For technicians

Field wiring reference

What to pull up when the diagram is burnt off the door. Outdoor unit, gas furnace, heat pump and air handler — with the faults that actually stop each one.

Confirm against the unit, and kill the power

These are the arrangements common across the trade, not any one manufacturer's diagram. Boards and colours vary; terminals and the order of operations do not. Discharge capacitors before you go near them — a run cap holds a charge long after the disconnect is pulled.

Outdoor unit

Contactor and dual run capacitor

Condenser wiring: contactor, dual run capacitor, compressor and fan Line voltage L1 and L2 land on the contactor line side. With the 24 volt coil energised from Y and C, T1 feeds the compressor common terminal and T2 feeds the compressor run terminal. T2 also feeds the C terminal of the dual run capacitor. The capacitor HERM terminal goes to the compressor start terminal and the FAN terminal goes to the condenser fan start winding, normally the brown wire. LINE 240V L1L2 CONTACTOR T1T2 24V COIL YC DUAL RUN CAP e.g. 45 + 5 MFD C HERM FAN COMPRESSOR single phase C S R T1 → Common T2 → Run HERM → Start FAN condenser FAN → brown other lead returns to line
C comes off contactor T2. HERM goes to the compressor start terminal, FAN to the fan's start winding — normally brown. On a "45 + 5" the first number is the compressor side.

Hard start kit: potential relay and start capacitor

Hard start kit: potential relay and start capacitor A potential relay has three terminals. The coil sits across terminals 2 and 5. The normally closed contacts are between 1 and 2. Terminal 1 goes to the start capacitor, terminal 2 goes to the compressor start winding alongside the run capacitor HERM lead, and terminal 5 goes to compressor common. As the motor reaches about seventy five per cent of speed the back EMF across the coil opens the 1 to 2 contacts and drops the start capacitor out of the circuit. POTENTIAL RELAY 1 2 5 N/C contacts 1 – 2 COIL 2 – 5 START CAP high MFD, momentary only 1 → start cap COMPRESSOR C S R 2 → start winding (with HERM) 5 → compressor common At roughly 75% speed the back EMF across the coil opens contacts 1–2 and the start cap drops out. A start cap left in circuit will not survive long.
Coil across 2 and 5, normally closed contacts between 1 and 2. Terminal 1 to the start cap, 2 to the start winding alongside HERM, 5 to compressor common. Back EMF opens 1–2 at roughly 75% of speed and drops the start cap out — a start cap left in circuit does not last.

Compressor terminals

Compressor terminal identification and resistance check The three compressor terminals are common, start and run. Measured winding to winding, common to start plus common to run should equal start to run. The highest reading of the three is always between start and run, and the terminal not involved in that reading is common. Any winding reading to the shell means the compressor is grounded and is scrap. COMPRESSOR terminal box C S R ON THE METER C to S + C to R = S to R The HIGHEST of the three readings is always S to R. The terminal left out of that reading is COMMON. Start winding reads higher than run. Any winding to the shell = grounded = scrap.
The highest of the three readings is always start to run, and the terminal left out of it is common. Any winding reading to the shell means it is grounded.

Gas furnace

Integrated control board

Representative integrated furnace control board A typical integrated furnace control. Line voltage connections on the left: L1 and neutral in, plus switched 120 volt outputs for the humidifier and electronic air cleaner. Blower speed taps on the right for heat, cool, fan and an unused park terminal. Low voltage thermostat terminals along the bottom. Component connections around the board for inducer, pressure switch, the limit string, igniter, flame sensor and gas valve. INTEGRATED FURNACE CONTROL layout varies by make — the groups do not LINE 120V L1NEUTRALHUMEAC HUM and EAC are switched 120V, live only while the blower runs BLOWER TAPS HEATCOOLFANPARK PARK holds an unused speed, it is not powered LOW VOLTAGE — THERMOSTAT RWYGC INDUCERPRESSURE SWLIMIT STRING IGNITERFLAME SENSEGAS VALVE
HUM and EAC are switched 120V and live only while the blower runs — worth knowing before chasing a "dead" humidifier. PARK is a landing spot for an unused speed tap and is not powered.

The safety string

Furnace safety string wired in series The safety devices are wired in series, so any one of them opening breaks the whole circuit and the furnace stops. Because they are in series, the board cannot tell you which one opened. Measure across each device in turn: the one with voltage across it is the one that is open. ALL IN SERIES — any one opens, everything stops DOOR SWITCHROLLOUTHIGH LIMITAUX LIMITPRESSURE SW The board sees one open circuit, not which device opened it. Measure ACROSS each one in turn — the open device is the one with voltage across it. The closed ones read zero.
All in series, so the board sees one open circuit and cannot tell you which device opened it. Measure across each in turn — the open one has voltage across it, the closed ones read zero.

Flame rectification

Flame rectification circuit and expected signal The board sends an alternating voltage down the flame rod. A burning flame conducts in one direction only, rectifying it to a small direct current of a few microamps which returns through the burner ground to the board. The board reads that current as proof of flame. A typical healthy signal is two to six microamps; many boards drop out below about half a microamp to one microamp, and the exact threshold is the manufacturer’s to state. CONTROL BOARD sends AC out, reads DC back flame rod lead FLAME ROD BURNER burner ground — the return path EXPECT 2 – 6 µA healthy drops out under ~0.5–1 A weak signal is usually a dirty rod, a cracked porcelain insulator, or a bad burner ground — the return path is half the circuit and gets blamed far less often than it deserves. Clean with a non-abrasive pad, never sandpaper.
A healthy signal is typically 2–6 µA; many boards drop out below about 0.5–1 µA, and the exact threshold is the manufacturer's to state. The burner ground is half the circuit and gets blamed far less often than it deserves.

Sequence of operation

Knowing the sequence is ordinary. Knowing what it means when it stops at step five is the job.

  1. 1

    Call for heat

    R to W closes at the thermostat.

    Nothing at all? Check 24V across R and C at the board before you touch the furnace.

  2. 2

    Inducer starts

    The board energises the draft inducer and begins pre-purge.

    Inducer silent: no 24V to it, failed motor, seized wheel, or the board has locked out.

  3. 3

    Pressure switch proves

    Draft pulls the switch closed, telling the board the flue is clear.

    Will not prove: blocked flue or intake, split or kinked hose, water in the trap, cracked inducer housing. The switch is usually the messenger, not the fault.

  4. 4

    Igniter warms up

    Hot surface igniter glows for roughly 30 to 45 seconds.

    No glow: open igniter (check resistance), no output from the board, or the limit string is open.

  5. 5

    Gas valve opens

    The board energises the valve and the trial for ignition begins.

    No gas: valve not energised, gas off, or the valve has failed. Confirm 24V at the valve before condemning it.

  6. 6

    Flame proven

    The flame rod passes a few microamps back to the board within the trial period.

    Lights then drops out: dirty rod, cracked insulator, weak signal, or a bad burner ground. The classic repeat no-heat.

  7. 7

    Blower starts

    After the heat-on delay the blower runs on the HEAT tap.

    No blower: failed motor or capacitor, open limit, or the wrong tap landed at install.

  8. 8

    Satisfied

    Gas valve closes, inducer post-purges, blower runs its off-delay and stops.

    Blower never stops: shorted G, stuck relay, or a limit open and calling for cool-down.

Heat pump

O versus B

Terminal Valve is energised Manufacturers
O Energised in COOLING Carrier, Bryant, Rheem, Ruud, Goodman, Amana
B Energised in HEATING Trane, American Standard

Set the thermostat's O/B configuration to match the equipment, not the colour of the wire somebody landed.

Defrost control

Heat pump defrost control A defrost board watches two things: elapsed run time since the last defrost, set by a jumper at thirty, sixty or ninety minutes, and the coil temperature read by a sensor clamped to the outdoor coil. Defrost begins only when both agree. In defrost the reversing valve switches to cooling, the outdoor fan stops, and auxiliary heat is brought on to temper the air. Defrost ends on coil temperature, or on the fail-safe timer if the sensor never satisfies. DEFROST BOARD TIME jumper 30 / 60 / 90 min COIL SENSOR clamped to the coil BOTH must agree starts defrost DURING DEFROST • reversing valve switches to COOLING • outdoor fan STOPS • aux heat comes ON ENDS ON • coil temperature satisfied, or • the fail-safe timer, if the sensor never does Steaming outdoor unit and warm air off the coil is NORMAL in defrost. A unit that defrosts every ten minutes is usually low on charge or has a sensor reading wrong — not a board fault.
Time and temperature must both agree before a defrost starts. A steaming outdoor unit is normal. Defrosting every ten minutes usually means low charge or a sensor reading wrong — not a board fault.

Auxiliary versus emergency heat

Heat pump auxiliary and emergency heat Auxiliary heat runs alongside the compressor when the heat pump cannot keep up, and the thermostat brings it in on W or W2. Emergency heat is different: it is a manual mode that locks the compressor out entirely and runs the electric heat or furnace on its own. A customer left in emergency heat all winter will have a very large bill, which is the single most common reason for a complaint that nothing is broken. AUXILIARY HEAT Compressor KEEPS RUNNING Thermostat adds it on W / W2 when the heat pump cannot hold setpoint, or during defrost to temper the supply air. Normal. Expected. Not a fault. EMERGENCY HEAT Compressor LOCKED OUT A manual mode the customer selects. The heat pump does nothing at all; the strips or furnace carry the whole load. Left on all winter = enormous bill.
Auxiliary runs with the compressor and is normal. Emergency locks the compressor out entirely. A customer left in emergency heat all winter is the most common "nothing is broken but my bill is enormous" call.

Air handler

PSC and X13 blower motors

PSC and X13 blower motor wiring compared On the left a PSC motor: line voltage plus a run capacitor, with several speed tap leads of which exactly one is landed on the board heat or cool tap and the rest parked. On the right an X13 constant torque motor: a five pin power connector carrying line voltage on pins four and five, and a sixteen pin control connector where the board energises one of taps one to five with twenty four volts. PSC — permanent split capacitor MOTOR RUN CAP HIGHMED-HIMED-LOLOW one tap landed, the rest parked X13 — constant torque ECM MOTOR 5-PIN POWER line on 4 & 5 + ground 16-PIN CONTROL taps 1–5 and C board sends 24V to ONE no run capacitor — the electronics do that
A PSC runs on line voltage with a run capacitor and whichever tap was landed at install. An X13 is constant torque: the board energises exactly one of taps 1–5 with 24V, with line voltage arriving separately on pins 4 and 5. Tap colours vary — confirm against the unit.

Electric heat strips and sequencers

Electric heat strips with sequencers Each heat strip is protected by a fuse link and switched by a sequencer. A sequencer is a heater element warming a bimetal contact, so it closes slowly, around twenty to sixty seconds after it is energised, and opens slowly as well. Staging them deliberately avoids pulling the whole load at once. The slow close is why a strip appears not to work if you measure too early, and the slow open is why the blower must keep running after the call ends. W2 / aux call SEQUENCER 1bimetal, slowFUSE LINKHEAT STRIP 1,SEQUENCER 2bimetal, slowFUSE LINKHEAT STRIP 2,SEQUENCER 3bimetal, slowFUSE LINKHEAT STRIP 3 Sequencers close 20–60s after energising and open just as slowly — measure too early and a good strip looks dead.
A sequencer is a heater warming a bimetal, so it closes 20–60 seconds after being energised and opens just as slowly. Measure too early and a perfectly good strip looks dead.

Control transformer

Multi-tap control transformer A control transformer has a primary wound for several supply voltages and a twenty four volt secondary. Only one primary tap is used; the unused leads must be capped off safely. Landing the common lead on the wrong tap gives either a transformer that runs hot and fails, or twenty four volts that sags under load and causes faults that look like anything but a transformer. PRIMARY 208V240V277V COM SECONDARY 24V R C core ONE primary tap only. Cap the unused leads — wrong tap means a transformer that cooks, or 24V that sags under load.
One primary tap only, and cap the unused leads. The wrong tap gives either a transformer that cooks itself or 24V that sags under load and causes faults that look like anything but a transformer.

Need the model-specific diagram? Manufacturers publish installation manuals free against the model number — the numbers to ask are on our support directory.

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