Running a Gas Furnace on a UPS: What Works and What Fails

When the power goes out in winter, a gas furnace stops even though the gas is still flowing. Everything that makes the burner safe and useful is electric: the control board, the draft inducer, the igniter, the flame sensor and the blower that moves heat through the ducts. A modest battery system can restore all of that, which makes a furnace one of the more worthwhile household appliances to back up. It is also one of the easiest to get wrong.

What is actually drawing power

A heating cycle in a modern induced-draft gas furnace runs in a fixed sequence. Knowing the sequence matters because the loads come on at different times, and your UPS has to handle the largest combination that overlaps.

1. Call for heat. The thermostat closes the 24 V circuit. The control board (a few watts to perhaps 15 W) starts the sequence.
2. Inducer prepurge. The draft inducer fan starts to clear the heat exchanger and prove venting through a pressure switch. Inducers typically draw on the order of 50 to 150 W.
3. Ignition. A hot surface igniter heats until it glows. Silicon carbide igniters commonly draw roughly 300 to 500 W for tens of seconds; newer silicon nitride igniters are generally lower. Older and some other designs use intermittent spark or pilot ignition instead, which draws little.
4. Flame proving. The gas valve opens, the burners light, and the board must detect flame within a few seconds or it shuts the gas off and retries, eventually locking out.
5. Blower on. After a heat-exchanger warm-up delay, the circulating blower starts. This is the big steady load: a PSC (permanent split capacitor) blower commonly draws roughly 300 to 700 W, while an ECM (variable speed) blower may draw from under 100 W at low speed to several hundred watts at high speed.

The igniter normally turns off before the blower starts, so the two biggest loads rarely overlap. Your peak demand is whichever is larger: inducer plus igniter, or inducer plus blower. A PSC blower also has a startup surge, typically a couple of times its running current for a fraction of a second.

The nameplate on the furnace lists full-load amps, which is a worst-case number. Multiplying it by 120 V will overstate the running load, sometimes by a lot. A plug-in wattmeter cannot be used on a hardwired unit, but a clamp meter on the furnace circuit during a heating cycle gives a realistic current figure. Our guide to measuring power draw covers the method.

Why the waveform matters more here than almost anywhere else

Most UPS guides say pure sine wave output is "preferred". For a gas furnace, treat it as mandatory.

Flame rectification

Most modern furnaces prove flame with a flame sensor rod. The control board applies an AC voltage to the rod; the flame conducts a tiny current, on the order of a few microamps, and because the burner surface is much larger than the rod, the flame conducts better in one direction than the other. The board looks for that rectified DC component, measured against ground. Anything that distorts the waveform or disturbs the ground reference can make a healthy flame read as "no flame". The result is a furnace that lights, runs for a few seconds, shuts off the gas, retries a few times, then locks out.

A stepped "simulated sine" output, with flat tops and abrupt transitions, is a common cause. So is a power source whose neutral is not bonded to ground. Many portable inverters and inverter generators have a floating neutral by design, and some UPS designs do not hold the same neutral-to-ground relationship on battery as on utility power. Behavior varies by model, so check the UPS documentation, and test before you need it.

Motors and controls

PSC blower and inducer motors run hotter and noisier on stepped waveforms. ECM motors contain their own electronics, often with power factor correction, and may refuse to start or fault on poor waveforms. Polarity matters too: furnace boards commonly check for reversed hot and neutral and will flag a fault. See pure sine vs simulated sine for the underlying differences.

Connecting the furnace: the legal and safe options

Most furnaces in the US are hardwired to a nearby service switch rather than plugged in. That is the first obstacle, and it is not one to solve with improvised cords.

Worked example: how long will it heat?

Take a mid-efficiency furnace with an ECM blower. Measured with a clamp meter during a cycle:

• Control board: 10 W
• Inducer: 80 W
• Hot surface igniter: 400 W for about 45 seconds
• ECM blower at heating speed: 250 W

Peak demand. Ignition phase: 10 + 80 + 400 = 490 W. Running phase: 10 + 80 + 250 = 340 W. The ignition phase is the peak, so the UPS must deliver at least 490 W with headroom. A 1500 VA / 1000 W pure sine unit runs at 49% during ignition: comfortable. A 750 W unit would work at 65%, but leaves less room if a PSC blower ever replaces the ECM.

Battery energy. Suppose the 1000 W unit has two 12 V 9 Ah batteries: 2 x 12 x 9 = 216 Wh nominal. At this discharge rate lead-acid delivers noticeably less than nominal, and the inverter loses some energy too. Using about 0.8 for rate derating and 0.88 for inverter efficiency:

The igniter adds little energy: 400 W for 45 seconds is about 5 Wh per start. Duty cycle is the real multiplier. On a moderately cold day the furnace might run 40% of the time, so the average load is about 0.4 x 340 = 136 W plus a few starts per hour. That stretches wall-clock heating to about 152 / 145 = roughly an hour. On a bitter night at 80% duty cycle, expect closer to 35 minutes.

Scaling up. To cover an eight-hour overnight outage at that 145 W average you need about 145 x 8 = 1,160 Wh delivered, or roughly 1,400 to 1,600 Wh of battery once losses and lead-acid derating are included. That is the territory of extended battery packs on a UPS that supports them, or a large lithium battery system. The runtime calculator can model your figures.

Choosing the hardware

• Waveform: pure sine wave, with no exceptions. Line-interactive is adequate; online double-conversion is excellent but costlier and runs warmer. See UPS types explained.
• Capacity: size against your peak (usually ignition, or a PSC blower start) and keep it under about 70% of the watt rating.
• Runtime expandability: if you want hours, buy a model that accepts external battery packs, or plan for a power station or generator on a transfer switch.
• Location: furnace rooms can be dusty and warm near the cabinet. Keep the UPS off the floor, away from the flue, and with clearance for ventilation.

Mistakes we see repeatedly

• Buying a simulated sine UPS because it was cheaper. It may run the blower and still fail to prove flame.
• Sizing from the blower alone. Forgetting the igniter phase undersizes the unit, which can trip an overload alarm at every ignition.
• Forgetting the condensate pump. High-efficiency furnaces shut down when the condensate float switch trips.
• Leaving the UPS on the floor of a basement prone to water. If the basement floods, the UPS is the last thing you want submerged.
• Assuming the generator will be fine. Many portable generators and inverter generators have a floating neutral, which causes the same flame-sense lockouts. A bonding plug or a transfer switch arrangement designed for it is often needed; ask an electrician, and see using a UPS with a generator.

When a UPS is the wrong tool

A desktop UPS is good at a specific job here: keeping heat going through short outages, and bridging the minutes until a generator starts. It is not an economical way to heat a house for a day. For multi-day outages, a properly installed standby or portable generator with a transfer switch, or a large home battery system, is the realistic answer. The UPS vs generator comparison walks through the trade-offs.