Heat Pump vs Radiator: Real COP Doesn't Match the Brochure

Heat pump or radiator: when the real COP doesn't match the brochure

Three heat pump units, same square footage, same temperature, same time slot, and outdoor temperatures between 0 and 10 degrees. The result, measured by a user at home rather than in a catalog, is that electrical consumption barely differs from that of a resistance radiator. This data challenges the commercial promise that each kilowatt of electricity converts to three or four of heat, raising a question many homeowners ask before spending: Has heat pump efficiency truly improved over the last two decades, or are we still selling the same machine with a different gas?

The short answer is that there has been improvement, but not what the brochure suggests. The long answer depends on the machine, insulation, and, above all, outdoor temperature. And on a detail almost no one checks: the consumption of the compressor sump heater when the unit is off.

What COP really measures for a heat pump

COP is the ratio between heat delivered indoors and electricity consumed. A modern unit with R32 or R410A gas can deliver 4 to 6 kilowatts of heat per kilowatt of electricity, but this figure is measured under laboratory conditions, with mild temperatures and continuous operation. When the thermometer drops below 5 degrees, performance falls. At -7 degrees, a 16-kilowatt unit may drop to 9, not because it consumes less: simply, it cannot deliver more.

The key is that a heat pump does not generate heat; it moves it. It extracts energy from outside air and injects it inside. The colder the outside air, the harder this transfer is, and the more electricity is needed for the same result. At temperatures near zero, the unit enters defrost cycles: it reverses the cycle to heat the outdoor unit and melt frost, effectively stopping heating the house for a few minutes.

Why insulation matters more than the machine

There is an uncomfortable consensus in the sector: the most cost-effective investment is not the heat pump, but insulation. Those who have renovated with rock wool, expanded polystyrene, and high-end windows report maintaining 24 degrees in summer with high outdoor temperatures and seeing consumption plummet. The machine becomes a complement, not the main cost driver.

The problem is that most installations are sized for the worst day of the year, not the average day. This leads to oversized units that start and stop every few minutes, destroying COP and shortening compressor lifespan. A 5-kilowatt unit with underfloor heating and continuous operation performs better than a 12-kilowatt unit with conventional radiators and intermittent use.

The phantom consumption no one tells you about

Here comes the surprising data. Modern heat pumps, to start quickly, heat the compressor oil sump. This heater consumes between 100 and 400 watts depending on the unit, and it does so continuously, even when the machine is off. Measured with a clamp meter, a unit turned off for months can consume 7 kilowatt-hours daily. This is a hidden cost not shown on efficiency labels, as tests are conducted with the unit running.

The story has its irony: the system for heating the sump for quick start is a technique Japanese engineers used in World War II submarines. Technology travels, but so does the bill.

Aerothermy, gas, and the 24-cent threshold

The calculation ordering this debate is simple. With a condensing gas boiler on regulated tariff, producing one kilowatt of heat costs about 6 cents. With aerothermy and a COP of 4, producing those same four kilowatts costs one kilowatt of electricity. If electricity is 24 cents, they break even. Above that price, aerothermy loses. Below, it wins.

The problem is that a COP of 4 is optimistic in winter. With outdoor temperatures of 0 to 10 degrees and conventional radiators, real performance drops, and the advantage narrows or disappears. Several homeowners who installed aerothermy with radiators report a 50% increase in electrical consumption and had to keep their old gas boiler as backup. The repeated conclusion: without underfloor heating and insulation, aerothermy is an expensive machine operating below its potential.

Mitsubishi, Daikin, Fujitsu: brand doesn't defy physics

Regarding manufacturers, consensus points to Japanese brands as the reference: Mitsubishi, Daikin, Fujitsu, and General. Toshiba and Carrier share technology, as do Fujitsu and General. Older LG units, like the 2003 model still running, hold up, but do not compete in efficiency with current models. Mundoclima and Saunier Duval generate more divided opinions.

The lingering question is whether it is worth replacing units that still work. With current performance differences, payback depends on electricity prices, home insulation, and whether underfloor heating can be installed. For those with gas on a regulated tariff, the margin is narrow. For those with an old heat pump, improvable insulation, and high electric bills, the calculation may lean toward replacement. But it should be done with data measured at home, not from the brochure.

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The prediction, with reservations: If electricity prices remain above 24 cents and gas stays on a regulated tariff, aerothermy will remain a gamble for those with underfloor heating and good insulation, and a bad deal for those trying to heat conventional radiators in a leaky house. Physics does not negotiate.