Short-cycling happens when your heat pump's indoor unit switches on and off in rapid, repeated bursts — often before it has had time to properly heat or cool the room. This puts extra strain on the compressor, wastes electricity, and leaves your home uncomfortable. The good news is that many of the early checks are straightforward and safe for a homeowner to carry out.
What Is Short-Cycling?
Short-cycling refers to a heat pump that completes its on/off cycle far more frequently than normal. A healthy heat pump typically runs for at least 10–15 minutes per cycle before shutting off. If yours is cutting out after just 2–5 minutes and then restarting again shortly after, that's a strong indicator of short-cycling. Over time this behaviour stresses the compressor and can shorten the life of the unit significantly.
Step 1 — Time the On/Off Cycles
The simplest first check is to time how long the unit runs before it shuts off, and how long it stays off before restarting.
- Set your heat pump to a consistent temperature (e.g. 20 °C in heating mode).
- Use your phone's stopwatch to record how long the indoor unit runs before it stops.
- Note how many minutes pass before it starts again.
- Repeat this three or four times to get a reliable pattern.
If cycles are consistently shorter than 8–10 minutes, short-cycling is likely occurring.
Step 2 — Check the Air Filter
A blocked air filter is one of the most common causes of short-cycling in NZ homes. Restricted airflow causes the unit to overheat or over-cool internally, triggering the safety cut-out.
- Turn the unit off at the remote and wait a few minutes.
- Open the front panel of the indoor unit (most clip open without tools).
- Remove the filter and hold it up to the light — if you can't see light through it, it needs cleaning.
- Wash the filter gently with warm water, let it dry completely, and refit it.
Most manufacturers recommend cleaning filters every 4–6 weeks during heavy use. Check your unit's manual for the specific recommendation.
Step 3 — Inspect the Indoor Unit's Airflow
Even with a clean filter, blocked airflow can cause problems. Check that:
- Nothing is obstructing the front or underside of the indoor unit (curtains, furniture, shelving).
- The louvres (the adjustable vanes) are not stuck in a position that directs air back into the unit.
- The room isn't sealed so tightly that there's no return air path — heat pumps need air to circulate freely.
Step 4 — Check the Thermostat Setting and Room Size
If the heat pump is oversized for the room, it will reach the set temperature very quickly and shut off — this is a form of short-cycling caused by incorrect sizing rather than a fault.
- Compare the unit's rated output (in kW, shown on the nameplate or in the manual) against the room size.
- As a rough guide, NZ homes typically need around 0.1–0.15 kW per square metre for a well-insulated room, though this varies with ceiling height, glazing, and climate zone.
- BRANZ publishes guidance on home heating that can help you assess whether your unit is appropriately sized.
Also check that the set temperature isn't too close to the actual room temperature — if the room is already 19 °C and you've set 20 °C, the unit will satisfy the demand almost immediately.
Step 5 — Look for Error Codes
Most modern heat pumps display fault codes on the indoor unit's display panel or flash them via the indicator lights. Check your owner's manual for the error code list. Common codes relate to:
- High-pressure or low-pressure faults (refrigerant issues)
- Temperature sensor faults
- Communication errors between indoor and outdoor units
Photograph any error codes before resetting the unit, as this information is valuable for a technician.
Step 6 — Inspect the Outdoor Unit
While you're investigating, take a look at the outdoor unit:
- Is the fan spinning freely when the unit is running?
- Is the unit surrounded by vegetation, fencing, or stored items that restrict airflow?
- In winter, is there ice build-up that isn't clearing during the defrost cycle?
Keep at least 30 cm of clear space around the outdoor unit as a general rule — check your installation manual for the manufacturer's specific clearance requirements.
Common Causes of Short-Cycling at a Glance
| Cause | DIY Check? | Likely Fix |
|---|---|---|
| Dirty air filter | ✅ Yes | Clean or replace filter |
| Blocked airflow | ✅ Yes | Clear obstructions |
| Oversized unit | ✅ Assess | Consult installer |
| Refrigerant leak | ❌ No | Licensed technician |
| Faulty thermostat/sensor | ❌ No | Licensed technician |
| Compressor fault | ❌ No | Licensed technician |
| Electrical fault | ❌ No | Licensed electrician |
When to Call a Professional
If cleaning the filter and clearing obstructions doesn't resolve the short-cycling, it's time to call a licensed heat pump installer or refrigeration technician. Refrigerant handling is restricted work under NZ law — only certified technicians holding the appropriate refrigeration trade qualifications (regulated by the PGDB) may legally handle refrigerants. Attempting to top up or check refrigerant yourself is both illegal and dangerous.
Similarly, any electrical work inside the unit or at the isolator switch must be carried out by a registered electrician — this is restricted electrical work under the Electricity Act 1992.
A qualified installer can also advise whether your unit is correctly sized for your space, which is a common root cause of short-cycling in NZ homes. You can find experienced local heat pump installers on tradedirectory.co.nz.
For rental properties, note that Tenancy Services Healthy Homes standards set requirements for heating capacity — a short-cycling unit may not be meeting its rated output, which could be a compliance issue for landlords.
ISO 9001
