Furnace Not Heating: Heat Exchanger and Safety Considerations

When a furnace quits producing heat on a cold night, the instinct is to blame a thermostat or a tripped breaker. Those cause plenty of service calls, but the more consequential failures often sit deeper inside the cabinet. The heat exchanger, an unassuming metal chamber that keeps flames separated from breathing air, is the heart of the safety conversation. When it fails, it is not just about comfort. It is about the risk of carbon monoxide mixing into your home’s air, and the judgment calls that follow.

I have spent enough late nights in basements and utility closets to see the pattern: a house goes cold, the family discovers a “furnace not heating” situation, and a technician faces a choice between coaxing life out of old steel or recommending a electric heater not working replacement. Understanding how the heat exchanger works, how it fails, and what your options look like is the difference between fast fixes and sound decisions.

The heat exchanger’s job and why it matters

Every gas furnace burns fuel to make heat, but it cannot let flame gases mix with the air you breathe. The heat exchanger is the barrier. Combustion happens on one best hvac in richmond ky side, your house air passes over the other side, and heat transfers through the metal. That separation is the entire safety premise of a forced-air furnace. The blower moves room air across the exchanger’s outer surface, picks up heat, and circulates it through the ducts. Meanwhile, the inducer motor pulls exhaust through the exchanger’s internal passages and out the flue.

Modern high-efficiency furnaces often use two heat exchangers: a primary exchanger of heavier steel for the initial burn, and a secondary, usually a condensing coil of stainless or coated metal, to harvest extra heat from water vapor in the exhaust. Older 80 percent furnaces use a single primary exchanger. Different designs fail in different ways. Secondary exchangers plug with condensate sludge. Primaries develop cracks at bends, welds, or where inshot burners blast heat.

Why does it matter? Because even hairline cracks can open under heat and negative pressure, letting flue gases bleed into the supply air. Carbon monoxide is colorless and odorless, and at the levels involved in small leaks it often makes people woozy rather than triggering an immediate crisis. I have seen houses where morning headaches disappeared after the furnace was condemned and replaced.

Telltale signs of a compromised heat exchanger

A crack is hvac lifespan comparison not the only reason for a “heater not working” call, but a few symptoms push me to inspect the exchanger right away. Flame behavior is the first. If the burners light, then roll out toward the front of the cabinet when the blower starts, something is diverting the flame. Sometimes it is a blocked heat exchanger or a cross-leak pulling air the wrong way. On sealed-combustion models, flame distortion when the blower engages can be one of the only visible clues.

Another clue is recurring trips of the rollout switch or high-limit switch. A blocked or cracked exchanger can overheat local metal, causing safety devices to cut power. On condensing furnaces, a rust-stained secondary drain, excessive white powder in the collector box, or persistent water inside the combustion chamber hints at a plugged secondary. Less obvious, but just as real, is soot inside the burner compartment. Properly tuned gas furnaces should not produce soot. If you see it, combustion is off, and that often ties back to exchanger restriction, venting, or gas mixture.

Some homeowners report the furnace will ignite, run briefly, then shut down repeatedly. That short-cycling can be caused by plenty of things, including airflow problems or a dirty flame sensor, but if the timing lines up with blower operation and you see flames wavering, your attention should shift to the exchanger. A CO alarm going off is obviously serious. If it sounds near a running furnace, air it out, shut the unit down, and call a pro.

How technicians verify the condition

There is no single perfect test. A credible inspection uses a few complementary methods. I start with a visual check using mirrors or a borescope. You need patience. Many exchangers hide cracks near bends or seams that only open when hot. I will often warm the unit to operating temperature and look again. Good borescopes catch what a flashlight will miss, but they also show false positives like surface rust or manufacturing seams. Experience helps tell the difference.

Pressure tests are more exact, but not always practical in a home. Some manufacturers allow a static pressure test on each cell by sealing burner inlets and applying a small pressure differential, then looking for loss. On condensing units, pulling the blower and inspecting the primary from the supply plenum side can reveal obvious breaches. Combustion analysis belongs in this mix. Elevated carbon monoxide in the flue, high excess air, or erratic O2 readings when the blower starts can suggest a breach or a restriction. It is not proof by itself, but combined with flame behavior it builds the case.

If I suspect the secondary heat exchanger is blocked, I check temperature rise across the furnace, static pressure in the ductwork, and the delta P across the heat exchanger cells if ports exist. When temperature rise is high despite clean filters and good blower speed, and the inducer amperage climbs, restriction becomes likely.

Safety controls that buy time, not immunity

Modern furnaces have layers of protection: pressure switches to verify draft, rollout switches near burners, a high-limit switch to prevent overheating, and in most models, a control board that will lock the system out after failed ignition attempts. These are critical, but they cannot guarantee safety if the exchanger is compromised. A pressure switch does not know whether exhaust is leaking into the air stream. A limit switch might stop a meltdown, but it does not stop low-level CO from blending with the supply air.

It is common for someone to say, “If it were dangerous, it would shut itself off.” That is not how these devices work. They watch operating conditions, not the integrity of a metal seam inside a chamber. If a professional documents a crack, most codes and manufacturer guidelines call for disabling the unit until repair or replacement.

When a crack means replacement

Homeowners often ask if a crack can be patched. The short answer, in residential HVAC, is no. The heat exchanger is a listed component. Cutting, welding, or applying sealants violates the furnace’s listing and usually the gas code. Manufacturers do not approve field repairs on heat exchangers. In practical terms, even if you could weld it, you risk warping, changing clearances, and inviting another failure nearby. If the furnace is under heat exchanger warranty, the manufacturer may supply a replacement exchanger. You still pay labor, which can run several hundred to over hvac maintenance service benefits a thousand dollars depending on access and model. When units are older, the labor and downtime push many owners toward full replacement.

Gray areas and judgment calls

Not every suspect exchanger is cracked. I have pulled blowers and found rust flakes that looked like holes but were superficial. I have also seen micro cracks that only revealed themselves with the burners roaring and the blower on high. If your technician presents photos, ask to see them hot if possible. If the call was after-hours and the photos are inconclusive, you might choose to run portable electric heat for a night and schedule a daylight inspection. That is not a comfortable option in a cold snap, but it is safer than guessing.

False alarms sometimes stem from flame disturbance caused by supply duct leaks or poorly sealed return air near the furnace. If the return pulls air from the burner compartment, the blower can distort the flame even with a healthy exchanger. Sealing duct leaks and correcting return paths can fix the flame issue and stop nuisance trips. That is why a solid diagnosis includes checking static pressure, duct integrity, and blower performance, not just peering at metal.

Why “furnace not heating” often starts upstream

A surprising number of heat exchanger failures trace back to airflow and venting, not just age. Restrictive filters, closed registers, undersized returns, or a blower misconfigured after a thermostat upgrade can drive temperature rise beyond spec. Metal expands and contracts more aggressively, welds fatigue, and seams split. Excessive temperature rise is silent, but the data plate on the furnace lists a target, often 35 to 65 degrees Fahrenheit depending on the model. When I measure 85 degrees of rise on a system designed for 50, I expect problems even if the house feels warm.

Venting matters too. On condensing furnaces, improper flue pitch traps condensate, and acidic water collects in the secondary exchanger. That sludge eats metal and blocks passages. On non-condensing units, a double-acting draft hood with a shared water heater vent can backdraft under certain conditions. That disturbs combustion, soots the exchanger, and accelerates corrosion. If a water heater was swapped after the furnace installation, the shared vent sizing might no longer meet code. Details like these separate a quick fix from a lasting solution.

The carbon monoxide problem in plain terms

People ask how serious the CO risk is from a cracked exchanger. The answer depends on the size and location of the breach, the blower speed, the venting, and the home’s leakage. In some cases, CO levels in the house never rise because the inducer’s negative pressure still wins. In others, CO drifts into the supply, and levels climb slowly. I have measured homes at 20 to 50 ppm in occupied areas from a compromised furnace, which is enough to cause headaches for sensitive occupants. Public health guidance varies by country, but a common threshold is that sustained levels above 9 ppm are undesirable, and above 35 ppm require action. It does not take a dramatic crack to pass those thresholds.

CO alarms are a last line of defense, not a diagnostic tool. Install one on each floor and near sleeping areas, ideally with a digital display. If it sounds, evacuate and ventilate. Do not reset the furnace and hope for the best. If the alarm does not sound but you suspect an issue, a technician with a combustion analyzer and a low-level CO monitor can provide better data.

What homeowners can safely check before calling

A few simple checks solve a large share of “furnace not heating” complaints. They also prevent the conditions that shorten an HVAC system lifespan.

Verify power and gas. Make sure the service switch is on, the breaker is not tripped, and the gas valve at the furnace is open parallel to the pipe. Replace the thermostat batteries if applicable.

Check airflow. Replace the filter if it looks loaded, open supply registers, and confirm return grilles are not blocked. Do not install the highest MERV filter on a system not designed for it. A moderately efficient filter changed regularly beats a restrictive filter left in all year.

Look and listen. When you call for heat, observe the ignition sequence. Inducer starts, pressure switch makes, igniter glows or sparks, gas valve opens, burners light, then the blower starts after a short delay. If the burners light and then go out when the blower starts, tell your technician. If the igniter glows but no flame appears, that points to gas supply or ignition rather than the exchanger.

These steps are safe. They do not involve opening combustion compartments or bypassing safeties. If anything seems off or you smell gas, stop and call a professional.

Repair versus replace when the heat exchanger fails

Once the exchanger is condemned, the choice is repair-by-replacement of the exchanger or replacement of the furnace. The right answer depends on age, efficiency, warranty, and broader system needs. If the unit is relatively young and the heat exchanger is under warranty, replacing the exchanger can be sensible. Labor can still be significant because that job often means removing the entire burner assembly, blower, and sometimes the coil case. On a 10 to 15 year old furnace, even with a warranty part, the labor cost and future risk tilt toward replacing the whole unit.

Consider the ductwork, flue, and coil. If your air conditioner is also old or “ac not cooling” has been a recurring issue, this is a chance to address both sides. Mismatched equipment undermines performance. A high-efficiency furnace might need a lined or PVC vent, a revised drain, and a different blower programming. If you upgrade the furnace but leave a leaky return, the benefits will disappoint. Replacing the coil and sealing ducts while the system is open costs less than doing it later.

Numbers help. A typical 80 percent furnace wastes 20 cents of each dollar of gas up the flue. A 95 percent model wastes 5 cents. In a cold climate burning $1,200 of gas annually, that difference can be a few hundred dollars a year. If the furnace is near the end of the typical 15 to 20 year HVAC system lifespan, investing in new equipment with modern safeties and efficiency pays off in both energy and peace of mind.

Maintenance that protects the exchanger

No part lasts forever, but you can extend life. Keep the temperature rise within the manufacturer’s range. That means clean filters, correct blower speeds, and open ducts. Measure or have measured the temperature at the return and supply during a heat call at least once a season. If you see it creeping above spec, address airflow.

On condensing furnaces, clean the condensate trap and hoses annually. That gunk that looks like coffee grounds will block passages if ignored. Confirm proper flue pitch so water drains back to the furnace where it belongs. Check the combustion air intake for lint and leaves. On sealed combustion units in laundry rooms, lint accumulation is real. Keep the area around the furnace clear. Storage can choke returns and prevent service.

Have a combustion analysis performed when the unit is serviced. A few minutes with a calibrated analyzer reveals mixture issues that the naked eye misses. A slightly overfired furnace runs hotter, which pushes the exchanger harder. If the gas pressure is off, adjust it. If the flame pattern looks lazy, check the orifices and burners for debris.

When the issue is not the furnace at all

I have been called to homes with “heater not working” only to find a perfectly fine furnace and a broken thermostat wire in the wall. I have also found return ducts disconnected in crawlspaces, sending most of the heat under the house. Those jobs remind me to take a whole-system view. A home with negative pressure from a strong kitchen hood can pull exhaust down a water heater flue, which trips the furnace’s rollout safety via ambient heat. A severely clogged A-coil sitting above the furnace restricts airflow to the point of overheating the heat exchanger. Even whole-house humidifiers, when set too high, can corrode sections in older furnaces and induce off-cycle condensation.

If your ducts are undersized or the home underwent a renovation that changed the load, a furnace that used to be adequate might now short cycle or overheat. These are the edge cases where a straight swap of the box is not good practice. A load calculation, some duct adjustments, and a staged or modulating furnace can transform comfort and longevity.

A note on heat pumps and dual-fuel setups

As heat pumps have become more common, some homes run a dual-fuel arrangement with a heat pump for most days and a gas furnace for colder snaps. If your furnace stands silent because the heat pump handles the load, a crack can go unnoticed until a deep cold day. Annual checks remain important even if the furnace runs fewer hours. Conversely, if your “ac not cooling” complaint leads a technician to find a frozen coil in summer, do not ignore the winter implications. Restricted airflow that freezes a coil also overheats a furnace heat exchanger in January.

What a responsible contractor will document

When a technician condemns a heat exchanger, they should be prepared to show you what they saw. Expect photos, preferably of the crack when hot, and notes on CO readings, temperature rise, and safety trips. The report should state whether the failure is in the primary or secondary exchanger and whether other defects contributed, such as a blocked coil or improper venting. If a warranty applies, they should help you navigate the claim. If replacement is advised, you should see a clear scope of work, including vent changes, drain routing, code upgrades like a new shutoff and drip leg, and any duct sealing or return modifications needed to keep the new exchanger within its temperature rise.

I have little patience for scare tactics. The risks are real and worth addressing without hyperbole. A cracked exchanger is not a place for wishful thinking, but it is also not a reason to accept a rushed, oversized furnace that short cycles for the next 15 years. Good contractors take time to right-size and to fix the root causes.

When to pull the plug immediately

There are a few red lines. If flames roll out of the burner area or trip the rollout switch, shut it down. If you or anyone in the home feels dizzy, nauseous, or develops a headache when the heat runs, shut it down and air out the house. If a CO alarm sounds, shut it down and call for help. If a technician shows a confirmed breach in the heat exchanger, do not run the furnace “just for the night.” Space heaters and extra blankets are inconvenient. They are also far safer.

The quiet rewards of getting it right

Nobody admires a heat exchanger when the furnace hums along in a closet, but when you keep airflow in check, venting correct, and combustion tuned, that steel rewards you with long service. I have serviced 20-year-old furnaces that still ran within their original temperature rise and combustion specs. The homeowners changed filters on time, kept return grilles open, and had the unit checked each fall. On the other hand, I have condemned five-year-old secondaries clogged by improper condensate management and filthy intakes. The difference was not luck. It was small habits and attention to details that rarely make it into brochures.

Treat “furnace not heating” as a prompt to look deeper, especially at the heat exchanger and the conditions that shape its life. The safety conversation is not an add-on. It is the center. When you and your contractor make decisions with that in mind, you avoid the worst risks and spend your money where it counts.

AirPro Heating & Cooling
Address: 102 Park Central Ct, Nicholasville, KY 40356
Phone: (859) 549-7341