Heat Pump Immersion Heater – What It Does, When It’s Used, and Common Problems
Most heat pump systems include an immersion heater as a backup or support heat source. It’s a simple electric heater fitted inside the hot water cylinder, but it’s often misunderstood and blamed for high bills or system problems.
This guide explains what the immersion heater does, when a heat pump uses it, when it should not be running, and how to spot common issues.
What is an immersion heater?
An immersion heater is a direct electric heating element inside the hot water cylinder. Unlike a heat pump, it does not move heat — it creates heat using electricity.
Immersion heaters are:
- Simple and reliable
- Expensive to run compared to a heat pump
- Usually intended as backup or occasional support
Why heat pump systems include an immersion heater
Immersion heaters are included for several important reasons:
- Backup hot water if the heat pump is unavailable
- Legionella protection (periodic high-temperature heating)
- Emergency heat during faults or extreme conditions
- Commissioning, testing, or initial setup
In normal operation, the immersion heater should run rarely.
When the immersion heater normally runs
It is normal for the immersion heater to operate:
- During a scheduled legionella cycle
- If the heat pump is locked out or faulted
- When manually enabled for a hot water boost
- During initial commissioning
Outside of these situations, regular immersion use is usually a sign of a problem.
When immersion heater use is NOT normal
You should investigate further if:
- The immersion heater runs every day
- Electricity bills are unexpectedly high
- Hot water works only when immersion is enabled
- The heat pump never heats the cylinder on its own
These issues are often caused by control settings, sensor problems, or commissioning errors — not a failed heat pump.
Common immersion heater problems
Immersion permanently enabled
The immersion heater may be switched on at the fused spur or controller and left running unintentionally. This is one of the most common causes of high running costs.
Incorrect control settings
Some systems are set to allow the immersion heater to assist too easily, especially during cold weather or hot water reheats.
Heat pump not reaching hot water temperature
If the heat pump struggles to reach target temperature, the system may automatically call for immersion support.
Faulty cylinder sensor
A sensor that reads incorrectly can cause the immersion heater to activate unnecessarily.
What homeowners can safely check
You can safely check:
- The immersion heater fused spur is switched off (unless intentionally using it)
- Whether the controller shows immersion or electric backup active
- How often legionella cycles are scheduled
- Hot water target temperature (typically 45–55°C)
Avoid enabling the immersion heater as a permanent solution — it should be used sparingly.
Immersion heater and running costs
Immersion heaters use electricity directly, typically at 3 kW or more. This makes them far more expensive to run than a heat pump producing the same amount of hot water.
If your immersion heater is running frequently, addressing the root cause can significantly reduce electricity bills.
When to call an engineer
Contact a qualified heat pump engineer if:
- The immersion heater runs regularly without obvious reason
- The heat pump cannot heat hot water on its own
- Sensor or control faults are suspected
- Error codes appear during hot water operation
- The system has never been commissioned correctly
In most cases, immersion overuse is a settings or setup issue, not a major component failure.
Key takeaway
The immersion heater is a backup tool, not the main heat source. Occasional use is normal; regular use is not.
If your heat pump relies heavily on the immersion heater, it’s a sign something else in the system needs attention.