Heat Pump Troubleshooting Guide Series (4)

A water tank that continuously overflows is a common but sometimes confusing problem in air source heat pump hot water systems.

The symptoms may appear simple:

The tank is already full, but water continues entering and eventually overflows.

However, the actual cause may not necessarily be the water tank itself.

In practical heat pump projects, water tank overflow problems can generally be divided into two categories:

1. Hydraulic or plumbing problems
2. Electrical control problems

The fastest way to diagnose the problem is to answer one important question:

Does the water tank continue overflowing after the heat pump system is powered off?

This simple test can quickly separate a mechanical/hydraulic problem from an electrical control problem.

1. First Diagnosis: Does the Tank Overflow When Power Is Off?

Before replacing sensors, solenoid valves or control boards, switch off the heat pump system and observe the tank.

There are two basic situations.

Situation A

The tank continues filling or overflowing even when the heat pump is powered off.

This normally indicates a mechanical, hydraulic or plumbing problem.

Situation B

The tank stops filling when power is off, but starts overflowing after the system is powered on.

This normally indicates an electrical control or water-level detection problem.

This distinction can significantly shorten troubleshooting time.

2. Situation One: Water Tank Still Overflows When the Heat Pump Is Powered Off

If water continues entering the tank even when the entire heat pump system has been disconnected from power, the electrical controller is unlikely to be the primary cause.

Technicians should first inspect the water supply and piping system.

Common causes include:

• Manual makeup water valve left open
• Float valve stuck open
• Excessive inlet water pressure
• Makeup water solenoid valve mechanically stuck
• Return water solenoid valve mechanically stuck
• Cold water entering the hot water return circuit

Let's examine these problems individually.

3. Check the Manual Makeup Water Valve

This should be one of the first checks.

If the manual water filling valve has accidentally been left open, water may continue entering the storage tank regardless of the controller status.

The water level will continue rising until it reaches the overflow outlet.

Check:

✔ Is the manual refill valve completely closed?
✔ Is the valve damaged internally?
✔ Does water still pass through the valve when it is supposedly closed?

Do not assume that a valve is closed simply because the handle appears to be in the closed position.

An internally damaged valve may continue allowing water through.

4. Check Whether the Float Valve Is Stuck Open

Some heat pump water tanks use a mechanical float valve to control the water level.

Under normal conditions:

Water level rises → Float rises → Valve closes → Water supply stops

If the float or valve mechanism becomes stuck:

Water level rises → Float cannot close valve → Water continues entering → Tank overflows

Possible causes include:

• Scale buildup
• Sediment
• Foreign objects
• Mechanical wear
• Damaged float mechanism

Troubleshooting:

Move the float manually and observe whether the inlet water stops.

If the float moves but the valve cannot shut off the water supply, the valve assembly may require cleaning or replacement.

5. Excessive Water Supply Pressure Can Cause Overflow

High inlet water pressure is another potential cause.

Inside some makeup water solenoid valves, a diaphragm and spring are used to control water flow.

If the incoming pressure is excessively high, the valve may not seal correctly.

The result can be:

High inlet pressure → Valve cannot close completely → Continuous water leakage → Tank overflow

Therefore, technicians should not only check whether water pressure exists.

They should also check whether the pressure is within the acceptable operating range of the valve and system.

If necessary, a suitable pressure-reducing device may be required.

6. Check the Makeup Water Solenoid Valve

The makeup water solenoid valve is one of the most important components in an automatic filling system.

Normally:

Controller requests water → Solenoid valve opens

and

Water level reaches target → Controller removes signal → Solenoid valve closes

But the valve may remain mechanically open even after electrical power is removed.

Possible causes include:

• Dirt trapped on the diaphragm
• Scale deposits
• Valve core stuck
• Damaged diaphragm
• Mechanical wear

This explains why some tanks continue overflowing even after the heat pump is switched off.

7. Why Installing a Y-Strainer Is Important

Water quality has a direct impact on solenoid valve reliability.

Sand, rust, scale and other debris can enter the valve and prevent the diaphragm from sealing correctly.

For this reason, installing a Y-strainer upstream of the makeup water solenoid valve is strongly recommended.

A suitable strainer helps protect:

• Solenoid valves
• Float valves
• Heat exchangers
• Pumps
• Other hydraulic components

The filter should also be inspected and cleaned periodically.

A filter that is installed but never maintained can eventually become another source of system problems.

8. Don't Forget the Return Water Solenoid Valve

In centralized hot water systems, the problem may not come from the normal makeup water inlet at all.

Water may enter the storage tank through the return water line.

If the return water solenoid valve becomes stuck open, water may continuously enter the tank through the return pipe.

Possible causes are similar to those of the makeup valve:

• Foreign objects
• Scale
• Stuck valve core
• Valve unable to return after opening

Therefore, when diagnosing an overflowing tank, determine:

Where is the extra water actually entering the tank?

Is it coming from the normal cold-water inlet?

Or is it entering through the hot-water return pipe?

This distinction is extremely important.

9. A Special Case: Cold Water Backflow Into the Hot Water Return Line

This is one of the more difficult problems to identify in larger heat pump hot water projects.

Imagine a centralized system serving:

• A hotel
• Apartment building
• Dormitory
• Resort
• Commercial facility

The building may contain:

• Heat pump hot water system
• Electric water heaters
• Mixing valves
• Hot water circulation lines
• Cold water supply lines

If there is an incorrect connection or a failed mixing valve, cold water can cross into the hot water piping.

The flow path may become:

Cold water → Hot water pipe → Return pipe → Water tank

The tank then receives water continuously from the return side and eventually overflows.

10. How to Identify a Cold-Water Crossover Problem

A useful symptom is:

The water tank is overflowing, but the incoming water is entering from the return pipe rather than the normal makeup water inlet.

Another clue may be abnormal return water temperature.

If the return water temperature remains unusually low, cold water may be entering the hot water circulation system somewhere in the building.

Technicians should inspect:

• Hot/cold water connections
• Mixing valves
• Electric water heater connections
• Return water piping
• Check valves
• Isolation valves

This type of fault is particularly common in systems that have been modified several times or contain multiple hot water sources.

11. Real-World Example: Hotel Hot Water System

Consider a hotel with more than 50 guest rooms.

During the peak tourist season, occupancy may reach nearly 100%, so the building requires a large hot water storage capacity.

During the low season, however, occupancy may fall significantly.

To reduce energy consumption, the heat pump system may sometimes be switched off while individual electric water heaters remain available in guest rooms.

If the plumbing arrangement allows cold water to pass through an electric water heater circuit into the common hot water network, cold water may eventually reach the return pipe.

The flow may become:

Cold water → Electric water heater circuit → Hot water pipe → Return line → Storage tank

The result:

The water tank keeps receiving water and eventually overflows.

In this situation, replacing the water-level sensor or heat pump controller will not solve the problem.

The real issue is the hydraulic connection between the cold and hot water networks.

This is why troubleshooting must consider the entire system, not only the heat pump itself.

12. Situation Two: No Overflow When Powered Off, But Overflow Occurs After Power-On

Now consider the opposite situation.

When the heat pump is switched off:

No overflow occurs.

After power is restored:

The tank begins filling continuously and eventually overflows.

This strongly suggests an electrical control or water-level detection problem.

Common causes include:

• High water-level sensor scaling
• Water-level sensor wiring failure
• Incorrect water-level signal
• Water-level relay failure
• PCB control output problem

13. Check the High Water-Level Sensor

The controller relies on water-level sensors to determine whether the tank needs additional water.

When the tank reaches the high water level, the sensor should tell the controller:

“The tank is full — stop filling.”

If this signal is incorrect, the controller may continue energizing the makeup water valve.

The result is:

Tank full → Controller detects low water → Solenoid valve remains open → Tank overflows

14. Water-Level Sensor Scaling

In areas with hard water, mineral deposits can accumulate around water-level probes.

Scale can interfere with electrical conductivity or sensor accuracy.

The controller may therefore receive an incorrect water-level signal.

Typical symptoms include:

• Tank is physically full
• Controller displays a lower water level
• Makeup valve remains energized
• Tank continues filling

Inspection:

Check the water-level probes for:

• Scale
• Corrosion
• Contamination
• Loose terminals

Clean or replace the sensor when necessary.

15. Water-Level Sensor Wiring Failure

Sometimes the sensor itself is working correctly, but the signal cannot reach the controller.

Possible problems:

• Broken wire
• Loose terminal
• Corroded connector
• Damaged high-level signal cable

If the high water-level signal is interrupted, the controller may interpret the condition as:

“Water level is too low.”

It then continues supplying power to the makeup water solenoid valve.

This causes continuous filling and eventually overflow.

16. Check the Water-Level Relay and Main PCB

If:

• The water-level sensor is normal
• Wiring is normal
• Solenoid valve operates mechanically
• Water pressure is normal

the next step is to inspect the electrical control system.

Possible faults include:

Water-Level Relay Failure

A relay may remain energized even after the controller should stop the refill command.

Main PCB Output Failure

The PCB may continuously output a refill signal because of:

• Component failure
• Signal processing error
• Output circuit failure

In these situations, professional electrical diagnosis is required.

17. Quick Troubleshooting Flowchart

For installers and service technicians, the following sequence can save considerable time.

Step 1 — Turn Off the Heat Pump

Observe whether water continues entering the tank.

Step 2 — If It Still Overflows

Focus on the hydraulic system:

Manual valve → Float valve → Water pressure → Makeup solenoid valve → Return valve → Hot/cold water crossover

Step 3 — If Overflow Stops

Focus on the electrical system:

Water-level sensor → Sensor wiring → Relay → Controller/PCB

Step 4 — Identify the Actual Water Entry Point

Determine whether water enters through:

Cold water makeup line

or

Return water line

This is especially important in commercial hot water projects.

18. Why You Should Not Replace the PCB First

One of the most common troubleshooting mistakes is replacing electrical components too early.

For example:

Water tank overflowing → Assume sensor failure → Replace sensor → Problem remains → Replace PCB → Problem remains

The actual cause may simply be:

• A stuck solenoid valve
• Excessive water pressure
• A failed mixing valve
• Cold water crossover
• Incorrect plumbing connection

A better approach is to diagnose the system based on physical behavior.

The first question should always be:

Does the tank still overflow when the heat pump is completely powered off?

That answer determines the direction of the next inspection.

19. How to Prevent Water Tank Overflow Problems

Good system design can prevent many of these failures.

Water-Side Installation

✔ Install Y-strainers before important solenoid valves
✔ Maintain appropriate inlet water pressure
✔ Use suitable check valves where required
✔ Avoid incorrect hot/cold water cross-connections
✔ Design the return water circuit correctly

Electrical and Control System

✔ Install water-level probes correctly
✔ Protect sensor terminals from corrosion
✔ Check water-level signals during commissioning
✔ Test automatic filling and shutoff functions

Routine Maintenance

✔ Clean Y-strainers
✔ Inspect solenoid valves
✔ Remove scale from water-level probes
✔ Check float valve movement
✔ Monitor abnormal return water temperature
✔ Inspect mixing valves in centralized hot water systems

20. Frequently Asked Questions

Why does my heat pump water tank keep overflowing?

The most common causes include a stuck float valve, excessive inlet pressure, a makeup water solenoid valve that cannot close, incorrect water-level detection, or water entering through the return line.

Why does the water tank overflow even when the heat pump is switched off?

If overflow continues with power disconnected, the problem is usually mechanical or hydraulic rather than electronic.

Check the manual filling valve, float valve, water pressure, makeup water solenoid valve and return water circuit.

Why does the tank only overflow when the heat pump is powered on?

This usually points toward the automatic control system.

Check the high water-level sensor, sensor wiring, water-level relay and PCB output.

Can a bad mixing valve cause a hot water tank to overflow?

Yes.

In centralized hot water systems, a faulty or incorrectly connected mixing valve can allow cold water to enter the hot water circuit.

That water may travel through the return line and continuously enter the storage tank.

Can high water pressure cause the tank to overflow?

Yes.

Excessive inlet pressure can prevent certain valves from sealing correctly, resulting in continuous water flow.

Always verify that the water supply pressure is within the permitted operating range of the system components.

Conclusion

When an air source heat pump water tank overflows, don't immediately assume that the controller or water-level sensor is faulty.

The problem can originate from either the hydraulic system or electrical control system.

A practical troubleshooting sequence is:

Power OFF test → Identify water entry point → Check valves and pressure → Check water-level detection → Check relay and PCB

And remember one particularly important rule:

If the tank continues overflowing after the heat pump is completely powered off, investigate the hydraulic system first.

For commercial heat pump hot water projects, technicians should also consider the complete building plumbing network because a failed mixing valve, incorrect piping connection, or cold-water crossover can create symptoms that appear to be a heat pump fault.

Good troubleshooting is not about replacing components one by one.

It is about identifying the actual cause before taking action.