Heat Alarms Where a Smoke Alarm Would Lie

Why this matters

Put a smoke alarm in a garage and it lies: car exhaust, sawdust, a cold January morning or a 115 F August afternoon set it off, and within a month someone takes it down. Leave the garage with nothing and the fire most likely to start there, a charging battery, a gas can, a workbench, has a head start on a family asleep on the other side of the house. A heat alarm is the device built for the spaces where a smoke alarm cannot be trusted. It is also the device most often misunderstood: put one where a smoke alarm is required and the house has quietly lost minutes of warning. This card covers what a heat alarm senses, where it earns its place, and the one thing it must never be used for.

What a heat alarm senses

A heat alarm responds to temperature at the ceiling, not to particles. Two methods, often in one unit:

Fixed temperature. The alarm sounds when the air at its sensor reaches a set temperature. Household units are commonly rated around 135 F; higher ratings exist for spaces that run hot. The rating is on the label and the data sheet, and it is the number that matters.

Rate of rise. The alarm sounds when the temperature climbs faster than a set rate, commonly about 15 F per minute, whatever the starting temperature. A fast fire in a cold garage can trip rate of rise well before the air reaches the fixed rating; a slow-heating space never trips it.

Single- and multiple-station heat alarms are listed to UL 539. A unit listed for interconnection with smoke alarms can sit on the same interconnect line, wired or wireless, and sound every alarm in the house when it trips.

Why it ignores what fools a smoke alarm

Exhaust, dust, steam, cooking smoke, aerosol sprays, insects: none of them raises the ceiling temperature by much, so none of them sets off a heat alarm. Temperature swings that move slowly, such as a garage warming through a summer day, do not trip rate of rise. A heat alarm is also commonly listed for a wider ambient temperature range than a household smoke alarm, which typically lists for a living-space range; the data sheet states both, and an unheated garage or attic can be outside a smoke alarm's.

That tolerance is exactly its weakness. A heat alarm waits until a fire has produced enough heat to warm the ceiling air, which means the fire is already flaming and growing. A smoldering fire, the kind that fills a bedroom with smoke for a long time before it flames, may not trip a heat alarm until the room is untenable.

The rule that does not bend: never a substitute

A heat alarm never takes the place of a required smoke alarm. The 2021 IRC, R314, as adopted, requires smoke alarms listed to UL 217 in each sleeping room, outside each sleeping area and on each story. A heat alarm does not meet any of those requirements, and putting one in a hallway because the smoke alarm there kept going off at breakfast has removed the early warning from the escape route. The nuisance has a fix on its own card (see related: Smoke Alarm Distances From Cooking Appliances and Bathrooms). The heat alarm is an addition, in a place no smoke alarm is required.

Some jurisdictions go further and require one. The 2021 IRC itself does not require heat alarms. Some states and cities amend it to require a heat alarm in an attached garage, or in new garages, interconnected with the house's smoke alarms. Where yours does, it is in your adopted code or local amendments, and the inspector will look for it. Elsewhere it is a recommendation you make.

Where a heat alarm earns its place

• Attached garage. The usual location. A fire in an attached garage can reach the house through the separation (see related: The Garage to House Fire Separation), and an interconnected heat alarm in the garage sounds in the bedrooms before it does.
• Unfinished attic, especially one with equipment. An air handler, a bath fan motor or old wiring in an attic is a fire source nobody is watching. Dust and summer heat rule out a smoke alarm.
• Utility, furnace or workshop rooms where dust or combustion products make a smoke alarm nuisance, as an addition to the smoke alarm required on that story.
• A kitchen, in addition to the smoke alarms nearby, never in place of them.

Not a heat-alarm location: anywhere people sleep, the hallway outside bedrooms, or any location R314 requires a smoke alarm.

Placement that keeps it honest

Mount it on the ceiling, where heat collects, following the maker's spacing and its distance from walls and peaks, and NFPA 72 in the edition your jurisdiction adopts. Keep it away from things that heat the ceiling without a fire: a garage door opener motor, a light fixture, a heater discharge, a skylight in direct sun. Do not mount it where the space routinely runs within a few degrees of the fixed rating; that is a nuisance waiting for the hottest day of the year.

Worked example: an attached garage and an attic

A homeowner asks an electrician, there for a panel upgrade, to add "fire protection" to the attached garage and the attic, where the air handler lives. The house has five interconnected, hardwired smoke alarms from one maker's family, all within date.

First check: the space temperatures. The homeowner has a logging thermometer and lends it out for a hot week before the job. Peak at the garage ceiling: 121 F. Peak in the attic near the air handler: 148 F. These are this house's readings, not general figures.

Garage. A heat alarm with a 135 F fixed rating has 135 - 121 = 14 F of margin above the hottest reading. That is enough room on this evidence, and the rate-of-rise element adds response to a fast fire. Mount it in the center of the garage ceiling, away from the opener motor, and on the interconnect so it sounds the whole house. A smoke alarm here would have faced car exhaust daily and temperatures that may sit outside its listed range.

Attic. At a 148 F peak, a 135 F unit would sit 148 - 135 = 13 F below the reading and alarm on a hot afternoon. A higher-rated unit is needed, and the family's compatibility list has to include one that can sit on the interconnect. The electrician checks: the maker of the existing smoke alarms lists a heat alarm only in the 135 F rating. So the choice is a higher-rated heat alarm from a family that cannot interconnect with the house (a stand-alone alarm the bedrooms will not hear), or a hardwired relay or a system detector arrangement that the maker supports. The electrician quotes the garage unit and writes up the attic as needing a design decision, rather than hanging a 135 F unit in a 148 F attic and leaving.

Interconnection check. After the garage unit is installed, he presses its test button: all five smoke alarms and the heat alarm sound, six devices. Because the interconnect was opened to land the new unit, he then presses the test on each of the five smoke alarms in turn: all six sound every time. Every unit can send and receive.

The note. Garage heat alarm installed, 135 F fixed and rate of rise, listed to UL 539, interconnected with five smoke alarms, tested from each of the six units, manufacture date recorded. Attic not done: peak 148 F exceeds the only interconnect-compatible rating; options given to the customer in writing.

What changes the answer.

• The garage had peaked at 132 F. Margin 135 - 132 = 3 F. Too close; the first heat wave would alarm the house. Higher rating, or a location in the garage that runs cooler, measured rather than assumed.
• The jurisdiction requires a garage heat alarm. It is no longer optional, it is a code item on the permit, and the location and interconnection follow the amendment.
• The customer wanted it in the hallway instead of the smoke alarm. No. That is the substitution this card rules out.

Explaining it to the customer

The question customers ask is why not just put a smoke alarm there, and the answer fits in a few lines:

CUSTOMER: Why not a smoke alarm in the garage?
TECH:     Car exhaust and dust would set it off, and
          you would end up taking it down.
CUSTOMER: So the heat one is better?
TECH:     For the garage, yes. It only goes off once
          a fire is putting out real heat, so it is
          later. That is fine out there because it is
          wired to the alarms by the bedrooms. It would
          not be fine in your hallway.

The last line is the one to keep. It stops the customer drawing the conclusion that a heat alarm is the cure for every nuisance alarm in the house.

The work itself

A hardwired heat alarm on the house's interconnect is electrician work, licensed where your state or locality licenses it: de-energize and lock out the circuit, and prove it dead on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with work practices at 29 CFR 1910.333(b)(2). Once the alarm is landed and mounted, remove the lock, turn the breaker back on, and confirm the new unit's power light is lit, the breaker holds without tripping, and the interconnect test from each unit passes before you leave. The attic adds its own hazards: heat stress on a summer afternoon, walking only on framing, and disturbing insulation, which releases fibers and dust, so a respirator under a written program per 29 CFR 1910.134 (applied to construction work by 29 CFR 1926.103) where the insulation type or condition calls for one, and stop if it looks like vermiculite until it is identified. A listed wireless heat alarm in a family that matches the house's wireless alarms is a battery device and avoids the circuit work. Testing a heat alarm is done with its test button; never test one with a flame or a heat gun, which can damage the sensor and is a fire in the customer's garage.

References

• International Residential Code, 2021 edition as adopted by your jurisdiction: R314 (smoke alarms), and any local amendment requiring garage heat alarms
• UL 539 (single and multiple station heat alarms) and UL 217 (smoke alarms); the manufacturer's data sheet for rating, spacing and listed ambient range
• NFPA 72, in the edition your jurisdiction adopts, for supplemental heat alarms in dwellings
• 29 CFR 1910.134, Respiratory Protection, applied to construction work by 29 CFR 1926.103
• See related: The Garage to House Fire Separation; Smoke Alarm Sensing Types and Which Belongs Where in a House; Hardwired, Interconnected and Battery Backup: What Each Word Means