Single vs dual hose portable air conditioners

Choose a single-hose portable air conditioner if the room is small, you will use it a few weeks a year and you want to spend as little as possible. Choose a dual-hose unit if the room is large, hot or poorly sealed, or you will run it most days in summer.

Every portable air conditioner must cool its condenser, the coil that gets hot when refrigerant is compressed. A single-hose unit does this with air from the room it is cooling.

1. Room air is drawn into the unit.
2. Part of it is cooled and returned to the room.
3. Part is blown over the hot condenser and pushed out of the window through the hose.
4. The room now has less air in it than before, so air from outside and from other rooms leaks in through gaps to replace it.
5. That replacement air is warm, and the unit has to cool it too.

This is the “negative pressure” problem. The unit is partly working against its own exhaust. The effect is largest in leaky rooms, in very hot weather and in rooms where the unit is large for the space.

A dual-hose unit has a second hose. One hose brings outdoor air in to cool the condenser, the other sends the heated air back out. The room air that passes through the cooling coils stays in the room.

Because no room air leaves, the room stays at normal pressure, nothing has to leak in to replace it, and all the cooling the unit produces is delivered to you. The price is a second hose, a wider window kit and a higher purchase cost.

Be wary of exact percentages. No UK test lab publishes head-to-head results by hose type, and manufacturer claims vary. What the physics and the regulators tell us is consistent:

• US efficiency rules for portable air conditioners account for the air a single-hose unit pulls into the room when they calculate its real cooling capacity, which is why a unit once sold as 14,000 BTU is now often listed at around 10,000 BTU on that basis.
• Commonly quoted estimates put the real-world advantage of dual hose at 10 to 30 per cent more cooling for the same electricity.
• The gap widens as outdoor temperature rises, as the room gets larger and as it gets leakier.
• It narrows to almost nothing in a small, well-sealed room on a mild day.

A hose-in-hose (or coaxial) unit puts two air paths in one thick hose: a tube inside a tube. It behaves like a dual-hose unit but needs only one hole in the window kit, which makes it easier to seal in a sash window. The hose is wider and stiffer, so check it reaches your window without tight bends.

Only partly. Suppose a dual-hose unit needs about 0.2 kW less power than a single-hose one to deliver the same cooling. At 26.32p per kWh, the saving is about 5p an hour.

Dual hose rarely repays its premium through electricity alone unless you run the unit heavily. The real case for it is comfort: a lower room temperature, faster cooling and less draught. Work out your own numbers with the running cost calculator.

A single-hose unit needs one hose-sized opening and usually sets up in 15 to 30 minutes. A dual-hose unit needs two openings, either two holes in a panel kit or a wider slider, and takes longer to seal. On a sash window, ask whether the kit has two apertures or choose a hose-in-hose model. See how to install a portable air conditioner for the step-by-step.

Still deciding? Our how to choose a portable air conditioner guide puts hose type in the context of size, noise and budget.