Heat pump boiler: eight electricity cost traps to avoid

The conventional electric boiler has long been regarded as an energy guzzler. Modern heat pump boilers, often referred to as “thermodynamic hot water heaters”, are intended to address that issue. They draw on ambient heat to warm water far more efficiently. However, poor planning, an unsuitable location or badly configured settings can still make this appliance use more electricity than most people would like – and that is where eight common electricity-cost traps arise.

Why hot water can account for up to one fifth of your electricity costs

When considering an electricity bill, most people first think of heating, the fridge or lighting. Hot water is easily overlooked. Yet, in many households, producing hot water represents 15 to 20 per cent of total electricity consumption.

A correctly sized and properly configured heat pump boiler can cut this share considerably. Compared with a standard electric boiler, it typically uses only one third of the energy. Measurements for a 200-litre cylinder provide the following example:

• Electric boiler: around 3,500 kWh per year (approximately €700 at 20 cents/kWh)
• Heat pump boiler: 800 to 1,300 kWh per year (approximately €160 to €260)

This can mean savings of up to €500 annually – provided the unit operates at its best. In practice, this is often precisely where things go wrong.

“A heat pump boiler only saves significant money when its location, size, temperature and operating mode are properly matched to the household and building.”

Electricity-cost trap 1: Choosing the wrong cylinder capacity

The first mistake is often made at the point of purchase: the cylinder is the wrong size. Both undersizing and oversizing can prove expensive.

A boiler that is too small: costly back-up operation

If the cylinder capacity is insufficient, it will regularly run out before everyone has showered. The electric back-up heater then takes over. It operates almost as inefficiently as an old electric boiler and consumes a great deal of electricity.

• A family of 4 taking frequent showers: 200 litres is often the minimum
• Frequent baths or a large bath: allow for 250 to 300 litres
• A single person or couple with typical showering habits: 150 to 200 litres is often enough

Manufacturers provide guideline figures that are useful as an initial reference point. Anyone who continually triggers “back-up operation” should have the cylinder size checked.

A boiler that is too large: hidden standby losses

An oversized cylinder must keep a large volume of water hot at all times, even when it is not used every day. This creates unnecessary standing losses. Heat gradually escapes through the insulation, while the boiler keeps reheating the water.

As a rule of thumb, it is better to be slightly short on capacity than substantially oversized – provided users are willing to adjust their showering habits a little.

Electricity-cost trap 2: The wrong room – location matters too

Heat pump boilers take energy from the surrounding air. The warmer that air is, the better their efficiency. This is an area in which many installations make crucial mistakes.

A cold corner instead of a moderately warm room

Installing the unit in an unheated, cold room or a draughty cellar wastes potential. The heat pump then has to work much harder to heat the same quantity of water.

More economical locations include:

• A utility room
• A garage with a base temperature above freezing
• A heated cellar room or an adjoining room to the heating plant room

The room must remain frost-free. In genuinely cold cellars, part of the system’s benefit may otherwise disappear.

Too little air: the appliance is starved

The built-in heat pump needs sufficient air volume to operate properly. Fitting it inside a tiny, airtight cupboard is problematic.

“If the heat pump boiler is installed in a very small, poorly ventilated room, its efficiency falls – and, in extreme cases, operational faults may occur.”

Keep the following in mind:

• Do not enclose the appliance in a completely sealed cupboard
• Fit grilles or ventilation openings in doors and walls
• Ensure there is adequate room volume; if unsure, ask the installer

Electricity-cost trap 3: Insufficient insulation for the cylinder and pipework

Heat that escapes without being used costs real money. This applies equally to the cylinder and the pipes.

• If the outside of the cylinder feels noticeably warm, an additional insulation jacket is often worthwhile.
• Uninsulated hot-water pipes in cold cellars or service voids lose substantial amounts of energy.

Simple pipe insulation from a DIY retailer can reduce hot-water costs by up to ten per cent, while requiring very little investment.

Electricity-cost trap 4: Setting the water temperature too high

Many users set the target temperature much higher because they worry about water being too cold – and pay extra for it every month.

Most manufacturers recommend a range of 50 to 55 degrees Celsius. This is sufficient for both comfort and hygiene. Every additional degree above this can increase electricity costs for hot water by around seven per cent.

“Anyone who permanently sets 60 degrees rather than 50 degrees can, in extreme cases, increase their hot-water costs by almost one third.”

For hygiene reasons, temperatures below 50 degrees should not be used permanently. Many appliances allow regular “hygiene cycles” to be programmed, briefly heating the water to a higher temperature to help prevent Legionella.

Electricity-cost trap 5: The wrong mode – permanent boost instead of Eco

Almost every heat pump boiler has several operating modes. These have a direct effect on electricity consumption.

• Eco or heat-pump mode: The standard setting, which mainly uses ambient heat. This is where the greatest savings are achieved.
• Boost or rapid mode: Heats water quickly using the electric back-up heater. Useful after many showers, but highly energy-intensive.
• Away mode: Lowers the temperature during holidays or longer absences.

A major mistake is leaving boost mode permanently switched on. The appliance then behaves almost like an ordinary electric boiler, making the additional investment in the heat pump barely worthwhile.

Electricity-cost trap 6: Unsuitable timer programmes

Time controls are designed to save electricity, but poorly configured programmes can have the opposite effect.

Many tariffs offer off-peak electricity or cheaper time windows. Heat pump boilers should preferably run during these periods. However, the air temperature in the installation room is often lower overnight, reducing the heat pump’s efficiency.

“The ideal approach combines cheaper electricity periods with the warmest possible ambient air, for example in late afternoon or early evening in a lightly heated room.”

A sensible arrangement could look like this:

• Schedule most heating for the cheapest tariff periods.
• Move a small proportion to time windows when the room is at its warmest.
• Align showering routines broadly with the heating schedule, so hot water is available when needed.

Electricity-cost trap 7: Neglecting maintenance

Heat pump boilers need less maintenance than traditional boilers, but they still require some care. The air filter is one frequently underestimated component.

When the filter at the air intake becomes blocked, the heat pump has to work much harder. Electricity use rises and service life may decrease. A brief cleaning routine every few months is usually sufficient:

• Switch off the appliance and isolate it from the electrical supply.
• Remove the filter from the upper section of the heat-pump unit.
• Clean it with lukewarm water and a little mild washing-up liquid.
• Allow it to dry before refitting it.

In hard-water areas, it is also worthwhile regularly checking for limescale deposits in the cylinder and inspecting the safety valve assembly. Limescale insulates the heating surfaces, increasing energy demand.

Electricity-cost trap 8: Everyday habits undermine the technology

Even the most efficient heat pump boiler has limits when hot water is needlessly sent down the drain. This is often where the greatest savings can be made entirely free of charge.

• Take short showers rather than frequent baths
• Use water-saving shower heads and tap aerators
• Repair small leaks and dripping taps promptly
• Roughly coordinate shower times with the cylinder’s heating periods

“Every litre of hot water saved directly reduces electricity consumption – without any loss of comfort when the technology is used intelligently.”

How the individual factors reinforce one another

The impact becomes particularly interesting when several measures are combined. For example, someone who lowers the temperature from 60 to 52 degrees, insulates the pipes, uses Eco mode permanently and takes only short showers is adjusting four factors at once.

The benefits add up: lower cylinder losses, a smaller temperature lift for the heat pump, fewer reheating cycles through consistent use of Eco mode, and simply less hot-water demand in daily life. On an electricity bill, these improvements can combine into a clearly noticeable sum.

Key terms explained briefly

Heat pump boiler / thermodynamic hot water heater: An appliance that, like a heat pump, uses energy from the air to heat water. It contains a small refrigerant circuit, comparable with a refrigerator but operating in reverse.

Coefficient of performance (COP): A measure of how efficiently the heat pump operates. A COP of 3 means that one unit of electricity produces three units of heat. This value depends heavily on ambient temperature and the selected settings.

Those who understand their appliance, choose the installation location carefully, check the settings and make small changes to their habits can make much better use of the technology – and noticeably reduce their electricity bill without having to go without hot water.