Cracked heat exchanger: the field decision tree

First week of September, first night under ten degrees, and every start-up call lands at once. Across the flat roofs of Greater Montreal, gas heating sections that have not fired since April light off cold, with five months of thermal cycling behind them and a summer of condensation inside. This is the week of the year when a cracked heat exchanger shows itself — and when the question reaches the technician in its most uncomfortable form: do we condemn this unit today, in the middle of the return-to-office ramp-up, or do we let it heat? That call is not made on judgment. It is made at the end of a sequence.

What you need on the truck

• A calibrated combustion analyzer with a working CO cell and a current calibration certificate.
• A personal CO monitor worn on you, separate from the analyzer.
• A videoscope (4 to 6 mm probe) that captures images.
• A differential manometer for draft and manifold pressure.
• An inspection mirror, a strong light, a camera.

⚠ Safety: what stops the diagnosis before it starts

Three situations end the sequence before step one. A CO alarm sounding in the building: that calls for the carbon monoxide alarm procedure, not a diagnostic tree. A CO reading climbing at breathing height in an occupied space: evacuate, shut down, ventilate. Occupants reporting headaches or nausea that track the heating hours: that symptom outranks every measurement, because it describes an exposure already under way.

In every other case, you diagnose — in order, writing down each value before moving on.

Step 1 — Rule out what imitates a cracked heat exchanger

Half the appliances suspected of a crack do not have one. Four checks clear the usual impostors before the exchanger is on trial.

Venting first: a partly blocked flue, a bird’s nest, a snow-buried termination or a disconnected joint produces exactly the picture you expect — high CO, odour, soot. Then combustion air, routinely choked off after a building air-sealing project. Then the burner: fouling, poor adjustment, delayed ignition. Finally supply airflow, because a loaded filter or a slipping belt overheats the exchanger and distorts the flame with no crack anywhere.

These four points are ordinary gas heating and combustion work and take about twenty minutes. Skip them and you condemn an exchanger over a blocked flue.

Step 2 — Measure ambient CO before you open a single panel

This reading comes first because it is the only one that vanishes the moment you open the cabinet. Three points, with the unit firing for at least ten minutes: the return air, the supply air at the nearest diffuser, and the space itself at breathing height.

The normal value in supply air is zero. For context, Health Canada recommends indoor air stay below 25 ppm over one hour and 10 ppm over 24 hours, while Quebec’s occupational health and safety regulation sets a worker exposure limit of 35 ppm averaged over eight hours and 175 ppm over fifteen minutes. Those thresholds frame exposure; they do not define a healthy appliance. A unit delivering a steady 6 ppm into the supply duct sits comfortably under every regulatory limit and still tells you combustion products are reaching the building air.

Step 3 — Combustion analysis, blower off then blower on

This is the most revealing test in the sequence and the one most often botched. Sample the flue with the supply blower off, then again with the blower running, under the same firing rate.

On a sound exchanger, the two readings look alike. If starting the blower moves oxygen, drives CO up or pulls stack temperature down, distribution air is getting somewhere it should not be. The direction of the shift matters less than the fact of it: a healthy appliance does not change its combustion because a fan started. A properly tuned natural gas appliance runs at CO levels in the tens of ppm at most in the flue; a reading that jumps by hundreds of ppm when the blower energizes earns the next step immediately.

Step 4 — What does the flame actually tell you at blower start-up?

The flame disturbance test is the oldest one in the trade and the most misread. You watch the flame at the exact instant the blower energizes: distortion, lifting off the burner, rolling yellow, being drawn backward.

A flame that moves is a serious indicator. A flame that stays put proves nothing. Many cracks only open hot, twenty minutes into a cycle, and some sit where the pressure differential is too weak to disturb combustion at all. That is why this test lands fourth rather than first — it confirms, it never exonerates.

Step 5 — Look inside before you tear anything down

When the measurements point at the exchanger, a borescope inspection turns a hypothesis into a dated photograph. The probe goes in through an observation port or the flue sampling hole, with nothing disassembled, and hunts the places cracks appear first: the bends of the heating section, weld seams, low points where condensate collects, and any surface carrying a bright burn mark.

An image of a crack closes the file. A clean image does not: the probe sees only part of the exchanger, and absence of evidence is not evidence of absence.

Step 6 — Condemn, replace, or return to service under watch

The verdict is built on indicators that agree, not on how many boxes you ticked.

Shutting an appliance down is not a technician’s opinion. The natural gas and propane installation code CSA B149.1, enforced in Quebec by the Régie du bâtiment, governs how a non-compliant appliance is brought back into service — and a breached exchanger is not something you fix with a torch. An original replacement exchanger or a new appliance are the only two exits.

Field case: three rooftop units in Anjou, one condemned

A light industrial building in Anjou, three natural gas rooftop units of the same model and vintage. The complaint came with the first cool snap in September: a combustion odour in the front offices, one employee reporting late-afternoon headaches. The owner had already braced for replacing all three.

The sequence produced three different answers. On the first unit: a steady 4 ppm of CO in the supply air, a combustion analysis that clearly shifted when the blower started, and a crack photographed through the probe in the upper bend of the heating section. Condemned on the spot, gas closed, written notice to the manager within the hour. The second unit showed no CO in the supply air but weak draft and elevated flue CO — the actual cause was a vent termination half-blocked by roofing debris. Cleared, retested, back in service the same day. The third was simply a dirty burner.

One unit to replace instead of three, and two reports explaining precisely why the others stay in service. That distinction is what the manager carried into the capital planning meeting — not a recommendation, but readings.

What the first heating start-up of the season should leave behind

Schedule your gas appliance start-ups before the cold makes them indispensable: a unit condemned in September gets replaced, while the same unit condemned in January leaves a tenant without heat. On every appliance, require three numbers on the start-up report — CO at the return, CO in the supply air, CO in the flue — even when everything reads normal. It is the series of those numbers, season over season, that makes a drift visible long before it becomes a crack, and it is what moves this check out of emergency work and into a preventive maintenance program.

For the technician, this discipline pays off in a language the owner already speaks: a unit condemned with two measurements and a photograph does not get argued with at a budget meeting, and a unit cleared the same way avoids a full replacement that was never warranted. That is the standard of proof behind every replacement Montréal Combustion recommends — and it is what separates a diagnosis from an instinct.