Crankcase heater warm-up before startup: why the wait is 6, 12, or 24 hours
Manufacturers publish six, twelve, and twenty-four hour waits before a compressor may start. What each figure covers, and why some units carry no heater band at all.
What this means
The wait lets the sump heater drive liquid refrigerant back out of the compressor oil before the compressor turns. No single figure governs. Amana publishes twenty-four hours for its package units, Copeland states twelve hours as the compressor minimum, and one ductless cassette manual sets the step at six hours during test operation. The number that applies is the one printed in the literature for the equipment you are standing in front of.
Equipment and model context
Cross-brand explanation; product figures come from the model documentation
- Air-source split systems, package units, and ductless equipment with a compressor sump heater
- Inverter outdoor units that heat the compressor through the motor windings instead
Each figure below belongs to the document it came from. A twenty-four hour instruction written for a package heat pump is not evidence about a ductless inverter unit, and a compressor manufacturer's minimum is written about the compressor rather than about any finished machine built around it. Where the equipment manual and a component figure disagree, the equipment manual is the one the installer is held to.
What this guide covers
- What the pre-start wait is protecting inside the compressor.
- Which published figure applies to a given piece of equipment.
- Whether a unit has a heater band at all, and what happens if it does not.
What changes the answer
- Refrigerant migrates toward the coldest part of a stopped system, and an outdoor compressor standing in cold weather can be that part.
- Refrigerant dissolved in the oil thins it, so the first revolutions after a long cold shutdown are the ones taken with the least lubrication.
What the heater is actually doing while nothing runs
A sump heater keeps the compressor warmer than the rest of the circuit. Carrier states the purpose plainly for its variable-speed outdoor units: the heater prevents liquid refrigerant accumulating in the compressor by ensuring the compressor is not the coldest part of the system. Nothing about that job involves running the machine, which is why the heater draws power during off cycles and why a unit that has been isolated all winter has had no protection at all.
Amana describes the same component from the wiring side on its package equipment. The crankcase heater is on whenever power is supplied to the unit and its own thermostat is closed. That thermostat closes at 67 degrees Fahrenheit and opens at 85, and the insert-type heater is self-regulating and wired to the contactor line terminals. Carrier gives a similar window on the units it documents, energizing below 65 degrees Fahrenheit and de-energizing above 85.
So the heater is not a start-up device that someone switches on. It is a standby load with its own temperature window, and the pre-start wait is a request to let that load do its work before anyone asks the compressor to turn.
Where each published figure comes from
Amana states it as a conditional instruction on its self-contained package air conditioners and heat pumps. If a crankcase heater is used, the unit should be energized twenty-four hours prior to compressor start up so the heater has sufficiently warmed the compressor. Compressor damage may occur if the step is skipped. The same manual repeats it as a line on the start-up checklist, verifying the heaters have been energized for twenty-four hours.
Copeland answers a narrower question and gets a smaller number. Its application engineering entry says the crankcase heater must be turned on a minimum of twelve hours prior to starting the compressor, and gives the reason as preventing oil dilution and bearing stress at start. That figure is about a compressor, not about the cabinet, the charge, or the pipe run around it.
The six-hour figure appears in equipment literature too. ECR International's cassette ductless manual puts it in the test operation sequence: open the gas side stop valve, open the liquid side stop valve, electrify the crank case heater for six hours, then start the unit from the remote controller. Read in order, that is an installer being told to wait before commissioning rather than a general rule for the trade.
Reading the three together, the instruction is stable and the interval is not. Every document is describing the same protection, and each one sets the interval its own product was validated against.
Some units have nothing to warm up
Carrier's service literature for its variable-speed outdoor units describes two arrangements. Some systems carry an external crankcase heater around the bottom of the compressor. Others use a stator heating function in which the drive passes direct current through the motor windings, heating the motor so that heat reaches the compressor shell and the sump. On equipment built the second way there is no band to find and no separate heater circuit to fuse.
Amana's wording carries the same caution from the other direction. Its instruction opens with a condition, if a crankcase heater is used, rather than an assertion that one is fitted. On smaller cooling-only equipment a heater is an accessory, and the crankcase heater kits sold for that purpose are described as required for low ambient and long line applications rather than for every install.
That is the practical trap in a twenty-four hour rule quoted without its source. Applied to a unit with no sump heat, the wait protects nothing. Applied to a unit that does have it, skipping the wait is the step the manufacturer named as a cause of compressor damage. Neither answer can be guessed from the outside of the cabinet, which is what makes the data plate and the manual the starting point.
The winter isolation habit this argues against
Turning off the outdoor disconnect for the season removes the standby heat along with everything else, and every documented pre-start wait then applies again in spring. On a heat pump the disconnect also has to stay closed for the machine to heat the building at all, so the habit belongs to cooling-only equipment and to owners who were told it saves standby power.
Newer equipment adds a second reason to leave the supply alone. Systems fitted with a refrigerant detection system for A2L refrigerant carry a caution stating the unit must be powered except for service, because the sensing and mitigation circuit is inoperative with the supply removed.
If power has already been off, the recovery is a wait rather than a repair. Close the supply, leave it closed, and give the equipment the interval its own literature names before calling for cooling or heating. Nothing else an owner can reach shortens it.
| Document | Interval it states | What it is written about |
|---|---|---|
| Amana package unit installation instructions | 24 hours before compressor start up | A finished package air conditioner or heat pump, where a heater is fitted |
| Copeland application engineering entry | 12 hours minimum before starting | The compressor itself, against oil dilution and bearing stress |
| ECR International cassette manual | 6 hours of crank case heater power | A commissioning step inside one ductless test operation sequence |
| Carrier variable-speed service manual | No interval stated | How sump heat is produced, by a heater band or by stator current |
Migration follows temperature, not gravity, so the coldest component wins the refrigerant whatever its height in the circuit. Sump heat works by taking the compressor out of that competition.
- Outdoor air fallsThe whole outdoor section cools toward ambient
- Compressor shell coolsIts mass holds it cold once the run heat is gone
- Vapour condenses thereLiquid collects and dissolves into the oil
- Sump heat appliedThe shell stops being the coldest surface
Questions people ask about this
Does every outdoor unit need a 24 hour wait?
No. Twenty-four hours is the figure Amana publishes for its package equipment where a crankcase heater is fitted. Copeland gives twelve hours for the compressor, one ductless cassette manual gives six hours during test operation, and Carrier documents equipment that heats the compressor through the motor windings instead of a heater band.
How do I know if my unit has a crankcase heater?
The literature for the model says so, and the wording matters. Amana writes the instruction conditionally, as if a crankcase heater is used, and crankcase heater kits are sold separately for low ambient and long line applications. Identifying the fitment means reading the manual for that model rather than looking at the cabinet.
Is the outdoor unit faulty if it feels warm when off?
Warmth at a stopped compressor is consistent with sump heat rather than with a fault. Amana documents the heater as on whenever the unit has power and its thermostat is closed, with that thermostat closing at 67 degrees Fahrenheit and opening at 85.
What happens if the compressor starts without the wait?
Amana names compressor damage as the consequence of skipping the step. Copeland describes the mechanism as oil dilution and bearing stress at start. Neither document quantifies the risk of a single occurrence, and neither should be read as a prediction about one particular start.
Should I switch the outdoor disconnect off for winter?
Doing so removes standby sump heat, so every documented pre-start interval applies again before the next run. Equipment carrying a refrigerant detection system for A2L refrigerant also carries a caution that the unit must be powered except for service.
Evidence recordHow this page was checked
service manual, official manufacturer support article, operation manual · checked 2026-09-07
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Amana, Copeland, ECR International and Carrier 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, official manufacturer support article, operation manual
- Scope of the definition
- Confirm against the exact model manual
- Last checked
- 2026-09-07