Heat pump short cycling

Short cycling is when the heat pump starts, runs briefly, shuts off, and starts again soon after, over and over. The cause sources name most consistently is equipment that is too large for the load. ENERGY STAR: "Oversized equipment may cycle too frequently, resulting in less comfort and a shortened lifespan."^{[Source 1]}

Why it matters

NRCan says that when a heat pump is oversized, "the unit cycles on and off leading to discomfort, wear on the heat pump, and stand-by electric power draw."^{[Source 2]} The DOE Building America Solution Center links rules-of-thumb sizing to "too-frequent on and off cycling."^{[Source 3]} In cooling, short cycles also remove less moisture: ORNL researchers note the "risks of poor dehumidification" and "increased cycling losses" with oversized single-speed equipment.^{[Source 5]}

Common causes

Cause	Who addresses it
Oversized equipment	Installer; see Manual J, Manual S and Manual D explained
Thermostat location (sun, drafts, doorways, windows)	Installer or electrician moves the thermostat
Thermostat cycle rate settings	Installer reviews thermostat setup
Restricted airflow (dirty filter, blocked registers, duct problems)	Homeowner for filter and registers; technician for ducts
Refrigerant or component problem	Technician

Details on each:

• Oversizing. In mild weather a single-stage unit's full output is far more than the house needs, so it satisfies the thermostat in a few minutes and shuts off. Variable-speed units can slow down instead: ORNL notes they "can often run at 30-40% of their rated cooling capacity to reduce cycling losses."^{[Source 5]} See Variable-speed vs single-stage heat pumps.
• Thermostat location. DOE says the thermostat "must be on an interior wall away from direct sunlight, drafts, doorways, skylights, and windows," and that poor placement can cause "ghost readings" and unnecessary cycling.^{[Source 4]}
• Thermostat cycle rate settings. Many thermostats have installer settings for cycle rate. The Resideo T6 Pro, for example, has separate compressor cycle rate settings for stage 1 and stage 2.^{[Source 8]} If these are set wrong for the equipment, cycling can be too frequent.
• Airflow and refrigerant. DOE notes that "too much or too little refrigerant... reduces heat-pump performance."^{[Source 6]} NIST's study of installation faults lists refrigerant undercharge, overcharge and low indoor airflow among the faults with "the most potential for causing significant performance degradation."^{[Source 7]} Whether a specific refrigerant or airflow problem causes short cycling on your system is something a technician has to diagnose with instruments.

What you can check

1. Filter. Replace or clean it. See Heat pump filters and maintenance.
2. Registers and returns. Open and unblock them.
3. Thermostat surroundings. Is there a lamp, TV, sunny window, kitchen or supply register near it? Is it on an outside wall?
4. Note the pattern. Write down how long each run and each off period lasts, the outdoor temperature, and whether it happens in heating, cooling or both. Give these notes to the technician.

When to call a technician

• Short cycling started suddenly on a system that used to run normally.
• The thermostat or unit shows an error code.
• The outdoor unit starts and stops within a minute or two repeatedly.
• You suspect a refrigerant problem. Refrigerant work requires an EPA Section 608 certified technician.^{[Source 9]}

If the cause is oversizing, ask the installer which thermostat or equipment settings can reduce cycling on your system. Sizing is set at purchase, which is why a load calculation before buying matters; see Heat pump buying checklist.