Aerotermia in Spain: When It Pays Off and When It Doesn't

Heat pumps: The €20 bill hiding years of investment

Filling a diesel tank twice a year cost around 800 euros when the liter was priced at 0.55. Today, that same expense exceeds 2,000, and those needing a third fill face costs near 3,000. This surge has led many homeowners to consider aerotermia (air-source heat pumps) with a mix of hope and skepticism about potential scams. Currently, Spanish autonomous communities are requesting substantial EU funds to subsidize these eco-friendly installations. The central question remains: is it a rip-off or a self-paying investment?

What is aerotermia and why does the budget skyrocket?

Essentially, it is an air-to-water heat pump: an inverter-type unit that extracts heat from outside air and transfers it to the heating circuit, using water tanks for energy storage. It is not instantaneous and depends on the building's thermal inertia. Here lies the first problem: the equipment is bulky. Some units take up more space than a large fridge, ruling them out for apartments without patios or houses lacking a boiler room.

Installation is also complex, not just a simple boiler swap. You must review contracted power, certifications, and often adapt existing systems. Those switching from aluminum radiators will feel the cost: air-to-water pumps work best with water at 40 ºC (underfloor heating temperature) and struggle if forced to push water to 70 ºC as required by classic radiators.

How much do you really save with aerotermia?

The analysis splits into two scenarios. The standout case: a home with 6 kW photovoltaic panels installed and 5.75 kW contracted power, generating 35-40 kWh daily on clear days, paid only 20 euros for electricity in February. The same house, maintained at 22.5-23 ºC despite outside temperatures of -3 ºC, performed well. Even during Storm Filomena, the heat pump did not fail.

In contrast, another detached house running aerotermia alongside a diesel boiler reported a daily electricity increase of 25 kWh, leaving the owner disappointed. For many participants, the difference between these cases lies not in the machine but in insulation, orientation, and installation quality.

Cold weather: Where efficiency drops

Heat pump performance decreases as outside temperatures fall. At 7-9 ºC outdoors, a unit consuming 5 kW of electricity can deliver up to 20 kW of thermal power; below zero, it delivers less precisely when needed most. This is physics, not a design flaw.

Consequently, many advise against using aerotermia as the sole system in harsh continental climates. Air-to-water pumps work well at 36 ºC supply temperature (39-40 ºC in severe cold), provided the house retains heat. If not, the unit runs all day, and the meter keeps spinning. Some opt to supplement with wood, pellets, or olive pits (used in rural Soria and Huesca), or stick to traditional diesel stoves, which are cheap to repair and have widely available parts.

Is the investment amortized or money wasted?

Honest calculations depend on the starting point. For those moving from diesel to solar-assisted aerotermia, the payback period is roughly ten years or more, and only if subsidies apply. The hidden cost nobody signs off on is repairs: a complex system with exchangers and electronics where a breakdown can wipe out savings accumulated over several winters.

A gaining alternative is simpler. Instead of air-to-water pumps, some advocate for standard air-to-air heat pumps (AC units with heating mode) due to easy installation and lower cost. They don't heat water or provide underfloor comfort, but they work. For rural residents with time and dry wood, closed stoves remain the cheapest option to maintain.

So, scam or viable option?

Short answer: It depends on the house, not the machine. With decent insulation, proper installation, and solar support, aerotermia lowers bills to figures once thought impossible. Without these conditions, it is a high expense yielding less than promised, taking longer to pay back than brochures suggest.

One data point confuses those considering the switch. The same technology generating a €20 bill in February causes a 25 kWh daily spike in another house, without either owner lying. Aerotermia doesn't fail; what surrounds it does.