FieldBench

Bryant

987M

Furnace · Evolution · modulating · communicating

Bryant Evolution modulating 98% furnace (factory twin of Carrier Infinity 59MN7). ABCD communicating bus. The wall control is part of the IFC. Capacity is a percent, not W1/W2. 31/42 is an inducer/pressure BAND.

Not 926T Preferred (24V + model plug, no ABCD). Not 986T / 987T Evolution two-stage (discrete 31/32, two-stage valve). A 926T will not run as Evolution by swapping the stat.

Walk this in Diagnose

This plate

Heat
Modulating gas valve · variable inducer · capacity is %
AFUE
Up to 98.5%
Bus
Evolution / Infinity ABCD (not InteliSense-on-926T, not ComfortBridge)
Blower
Evolution VS ECM — CFM follows the wall control
Sister SKU
Carrier 59MN7

This board

Evolution IFC + ABCD wall control

  • Read the Evolution thermostat Service / Last 10 events first — English names.
  • Board two-digit codes (13, 14, 24, 31, 33, 34, 41) still apply, but 31/42 is a pressure band, not 926T 32.
  • There are not two discrete pressure switches to ohm as low vs high the way 986T / 926T do.
  • ABCD polarity, one Evolution control. D is not W. C is required.

Sequence

  1. Call. Wall control requests a % capacity over ABCD. Valve and inducer modulate. No W1/W2 staging to watch.
  2. Prove. Variable inducer must hit the pressure band for that firing rate. 31/42 is a band miss.
  3. Ignition. Still HSI + flame rod. 34/14 still mean no prove.

Faults on this IFC

  • 24

    Secondary fuse

    warn

    24 V fuse. Evolution still has a 24 V secondary.

    • Short on HUM/EAC, valve, or 24V landed on ABCD.
  • 31 / 42

    Pressure / inducer band

    lockout

    Modulating inducer did not hit the band for the commanded rate.

    • Vent, trap, inducer, transducer/hose — do not ohm a ‘low’ and ‘high’ switch that are not there.
    • 42 = inducer RPM out of the commanded band.
  • 33 / 13

    Limit circuit

    warn

    Airflow or rollout. 13 is the hardened 33.

    • Evolution heat CFM, filter, coil. Rollout is manual reset.
  • 34 / 14

    Ignition

    lockout

    No flame prove.

    • HSI, gas, rod. Then the modulating valve offset — a valve stuck on minimum looks like 34.
  • 41

    Blower RPM

    lockout

    Evolution VS motor not at commanded CFM.

    • Static, wheel, harness. Do not swap a 926T model plug onto this IFC.
  • ABCD

    Communication

    lockout

    Wall control or outdoor dropped.

    • A/B/C/D, polarity, C present, no 24V on D.
    • One Evolution control only.
  • COMM

    No communication with indoor

    lockout

    Evolution wall control lost the IFC.

    • R to C. A/B polarity. D is not W.
    • 926T + this stat is the wrong bus.
  • 32

    32 is not this IFC

    info

    987M modulating has no discrete 32 low-fire switch. 31/42 is a pressure BAND. Two discrete switches = 986T. Model plug + 24V = 926T.

    • Count transducers/hoses.
    • Band miss: trap/PVC, then inducer RPM (42).
    • Do not order a low-fire switch for this cabinet.

Workflows

Evolution control blank, furnace idle

Homeowner says the thermostat died.

  1. 24 VAC at C and R on ABCD? Evolution still needs C.
  2. Do not land a 24V Preferred stat on ABCD. It will not modulate.
  3. Board two-digit if the stat is blank. Believe the wall-control text when it is alive.
  4. Do not ‘convert it to 24V’ by landing W on a random terminal.

31 / 42 on a 987M

Someone already ordered two pressure switches.

  1. This is a pressure BAND, not 926T/986T low vs high. Count the transducers before you buy switches.
  2. Trap and PVC first.
  3. Then inducer ramp. 42 is RPM, not a discrete high PS.

Test points

ABCD
OEM communicating. D is not W. C is required.
Flame sense
µA per this Evolution / 59MN7 door card.

Gotchas

  • 926T InteliSense is not Evolution. Different stat, different bus, different board.
  • 986T / 987T is Evolution two-stage — same ABCD plug, different valve and different 31 meaning.
  • Carrier 59MN7 is the factory twin.

Related plates

  • carrier-59mn7 — Carrier Infinity factory twin. Same modulating ABCD IFC.
  • bryant-986t — Evolution two-stage. Discrete 31/32, two-stage valve.
  • bryant-926t — Preferred 24V two-stage. No ABCD. Swapping the stat does not make it Evolution.
  • bryant-280anv — Evolution Greenspeed outdoor that belongs on this ABCD bus.