order.

Reassemble the flue pipe.

With the burner running, have the combustion analyzer warmed up and ready for a test. Connect a jumper across FF, and connect a meter to the cad cell eye leads. Watch the ohm reading, the O2 and the CO numbers with burner running and the blower off. When the blower comes on, look for a change in the ohms, the O2, and CO readings. A compromised heat exchanger will cause the flame characteristics to change.  You would be looking for a significant change in ohm readings between blower on and off, but the O2, and CO number will change as well, depending on where the compromised location in the exchanger is. If the clean outs are properly sealed, and you discovered a change in ohms, O2, and CO with the blower off, then on, then off again, The heat exchanger is compromised.

On some units, it’s worth performing an exchanger replacement. With most units, its new furnace time.

Ok, there is a confirmed fault, but you’re not done yet.

It is time for a visual inspection and confirmation. On most furnaces this is difficult, but if you remove the oil burner, remove the cage on your drop light and place the light in the combustion area, you can then look for light where the air movement is. Confirming that the unit is de energized, remove the blower.  You can usually view around most of the heat exchanger in this manner.  With the drop light in the combustion area, and the burner removed, you can get better access with inspection mirrors, or if you have one, a flexible probe inspection camera to find the fault.  If no light is seen from where the blower motor is, then make access in the duct to view the exchanger from that side. Look for a confirmation of at least two failed tests, and preferably a visual, before taking a unit out of service.

See other similar Articles in the Call Backs Category.