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I know that the following methods may be a little more intense than those done by Home Inspectors but it is always good to know the information.


Did I think up all these methods? No! But I did actively search on heat exchanger inspection methods and the following are methods that I found. For those that want one single never fail method for inspecting heat exchangers. There is none. 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 ? No you wouldn't nor should you. With equipment, knowledge and training you can become more proficient at inspecting heat exchangers.

Also check your local building codes. They may list further defects other than the ones listed requiring heat exchanger or equipment replacement.

Types of Heat Exchanger Failures

1. Cracks.

2. Rust perforations.

3. Over 50 percent of the heat exchanger wall thickness has rusted away.

4. Leaking heat exchanger seams.

5. Loose or missing screws securing cells.

6. Broken crimped rings

7. Broken or leaking seals or gaskets, including cemented seals.

8. Missing factory welds on seams.

9. Clean out port gaskets missing or leaking.

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 as at the top. Jet engine mechanics use high candled power lights to inspect turbine blades and illuminate cracks.


Flexible lights are good for getting a light right on a suspected area. Having a light almost on the crack will highlight the defect. Flex-A-Light from Sears or the inspection light from SnapOn Tools Stock#: GA481B


Sometimes you can even pickup odd looking lights such as this one that uses a rechargeable Makita battery.


By shining a light on the outside or the inside of the heat exchanger area and examining the opposite side look for light penetration.

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. Large mirrors can be used for drum type heat exchangers.


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. Use a male threaded cap for one end. Install a small piece of foam such as rubatex in both ends. Tool plastic coating can be sprayed on the back side for protection and at one end for a hand grip.



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.

Don't eliminate the need to pull and fill a heat exchanger with water. Had to do that once on a brand new secondary heat exchanger to find that it had several leaking rivets causing the pressure switch not to close.

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/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 of the heat exchanger. A detailed fact sheet on the AGA test procedure is available at http://www.aga.org/pdf/publicinfo/codesstandards/twfacts8612b.pdf

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. http://www.testproductsintl.com/gas.html



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 the heat exchanger either from cracks, rust perforations or leaking seals or gaskets.


Photo courtesy of the Baldloonie

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.

Vapco http://www.vapcoproducts.com/catalogPDF/P19-outline.pdf

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 this test. Magna Flux cleaner, developer , red penetrant and zyglo fluoroescent penetrant is available through welding supply stores. It was originally developed by the military and has a mil standard labeled on the cans

9A. A flourescent dye spray and UV light test has been devloped by Visible Defects. The dye is sprayed on the exterior of the heat exchanger. The interior is then checked with the UV light. Picks up cracks, rust holes and faulty seams and welds. They also have a camera system. The lens head diameter is smaller than a dime. 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.

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

Visible Defects http://www.visibledefects.com

12. Optical boroscopes can also get you access to some narrow openings. Yet do not offer the ability to tape the inspected areas.



By supergluing a larger mirror to the Testo mirror you can improve the quality of the view.


Be sure to properly attach the mirror. The mirror support should be on the side of the head that has the large optical opening. The two smaller openings are the lights.

Testoview http://www.testo.com/testoview.htm

13. Pressure Testing - Lennox Pulse Furnaces using kit part number 74K96 available through Lennox.


This kit comes with the pressure gauge fixture, plugs, instructions and a furnace wall patch plate.Additional patch plates available in a pakage of 5 using part number 16L98 can be ordered through Lennox. The plugs can also be used to block off standing tees on air conditioning drains so you can use a sludge sucker at the drain end to clear plugs.

The carrying case and inlay is something I did on my own to protect the gage assembly as well as provide quick inventory to insure that plugs are not left behind. This case was obtained through Sears.

Most failures on the Pulse occur at the solder joints at the condenser (secondary hx) outlet tube. However failures can occur at other areas as the following photo shows.


14. Global Leak Detection Corporation--- LeakChek Hawk pressure test kit. It can record and report using new industry standards in testing procedures as developed by Alberta Mechanical Officials Society . Click on products link.


15.Here's an old timer's method for oil furnaces. According to the Timken Silent Automatic Oil Burner Serviceman's Guide, 1926 Edition:

A serviceman can easily check whether smoke is passing through the heat exchanger due to a crack is to shut off the burner and place a burning piece of TARPAPER in the combustion area. By then placing his nose next to a register if the smell of asphalt is detected in the home then there is probably a defective heat exchanger.

16. Here's a test that is recommended by someone in the land of natural gas, Alberta, Canada. It's called a sulphur test. We bring the unit up to full operating temp. then a small amount of sulphur is placed on a tiny collectors spoon which is fastened to a piece of soft 1/4" copper tubing about 15 and 22" long (I made 2 for different applications) and then inserted into the burner area. I let it burn in each chamber for about 15 seconds, moving it slowly back & forth, side to side. The amount of sulphur should do 3 or 4 chambers.NOTE: if you need additional sulphur, make sure you cool off the spoon in some water, or you'll have one stinky fire on your hands!! And don't lay the hot spoon on a lino floor or something until it cools off.

The principle being... if the exchanger is good, the rotten egg smell will be taken out of the home via the chimney, if it has a hole or crack anywhere in it, the smell will be distributed into the living space via the heating ducts, just like CO would be. I still monitor for CO with a digital 4 gas detection unit as an addition back-up, but as you have already heard, there are many conditions that can cause false or no readings at all.

The test is very cost effective, quick to perform, and quite reliable. Most HVAC company's around here use it. The sulphur is available at Pharmacy Drug Co. and at some Veterinarian places. It's bright yellow in color, and can be in powder or pellet form. It's called Precipitated Sulphur and sells for ~$15. for a lb. which lasts a long time.

Hint: try and be quick if you have to pull the spoon completely out past the heat shield to get it into the next chamber, so you don't smell up the area your working in. It's pretty smelly stuff!

Want additional sources for heat exchanger testing and inspecting which include detailed methods and photographs of various heat exchangers showing locations of defects? Check the following.

RSES Members check your SAM manual "Residential Gas Furnace Heat Exchanger Testing" Douglas DeWerth, P.E. American Gas Association Laboratories manual number 630-92 9/86.

If you want to buy a copy call RSES (800)297-5660 http://www.rses.org

Contractor's Advantage


Furnace Safety Consultant's


Heat Exchanger Experts


Residential Furnace Heat Exchangers BOOK

http://www.ahit.com/products/books/heatexchanger.htm or


Gas Appliance Service Training and Consulting, 22 Griffith Drive, Riverside, Rhode Island 02915 phone 401.437.0557

Also a good book source for combustion and carbon monoxide testing "Carbon Monoxide a Clear and Present Danger "


Training Classes

Kansas City Building Institute


Heat Exchanger Experts


Carbon monoxide is not the only thing to be concerned about from furnaces. Check this site about the toxicity of natural gas and other toxins. http://www.geocities.com/RainForest/6847/report1.html#6.0

Bottom line is never stop thinking you may think of a better way to test and inspect heat exchangers.

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