Cracked Heat Exchanger: Symptoms and How It's Confirmed
Signs of a cracked furnace heat exchanger, how a technician confirms it with a camera and CO readings, what to do after a red tag, and repair vs replace.
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Mahtomedi furnace repair costs a $79 trip charge that's waived when you approve the repair, and most houses here date from 1970 to 2000. Near the lake, a low basement furnace may still share a chimney with the water heater, so we check draft too.
In the houses built between 1970 and 2000, which make up most of Mahtomedi, the furnace in the basement is often the second or third in that spot and the ductwork is original. Failures are the everyday kind: hot surface igniters that crack, flame sensors that coat over, inducer motors that grind, and blower motors running out of bearing life. A replacement furnace that was never matched to the old ducts can also trip its high limit when the returns are too small.
Near the lake, older homes bring different puzzles. A furnace in a low basement may still vent up the same chimney as the water heater, which puts a draft test and a look at the flue connector first on the list. Cottage-era sections and additions sometimes hang off the main house on one long duct run, and those rooms go cold first, which owners take for a furnace problem. Xcel Energy supplies the gas throughout Mahtomedi.
Updated October 27, 2026
National averages (HomeGuide, 2026), not our prices: an inducer motor $400–$1,100, a PSC blower motor $500–$1,400, a variable-speed (ECM) blower motor $800–$2,300. More in the furnace repair cost guide.
Symptom guides: Won't ignite · Short cycling · Blinking light codes · Pressure switch trips · Signs your furnace needs a technician
Often airflow. When the returns are small or the filter is loaded, heat builds in the furnace until the high-limit switch shuts the burners off, the blower cools things down, and the cycle repeats. That's common where a newer furnace sits on original 1970s or 1980s ducts. A dirty flame sensor or an oversized furnace can cause short cycles too. We check temperature rise and static pressure to tell which.
Once is reasonable. Turn the furnace off at its service switch for about 30 seconds, then back on, and make sure the filter isn't clogged. If it locks out again, count the blinks and match them to the code chart inside the door, then call us with that code. Resetting over and over on a flame or pressure fault can hide a venting or gas problem that needs a technician.
Signs of a cracked furnace heat exchanger, how a technician confirms it with a camera and CO readings, what to do after a red tag, and repair vs replace.
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