Originally Posted By: Gary Reecher
This post was automatically imported from our archived forum.
The following information is gleaned from over 15 years as a residential HVAC service technician.
If you find co present in the plenum then the heat exchanger is defective. With that said you can also have a defective heat without having CO present in the supply plenum. So you can never say that a heat exchanger does not have a defect based soley on the absence of Carbon Monoxide.
There is no single method of inspecting heat exchangers. Think about it. Would you inspect a Pulse heat exchanger the same way as a clamshell heat exchanger , a clamshell heat exchanger the same way as a tubular heat exchanger ?
Types of Heat Exchanger Failures
2. Rust perforations.
3. Leaking heat exchanger seams.
4. Loose or missing screws securing cells.
5. Broken crimped rings
6. Broken or leaking seals or gaskets, including cemented seals.
7. Missing factory welds on seams.
Inspecting Heat Exchangers
1. Flame deviation test. Burner flame moves when the circulator blower comes on. If you do not locate a crack or perforation the heat exchanger seams can be leaking or the metal may have pinholes rusted through.
2. Visual inspection of burner chamber area and exterior of heat exchanger using a flashlight with high candlepower such a rechargeable Mag Lite.
Use a variety of mirrors. Small round and oval mirrors that have long telescoping reach are available at most welding supply stores. A large round mirror with long reach is also available through Sears.
For older heat exchangers have a mirror made at local glass shop 1/4" thick x 1-3/16" width x 24" length. A suitable carrying case can be made from 1-1/4" PVC pipe and fittings.
3. Water Spray Test.
Remove furnace blower assembly and the fan/ limit to prevent damage to these components. Spray the exterior of the heat exchanger with a water/soap solution 1 gallon water/ 1 tablespoon soap using a garden sprayer. Inspect the interior of heat exchanger with mirror and flashlight observing for evidence of water indicating a crack, rust perforation or crimp leak.
By shining a light on the outside or the inside of the heat exchanger area and examining the opposite side look for light penetration.
4. Measuring for CO in the plenum.
A CO detector cannot tell you if a heat exchanger is good. A CO detector can indicate a heat exchanger is cracked only if all of the following conditions occur simultaneously:
a. The flame generates enough CO (lack of oxygen, excess fuel, high temperature).
b. Enough exhaust gases are emitted from the heat exchanger crack or perforation.
c. The exhaust gases from the crack are not diluted too much before coming in contact with the sensor. A cracked heat exchanger may leak CO in a small stream. You may measure high concentrations only an inch away.
d. The heat exchanger is the only possible source for the CO detected.
Note: I have not tried this but have heard of some that will spray WD-40 or Gunk penetrating oil into the burner chambers (not the outside) of heat exchangers of furnaces with inshot burners to generate CO to see if CO will leak to the outside of the heat exchanger.
5. Combustion meter measuring undiluted flue gases.
One sign of a cracked heat exchanger is a change of oxygen concentration in the flue gases of greater than 1 / 2 % oxygen, or a change in the carbon monoxide level greater than 25 ppm. This change is measured by comparing readings before and after the circulation blower has turned on. For this test to be valid , CO levels must be present in the flue gas.
Check Bacharach's site Checking for Cracked Heat Exchangers
6. The American Gas Association developed FURNACE HEAT EXCHANGER LEAKAGE TEST injecting a nitrogen and methane gas mixture into the burner chamber. The heat exchanger outlet of the heat exchanger is plugged and a combustible gas detector is used to check for gas leakage on the exterior. A detailed fact sheet on the AGA test procedure is available at
Test Products International advertises the J&N Associates H.E.T. Kit and HXG-2 Combustible Gas Detector on their web site which does the above AGA test.
7. Magnehelic gauge test. Tape shut burner chamber openings and flue outlet. Connect magnehelic gauge to inducer pressure sensing port. Operate circulator blower. Movement of gauge needle indicates leakage into chamber.
8. Vapco H.E.A.T. Spray catalog number HT-1Q. Sprayed into the blower compartment on opposite side of motor. If burner flame changes color there is a leak in the heat exchanger.
9. Magna Flux dye penetrant test. Using cleaner then penetrant and then cleaner over suspected area developer is then sprayed. Penetrant which has seeped into cracks, perforations or pinholes is pulled out by the developer agent. Full length of crack is seen not just what may be seen without theis test. Magna Flux cleaner, developer , red penetrant and zyglo fluoroescent penetrant is available through welding supply stores.
9A. Flourescent Dye with UV light inspection system from Visible Defects http://www.visibledefects.com/
10. Smoke Bomb test. Smoke bomb is placed inside burner chamber and lit. Evidence of smoke on the exterior of the heat exchanger indicates leakage.
10A. Smoke puffer test at the burner inlet and watching the smoke if it sprays away from the burner opening you may have a leak in the heat exchanger. [url] http://www.comfortinstitute.org/Brochures/smokepufferv2.htm
11. Camera systems can gain you access to some narrow openings and the inspection can also be taped for viewing by the home owner or for training. A couple sources for cameras are:
The Inspector http://www.shamrockindustries.com/home.html
IC Cam http://www.rotobrush.net/IC%20Cam.htm
Abatement Technologies http://www.abatement.com/residential/hvac_video.htm
12. Optical boroscopes can also get you access to some narrow openings. Yet do not offer the ability to tape the inspected areas.
13. Pressure Testing - Lennox Pulse Furnaces using kit part number 74K96.
Most failures on the Pulse occur at the solder joints at the condenser (secondary hx) outlet tube. However failures can occur at other areas[/b]
Gary Reecher, CM
HVAC Service Technician
MechAcc's Carbon Monoxide Site Links