Quick answer
A furnace that stops working after a renovation has usually been choked by dust or cut off by the trades. Drywall and sanding dust can plug a filter in days, then coat the flame sensor, burners, pressure-switch port and secondary heat exchanger. Check the breaker, furnace switch, thermostat wiring, taped registers and blocked returns first. Manufacturers exclude construction-dust damage from warranty, so have it cleaned before another heating season.
If your furnace stopped working after a renovation, the timing is not a coincidence. A furnace is a large fan with a flame in it: for the whole job it has been pulling in the air of a construction site, and it has been worked around by trades who each needed something switched off, taped over or moved. The fault is nearly always one of two kinds. Either something was left disconnected, or dust has reached parts it should never touch.
Start with what the trades may have left off, taped or unplugged
Check these before assuming the furnace is damaged. They cost nothing to fix and take ten minutes to rule out.
- Breaker and furnace switch. Electricians shut circuits off, and the furnace switch looks like a light switch on a joist or wall that has just been drywalled or painted around. Confirm both are on.
- Thermostat wiring. A thermostat taken off the wall for painting, or moved to a new wall, can go back with a wire on the wrong terminal or not fully seated. A drywall screw or finishing nail through the thermostat cable shorts the 24-volt circuit, the subject of why a furnace keeps blowing its control board fuse.
- Registers taped or covered. Painters and floor finishers mask supply registers and return grilles, and new flooring or cabinets sometimes bury a floor register for good. A furnace that cannot move air overheats and shuts down on its high-limit switch.
- Blocked returns. A return grille behind a new built-in, a sofa wall or stacked boxes of leftover tile starves the blower.
- Ducts disconnected. Framers and plumbers pull down a branch duct to get past it. Look along the basement ceiling for a run hanging open, or a new bulkhead with no duct inside it.
- Gas valve closed. If a range, fireplace or dryer was moved, the gas fitter may have closed the furnace's appliance valve. The handle sits in line with the pipe when it is open.
- The blower door. If someone opened the furnace to vacuum it, the door may not be pressing its safety switch.
Where drywall and sanding dust does its damage
Gypsum dust is finer than house dust, and it sets like plaster when it meets moisture. It moves through a furnace in a predictable order.
- Filter. A new one-inch filter can load solid after a few days of sanding. Airflow drops, the heat exchanger overheats and the limit switch cycles the burners off.
- Flame sensor. A white coating insulates the rod so the board cannot detect flame. The burners light for a few seconds and drop out.
- Burners and orifices. Dust in the burner ports gives uneven, lifting or noisy flames and a delayed, thumping ignition.
- Pressure-switch port. The small port and rubber tube that prove the inducer is drafting plug with dust paste, the switch will not close, and the furnace never tries to light.
- Secondary heat exchanger. On a high-efficiency furnace the finned coil above the blower catches what got past the filter. A blocked secondary is the costliest result of heating a house through a renovation.
- Condensate trap. On furnaces that take combustion air from the room, dust goes through the burners, meets the condensate and turns to sludge that blocks the drain.
- Blower wheel and motor. Caked blades and dust in the motor bearings shorten its life.
Construction dust and your furnace warranty
Most furnace manufacturers state in the installation manual that the furnace is not to be used as a construction heater, or allow it only under strict conditions such as sealed return air, filters in place and a full cleaning afterwards. Damage traced to construction dust is treated as a site condition and not a defect, so a parts claim for a plugged secondary heat exchanger or a failed blower can be refused on that basis. Keep the renovation dates and any cleaning invoice with the furnace paperwork. The wider list is in what voids a furnace warranty.
For the next project the rule is short. Furnace off and registers sealed during sanding, an inexpensive filter changed weekly while trades are working, and temporary heat from another source if the house must stay warm.
What a post-renovation furnace clean and combustion check covers
This is a longer visit than a standard tune-up, because the dust has to come out and not just be measured.
- Blower assembly pulled, wheel and housing cleaned.
- Flame sensor cleaned, burners removed and brushed, igniter inspected.
- Pressure-switch port and tubing cleared, condensate trap flushed.
- Secondary heat exchanger face inspected and cleaned where it can be reached.
- New filter fitted and checked for size and seal.
- Combustion analysis at the vent, gas pressure checked, temperature rise and duct static pressure measured against the rating plate.
- A walk of the registers, returns and ducts to find anything taped, buried or disconnected, plus a look at whether new walls have closed in the furnace room and cut off combustion air to a water heater or older furnace.
A standard furnace tune-up is $120–$250, a flame sensor cleaning or replacement is $150–$300, and blower compartment cleaning is listed at $75–$150 as an add-on. A heavy post-construction clean is quoted in writing once we have seen the furnace. If the ducts are full of drywall dust, duct cleaning at $300–$700 for a typical home is worth doing once, after the last coat of paint.
When a post-renovation furnace fault cannot wait
- Call now if you smell gas or exhaust, or a CO alarm has sounded since the work finished.
- Call now if the furnace bangs on ignition or the flames are yellow and lifting off the burners.
- Call before using the heat if the furnace room was framed in around a gas water heater or a mid-efficiency furnace with no grille or louvred door for combustion air.
- It can wait a day if the furnace runs but short-cycles. Fit a clean filter, untape every register and book a cleaning.
ZK Mechanical cleans and re-commissions furnaces after basement finishing, kitchen and whole-house renovations, with a written price before the work starts. A furnace that quit right after a routine filter swap, with no construction involved, is a separate fault covered in why a furnace stops after a filter change. Call (647) 801-1252 or request a visit for furnace repair in Burlington, Oakville, Milton, Hamilton and nearby.