Air-to-water heat pumps are designed to deliver reliable, energy-efficient heating for residential and commercial buildings. However, one common issue homeowners and facility managers encounter is low water temperature, which can lead to discomfort and poor system performance.

In this comprehensive guide, we’ll explore the main causes of low water temperature in heat pump systems, provide actionable troubleshooting steps, and offer tips to restore optimal operation.

Common Causes of Low Water Temperature in Heat Pumps

If your heat pump isn’t heating water to the expected setpoint (typically between 35°C to 55°C), the issue may stem from one or more of the following:

1. Improper System Settings

• Low flow temperature setpoint

• Incorrect thermostat configuration

• Zone control issues

2. Undersized Heat Pump or Poor System Design

• Heat pump capacity is too low for heating demand

• High heat loss from building envelope

• Poorly sized buffer tank or underfloor circuit

3. Low Ambient Temperature

• Cold outdoor conditions reduce heat extraction efficiency

• System may struggle if not designed for low-temp climates

4. Flow Rate Problems

• Blocked filters, airlocks, or pump faults

• Closed valves or partially obstructed piping

5. Refrigerant or Compressor Issues

• Low refrigerant charge

• Compressor malfunction or reduced performance

• Faulty expansion valve

6. Sensor or Control Board Malfunction

• Inaccurate temperature readings

• Broken or poorly positioned temperature sensors

• Software/firmware faults

️ Step-by-Step Troubleshooting Guide

✅ Step 1: Check the Set Temperature

• Access the control panel and ensure the flow temperature setting is correctly configured.

• Verify zone temperatures in multi-zone systems.

✅ Step 2: Inspect System Pressure and Flow

• Check pressure gauge (typically 1.0–1.5 bar)

• Look for signs of air in the system (gurgling or cold spots in radiators or underfloor loops)

• Bleed the system if needed

• Inspect circulating pumps and flow sensors for correct operation

✅ Step 3: Examine Outdoor Conditions

• If outdoor temperatures are very low (e.g., below -5°C), the heat pump may need to run longer or use a backup heater.

• Make sure defrost mode is functioning properly to prevent coil icing.

✅ Step 4: Verify Sensor Readings

• Compare displayed water temperature to actual measurements with a thermometer.

• Replace or recalibrate faulty sensors.

✅ Step 5: Clean Filters and Heat Exchangers

• Dirty filters or fouled heat exchangers can restrict flow and reduce efficiency.

• Schedule regular maintenance for cleaning.

✅ Step 6: Review the Compressor and Refrigerant Circuit

• If the heat pump runs continuously but cannot raise water temperature, the issue may lie with:

• Low refrigerant levels

• Worn compressor

• Faulty expansion valve

• A certified HVAC technician should inspect these components.

⚠️ When to Call a Professional

Call a certified heat pump technician if:

• Water temperatures stay below 30°C despite correct settings

• The system shuts off or throws fault codes

• You notice refrigerant leakage or unusual noises

• Defrost mode fails to activate in icy conditions

️ Tips to Maintain Optimal Water Temperature

✅ Conclusion

Low water temperature in a heat pump system is usually caused by incorrect settings, poor flow, extreme weather, or component malfunctions. By following a systematic troubleshooting approach—starting with user settings and moving toward more technical checks—you can often identify and resolve the issue efficiently.

Preventative maintenance and professional diagnostics are key to long-term heat pump performance and comfort.