Cracked Heat Exchanger: Symptoms, How It's Confirmed and What to Ask For
Signs of a cracked furnace heat exchanger, how a technician confirms one, what to ask for if your furnace is red-tagged, and when to repair or replace it.
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Many Columbia Heights houses date from the 1910s through the 1950s, with cramped basements and converted gravity ductwork, and we repair both their furnaces and their hot-water boilers. The $79 trip charge is waived if you approve the repair, the flat-rate price comes in writing first, and we're open 7 days, 7am–9pm.
Columbia Heights houses are older and smaller than most in Anoka County, and the furnace often sits in a cramped basement with a low ceiling and the laundry crowding it. Many replaced a gravity furnace years ago, so the ductwork is a converted gravity system: big, low-velocity trunks and sometimes a single large return. A modern furnace on those ducts can overheat and trip its limit switch if the return is restricted, so a limit code isn't always a bad switch.
The other common setup is hot-water radiators on a boiler, especially in pre-1930 houses on the old streetcar blocks near Central Avenue. Boiler calls here involve circulator pumps, zone valves, air in the upper radiators and waterlogged expansion tanks. On furnaces and boilers alike, the chimney matters. These houses mostly vent into a masonry chimney, and we check draft and look for spillage at the draft hood on every call.
Updated September 23, 2026
Every furnace repair call in Columbia Heights starts with a $79 trip charge, waived when you approve the repair, and a written price before any work. Same prices in Columbia Heights as in every town we serve. National averages (HomeGuide, 2026), not our prices: a pressure switch $150–$400, a limit switch $100–$375, a control board $300–$650. More in the furnace repair cost guide.

More detail on our furnace repair page.
Symptom guides: Blowing cold air · Won't ignite · Short cycling · Blinking light codes · Pressure switch trips · Strange noises · Leaking water · Burning smell · Red tag · Carbon monoxide
Sometimes, but in older houses the switch is usually doing its job. A converted gravity system or a single central return can starve the furnace of air, and a packed filter or closed registers make it worse, so the heat exchanger overheats and the limit opens. We measure temperature rise and static pressure. If airflow is the problem, a larger filter, another return or a blower speed change fixes it; a new switch alone won't.
Most calls come down to a few parts: a circulator pump that hums without moving water, a zone valve stuck shut, air trapped in the upstairs radiators, or an expansion tank that has lost its air charge, which makes the relief valve drip. The boiler's own ignition and controls fail much like a furnace's. We bleed, test and price the repair in writing. Never cap a dripping relief valve.
Signs of a cracked furnace heat exchanger, how a technician confirms one, what to ask for if your furnace is red-tagged, and when to repair or replace it.
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