A2L refrigerant detection: why the blower runs and the compressor will not start
On R-454B equipment a leak sensor can run the indoor blower and refuse every heating and cooling demand. What the published mitigation sequence does, and what it does not prove.
What this means
A refrigerant detection system sits between the thermostat and the equipment, and it can act without the thermostat knowing. Allied Air documents the detected sequence as removing the 24 volt R input to the thermostat, which de-energizes the outdoor compressor and any gas or electric strip heat. The blower then runs at high speed until concentration falls, and for seven minutes after that. Rheem documents the same shape with a five minute minimum. A blower running on its own is the mitigation state, not a diagnosis of the leak.
Equipment and model context
Indoor sections carrying a refrigerant detection system for A2L refrigerant, documented here for the Allied Air HWC8, HWC9 and EWC kit and for Rheem (-)H1P and (-)H2T air handlers
- Allied Air MagicPak V-Series equipment fitted with the HWC8, HWC9 and EWC refrigerant detection kit
- Rheem (-)H1P and (-)H2T air handlers supplied for R-454B
The sequences and thresholds here are published for the two products named above. Detection boards differ between manufacturers and between model years, and the interval one board runs the blower for is not evidence about another. A unit with no refrigerant detection system fitted cannot enter any of these states, and a system charged with R-410A has no sensor to enter them with.
What you may notice
- The indoor blower starts by itself, runs at full speed, and does not stop when the thermostat is set to off.
- The thermostat screen has gone blank, lost its schedule, or reverted to a setpoint nobody chose.
- Neither heating nor cooling answers a demand, while the air moving through the registers is at room temperature.
Likely causes
- A sensor in the indoor cabinet has reported refrigerant, and the detection board has taken control of the blower and cut the thermostat supply.
- The detection board is in its fault state, which Allied Air documents as running the blower at constant output until the fault is cleared.
- The board is completing the five minute purge it runs at power-up, which happens after any interruption to the supply.
Diagnostic paths
The blower started within a few minutes of power being restored
Somebody reset a breaker, a power cut ended, or a service visit finished, and the fan came on shortly afterward with no demand behind it.
Allied Air describes an initializing mode in which the board establishes contact with its sensor and completes a five minute purge. Let that interval pass before treating the fan as a fault, and note the time it began so a technician can tell an initialization from a detection.
The blower runs continuously and the thermostat has gone dead
The fan will not stop from the thermostat, and the thermostat itself is blank or has lost the settings it held yesterday.
That combination matches the documented detected sequence, in which the board removes the 24 volt R feed the thermostat runs on. Allied Air lists the consequences in its own literature: heating lost on a cold night, cooling lost on a hot day, and a thermostat that comes back on a setpoint nobody selected. Leave the supply on, ventilate the space, keep flames and other ignition sources away from the equipment, and call for service.
The blower runs but heating and cooling still work between times
The system answers demands normally and the fan also runs at times no demand explains, with no loss of the thermostat.
Record when the fan runs and for how long. A completed detection leaves a trace at the board rather than at the thermostat, and Allied Air documents a monitoring indication that carries a blue flash once its sensor has previously detected refrigerant and run its sequence. Reading that indication means opening the equipment, which is service work.
| Board state | What the equipment does | What it establishes |
|---|---|---|
| Initializing | Five minute purge while the board finds its sensor | Only that the supply was interrupted recently |
| Refrigerant detected | Thermostat R feed removed, compressor and heat de-energized, blower at high speed | That the sensor reported refrigerant above the board threshold |
| Fault | Blower held at constant output until the fault is cleared | A problem in the detection circuit, not the presence of refrigerant |
| Monitoring after an earlier event | Normal operation, with the earlier sequence recorded at the board | That a detection happened and cleared, with no date attached |
Every demand from the thermostat passes through the detection board before it reaches the equipment, which is why the board can stop a system that has nothing wrong with its own controls.
- Sensor in the cabinetWatches the air stream inside the indoor section
- Detection boardHolds the thermostat supply and the blower output
- ThermostatLoses its 24 volt feed when the board acts
- Compressor and heatDe-energized together, including strip and gas heat
The threshold is set well below the flammable limit
Allied Air states the number its board acts on. The control activates the blower if concentration reaches 12 percent of the lower flammability limit, against a UL standard figure of 25 percent, and it also interrupts power to ignition sources in the system when it detects refrigerant. Those two figures are the reason a mitigation event is not the same thing as a dangerous room.
Reading it the other way round is what causes panic. The board acted at a small fraction of the concentration that would burn, and the sequence it ran exists to keep the fraction small. The correct response is to leave the mitigation running and get the leak found, not to evacuate the building or to pull the disconnect.
A continuous blower does not prove a leak
Two documented states produce the same visible behaviour. In the detected state the blower runs because the sensor reported refrigerant. In the fault state, which Allied Air describes as a problem within the board itself, the indoor blower engages and remains engaged at constant output until the fault is cleared. From the hallway those look identical.
The board separates them at its own indicator rather than at the thermostat. Allied Air publishes a multicolour LED inside the enclosure. Flashing blue marks a detection, and solid blue interrupted by a flash code marks a fault or a service condition. One of its red codes reports that the R input is missing rather than that refrigerant is present. None of that is visible without opening the cabinet.
So the honest position for an owner is that the system has entered a protective state, that two documented causes fit, and that telling them apart is a service task. Guessing between them changes nothing an owner can act on.
Why cutting power is the wrong instinct
The reflex when a fan will not stop is to pull the breaker. On this equipment that removes the mitigation as well as the noise. Allied Air puts a caution on the unit stating that a refrigerant detection system is installed and that the unit must be powered except for service. One of its own diagnostic codes exists for the case where the R input is not available.
Rheem documents the recovery in a way that also depends on the supply staying on. Its air handler literature says the system resets after it no longer detects refrigerant, with the blower held for a minimum of five minutes after that reset. A board without power cannot make that decision, and a system restored to power later starts again from its own initialization.
The reflex carries a second cost. Interrupting the supply discards the state the board was in, which is the one piece of evidence separating a detection from a fault. Leaving it alone keeps the answer available for the visit.
What the sizing tables mean for a room that trips more than once
Both manufacturers tie the sensor to a room-size calculation rather than treating it as an optional extra. Allied Air publishes a minimum conditioned area against system charge, listing 60 square feet at 4 pounds, 90 at 6, 120 at 8, and 150 at 10, with an altitude adjustment factor applied above roughly 2,600 feet. Rheem publishes an equivalent determination table relating total system charge to a minimum indoor airflow and a minimum conditioned room area, given separately for installations with and without a leak detection sensor.
Those tables are an installation input, not a diagnostic. They matter here because they explain why the sensor exists at all on a particular job, and why a repeat event in a small closet or a small conditioned room is worth raising with whoever commissioned the system.
What to give the technician
Give the technician the indoor and outdoor model numbers, the time the blower started, whether the supply had been interrupted in the hour before, whether the thermostat lost power or settings, and whether any heating or cooling demand has been answered since. That set is what separates an initialization from a detection and a detection from a board fault before anyone opens the cabinet.
Safe homeowner checks
- Note the time the blower started and whether anything interrupted the supply shortly before it, because a five minute purge at power-up looks the same from outside as a detection.
- Photograph the thermostat screen, including a blank one, and write down whether its schedule and setpoint survived.
- Open a window or a door to ventilate the space and keep flames, smoking materials, and other ignition sources away from the equipment and its ductwork.
- Leave the supply to the indoor unit switched on, and leave the equipment doors closed.
Is a reset appropriate?
Both manufacturers describe the state clearing on its own once refrigerant is no longer sensed, so there is no owner reset to perform and no benefit to cycling the breaker. Removing the supply deletes the board state that would have told a technician which of the documented conditions occurred.
When to call a technician
- Reading the detection board indicator, pressing its test and reset button, or clearing its state means working inside an energized cabinet and belongs to a qualified technician.
- Locating and repairing a leak in an A2L circuit requires certification and training specific to flammable refrigerant, and both manufacturers restrict this work to trained service personnel.
- Bypassing, disconnecting, or removing a refrigerant detection system to stop the blower defeats a safety function that the equipment was certified with.
Questions people ask about this
Is my house unsafe if the leak sensor tripped?
The documented threshold is well below the flammable limit. Allied Air states its board acts at 12 percent of the lower flammability limit against a UL figure of 25 percent, and that it also cuts power to ignition sources in the system. Ventilate the space, keep ignition sources away, leave the equipment powered, and arrange service.
Why did my thermostat go blank at the same time?
Allied Air documents the detected sequence as removing the 24 volt R input that feeds the thermostat. Its own literature lists the consequences, including heating lost on a cold night, cooling lost on a hot day, and a thermostat that returns on an incorrect setpoint.
Can I turn the breaker off to stop the fan?
That removes the mitigation with the noise. The equipment carries a caution stating it must be powered except for service, and cutting the supply also discards the board state that separates a detection from a board fault.
How long does the blower keep running?
It depends on the board. Allied Air continues the blower for a further seven minutes after concentration falls below its threshold, then returns the system to normal operation. Rheem states a minimum of five minutes after its detection system resets.
Does this happen on R-410A systems?
No. A refrigerant detection system is fitted because the refrigerant is classed A2L, so equipment charged with R-410A has no sensor and no mitigation board to enter these states.
Does a sensor have a replacement interval?
Rheem publishes a fifteen year life expectancy for its refrigerant detection system and says it should be replaced at end of life, directing the reader to its parts department. Treat that figure as belonging to Rheem equipment rather than to detection systems in general.
Evidence recordHow this page was checked
service manual · checked 2026-09-07
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Allied Air Enterprises and Rheem technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- service manual
- Scope of the definition
- Indoor sections carrying a refrigerant detection system for A2L refrigerant, documented here for the Allied Air HWC8, HWC9 and EWC kit and for Rheem (-)H1P and (-)H2T air handlers
- Last checked
- 2026-09-07