"My Electricity Bills Doubled" – The Heat Pump Was Running At 55°C In Mild Weather

“My Electricity Bills Doubled” – The Heat Pump Was Running At 55°C In Mild Weather

The Problem

A homeowner contacted us after noticing their electricity bills had nearly doubled from one season to the next. The house was warm and comfortable throughout. The heat pump showed no fault codes. There was nothing obviously wrong — yet their electricity consumption had surged in a way that could not be explained by the weather or their usage patterns alone.

After reviewing their bills and asking a series of questions about how the system behaved day to day, a clear starting point emerged. The investigation needed to focus on what the heat pump was actually doing in terms of operating temperatures — not whether it had a fault.

Most homeowners in this situation assume the heat pump unit has developed a fault. In our experience, the unit itself is rarely the primary cause of high running costs.

The right starting point is always what the system is actually being instructed to do — and whether those instructions are appropriate for the conditions.

The First Clue

We began by looking at the operating temperatures during a period of mild autumn weather. The readings from the controller provided an immediate answer:

• Flow temperature: 55°C

• Return temperature: 53°C

• Outdoor temperature: 10°C

• Indoor temperature: 21°C — the house was comfortably warm

A 55°C flow temperature in 10°C outdoor conditions is significantly higher than a well-designed system needs. At that outdoor temperature, most UK heat pump installations should be supplying water somewhere between 35°C and 45°C. Running at 55°C means the heat pump is working considerably harder than the conditions require consuming substantially more electricity in the process.

This is the core reason bills had doubled. When flow temperature is unnecessarily high, the heat pump’s coefficient of performance (COP) the ratio of heat output to electricity consumed falls sharply. A system running correctly at 40°C might achieve a COP of 3.5 or higher. Running unnecessarily at 55°C, that same system might achieve only 2.0 or below. That difference alone can more than double electricity consumption.

Why Was The System Running At 55°C?

At 10°C outdoors, there is no engineering reason for a well-designed heat pump system to be producing 55°C water. When weather compensation is active and correctly configured, the controller automatically adjusts flow temperature downward as outdoor conditions become milder. On a 10°C day, it should be supplying cooler water than on a minus 3°C day. The curve ensures the system always works at the lowest flow temperature that will still heat the building effectively.

The immediate question was therefore clear:

Was weather compensation enabled, and if so, was it configured with a curve appropriate for this property?

In some systems, weather compensation is disabled entirely at commissioning and the controller is left targeting a fixed flow temperature regardless of outdoor conditions. In others, it is active but set to a curve that keeps temperatures unnecessarily high even in mild weather. Both produce the same result: high electricity bills that cannot be explained by the weather.

Looking Beyond The Heat Pump

With the operating temperature confirmed as the primary driver of high bills, we worked through several possible causes to understand where the misconfiguration had originated:

Weather Compensation Settings

The most common cause of sustained high flow temperatures in mild weather is incorrect weather compensation settings. When properly configured, the flow temperature should reduce proportionally as the outdoor temperature rises. A correctly set curve holds the balance between heating the building adequately and maintaining the efficiency the heat pump was designed to deliver. Our guide on what weather compensation does and why it matters explains the mechanics and why this single setting has more impact on running costs than almost anything else.

A system with weather compensation disabled, or with a curve configured too aggressively, maintains high flow temperatures regardless of outside conditions. The heat pump continues to produce far more heat than necessary, running at low efficiency, while the house stays at a comfortable temperature simply because the excess heat has to go somewhere.

Original System Design

We also considered whether the original installation design had specified a higher flow temperature than the property actually needed. In some cases this happens when the heat loss calculation was conservative, or when radiators were carried over from a previous boiler installation without being re-assessed at heat pump temperatures.

• The original heat loss calculation and the flow temperatures it assumed

• Whether radiators were re-sized for heat pump operation or carried across from the previous boiler installation

• The design flow temperature formally specified for this system

• The control strategy.

If the system was designed for a lower flow temperature, there is no reason the controller should be targeting 55°C during mild weather regardless of how wide that radiator safety margin might be. Our guide on what flow temperature a heat pump should run at explains the appropriate range for different property types and the efficiency cost of running unnecessarily high.

Thermostats And TRVs

We also examined how the room controls interacted with the system. In some cases, the thermostat and TRV configuration can indirectly contribute to sustained high flow temperatures by creating demand patterns that the controller cannot resolve efficiently.

A configuration that creates problems in this way typically looks like:

• A room thermostat set higher than the room can realistically reach given the building’s heat loss

• Because the thermostat is never satisfied, the heat pump continues heating without reaching steady state, maintaining high flow temperatures throughout

The heat pump is not at fault in this scenario. It is responding correctly to the instruction it is receiving. The problem is that the instruction does not reflect the actual heating demand — and correcting the thermostat configuration resolves it.

Another Clue: The Delta T

The difference between the flow (55°C) and return (53°C) temperatures was only 2°C. This is an unusually small Delta T and it carries diagnostic significance. Our article on what Delta T means on a heat pump system explains why this reading is one of the most useful indicators of what the heating circuit is actually doing.

A 2°C Delta T at 55°C told us the circuit had good circulation water was moving quickly around the system. It also told us the water was not giving up much heat per pass through the radiators. At that flow temperature, the radiators were clearly not undersized; they were managing to emit heat adequately at 55°C. The question was whether the system could emit the same amount of heat just as effectively at a lower flow temperature.

The Delta T reading directed investigation toward:

• TRV positions and whether any circuits were being partially restricted

• System balancing and whether heat was distributed proportionally across all radiators

• Whether any bypass arrangements were diverting flow away from heating circuits

• Circulation pump speed settings and their effect on heat distribution

• Heat distribution around the property.

These are hydraulic and control factors. None of them require the heat pump to be replaced or repaired they require the system operating around the heat pump to be correctly set up.

The Outcome

The investigation confirmed that the heat pump unit was operating correctly throughout. It was doing exactly what it was being instructed to do. The doubled electricity bills were a direct consequence of the flow temperature being held at 55°C regardless of outdoor conditions a result of how the weather compensation was configured rather than any fault with the heat pump itself.

With the weather compensation curve corrected for this property, the flow temperature dropped to an appropriate level for mild weather conditions. The heat pump began operating at a COP much closer to its designed efficiency, and electricity consumption fell substantially. Our detailed guide on how to set weather compensation correctly for a heat pump walks through the process of adjusting the heating curve for different property types and emitter configurations.

We see this pattern regularly across UK heat pump installations. A closely related example is a homeowner who had been fighting rising bills and accidentally made the problem worse by making changes without understanding the interaction between weather compensation and flow temperature. Another example is a property in Berkshire where weather compensation had been disabled at installation handover. In both situations, the heat pump unit was fully functional the control configuration was the problem.

The most directly comparable situation is a family home in Birmingham where the heat pump ran at 55°C all winter with higher bills than expected. The investigation followed an almost identical path and reached the same conclusion: no fault with the heat pump, but a significant and correctable misconfiguration in how flow temperature was being managed.

What This Shows

High electricity bills from a heat pump are almost never caused by a fault in the unit itself. In the large majority of cases, they are caused by the system operating at a higher flow temperature than the conditions demand and the most common root cause is weather compensation being absent, disabled, or incorrectly configured for the property.

Getting flow temperature right is the single most impactful thing that can be done to reduce heat pump running costs. Our article on why heat pump running costs are higher than expected covers every common driver of excessive electricity consumption, including flow temperature, weather compensation, immersion heater use, and system design.

Almost every case we investigate of this type requires no new hardware. It requires a thorough review of the system, a clear understanding of what the controller settings actually do, and the correct adjustments applied in the right sequence.

Need Help Understanding High Running Costs?

If your heat pump is using more electricity than expected, or if bills have increased significantly without a clear explanation, our Fix My Heat Pump service reviews your operating temperatures, controller settings, weather compensation configuration, and system data to identify precisely what is driving the higher costs.

You may also find our guide on how to reduce heat pump electricity bills useful — it explains the practical adjustments homeowners can make to bring running costs back to appropriate levels without specialist assistance.

Planning A Heat Pump Installation?

Many of the running cost problems we investigate originate in decisions made before the heat pump was installed specifically an inadequate heat loss calculation, an incorrect flow temperature specification, or weather compensation settings that were never calibrated for the property.

Our Pre-Installation Design Review provides an independent assessment of the proposed system design including the flow temperature strategy, weather compensation approach, and heat pump sizing before any installation work begins.