Crankcase heater

For homeowners

A crankcase heater is a small electrical heater attached to the bottom of the outdoor unit’s compressor, where the oil sump is located. It keeps the oil warm — typically 10–20°F warmer than ambient — during the periods when the compressor is not running.

The problem it solves.

When the compressor is off and outdoor temperature drops, refrigerant migrates through the system to the coldest point. In many installations, that coldest point is the compressor itself — sitting outside in cold air. Refrigerant vapor cools, condenses, and dissolves into the compressor oil.

When the compressor then starts, this refrigerant-saturated oil causes several problems: foaming at startup (as suction pressure drops, refrigerant rapidly boils out of the oil, pumping oil out of the sump into the suction line and the rest of the system); bearing starvation (with most of the oil in the wrong places, bearings get no lubrication during the first minutes of operation); and liquid slugging (if refrigerant condensed as actual liquid in the sump, startup can pump liquid refrigerant into the compression chamber, damaging valves and pistons).

The crankcase heater keeps the oil above the refrigerant’s saturation temperature at typical off-cycle pressures, so refrigerant stays as vapor and doesn’t accumulate in the sump.

Two common types.

Band heater (most common). A flat heating element wrapped around the bottom of the compressor housing, secured with a metal strap. Resistance heating — typically 25–100 watts at 240V.

PTC (Positive Temperature Coefficient) heater. A self-regulating heating element inserted into a well at the bottom of the compressor or wrapped against the housing. Resistance increases with temperature — when cold, low resistance and high heat output; as it warms up, resistance rises and output drops. Self-limiting, doesn’t overheat. Common in newer equipment.

Operating logic. Two common wiring schemes: always-on (the heater is connected to line voltage continuously regardless of compressor state — simplest wiring, continuous energy use) and contactor-switched (a normally-closed relay powers the heater only when the compressor is off — more efficient but more complex).

When does it matter? In warm climates like Florida, outdoor temperatures rarely drop low enough for significant refrigerant migration, and crankcase heaters rarely operate. In cold climates, the heater is essential — without it, every overnight off-cycle in below-40°F weather leads to migration and startup damage.

Common failures.

• Heater burns out (open circuit) — most common failure. Element fails like any resistance heater eventually does. Symptom: cold sump, condensed refrigerant in oil at startup, long-term compressor problems.
• Connection corrosion at the terminals — outdoor exposure attacks the high-voltage connections.
• Heater shorts to ground — internal insulation breaks down, current finds path to compressor housing, trips the breaker.

Testing. With power off and heater leads disconnected:

• Measure resistance across the two leads: should be 500–2,500 ohms typical
• Measure resistance from each lead to the compressor body: should be infinite (no ground path)
• Feel the bottom of the compressor housing: should be warm (10–20°F above ambient) after several hours of operation

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For technicians

The physics of off-cycle migration.

When a refrigerant system is off and reaches equilibrium, all parts of the system reach the same pressure. Refrigerant distributes itself between liquid and vapor phases based on temperature — the coldest point in the system attracts vapor that then condenses to liquid there.

If the outdoor compressor is at 20°F while the indoor coil and line set are at 70°F: refrigerant in the warm indoor portions evaporates; refrigerant migrates from warm areas to cold areas; it condenses at the cold compressor; eventually most of the system charge ends up as liquid in the compressor sump and bottom of the housing. In a heat pump where the compressor is outdoors at sub-freezing temperatures, migration can be severe — a 5-ton heat pump with 12 lbs of refrigerant charge might end up with most of that charge near the compressor.

The dissolved refrigerant problem. Even when refrigerant remains as vapor, it dissolves in oil: POE oil at 20°F can hold roughly 50% refrigerant by weight in solution; at 50°F, about 25%; at 100°F, under 10%. So even modest oil cooling causes significant refrigerant dissolution.

The foaming problem at startup.

When the compressor starts, suction pressure drops rapidly — from the equilibrium pressure (perhaps 100 PSIG for R-410A at 20°F) to the operating low-side pressure (perhaps 40–50 PSIG at cold-weather operation). For oil with dissolved refrigerant, this pressure drop causes the refrigerant to come out of solution rapidly — essentially boiling. The oil foams violently.

Oil pumping. Foamed oil has reduced density and increased volume. The compressor’s oil pump draws foam instead of liquid oil. Foam can’t lubricate. Bearings get no oil during the first 30–90 seconds of startup.

Oil migration through the system. The foaming oil bursts up out of the sump and into the suction and discharge lines. Once in the line set, oil doesn’t easily return — it must travel with refrigerant through the entire circuit. In severe cases, the compressor runs with the sump nearly dry for hours after startup.

Audible noise. Severe foamed-startup events produce loud rattling or banging from the compressor as bearings clatter. The sound diminishes within a minute or two as foaming subsides and oil returns. Customer reports of “loud startup noise” are often this exact phenomenon.

Slugging. If migration was severe enough to fill the sump with liquid refrigerant (not just oil with dissolved refrigerant), startup pumps that liquid into the cylinder or scroll. Reciprocating compressors are most vulnerable — liquid slugging can break valves or rods on a single startup.

The crankcase heater solution.

The heater keeps the compressor oil warm enough that: refrigerant vapor doesn’t condense in the compressor (sump is warmer than indoor/line set temperatures); dissolved refrigerant content in the oil stays low; startup doesn’t produce significant foaming.

The temperature differential needed is typically 10–20°F above the warmest part of the system:

• 25W heater raises sump temperature 5–10°F above ambient — adequate for mild climates
• 50–75W heater raises sump temperature 10–20°F above ambient — typical for moderate climates
• 100W heater raises sump temperature 20–30°F — for severe cold climate heat pumps

Band heater construction.

• Resistance wire wound on insulated form, flat cross-section for good thermal contact with cylindrical compressor housing
• High-temperature insulation (silicone or fiberglass) and two electrical leads
• Steel strap to clamp against compressor lower section (the oil sump area)

Specifications: voltage typically 240V single-phase; power 25W, 40W, 60W, 75W, or 100W depending on application; resistance = 240V² / Power (60W = 960Ω, 100W = 576Ω).

PTC heater construction.

Ceramic elements with low resistance when cold and high resistance when warm. As the heater warms up, its own resistance rises, limiting current and stabilizing temperature — self-limiting, can’t overheat. More compact than band heaters, can be inserted into a well in the compressor base. Better life expectancy, more expensive, less commonly stocked.

Operating logic in detail.

Always-on wiring (most common). Heater connected to L1 and L2 of the 240V supply at all times when the disconnect is closed. Simple wiring (two leads to the heater), always on when needed, no control circuit to fail. Disadvantage: wastes energy when compressor is running (compressor heats itself plenty) and continuously draws 25–100W even in summer.

Typical wiring: heater lead 1 → L1 terminal of contactor (always energized); heater lead 2 → L2 terminal of contactor (always energized). Heater runs whenever disconnect is closed.

Contactor-switched wiring. Heater powered through a normally-closed auxiliary contact on the compressor contactor. When compressor runs, contactor closes and auxiliary contact opens (heater off). When compressor stops, contactor opens and auxiliary contact closes (heater on). More efficient but more complex — auxiliary contact or relay can fail. Less common in residential, more common in commercial systems with energy management requirements.

Some systems use an outdoor thermostat to control the heater — only powers it when outdoor temperature drops below a threshold (typically 50–65°F). Saves energy in mild weather but adds another component that can fail.

Climate considerations.

Warm climates (Florida, Gulf Coast, Southern California, Arizona). Outdoor temperatures rarely drop below 50°F. Off-cycle migration is minimal. Crankcase heaters are sometimes omitted from cooling-only systems in these climates. Heat pumps in warm climates still benefit from crankcase heaters for occasional cold snaps.

Moderate climates (Mid-Atlantic, central US, Pacific Northwest). Off-cycle migration is a real concern during winter months. Crankcase heaters are standard equipment. Always-on wiring is most common.

Cold climates (Northeast, Midwest, Mountain States). Crankcase heaters are critical. Heat pumps without functional crankcase heaters experience compressor failures within a few years.

Common failures and symptoms.

Heater open circuit (most common failure). The heating element burns out. Resistance reads infinite. Heater produces no heat.

Detection: touch test (compressor bottom should be warm; if cold, heater isn’t working); electrical test (measure resistance across heater leads — should be 500–2,500Ω; infinite means open); visual inspection (heater band may show visible damage if extreme).

Symptoms in cold climates: compressor noisy at startup (foaming); customer reports “loud noise when unit starts in winter”; eventual compressor failure from bearing or valve damage.

Heater shorted to ground. Insulation breakdown allows current to find ground through the compressor housing. Trips breaker or GFCI. Detection: resistance from heater lead to compressor housing should be infinite; any reading less than 10MΩ indicates ground fault. Symptoms: breaker trips when unit is energized; burn marks on heater housing or compressor surface.

Loose connection at heater terminals. Terminal corrosion or loose crimp causes intermittent operation. Detection: resistance check varies with terminal movement; visual corrosion or oxidation at terminals.

Replacement.

For band heaters:

1. Disconnect heater leads at the contactor terminals
2. Loosen and remove the strap/clamp holding the heater
3. Slide the old heater off the compressor
4. Verify the compressor housing is clean and undamaged
5. Install new heater positioned correctly around the sump area
6. Apply thermal compound if recommended (improves heat transfer)
7. Tighten clamp/strap firmly
8. Reconnect leads to original terminals
9. Verify operation with multimeter and physical warmth check

Most band heater replacements take 30–45 minutes if the compressor is accessible. Cost: $30–80 for the heater, plus 1–2 hours labor.

For PTC heaters in wells: disconnect leads, slide PTC element out of the well, insert new element, reconnect leads. Simpler than band heater replacement when the compressor is designed for it.

Florida considerations.

Florida systems typically have crankcase heaters that rarely operate but exist for the occasional cold snap.

Heater fails undetected. The heater can fail and not be noticed for years because Florida’s warm air masks migration problems. When the first cold snap arrives, the compressor experiences violent startup conditions — but the customer or tech may not connect the dots.

Coastal corrosion attacks the heater terminals and heater body itself. Salt air accelerates element failure.

Annual fall maintenance should include verifying crankcase heater function: resistance check; touch test for warmth; visual inspection of heater body and connections.

For Florida coastal installations: replacing the heater proactively every 5–7 years (regardless of failure) is cheaper than the eventual compressor damage from a failed heater discovered during a cold night.

Diagnostic guideline for compressor problems.

When diagnosing compressor failures (bearing noise, slugging damage, premature failure) in heat pumps, always verify: crankcase heater functioning correctly; heater wired to proper supply (not through contactor in always-on systems); heater installed correctly (proper contact, proper position); adequate wattage for the climate.

A failed compressor that’s a year old in a system with a non-functioning crankcase heater suggests the heater failure caused the compressor failure. Replace both.