Old methods vs. new for finding cracked heat exchangers

May 12, 2000
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Checking for cracked heat exchangers is one of the most important inspections a technician can make when servicing a heating system. Miss a cracked heat exchanger and you put the homeowners at risk. Find them too often, and your company could get a rep for using scare tactics.

There are older, traditional ways for finding cracked heat exchangers, such as smoke bomb methods, which contractors have relied on for years. But surely technology offers better means.

Recently at Area51hvac.com, contractor Bill Bauer posed this question:

“In the old days we used smoke bombs, then we went to a salt solution which I am not comfortable with and check them visually anyhow. Has anyone seen a simple solution for detecting a bad one? Do any of these new gadgets work?”

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According to Rudy Leatherman, who runs training seminars for Bacharach, “One method I have used with pretty good success is with an electronic combustion analyzer with a continuous digital readout.

“You watch the O2 or CO reading when the blower comes on and if a change is noted, there is a good chance either the heat exchanger is cracked; or, if it’s oil, there is also the possibility that a cleanout port is leaking.”

He added that “This also provides some information on how dangerous the crack is and whether or not, for example, the residents need to leave the house until the unit can be replaced (i.e., when the blower comes on, the CO reading elevates to dangerous levels).

“I guess the bottom line is that I have never been able to find a gadget that is effective 100% of the time, and since I always use a combustion analyzer anyway, it’s always with me.”

Some other types of test kits are potentially more accurate but time-consuming — meaning that technicians probably won’t use them, said Leatherman. “Plus, the combustion analyzer tests under actual operating conditions.

“I think we would all agree that there is a chance a crack would not open up until the metal gets hot,” he continued. “Another test I found good with oil burners is to plug up the breech area through the baro with a bag of rags, duct tape up the burner air band and place another ‘bag of rags’ in the observation port to plug it off.

“Then take a draft gauge, slide it in around the bag in the observation port and (assuming the fan/limit has a manual on switch), turn on the blower and watch the draft gauge to see if any positive or negative pressure changes are noted. If so, there is some sort of communication between the fire side and distribution side.

“Do remember to take off the duct tape before you leave,” he said. “I have yet to forget to take the bag of rags out of the stack, but did end up spending the better part of a morning cleaning the soot out of a unit that had been in pretty good shape the day before until I sealed the combustion air inlet tight as a drum!”

Induced draft testing method

According to John Gates, owner-operator of a service company that specializes in oil-fired hydronic boilers, “With the induced-draft furnace, observing the inducer draft with an incline manometer or magnehelic gauge (suitable range) through the cycle from inducer on to indoor fan on should also be effective.

“If the blower pressurizes the combustion process, the inducer negative will decrease. Combine that with the combustion analyzer suggested by Rudy and you are shooting with both barrels.”

He continued: “If it is an 80% unit it will also give some indication of ‘stack action.’ After the burner comes on, the negative should increase as the vent begins to warm and work.

“If you shut off the burner and block the flue, your manometer will tell you more when you manually start the blower. Any change up or down means a cracked heat exchanger.”

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