Single hose air conditioners can't cool a room: Convert them to dual hose

Single hose air conditioners (AC) should be banned. By design, they are inefficient; they can blast cold air at a person sitting directly in front, but will never be able to cool the rest of the room. Single hose ACs are bad news for the environment, energy bills and your heatwave comfort!

The problem for UK residents is that single hose ACs are the only type of portable model available to buy. Dual hose models or window units are not available, despite their drastically better cooling performance. But what makes single hose ACs so bad?

Background: Air conditioners are just refrigerators with fans

Most air conditioning systems work by exchanging heat from the inside air to the outside air, using vapor-compression refrigeration. All efficient designs (anything that’s not a single hose) do this without ever mixing the separate air masses or causing any pressure differential between the outside and inside. It doesn’t matter whether it’s a split AC unit in a commercial property, the AC in your car, or your kitchen refrigerator, they follow this operating model:

Dual hose portable air conditioners follow the same principle, but incorporate the outside unit into a single inside unit; instead using ducting hoses to draw outside air over the condenser coils and back to the outside again. They are more convenient as no permanent installation is required, but are marginally less efficient due to heat loss through the hoses and the additional heat generated from the compressor which is now inside the room that needs to be cooled.

What makes single hose ACs so bad?

Single hose models are the only type that violate this principle, which is why they are so inefficient. Instead of using two hoses to keep the air masses separate and drawing outside air to pass over the condenser coils, they ditch the intake hose and just draw the inside air instead. Why? Probably because they’re cheaper to manufacture and easier for customers to install.

But this design has a huge performance cost. With the inside air now being forced to the outside through the exhaust hose, the inside is now always at a lower pressure. No room is perfectly sealed, so to equalise the pressure, hot air from outside is continuously drawn through door frames and a multitude of other tiny gaps in the room. This is why single hose units will never cool an entire room, they are constantly fighting against hot air rushing back into the room due to the negative pressure.

Don’t believe me? Turn the AC on in a small room, use a good seal between the exhaust hose and the window, then close all doors and feel around the gaps between the door and the frame. You’ll be able to feel the air rushing in to the equalise the negative pressure in the room you’re trying to cool.

Solution: Convert the single hose AC to dual hose

Thankfully, many single hose ACs can easily be converted to dual hose systems. Single hose systems are often designed with two separate intake ports just like dual hose units, despite only needing one; presumably to reduce design and manufacturing costs. By fashioning an adapter to duct the condenser intake port so that it draws from the outside air, it will perform with the same efficiency as a dual hose unit.

There are two requirements that a single hose air conditioning unit must meet to make conversion possible:

  1. There must be two separate air intake ports for the evaporator and condenser sides.
    1. The unit likely meets this requirement if it has three separate air grilles in addition to the exhaust hose.
  2. There must be an internal separation between the evaporator and condenser side air flows. There are two ways to find this out:
    1. Find photos or designs from others that have explored the internals of your model.
    2. Open the unit up and investigate yourself. Do not do this unless you are confident about electrical and refridgerant safety and are prepared to invalidate the warranty of your machine.

To convert the unit from single host to dual hose:

  1. Source an additional air conditioning hose or equivalent.
  2. Identify which air grille on the unit is the condenser (hot coil) air intake.
  3. Create an adapter to duct this air grille into the additional air conditioning hose, so that it can be made to draw air from the outside.

My conversion example

I purchased a “Challenge 5K Air Conditioning Unit” in 2019 for £289.99 from Argos. Despite only being 5000 BTU, after conversion to dual hose it was able to cool a medium sized room from 34 C outside temperatures down to 22 C in only a couple of hours.

The conversion was performed in minutes, using items easily purchased at a hardware shop or found around the house:

  1. A tumble drier hose.
  2. Discarded polystyrene packaging foam and cardboard.
  3. Spare foam strips that came along with the air conditioner to provide a better seal between the polystyrene and the unit.
  4. Plenty of duck tape.

This isn’t an ideal set of materials, but works well enough to tide over for the summer.

On the Challenge unit, the condenser air intake is the lower grille underneath the exhaust hose port. After ducting this grille into the tumble drier hose, this is the result. Not aesthetically pleasing, but very effective for the three days of summer that the UK gets every year:

Further tips for optimum efficiency

  • Purchase a window sealing unit designed for your type of window.
  • Keep the intake and exhaust hoses away from each other when outside of the window. Having them close to each other increases the change that the intake hose is drawing in freshly exhausted hot air. If stacking vertically, I like to keep the intake hose below the exhaust hose for this reason too, as hot air rises slightly.
  • Keep the length of the hoses short, to minimise losses back into the room. Move the unit as close to the window as possible.