Furnaces rarely die without warning. In the weeks before a breakdown they usually get louder, run in short bursts, leave one bedroom cold or start pushing your gas bill up for no obvious reason. Most people only notice in hindsight, on the first freezing night when the house will not warm up.
This guide walks through the symptoms our technicians hear about most each fall, what usually causes them, which ones are urgent, and what you can safely check yourself before you book a visit.
Short cycling: on and off every few minutes
A healthy furnace runs in steady cycles. If yours fires, runs for two or three minutes, shuts down and repeats, something is cutting the cycle short. The most common culprits are a dirty flame sensor, a clogged filter that lets the heat exchanger overheat, or a thermostat placed too close to a supply vent.
Short cycling wastes gas, wears out the igniter and blower motor, and is often the last stage before a no heat call. A dirty flame sensor is a quick cleaning on a service visit; ignoring it usually means a cold morning later.
Rooms that never keep up
If the main floor is comfortable but the upstairs or the room over the garage is always a few degrees behind, the furnace may be struggling to move enough air. Restricted filters, closed or blocked registers, leaky ductwork and a weak blower motor all show up this way.
Uneven heat can also mean the furnace was undersized for the house or for an addition that was built later. A technician can measure temperature rise and static pressure to tell the difference between a maintenance issue and a sizing issue.
Smells you should not ignore
A light dusty smell on the first cold day is normal; dust on the heat exchanger burns off within an hour. A smell that persists, or anything that resembles burning plastic, hot electrical wiring or rotten eggs, is different.
If you smell rotten eggs (natural gas), leave the house, do not touch light switches, and call your gas utility's emergency line from outside. Once it is safe, call a licensed gas technician for the repair.
Call (289) 801-7262Noises and what they usually mean
Furnaces make some noise, but new sounds are information. Each one points to a different part of the system, and catching it early is usually the difference between a small repair and a big one.
- Banging or a loud boom at startup: delayed ignition, often from dirty burners.
- Squealing: a worn blower belt or failing motor bearings.
- Rattling: a loose panel, a duct connection or, in older units, a cracked component.
- Clicking with no flame: an igniter or gas valve that is not doing its job.
Not sure it's a DIY fix?
Describe the symptom over the phone and a HeatNest technician will tell you what it likely is and what a repair involves.
Call (289) 801-7262Flame colour and carbon monoxide
A properly burning gas flame is mostly blue. A yellow or orange flame that flickers lazily usually means incomplete combustion, which can produce carbon monoxide. Soot around the burner area or on the cabinet is another sign.
Every home with a gas furnace should have working carbon monoxide alarms on each level. If an alarm sounds, get everyone outside and call emergency services. During a maintenance, a technician checks combustion and inspects the heat exchanger for cracks that could let combustion gases mix with your air.
Repair or replace?
Age, repair cost and efficiency drive the decision. A common rule of thumb: if the furnace is past 15 years and the repair costs more than a third of a new system, replacement deserves a serious look. A cracked heat exchanger on an older unit is usually the end of the road.
If the furnace is younger and has been maintained, most repairs are well worth doing. We give you both numbers in writing so you can decide without pressure.
What you can safely check yourself
Before you book, a few checks can save a visit. None of them require opening the burner compartment.
- Replace or clean the filter if it is grey or clogged.
- Confirm the thermostat is set to Heat and has fresh batteries.
- Check that the furnace power switch and breaker are on.
- Make sure supply and return vents are open and not blocked by furniture.
- Clear snow or debris from outdoor intake and exhaust pipes on high efficiency units.
Common questions
+How often should a furnace be serviced?
Once a year, ideally in early fall before the heating season starts. Annual service catches dirty sensors, worn parts and combustion problems early.
+Is short cycling dangerous?
It is not usually dangerous on its own, but it can be caused by overheating, which is a safety shutdown. It also wears parts quickly, so it should be checked soon.
+How long does a furnace maintenance take?
Most maintenance visits take about an hour, longer if the unit needs a deep cleaning or a part replaced.
+Can I clean the flame sensor myself?
Some handy homeowners do, but it involves cutting power and removing components inside the furnace. If you are not comfortable with that, it is a quick job for a technician.
+What is a normal furnace lifespan?
Roughly 15 to 20 years with regular maintenance. Units that skip service tend to need major repairs sooner.