If your home feels warmer or cooler than the temperature you've dialled in on your heat pump, it's worth doing a few straightforward checks before calling a technician. In most cases, you can test your heat pump's performance with a basic thermometer and a bit of patience — no special tools required. Here's how to do it properly.
Understand What 'Set Temperature' Actually Means
Your heat pump's set temperature (also called the target or setpoint temperature) is the air temperature it's trying to maintain in the room, not the temperature of the air blowing out of the unit. Many homeowners confuse the two. The unit will blow warm or cool air until the room reaches the setpoint, then cycle down — so the outlet air will always feel hotter or cooler than the room target. This is completely normal.
What You'll Need
- A reliable digital thermometer (a basic indoor thermometer from a hardware store is fine)
- A notepad or phone to record readings
- About 30–60 minutes of patience
Avoid using the temperature display on the heat pump's remote or wall controller as your sole reference — these sensors can be affected by direct sunlight, draughts, or being mounted near a heat source.
Step-by-Step: How to Test Room Temperature Accuracy
1. Set Up Your Thermometer Correctly
Place your digital thermometer at seated head height (roughly 1–1.2 m off the floor), away from windows, exterior walls, direct sunlight, and the heat pump's airflow. This gives you a representative reading of the occupied zone.
2. Set the Heat Pump and Wait
Set your heat pump to a specific temperature — say, 20°C in heating mode. Close doors and windows to the room. Allow at least 30 minutes for the room to stabilise, especially in winter or if the room was very cold to start.
3. Compare Readings
Once the heat pump's display shows it has reached setpoint (many units show a steady light or change their fan speed), compare your thermometer reading to the set temperature. A difference of ±1–2°C is generally acceptable and within normal operating tolerance for most residential heat pumps. A difference of 3°C or more is worth investigating.
4. Check the Return Air Sensor
Most split-system heat pumps have a return air temperature sensor inside the indoor unit that tells the system when the room has reached setpoint. If this sensor is blocked by dust or a dirty filter, it may give a false reading and cause the unit to under- or over-heat. Check and clean your filter — this is the single most common cause of temperature inaccuracy.
Check the Airflow and Filter
A clogged filter restricts airflow and reduces efficiency dramatically. Most manufacturers recommend cleaning filters every 4–6 weeks during heavy use. To clean:
- Turn the unit off at the remote.
- Open the front panel and slide out the filter.
- Vacuum or rinse with warm water, then allow to dry fully before reinserting.
For guidance on healthy indoor temperatures and ventilation, Energywise has useful information on home heating efficiency.
Test Heating vs Cooling Mode Separately
If you suspect the unit is underperforming, test it in both heating and cooling modes on separate days. A unit that heats well but cools poorly (or vice versa) may have a refrigerant issue or a faulty reversing valve — both of which require a licensed technician.
Environmental Factors That Affect Performance
NZ homes vary widely in insulation and airtightness. A heat pump is sized for a specific room volume and insulation level — if your room has poor insulation, draughty windows, or high ceilings, the unit may struggle to reach setpoint in extreme weather. BRANZ has research on how building performance affects heating system efficiency.
Also consider:
- Outdoor temperature extremes: Most heat pumps lose efficiency below around -5°C to -10°C (check your model's rated operating range in the manual).
- Room size vs unit capacity: An undersized unit will run continuously without reaching setpoint.
- Door and window sealing: Even small gaps cause significant heat loss in NZ's cooler climates.
Simple Checks at a Glance
| Check | What to Look For | DIY or Pro? |
|---|---|---|
| Filter cleanliness | Dust build-up blocking airflow | DIY |
| Room thermometer vs setpoint | Difference >2°C | DIY first, then Pro |
| Outdoor unit airflow | Blocked by plants, fencing, debris | DIY |
| Refrigerant level | Unit icing up, poor output | Licensed technician only |
| Sensor fault codes | Error codes on display | Pro |
When to Call a Professional
Some issues are beyond DIY checks and must be handled by a qualified heat pump technician. Call a professional if:
- The unit is displaying error or fault codes
- The outdoor unit is icing up heavily (beyond normal light frost in winter)
- You suspect a refrigerant leak (hissing sounds, oily residue near pipework, sudden drop in performance) — handling refrigerants is restricted work under NZ regulations and must only be done by a certified technician
- The temperature difference is consistently more than 3°C despite a clean filter and good room sealing
- The unit is more than 8–10 years old and performance has declined noticeably
For electrical faults or wiring concerns, any work on the fixed wiring must be carried out by a registered electrician — see EWRB for more on restricted electrical work.
You can find qualified local heat pump installers and service technicians on tradedirectory.co.nz.
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