When the furnace will not come on, find out whether power is reaching it

The first night cold enough to turn the heat on is when a lot of Lockport furnaces run for the first time since spring. Most of them start. The one that does not leaves somebody standing at the top of the basement stairs, unsure whether this is a job for the furnace company or for an electrician. The answer depends on where the power to the furnace stops, and most of that path can be seen without taking anything apart.

Why does a gas furnace stop when the power to it does?

A gas furnace burns gas, but everything that makes it burn runs on an ordinary 120-volt household circuit: the control board that decides when to start, the inducer fan, the igniter and the blower that pushes warm air through the ducts. With no power, the gas valve stays shut and nothing happens. A boiler works the same way, with its controls and the circulator pump on the circuit instead of a blower.

That circuit is where the line between the two trades falls. The breaker or fuse in the panel, the wire to the furnace and the switch beside it are electrical work, and when that side fails on a cold night it is a job for an emergency electrician in Lockport. Everything inside the cabinet belongs to the furnace company, and so do the thermostat, the filter, the igniter and the flame sensor.

Some of the older Fairmont foursquares and bungalows heat with a boiler rather than a furnace, and some still have a fuse box in the basement. The path is the same either way, from the panel through a switch to the equipment.

The furnace is silent. What can you check without opening it?

Four things outside the furnace can each cut its power, and none of them needs a tool or a cover removed. Look at them before calling anyone, and stop at the first one that is off. If there is water on the floor near the furnace or the panel, leave all of it alone and call.

• The service switch, which looks like a light switch and sits on the side of the furnace, on a joist next to it or at the top of the basement stairs. People flip it off by mistake while reaching for the stair light.
• The breaker for the furnace in the panel, sitting in the middle or the other way from its neighbors, or in an old fuse box, a plug fuse with a dark or broken window.
• On a high-efficiency furnace with a condensate pump, the outlet the pump plugs into. That small pump carries the furnace’s water to a drain. If its outlet is on a GFCI that tripped, the pump stops, and on many installs a safety switch wired from the pump to the furnace shuts the furnace off before the water can spill.
• The rest of the house. If the kitchen and the hall lights are dark too, it is a house or street outage, and the furnace is only one of the things that stopped.

A tripped breaker can be reset once, and so can the GFCI on the pump’s outlet. A blown furnace fuse gets one replacement of the same size, never larger. If the furnace starts and holds, keep an ear on it for the rest of the night. If it trips or blows again, leave it off. None of this involves the furnace’s own doors, switches or reset buttons.

What makes a furnace breaker trip again right after a reset?

A breaker that trips the moment it is reset, or as soon as the furnace tries to start, means something on that circuit is drawing more than the breaker allows, or there is a fault somewhere on it. On a furnace circuit that is usually the wiring between the panel and the furnace, a motor on its way out, or other things sharing the circuit with the furnace. Finding which one is the electrician’s half of the job.

When it trips tells the electrician a lot. Tripping instantly, before the furnace has done anything, points at a short, in the wire or in the furnace itself. Tripping a few seconds in, as the inducer or the blower starts, points at a motor. A blower that has worked through a dozen winters and then sat all summer can start hard enough on the first cold night to take the breaker with it. The electrician can confirm that the breaker and the wire are sound and that the extra draw is coming from inside the cabinet, so the furnace company starts with the motor.

In the 159th Street two-stories, the furnace is in the utility room that stayed unfinished when the rest of the basement became a family room, and the finishing sometimes borrowed power from whatever circuit was closest. A furnace that shares its breaker with the basement outlets can trip when the treadmill or the second fridge down there runs at the same moment as the blower. A furnace is supposed to have a circuit of its own, and giving it one is the fix, quoted once the circuit has been traced.

In the oldest Fairmont houses the furnace or boiler may sit on a plug fuse instead of a breaker. A furnace fuse that blows again after one replacement is the same signal as a breaker that trips again. Read the rating printed on the fuse before anything else goes in. A previous owner may have screwed a larger one into that socket to make the blowing stop, and then the wire to the furnace is protected for more current than it was sized to carry. That one goes to an electrician.

Tripping on other circuits in the house has its own set of causes, and why a breaker keeps tripping goes through them.

Is a dead furnace a night call, or can it wait until morning?

It depends on the weather and on what else is in the basement. A January night with no heat and a dead furnace circuit is a call now, at the after-hours rate. A mild October evening when the house will be fine under a sweater until morning is a 7 a.m. call at the daytime rate.

Deep cold makes it urgent for reasons beyond comfort. The furnace shares the basement with the water lines and the water heater, and a house that goes a whole cold night without heat can freeze a pipe along an outside wall. In a wet stretch, the sump in the same corner needs its power too, and a panel problem that has already taken the furnace is worth sorting the same night.

When it is clearly a night call, say on the phone what you already checked. A breaker that trips again points at the circuit. A furnace with its status light on points at the furnace company, and the electrician will say whether it is a tonight problem or a 7 a.m. one.

The daytime trip charge is $89 to $149 and covers the first half hour. Tracing a dead circuit usually comes to $150 to $250 for the first hour, with that trip charge included. Evenings, weekends and holidays, the trip charge is $199 to $299 plus an hourly premium. A repair, such as a new circuit for a furnace that has been sharing one, is quoted once the cause is found and before any of it is done. What an after-hours visit costs sets those numbers beside the rest of the rates.

Power is reaching the furnace and it still will not light. Whose job is that?

The furnace company. If the breaker is on, the service switch is on and the furnace shows signs of power, the electrical side has done its part. A status light blinking through the small window in the lower door, or a blower that runs and moves cold air, means power is there and the trouble is inside the cabinet.

If an electrician was the first call and the circuit turns out sound, that is still worth knowing. The furnace company starts with confirmed power, and if the visit found the furnace on a shared circuit or an old breaker, that can be put right while the electrician is there, quoted before the work.