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Is the Heat Exchanger Cracked?

By Paul J. Turek, CM

[Editor’s Note: With any technical advice, always be sure to wear proper personal protective equipment, and follow proper safety procedures and manufacturer’s guidelines.]

It can be difficult for an HVACR professional, even one with years of experience, to tell if a homeowner’s heat exchanger is cracked. It seems that in almost any situation—whether it is an RSES meeting, a supply house or just in conversation with other technicians—many in the industry cannot readily tell when a heat exchanger is cracked and needs to be replaced. While every situation is unique—and HVACR service professionals should confirm any initial findings with further testing—following these simple steps can help determine if a customer’s heat exchanger is cracked.

There are two paths of air that come in contact with the heat exchanger: the indoor blower pushes the first path of air over the outside of the heat exchanger; and the second path of air moves through the combustion chamber, furnace flue passes and flue-collection box.

When the heat exchanger is faulty or cracked, each pathway of air has the potential to cross over into the other pathway. A cracked heat exchanger can cause inefficient combustion and produce high amounts of carbon monoxide. This can be dangerous both in terms of system operation, as well as for the inhabitants of the dwelling.

Finding a crack
The following steps are the fastest and most efficient way to check for a cracked or faulty heat exchanger. [Editor’s Note: It is important to mention that no test methodology is 100% effective. Service professionals should keep in mind that, depending
on where a crack or hole has formed, they may not see the same static pressure as in other locations. This might cause a tech to say that the exchanger is good even with a crack. If a crack is suspected, the tech should take CO and CO2 levels in the supply/return ducts before and after the unit is put into the heat mode. The use of safe tracer gases makes testing even more reliable. These may require a little more expense, but they provide the technician with actual values to prove the results.]

To run these tests, the technician will need a roll of either foil or duct tape (foil tape works best). The technician also will need a u-tube manometer; an electronic manometer; a magnehelic gauge; a draft gauge from a combustion analyzer or any other gauge that can measure 1/100-in. water column; basic hand tools; and two pipe wrenches.

The following instructions are categorized by furnace type, but the basic steps are similar for all furnaces. [Editor’s Note: It is important to again note that the heat is to be turned off for all of these procedures. Be sure to have a copy of the manufacturer’s guidelines available in case any equipment questions arise.]

80% AFUE propane, natural-gas and oil-fired equipment:

  • Step 1. Turn the thermostat to the OFF position, the fan switch to the ON position and the unit service switch to the OFF position.
  • Step 2. Remove the flue piping from the unit and cover the flue pipe opening with foil or duct tape. If the unit is a gas-burning appliance, the tech also will need to cover the draft diverter with duct tape as well.
  • Step 3. Remove the atmospheric burners from the unit. Note: The technician may have to remove the burner manifold to do this, and then cover the heat-exchanger openings with duct tape.
  • Step 4. Insert the test hose into the tape on the flue pipe opening of the furnace (A manometer used in the incline should be used to measure 1/100-in. water column for this).
  • Step 5. Turn on the power supply to the furnace. The indoor blower should run, and the heat should stay off. If the tech’s test instrument shows a pressure reading with the sensing tube positioned in the vent-piping outlet, then the heat exchanger is faulty. Note: The pressure reading referred to above will be the same static-pressure reading that a technician would get if they put their test hose into the supply-air main trunk line or plenum. If there is no change in the reading on the test instrument, then the heat exchanger is fine.

90% AFUE condensing propane and natural-gas fired equipment:
  • Step 1. Turn the thermostat to OFF position, the fan switch to ON position, and the unit service switch to the OFF position.
  • Step 2. Tape up the combustion air inlet and vent outlet. Note: If this is a two-pipe system, perform this step outside, providing the unit does not terminate through the roof. Otherwise, this can be done at the unit.
  • Step 3. Set up the test hose from the gauge so that it is inside the combustion-air inlet to the furnace or vent-piping outlet.
  • Step 4. With all the panels replaced, restore power to the unit. The indoor blower should come on. If the test instrument shows pressure in the combustion-air inlet or the vent outlet of the furnace, the tech either has a bad primary or secondary heat exchanger. If there is no change in the test instrument, then both heat exchangers are fine.
Rooftop package units:
  1. Step 1. This is extremely important: Turn off the power supply to the unit using the service disconnect.
  • Step 2. Access the unit control low voltage terminal strip and disconnect all the wires for the heating stages W1, W2 and W3. Then, install a jumper wire from the R terminal to the G terminal.
  • Step 3. Tape up any combustion-air inlets and the combustion-vent motor outlet, and then replace all the unit panels.
  • Step 4. Place the test instrument’s test hose into the taped vent outlet.
  • Step 5. Turn on the service disconnect to the unit. The blower should come on and the heat should stay off. If the test instrument shows pressure in the combustion-air outlet, then the technician has a bad heat exchanger on their hands. If there is no change in the pressure reading on the test instrument, then the heat exchanger is fine.

Conclusion…and a reminder
These methods have been employed for years without fail, and will work on virtually all warm-air furnaces and rooftop package equipment, in addition to oil, propane, and natural- and condensing-gas furnaces. Having confidence in diagnosing heat-exchanger problems inevitably makes for a better technician and a more profitable company.

However, it is imperative that in all the examples given that the tape is totally removed from the unit before the unit is put back into service. In addition, it is strongly recommend that the tech take a reading of the supply ductwork static air pressure before beginning the test. This way the technician will know what to expect from their test meter if they have a bad heat exchanger.

Paul J. Turek, CM, is the owner of Zoned Comfort Systems, LLC in New Britain, CT. For more information, call 860.620.2699 or e-mail

From the September 2009 issue of RSES Journal.

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