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Commercial · Commercial Furnace Repair & ReplacementCommercial Furnace Repair & Replacement
Commercial furnace repair and replacement covers the residential-style gas furnaces that heat plaza units, churches and small offices, the twinned pairs that share one duct system, and duct furnaces set into the ductwork. Most failures are the same parts found in a house furnace and fall in the same published repair band of $150–$800, but the cause is often commercial: raw outdoor air, a filter nobody owns, or a renovation that choked the return. Our rule is to repair when the heat exchanger is sound and the furnace is under 15 years old, and to replace when the exchanger is cracked or a major part fails on an older, hard-run unit.
Furnaces turn up in commercial space more often than owners expect. A plaza unit commonly has an upflow furnace in a back-room closet with a cooling coil on top, a church has two or more furnaces feeding the sanctuary and the hall, and a house converted to an office keeps the furnace it was built with. Larger rooms use twinned furnaces: two identical units on one plenum, tied together with a twinning kit so both blowers start and stop together. Duct furnaces are the third type, a gas heat exchanger with no blower of its own, mounted in the ductwork downstream of an air handler.
Run hours age a commercial furnace, not calendar years. A house furnace cycles around a family's day; a store furnace fights an entrance door that opens from morning to night, often with the fan set to run continuously for ventilation, so the blower motor, inducer and igniter see far more starts and hours. When deciding what to spend, treat a 12-year-old furnace in a busy retail unit like a much older one in a home.
Three causes show up on commercial calls and almost never in houses. The first is raw outdoor air ducted straight into the return: when the air reaching the furnace is colder than the minimum return-air temperature in the installation manual, flue gases condense inside a heat exchanger that was not built for it, and it rusts and cracks. The second is filter neglect, since nobody in a leased unit believes the filter is theirs, and a plugged filter overheats the exchanger until the high-limit switch trips again and again. The third is a renovation that added walls and closed doors without adding return air, which pushes the temperature rise past the range printed on the rating plate.
Warranty is the surprise on the replacement side. Many residential furnace warranties state shorter heat-exchanger and parts coverage, or different terms altogether, when the furnace is installed in a commercial or non-residential application, and the long registered coverage advertised to homeowners may not apply to a store or a church. The terms differ by brand and by model, so read the warranty certificate for the exact model number before assuming a part is covered, and keep the installation date and maintenance records. We tell you what the certificate says before quoting a repair.
The parts are the same ones a house furnace uses, so the published bands apply to a residential-style furnace in a commercial unit: flame sensor cleaning or replacement $150–$300; hot surface igniter $200–$450; pressure switch $250–$500; draft inducer motor $400–$900; control board $500–$1,200; gas valve $500–$1,000; blower motor $500–$1,500. The diagnostic visit is $90–$150 and is credited toward the repair if you go ahead. Twinned pairs, duct furnaces and anything reached by ladder or roof hatch are quoted in writing after we have seen them, and the full list of bands is on the pricing page.
Replacement brings one commercial complication: a gas furnace of this type sold in Canada today is a condensing unit at 95% AFUE or better. Swapping out an old mid-efficiency furnace means new plastic vent and intake pipes through a wall or the roof and a condensate drain, and in a leased unit the landlord has to approve the penetration. Heat that sits on the roof is a packaged rooftop unit, a different machine, and a suspended heater in a shop bay belongs on the commercial unit heater page. For parts-level detail see furnace repair, start at commercial HVAC for the overview, or call the shop if the building is cold now.
Equipment we install and service
A single upflow or horizontal gas furnace with a cooling coil and an outdoor condensing unit, the same equipment found in a house. They fail on igniters, flame sensors, pressure switches and inducer motors, and those are the parts we stock.
Two matched furnaces on a common supply plenum and return, wired through a twinning kit so the blowers run as one. When one blower runs alone, air short-circuits backward through the idle furnace. A failed twinning control or a mismatched replacement board is the usual reason, and a replacement furnace has to match its partner in model and blower set-up.
A gas-fired heat exchanger section mounted in the supply duct, relying on a separate air handler or fan for airflow. An airflow-proving switch stops the burner when the fan is off, so weak airflow from a slipping belt or a dirty coil upstream shows up here as limit trips.
Furnace closets in commercial units get used for storage, and boxes stacked against the furnace block combustion air and service access. On condensing furnaces the plastic vent, the intake and the condensate trap and drain cause many of the lockouts, especially where the drain runs through an unheated space and freezes.
What goes wrong in these buildings
- The furnace in the back room is nine years old and already on its second heat exchanger.
- One of the two furnaces in the hall runs and the other just blows cold air backward.
- It trips out by mid-morning on every cold day and comes back when someone flips the switch.
- The supplier says the warranty does not apply because the furnace is in a store, not a house.
- After we built the two new offices the furnace got loud and the front of the unit stopped heating.
- Nobody has changed the filter since we moved in because the lease does not say whose job it is.
How it works
We read the fault history on the control board, test the ignition sequence and the safeties, and then measure what a parts-changer skips: return-air temperature at the furnace, temperature rise across the heat exchanger against the rating plate, and static pressure in the ductwork. Those three readings say whether the failed part is the problem or the symptom.
You get the price in writing before work starts, with the warranty position on the part stated. Common ignition and airflow parts are carried on the truck, so many repairs on residential-style furnaces finish on the first visit; twinning controls and brand-specific boards may have to be ordered.
Outdoor air is tempered or cut back to what the furnace can take, return air is added where a renovation removed it, and the filter schedule is assigned to a named person. If the heat exchanger is cracked, a licensed gas technician has to shut the furnace off and tag it, and we quote a replacement that includes the vent route and the landlord approval it needs.
Common questions
Is it safe to keep resetting a commercial furnace that trips out?
Not for long. A furnace that trips on its high limit is overheating its heat exchanger on every cycle, and repeated overheating is how exchangers crack. One reset to get through a morning is reasonable; a daily reset is a fault that needs a technician. Shut it off and call right away if you smell gas, see soot or scorch marks around the burner door, or a carbon monoxide alarm sounds.
Does a residential furnace keep its warranty in a commercial building?
Often with shorter terms. Many manufacturers write separate, reduced coverage for commercial or non-residential installations, and some registered-homeowner benefits do not carry over at all. It depends on the brand, the model and the installation date, so the answer is on the warranty certificate, not in the brochure. Send us the model and serial number and we will check what coverage is left before you approve a repair.
Why does one of our twinned furnaces run without the other?
Twinned furnaces are meant to run their blowers together through a twinning kit. When one runs alone, the kit or one control board has usually failed, or a past repair fitted a board or motor that does not match its partner. Running that way pushes heated air backward through the idle furnace and starves the ductwork, so it should be repaired, not lived with.
Do you repair commercial furnaces after hours?
Yes. The emergency line at (647) 801-1252 is answered around the clock, with no overtime, weekend or holiday surcharge. Tell us the temperature inside, what the furnace is doing and any flashing code on the board. Triage for a cold building, including when to start protecting water pipes, is on the emergency commercial HVAC repair page.
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