Every year, when it gets hot, I think the same thing: it is time to ban single-hose portable air conditioners. (Yes, I’m boring like that.)

This isn’t some crusade against air conditioning; quite the opposite. Cooling is great, and well-designed and implemented cooling will make people’s lives better and sell more heat pumps. The problem is that many of the portable air conditioners purchased are terrible.

A single-hose portable air conditioner has a heat pump. It has a compressor that makes some refrigerant very hot. It pulls in air from the room and blows it across the heat exchanger, and then blows that hot air out of a window through an exhaust hose. That process is absurdly illogical.

These units take some of the air from the room they are cooling and eject it, inducing negative pressure inside the room. So to equalise the pressure, air must be drawn into the room from elsewhere. Ultimately this must be from outside: the warm place…

So a single-hose portable air conditioner is doing something fundamentally absurd: it is trying to cool your room while actively sucking hot air back into the building.

This creates a hilarious inefficiency: the unit has to work extra hard to make up for the fact that it is drawing in warm air from outside.

There are portable aircon systems that don’t have this design flaw; they are just uncommon in the UK. The first type is dual-hose portable units, which have an intake and an exhaust. In this case the indoor and outdoor air streams are kept separate. Intake air from one hose is passed over the heat exchanger, then promptly ejected through the exhaust hose. So no negative pressure is created, and only the air in the room needs to be cooled.

With a single-hose unit, you are exhausting cooled indoor air. With a dual-hose unit, you are using outdoor air as the heat-carrying medium.

And of course, a proper split air conditioner is better again. The hot side is outside, the cold side is inside, and there is no large hose pumping room air out of the building. There are now portable versions of these, such as the Midea Portasplit. I’ve no idea how good these are.

As an anecdotal example of how inefficient the portable units are: in one house where I installed a proper 3.5 kW split air conditioner, it used around 800 W to maintain roughly 24°C across about three-quarters of the house when it was 35°C outside. In the same broad conditions, a portable single-hose unit was using around 800 W just to cool one bedroom. Obviously this isn’t a laboratory test, but I’m relatively confident in the efficiency penalty.

In a way they do need to be banned, as people have no concept of how inefficient they are, or “WHY” they are inefficient. It is only something you will think about when you become deeply involved in the cooling world.

Long term, if the UK is going to use electricity for cooling, we should use it well. We should not be filling homes with machines that dump cooled room air out of the window and drag hot air back in to replace it.

We do not need to ban air conditioning.

We need to stop pretending that the worst version of air conditioning is good enough.