ElectricalToolbox

Will a Halloween Fog Machine Set Off a Smoke Alarm?

It can, and indoors it often does. A fog machine does not burn anything, so there is no real smoke and no fire. But the fog is a cloud of tiny fluid droplets, and a smoke alarm cannot tell those droplets apart from smoke particles. When enough fog drifts up to a detector, the alarm reads it as a fire and goes off. The thicker the fog and the closer it gets to the detector, the more likely the false alarm. Outdoors it is almost never an issue because the fog disperses; the problem shows up in a garage haunt, an indoor party, or a closed room where the fog builds up and reaches a ceiling detector. The fix is to run the fogger away from alarms and keep the air moving, never to pull the battery or bag the detector and forget it.

The alarm question is about where the fog goes, not about the circuit, but the fogger is also the heaviest single thing most people plug into a Halloween circuit. Total it against the rest of your display on the Continuous Load Calculator so a 1,000-watt heater does not quietly fill the circuit while you are worried about the ceiling.

The short version

  • Yes, dense fog can trip a smoke alarm. The detector reads the suspended fog droplets as smoke particles.
  • There is no actual fire. A fog machine heats fluid into vapor; it does not burn fuel, so the alarm is a false one.
  • Distance and density decide it. A light haze far from a detector is usually fine; a heavy effect under a ceiling alarm is the trouble.
  • Outdoors it rarely matters. The fog disperses before it reaches anything; this is an indoor and garage problem.
  • Never disable the alarm and walk away. A covered or unpowered detector is how a real fire goes unnoticed. Move the fogger instead.

Why the alarm goes off when nothing is burning

A home smoke alarm is a particle detector. The two common kinds work in different ways. A photoelectric alarm shines a small light beam inside a chamber and watches for particles that scatter the light into a sensor; a lot of smoke means a lot of scatter, so it alarms. An ionization alarm runs a tiny current across a chamber and watches for particles that disrupt that current. Neither one knows what a particle is made of. It only sees that the air is suddenly full of fine suspended matter, which is exactly what a fire produces and also exactly what a fog machine produces.

Fog fluid is mostly water with a little glycol or glycerin. The machine heats it until it flashes into a cloud of microscopic droplets, which is the fog you see. Those droplets hang in the air and drift on the room currents, and warm air carries them up toward the ceiling, which is where detectors live. Photoelectric alarms, the light-scatter kind, tend to be the most easily fooled by this because the droplets scatter light much the way smoke does. A dense enough fog can set off either type. The machine never caught fire; the detector simply did its job on the wrong cloud.

How to run a fogger without the false alarm

The whole game is keeping dense fog away from the detector and not letting it pile up. A few things that work, in order of how much they help:

  • Put distance between the fogger and the alarm. Aim the output low and away from the room with the detector, and keep the machine out of the same small space.
  • Keep the fog low. Low-lying fog, chilled over ice or run through a ground chiller, stays near the floor instead of rising to a ceiling detector. A fogger blasting upward is the one that trips alarms.
  • Ventilate. A cracked door or a window keeps the fog from building to the density that trips a detector. A sealed room is the worst case.
  • Run it in bursts. A timed fogger that puffs and rests lets the fog thin out between cycles rather than filling the room.
  • Favor water-based fluid and a lighter effect. You rarely need the room packed solid; a thinner haze reads the same in the dark and is far less likely to alarm.

What not to do is take the battery out of the smoke alarm, or wrap it in a bag, and then forget about it. People do this for one evening and leave it that way for weeks, and that is how a kitchen fire or a space-heater fire goes undetected in November. If you genuinely must cover a single detector for a few hours during a party, set a loud reminder and uncover it the moment the fog is done, and keep the rest of the home's alarms live. Your life safety system is worth more than the effect.

The other fog machine question: the circuit

While the alarm is about where the fog goes, the fogger is also an electrical load worth watching, because its heater is one of the biggest draws in a typical display. A fog machine pulls its power when the heater is energized, and you size the circuit for that full draw, not for the quiet moments between puffs. Read the nameplate for the wattage and divide by 120 to get amps.

A common 1,000-watt home fogger draws 1,000 divided by 120, about 8.3 amps. A big 1,500-watt unit draws 1,500 divided by 120, about 12.5 amps. That matters because a display left running all evening is a continuous load, and the rule of thumb for a continuous load is to stay under 80 percent of the circuit rating: 12 amps on a 15-amp circuit and 16 amps on a 20-amp circuit. So a single 1,000-watt fogger at 8.3 amps fits a 15-amp circuit with about 3.7 amps to spare for lights and props; a 1,500-watt fogger at 12.5 amps is already over the 12-amp ceiling of a 15-amp circuit all by itself and wants a 20-amp circuit or a circuit of its own. Add your fogger to everything else on the same circuit on the Continuous Load Calculator before Halloween night, not after the breaker trips.

Smoke alarm, not carbon monoxide alarm

One point that confuses people: the fog can trip a smoke alarm but it will not trip a carbon monoxide alarm, because those two detectors look for completely different things. A CO alarm senses carbon monoxide gas, which only comes from burning fuel, and a water-based fog machine burns nothing. If you are wondering about the CO detector, or you are thinking about running a generator to power a yard display, that is its own question with a real safety answer, worked through in do Halloween fog machines set off carbon monoxide detectors.

NEC reference

NEC 2020. Smoke alarms themselves are governed by NFPA 72 and the manufacturer's instructions, not by the NEC, so this article treats the detector as a life-safety device to keep working rather than a code calculation. The circuit side is the NEC's: a display running all evening is a continuous load, and under 210.20(A) the branch-circuit overcurrent device must be rated at least 125 percent of that load, which is the same as keeping the load under 80 percent of the breaker (12 amps on a 15-amp circuit, 16 on a 20-amp). Outdoor receptacles at a dwelling must be ground-fault protected under 210.8(A), and temporary holiday power is permitted for up to 90 days under Article 590 (590.3(B)). Converting watts to amps (watts equal volts times amps) is the power relationship, not a code figure.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

For the whole season

The Holiday Electrical Safety Handbook

This guide answers one question. The Handbook covers Halloween through New Year's: how many amps a circuit can really carry (a 15 amp breaker means 12 amps, a 20 means 16), extension cords, GFCI outlets, outdoor displays in the weather, the Thanksgiving kitchen, tree lights, space heaters, and what to do when something goes wrong. Every figure is pre-converted to amps, so you add instead of calculate.

Read on Kindle Paperback on Amazon Free circuit map and load worksheet

Get the next seasonal safety check by email

Optional. Short emails before each holiday, written for homeowners.

Related