Cracked Heat Exchangers in Rooftop Units: What GTA Property Managers Need to Know This Heating Season
It's the first cold week of the season. The rooftop units have been switched over to heat, and a technician on a fall start-up calls with bad news: the heat exchanger in one of your gas-fired units is cracked, and the unit has been shut off and tagged. Your first question is usually the same one: "It was heating fine yesterday. Why can't it keep running?"
This post covers what a heat exchanger does, why heat exchangers crack, what happens when a technician finds a cracked heat exchanger in a rooftop unit, and what you can do to make it less likely in your building.
What a Heat Exchanger Actually Does
In a gas-fired rooftop unit, the burners fire into a set of metal tubes or chambers called the heat exchanger. The hot combustion gases stay inside those tubes and go out through the flue. The building's air is blown across the outside of the tubes, picks up the heat, and goes into the ductwork.
That thin metal wall is the only thing between the products of combustion and the air your tenants breathe. While it's intact, the two never mix. Once it cracks, that separation can no longer be relied on.
Why Heat Exchangers Crack
Heat exchangers don't usually fail all at once. Several conditions wear them down over time:
Age and thermal cycling. Every heating cycle heats the metal and lets it cool again. Over thousands of cycles and many winters, that expansion and contraction fatigues the metal, especially at seams and bends.
Overheating from low airflow. Dirty filters, a slipping or broken belt, a weak blower motor or closed dampers mean less air across the heat exchanger. It runs hotter than it was designed to, and the high-limit switch trips over and over. Each overheat speeds up the wear.
Condensation and corrosion. If flue gases cool too much inside a heat exchanger that isn't built for condensing, moisture forms and slowly corrodes the metal from the inside. Short cycling, very cold return air and large amounts of outdoor air all make this more likely.
Burner and gas pressure problems. Over-firing, wrong manifold pressure or misaligned burners can push flame directly onto the metal and create hot spots that crack.
Why It's a Safety Issue, Not Just a Repair
The concern is carbon monoxide. CO is colourless and odourless, and you can't tell it's there without a detector. A crack can let combustion products into the supply air, or it can disturb the flame enough that the burners make more CO than they should.
A cracked heat exchanger isn't always leaking a measurable amount of CO. It may leak only when the blower is running, only when the metal is hot, or only once the crack has grown. Because nobody can predict when that will happen, a crack is treated as a hazard as soon as it's found, not after something goes wrong.
What Happens When a Technician Finds One
In Ontario, gas appliances fall under the Gaseous Fuels regulation (O. Reg. 212/01), which the TSSA enforces, and only TSSA-certified gas technicians can work on them. When a technician finds a gas appliance in an unsafe condition, the regulation spells out what they must do, and it depends on whether the condition is an immediate hazard:
Immediate hazard: the technician must shut off the gas right away, tell the gas utility, give the user a written notice describing the problem, and attach a notice to the unit. The unit can't be used again until it's repaired, and only a certified technician may remove the notice.
Not an immediate hazard: the technician must tell the gas utility, give the user a written notice and attach a notice to the unit. The problem then has to be fixed by a deadline of no more than 90 days, or the utility shuts off the gas.
Whether a cracked heat exchanger counts as an immediate hazard is the technician's call, based on what they find. When it does, the unit can't stay on because the space is cold or a tenant is unhappy. That's hard to hear in November, but the rule protects the people in your building, and it protects you too.
What you can do right away: ask whether other units can cover the affected area, whether temporary heat makes sense, and which repair or replacement options are available, with lead times.
How to Spot a Cracked Heat Exchanger in a Rooftop Unit
No single test finds every crack, so a good inspection uses several methods:
Visual inspection, including a camera or mirror where the tubes can be reached, looking for cracks, rust, scaling and soot.
Watching the burner flame. If the flame lifts, rolls or changes shape when the blower starts, air may be getting into the combustion side through an opening.
Combustion analysis. Measuring CO and oxygen in the flue gas, and comparing the readings with the blower off and on.
Checking the supply air for CO at the unit and in the space.
Looking for warning signs, such as scorch marks around the burner area, rollout switches that have tripped, or a history of high-limit trips.
Repair or Replace?
Once a crack is confirmed, there are usually two options: replace the heat exchanger, or replace the unit.
Check the warranty first. Heat exchangers often carry a much longer warranty than the rest of the unit, commonly 10 years or more compared with about a year for most other parts. Labour is usually not covered, and many warranties require proof of regular maintenance, so keep your service records.
Think about the labour. Replacing a heat exchanger usually means taking much of the unit apart. On some roofs it can also mean a crane lift. Parts can take a while to arrive, especially for older models.
Look at the unit's age and condition. If the unit is near the end of its life, has an aging compressor, or runs on an older refrigerant, the money may be better spent on a new unit than on fixing the old one.
How to Lower the Odds in Your Building
Do a fall start-up with combustion testing every year, before the heating season starts, not after the first complaint.
Keep the air moving. Change filters on schedule and check belts and blowers. Good airflow is the best protection a heat exchanger has.
Don't ignore repeated trips. High-limit or rollout switches that keep tripping are warning signs. Resetting them and moving on is how small problems turn into cracked heat exchangers.
Have CO alarms where they're required, and treat every alarm as real until it's been investigated.
Keep service records so trends, such as a unit that has tripped on high limit three winters in a row, get noticed before they turn into a failure.
If a CO alarm goes off or people in the space have headaches, dizziness or nausea that ease when they go outside, get everyone out and call 911 first. Then call your HVAC contractor.
Getting Ahead of Heating Season
Burban Air Systems services gas-fired rooftop units, make-up air units and boilers across the Greater Toronto Area. Our fall start-ups include combustion checks. If we find a problem, we explain it plainly and give you your options.
To book a heating-season inspection or ask about a maintenance agreement, call (416) 757-3271 or email contact@burban.ca.
