FieldBench

Lennox

SLP99V

Furnace · Signature · modulating · iComfort communicating

Lennox modulating communicating furnace. iComfort RS bus is the system, not an option. E105 is a first-class native code. Capacity is a percent, not W1/W2. The wall control names the same SureLight E2xx pressure / flame / limit family in English.

Not EL296V 24V two-stage. An iComfort stat on an EL296 is optional; on SLP99 it is the system. Do not convert this furnace to 24V as a ‘fix.’ Not SL280 (80% two-stage).

Walk this in Diagnose

This plate

Heat
Modulating valve 35–100% class
Bus
iComfort RS (i+ / i−)
Blower
Variable-speed serial ECM
IFC
Communicating SureLight — stat event list is primary

This board

iComfort IFC

  • Stat event list is primary. It names the E-code in English. Write the E-number anyway.
  • E105 = RS bus. Native here — not the ‘someone landed data wires’ story from EL296V.
  • E2xx pressure / limit / flame codes still mean the same physics as the EL296 SureLight table. The stat just names them.
  • Capacity on the display is a percent, not stage 1 / stage 2. Do not land a non-communicating stat on i+/i−.

Sequence

  1. Call. Stat requests %. Valve and inducer modulate. Blower CFM follows. There is no W1/W2 prove pair like EL296V.
  2. Prove. Inducer must hit the pressure band for that firing rate. E223-family on the stat is a band miss, not ‘swap the low PS and leave.’
  3. Ignition. Still HSI + flame rod. E207 / E240 / E270 still mean ignitor / µA / retries.

Faults on this IFC

  • E105

    RS communication

    lockout

    Device off the iComfort bus. Native, expected, first-class on SLP99.

    • i+/i− polarity, one communicating control, indoor and outdoor on the bus. Data, not 24 VAC.
    • Do not convert to 24V. Isolating devices one at a time finds the bad node.
  • E201

    Indoor blower communication

    lockout

    Serial blower dropped off the IFC.

    • Harness, motor power, then motor. This is a communicating VS — same family as EL296V E201, native here.
  • E223 / E225

    Pressure / inducer band

    lockout

    Modulating prove failed at that rate. Stat may say low/high PS even though the hardware is a band, not two EL296 discs.

    • Vent, trap, inducer, hose. 96% PVC physics.
    • Do not treat it as a two-stage W1/W2 swap.
  • E240 / E270

    Flame current / retry lockout

    lockout

    Weak µA or max retries with no flame current.

    • Watch a try. Rod, HSI, gas. Same SureLight ignition as EL296V.
  • E250

    Limit switch circuit open

    warn

    Limit / rollout string. Stat names it in English.

    • Heat CFM follows %. Filter, coil, rise. Rollout is still manual reset.
  • E207 / E290

    HSI / ignitor circuit

    lockout

    Ignitor open or trigger fault.

    • Ohms and 120 VAC during trial. Confirm on the SLP99 door card.

Workflows

E105 on SLP99

No heat, stat shows communication.

  1. This is native iComfort. Do not convert to 24V as a ‘fix.’ You lose modulation and you may lock the IFC.
  2. Isolate devices on the RS bus one at a time — stat, indoor, outdoor, damper module.
  3. Outdoor communicating coil/HP can take the bus down and look like a furnace IFC failure.
  4. Soft-disable (two horizontal bars) means the thermostat does not recognize a device. Finish commissioning; do not jump i+ to R.

It will not come off minimum %

Long run, cold rooms, stat shows a low percent forever.

  1. Capacity is %. There is no W2 to jump. If the stat is asking 35% the valve should be at 35%.
  2. If the stat is asking 80% and the furnace stays at minimum, look at pressure-band / limit cutback, not a second-stage solenoid.
  3. Do not land a 24V W on a random terminal to ‘force it.’

Test points

i+ / i−
Data, not 24 VAC. One communicating control.
Capacity
Percent on the stat / IFC. Not W1/W2.
Flame
µA per SLP99 card — same F-menu idea if the board exposes it.

Gotchas

  • SLP98V is the older modulating sibling — similar idea, confirm that IO manual.
  • EL296V with an iComfort stat bolted on is still a two-stage 24V furnace that happens to speak RS. SLP99 is not that.
  • Do not convert to 24V.

Related plates

  • lennox-el296v — Elite two-stage 96% VS. E105 is optional there (24V jobs are normal). Capacity is W1/W2, not %.
  • lennox-sl280 — Elite 80% two-stage. Metal vent. Not a modulating iComfort furnace.