Cheap heating: portable heat pumps and their real limit

Portable heat pump: cheap heating below 5 degrees

Cheap heating this winter has an awkward name: the portable heat pump. And a limit almost nobody disputes: below 5 degrees outside, the contraption starts to struggle. The debate begins with a simple question —which device works in the north when the outdoor temperature drops below 5 degrees— and ends up as a treatise on domestic thermodynamics, complete with COP ratings, exhaust tubes and 20-euro electric underblankets.

The technical consensus that emerges is uncomfortable for miracle sellers: a single-tube portable unit expels air from the room, depressurizes it and forces cold outside air in. The result is that performance collapses just when you need it most. The COP of these units typically sits around 2.6, compared with 3.0 to 5.0 for fixed splits. In plain terms: for every kilowatt of electricity, you get about 2.6 of heat. Neither magic nor scam — just physics.

The 5-degree limit isn't what it used to be

The classic 5-degree threshold has aged badly. Two decades ago, the machines simply stopped heating when the outdoor thermometer dropped. Today there are units that work at -10 °C, losing efficiency but not giving up, and some that hold out down to -15 °C. The key lies in the refrigerant, the pressure and, above all, choosing carefully where the outdoor unit is placed so snow doesn't block the airflow.

The practical case described is that of a second home in northern Burgos, Spain: arriving to find the house at 6 °C after weeks empty and having a livable zone at 18 °C within an hour. A hot-water radiator can't do that — it needs 24 hours to shift the thermal inertia of a house. Here the heat pump wins hands down: it heats the air right away, even if the walls are still cold.

The tube trick and the hole in the window

The most common objection is about the work involved: do you have to drill through a wall or a triple-glazed argon window to vent the air? The sensible answer is yes, or you fit a plate accessory in the opening. And anyone who doesn't want to do that is left with the problem of the depressurized room. Cold air gets in wherever it can.

Some argue that in heating mode the portable unit's evaporator is in contact with indoor air, so the machine doesn't sense the outdoor temperature. True, until it's time to expel that air. That's where the romance ends. The full setup, with the heat-loss calculation and the tube workaround, is where you really find out whether the thing performs or just makes noise.

The 20-euro underblanket that competes with everything

Against all the technical paraphernalia, the most prepper-friendly winter alternative costs 20 euros at a bazaar-style store: an electric underblanket. Plug it in for a minute, literally, and it gives off heat for an hour and a half or two. With a blanket on top, you pay zero on the heating bill. Fire or carbon monoxide risk: none. For anyone who is genuinely cold because they can't afford the electricity bill, it's the most useful advice there is.

The comparison with an electric bed blanket has a catch: some claim it uses more power. There's no measurement to back that up, just household experience. And watch out for the fabric cover detail, because without it, it burns your hands.

The honest list of options and their fine print

The rundown of systems leaves little room for optimism. Butane or propane works, but in an apartment it's a hassle and in humid climates it leaves everything damp. Pellet prices keep rising and pellets don't work in an apartment; in a detached house, the flue can end in a neighbor complaint. Kerosene heats up fast and is expensive as a sole heating source. Firewood needs a cassette stove and doesn't always meet regulations.

The practical conclusion: there is no single system, there are systems for every home and every budget. And the portable heat pump, seams and all, remains the most efficient option for anyone who can't touch the installation.

Given all this, the forecast is that portables will keep flying off the shelves every autumn and the debate over their real COP will never be settled. Physics has the last word, but each of us writes the bill.