Heat pump and electric backup: when and why?

Understanding the role of the electric backup on a heat pump

Difference between backup, resistive element and boiler relief

The electric backup is a standby heat source built into the heat pump. It activates when the machine can't cover demand on its own. The electric element is the part that produces this heat, like a radiator. Boiler relief, meanwhile, switches over to a gas or oil boiler based on a bivalence temperature, to keep up power output without drawing too much on the electricity supply.

Situations where a heat pump reaches its limits (cold weather, defrost, high-temperature regime)

In extreme cold, available power output drops, and so does efficiency. During defrost, the heat pump reverses its cycle and heat delivery can be interrupted for a few minutes. Finally, in a high-temperature regime — radiators designed for 60°C, for example — the heat pump has to strain, and the backup can trigger more often.

What the backup changes for comfort and for the bill

Steady comfort is the payoff, since the backup prevents temperature drops during peak demand. On the bill, you need to stay vigilant. A resistive element heats with an efficiency close to 1, while a well-tuned heat pump can deliver several kWh of heat per 1 kWh of electricity. If the backup runs often, consumption climbs and seasonal performance drops.

Precisely identifying when the backup triggers on site

The most common triggers: outdoor temperature, setpoint, heating curve

On a heat pump, the (often electric) backup starts when the machine can no longer reach the requested flow temperature. Classic causes are an outdoor temperature below the bivalence point, a setpoint that's too high, or a heating curve that's too steep and quickly demands high temperatures. To fine-tune this, refer to the heating curve.

Settings and controls: heating curve, hysteresis, time schedules

Check the heating curve, then the hysteresis. Too narrow, and it triggers the backup at the slightest deviation. Also look at the time schedules and any permanent comfort mode, which blocks the night setback. A reading from the controller or the app gives the exact moment of switchover.

Signs to check when troubleshooting: faults, short cycling, low flow, low refrigerant charge

If the backup activates without notably cold weather, look for an alarm, short cycling, or low flow (filter, circulator, sludge build-up). A low refrigerant charge or a fouled heat exchanger causes power to drop. Check delta T, flow rate, and backup current draw.

Sizing: avoiding an oversized heat pump… and above all an undersized one

What sizing tells you: heat loss, bivalence point, power output at -7°C

The right starting point is the home's heat loss. The heat pump is sized to cover most of the need at the reference cold temperature (often power output read at -7°C). The bivalence point indicates the outdoor temperature below which the backup takes over. If it's poorly placed, the element switches on too early, or the house struggles to keep up.

Typical cases that overwork the element: radiators, domestic hot water, insufficient insulation

The element is triggered mainly when temperature needs to climb high. Radiators designed for 70°C, overly ambitious settings, or insulation that's still "leaky" in winter. Domestic hot water can also create demand spikes. The result: the electric element runs more than expected, and the bill climbs.

A simple check method: measurements, readings and power-output consistency

On site, cross-check three things. 1, the power output calculated in the heat-loss study. 2, the heat pump's actual available power output at -7°C per the manual. 3, your readings (run time, kWh dedicated to backup, flow temperatures). If the backup activates while the setpoint is modest, look for an inconsistency in settings, emitters, or insulation. To go further, also see how to avoid heat pump sizing mistakes on the job.

Backup and resistive element: best-practice settings to limit consumption

Recommended settings: bivalence threshold, power limitation, backup authorisation

On an air/water heat pump, the electric backup mainly serves very cold days or as a safety net. Set a bivalence threshold consistent with the local design temperature. Limit backup power to the strict minimum and authorise it only when the pump can no longer cover demand. Also think about time schedules and giving priority to the heat pump, to avoid a backup that "takes over" too early.

Site tips: balancing, sludge removal, pipe insulation, emitter settings

Before touching the menus, secure the hydraulics. Balancing the loops, sludge removal if the network is old, and good pipe insulation in unheated spaces prevent the element from making up for losses. Set the heating curve, check flow rates, and adjust the emitters (valves, taps, controls) to aim for the lowest possible flow temperature.

How to explain backup usage to the client without raising concern

Present the backup as a safety belt. It's there to guarantee comfort, not to heat the whole season. Show where to check its operation, set a monitoring point for the first winter, and give a reassuring message about normal triggering cases (extreme cold, reheat after absence, anti-legionella depending on the domestic hot water setup).

What's changing in 2026: client expectations, electricity costs and quality requirements

In 2026, why backup restraint becomes a selling point

With electricity prices staying under scrutiny, your clients want a heat pump that heats without triggering the backup at every turn. Promoting less backup use comes down to good insulation, well-matched emitters and controls that are simple to understand. The result: a clearer bill and fewer "it uses too much" callbacks.

RGE, support schemes and inspections: points of vigilance on sizing and settings

Between MaPrimeRénov' and the CEE certificates, quality is no longer a promise — it's a condition. Inspections often target correct sizing and baseline settings. A heat-loss estimate, a consistent flow temperature and a well-calibrated heating curve avoid overconsumption and comfort gaps. To go further, use the method for estimating heat loss.

Documenting your commissioning: documents, readings and proof of correct settings

To secure your files, keep simple proof. A signed commissioning sheet, photos of the settings, heating instructions, temperature readings and, where possible, a short operating log after 24 to 48 hours. It's your logbook — useful for inspections, and valuable for after-sales service.