Ducted heat pump burns through 1,000 kWh a month in a Spanish duplex

The ducted heat pump that devours 1,000 kWh a month

A brand-new 120 m² duplex, all-electric, with no gas pre-installation, in a village in inland Barcelona province with fog and damp nine months a year. The heating: a Mitsubishi ducted heat pump on the ground floor —model SEZ-KD71VAQR2, with a stated COP of 3.57 and electrical consumption of 2.26 kW in heating mode— plus a split unit upstairs. The owner signs the mortgage and, two weeks after moving in, is already at 400 kWh. Per month: 1,000. His conclusion, written without filters: "What a piece of sh_t heating system."

The question that brought him here was a different one: is ducted heating really efficient in a home? The short answer is yes, on paper. The long answer depends on three variables almost nobody checks before signing: the climate where it's installed, the contracted electrical capacity, and whether the hot air actually stays where it should.

What the spec sheet says and what the bill says

The manufacturer's numbers are good. The larger unit delivers 8.1 kW of heat with a consumption of 2.26 kW, which works out to a COP of 3.57. In cooling mode, 7.1 kW with an EER of 3.01. On paper, for every kilowatt of electricity that goes in, more than three kilowatts of heat come out. That is, technically, efficiency.

The problem is that COP isn't a constant. It's measured under laboratory conditions and plummets when the outdoor temperature drops below 3 degrees. In an inland village with persistent fog and thermometers swinging between +6 and -6 for nine months, the unit works many hours near its limit. The result: 1,000 kWh a month with the house at 17 degrees at night and 20-21 during the day.

Some argue the problem isn't the machine but how it's used. An installer told the owner he was doing it wrong: constantly lowering and raising the temperature forces the compressor to work harder. The recommendation was to keep a stable temperature 24 hours a day. Two days at a fixed 21 degrees cut consumption by 3 to 5 kW, according to his own records. Less consumption and more heat. Counterintuitive, but consistent with how an inverter works.

The problem with hot air: hot head, cold feet

The most common complaint isn't consumption. It's comfort. Hot air rises, stratifies, and leaves your feet cold and your head hot. You suffer it daily in an office. At home, too. One participant sums it up precisely: the moment you switch the unit off, five minutes later you already feel cold. There's no thermal inertia. Water radiators, with their mass, keep giving off heat after the burner shuts off. Air doesn't.

The technical fix exists: motorized zone dampers that let you heat only the rooms in use. Another is placing the return grilles low, near the floor, so cold air flows back to the unit instead of staying stratified. In new construction it can be planned for. In a finished building, it's an expensive nightmare. The owner of this duplex can't afford it: he just signed the mortgage.

There's one detail almost nobody mentions that an installer revealed: builders sometimes seal the return out of excessive zeal, leaving the unit drawing stale air from a bathroom instead of air from the hallway. A couple of whacks with a mallet on the false ceiling and the problem is solved. It costs zero euros. Nobody checks for it.

Contracted capacity: the mistake you pay for every month

The electricity supplier recommended contracting 9.9 kW. The owner refused and contracted 6.9 kW on a time-of-use tariff. Given the consumption figures, the decision seems reasonable: the larger unit draws about 2 kW, the upstairs split about the same, and the rest of the appliances don't all run at once. With 5.5 kW, according to several calculations, a 60-90 m² apartment with an inverter heat pump manages fine as long as nothing is switched on all at the same time.

The supplier's recommendation is no accident. Contracted capacity is the fixed component of the bill that has risen the most. Paying for 9.9 kW when you need 6 is throwing money away every month of the year, including the months you don't turn on the heating. The advice that keeps coming up: match the contracted capacity to actual consumption, not to the theoretical peak of every appliance running at once.

The alternative nobody wants to hear: firewood, pellets or butane

In cold regions, a heat pump alone isn't enough. The consensus among people who live in harsh climates is clear: you need a mixed source. A ductable pellet stove, a wood-burning insert, or even a butane heater for that extra boost at peak hours. The owner had an insert in his previous apartment and knows it works. The problem is that the duplex is new-build and he doesn't know whether the homeowners' association will let him drill through the roof for the flue, even if the roof is his.

The full calculation, itemized line by line between what it costs to heat with a heat pump at peak tariff and what it would cost with pellets, yields a difference that surprises anyone who only looks at the COP in the brochure. Electricity carries taxes, a capacity charge, meter rental and VAT. Firewood doesn't.

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One figure remains that throws everything off. The electric water heater with evacuated-tube solar panels —the kind that "work better than the typical flat ones"— switches on for one hour a day on the night tariff, and the owner doesn't know whether the panel does anything at all. In an area with fog nine months a year, probably very little. But that's another bill.