Heat Pump Troubleshooting Guide Series (3)

During winter operation, air source heat pump systems often run continuously for heating and domestic hot water production.

Besides common refrigeration faults such as high pressure alarm and low pressure alarm, another frequently reported issue in heat pump projects is:

“The water tank cannot refill automatically.”

When the water level decreases, the system should automatically replenish water through the makeup water system.

However, some users may find:

• The water tank level keeps dropping
• Automatic water filling does not work
• Manual filling works normally but automatic filling fails
• The system shows water shortage alarms

This problem is usually related to two major areas:

1. Water supply pipeline and water pressure problems
2. Electrical control system faults

A systematic diagnosis can quickly identify the actual cause.

1. Understanding the Automatic Water Refill System

In a typical air source heat pump system, the water tank is equipped with:

• Water supply valve
• Makeup water pipe
• Float switch or water level sensor
• Solenoid valve
• Control relay
• Main PCB control system

The operating logic is:

Water level decreases
↓
Water level sensor detects low water level
↓
Controller sends refill signal
↓
Solenoid valve opens
↓
Water enters the tank
↓
Water level returns to normal

If any step fails, the tank may not refill.

2. Common Cause One: Water Supply System Problems

Situation 1:

Manual Water Refill Valve Open, But No Water Enters

If opening the manual refill valve does not allow water into the tank, the problem is usually related to the water supply side.

Possible causes:

1. Insufficient Water Pressure

The incoming water pressure is too low.

Result:

• Water cannot overcome system resistance
• Refill speed becomes extremely slow
• Tank cannot reach normal water level

Inspection:

Check:

✔ Building water pressure
✔ Water supply pressure
✔ Water inlet pipeline condition

2. Cold Water Supply Valve Closed or Stuck

The makeup water valve may be:

• Closed accidentally
• Blocked by debris
• Mechanically damaged

Solution:

Check the valve opening condition and confirm normal water flow.

3. Float Valve Stuck

Some water tanks use mechanical float valves.

If the float:

• Gets stuck
• Cannot move freely
• Remains in closed position

The water supply will stop even when the water level is low.

Inspection:

Move the float manually and check whether the valve responds correctly.

3. Situation Two: Manual Filling Works, But Automatic Refill Does Not Work

This situation is more complicated because the water supply system itself may be normal.

Possible causes include:

• Water level sensor problem
• Solenoid valve failure
• Electrical signal failure
• Controller malfunction

4. Water Level Sensor Problems

The water level sensor is responsible for detecting the tank water condition.

Common failures:

1. Sensor Terminal Oxidation or Poor Contact

Symptoms:

• Incorrect water level display
• No refill signal
• Error code appears

Solution:

Check:

✔ Sensor wiring
✔ Terminal connection
✔ Signal stability

2. Water Level Sensor Installed Incorrectly

If the sensor position is incorrect:

The controller may receive wrong information.

Examples:

• Tank already has water but system detects shortage
• Tank is empty but system does not start refill

Confirm:

• Sensor installation height
• Sensor position
• Wiring sequence

5. Automatic Refill Solenoid Valve Problems

The solenoid valve controls whether water enters the tank.

Common failures:

1. Valve Core Stuck

Causes:

• Water impurities
• Rust
• Long-term inactivity

Symptoms:

• Control signal exists
• Valve does not open
• No water enters

Solution:

Clean or replace the solenoid valve.

2. Solenoid Coil Burned Out

If the coil fails:

• Valve cannot open
• Automatic refill stops

Check:

• Coil resistance
• Power supply voltage

6. Electrical Control System Failure

If:

• Water level sensor works normally
• Solenoid valve works normally
• Water supply is normal

The problem may come from the heat pump control system.

Possible components:

• Relay
• Main PCB
• Control signal output

Diagnosis Process:

Step 1:

Check whether the controller sends the refill signal.

↓

Step 2:

Check whether the water level relay operates.

↓

Step 3:

Check whether the PCB outputs voltage to the solenoid valve.

If:

No refill signal from PCB:

Possible causes:

• Relay failure
• PCB failure
• Control logic error

Solution:

Replace relay or main control board if necessary.

7. Special Situation: Water Level Sensor Removed but No Automatic Refill

In some troubleshooting cases:

The water level sensor is manually lifted out of the water, but automatic refill still does not happen.

Possible reasons:

Case 1:

The controller is not in refill condition.

For example:

• Water temperature has not reached the refill setting
• System logic prevents refill

Case 2:

Sensor terminal failure

The controller cannot correctly recognize the water level signal.

8. Step-by-Step Troubleshooting Procedure

For installers and service engineers:

Step 1: Confirm Water Supply

Check:

✔ Water pressure
✔ Water inlet valve
✔ Pipeline blockage

↓

Step 2: Test Manual Filling

Open manual refill valve.

If no water enters:

→ Check water supply system.

↓

Step 3: Check Water Level Sensor

Confirm:

• Sensor signal
• Wiring connection
• Installation position

↓

Step 4: Check Solenoid Valve

Confirm:

• Power supply
• Coil condition
• Valve opening

↓

Step 5: Check Controller Output

Inspect:

• Relay
• PCB
• Control signal

9. Prevention Tips for Heat Pump Installers

To reduce water refill failures:

During Installation:

✔ Ensure sufficient water pressure
✔ Install proper filtration before valves
✔ Check sensor wiring carefully
✔ Test automatic refill function before delivery

During Maintenance:

✔ Clean water inlet filters regularly
✔ Check solenoid valve operation
✔ Inspect electrical terminals
✔ Verify water level sensor accuracy

Conclusion

A water tank not refilling problem is not always caused by the water supply itself.

The root cause can come from:

• Low water pressure
• Closed or damaged valves
• Float valve failure
• Water level sensor problems
• Solenoid valve faults
• PCB control issues

The correct troubleshooting sequence is:

Check water supply → Check sensor → Check valve → Check electrical control system

A properly designed and maintained water replenishment system is essential for reliable air source heat pump operation, especially during long winter heating periods.