One Weird Trick to Make Your Air-Conditioner Better

Those of you who read my last post know that I find summers a bit… hot. Ever since my first summer in Germany I’ve considered my trusty air cooler essential for my survival. Well, this week, with temperatures soaring to 35 degrees celsius, I had to get an upgrade.

Except that I didn’t buy the Commando 8 with 12.000 BTUs of raw cooling power, but the Comfee MPPH-07CRN7, a portable air conditioner with only 7.000 BTUs (roughly 0,5 Refrigeration Tons) of raw cooling power, for the princely sum of 199 EUR from Media Markt. It was a decent deal, exept that I thought I could carry home myself, before I saw the size and weight of that thing and had to pay more for a taxi to come and pick me up and drive the both of us home.

Setting it up just enough to get it to run was not incredibly difficult. The unit comes with a small plastic pipe to drain condensate water, and a big white flexible plastic air duct that you’re supposed to use to blow the hot exhaust air straight outside your house. I put a big bowl beside the unit where the condensate could drain out to, but the exhaust hose was a bit more tricky.

The unit comes with two ways to mount the exhaust hose: a wall mount for the ducting pipe, to use which you literally drill a 15cm diameter hole through your exterior wall and connect the duct there, and a window adapter, which you can fit into the frame of a sliding window and connect the duct to that, for a less violent solution to your exhaust woes. Unfortunately, I have a casement window (ones that swing open), so neither was going to work for me.

The first couple of nights, I used the window mount panels to cover up just enough of the window so that I could jam the exhaust pipe between the panels and the window frame, closed the window shutters above it, and stuffed a bedsheet to cover up the remaining gaps. I did however, order this: a casement window sealing kit. It’s basically a sheet of polyester that attaches to the frame and casement of your window using velcro, and then has a zipper down the middle which you can open to let the exhaust pipe through. Once that arrived, I attached it to my window, and I was ready to go.

Except not quite.

Unfortunately, there’s one giant problem with this setup - it’s grossly inefficient. And I absolutely hate inefficiencies. And because I know something’s not quite right, I can’t rest until I’ve fixed it.

Without going into too much detail about how air conditioners work - there’s an excellent video on that - there’s an important detail. Air conditioners have a hot side and a cold side. In a window AC, the hot side sits outside your window, drawing in air from the outside atmosphere, running it through the radiator to draw away the heat, and then dumping that hot air into the outside atmosphere, again. In a split AC, the hot side is a separate unit that sits somewhere outside your house, and connects to a cassette or indoor circulator through sealed coolant pipes.

The cold side, importantly, simply recirculates air inside the room, passing it through the cooling fins to take away heat from the air and thus cool it down. The cold air inside the room does not leave the room.

Except in a single-duct portable air conditioner.

Now portable air conditioners necessarily have to have both the hot and cold side in the same unit, and keep them both indoors. This is fine, as long as the unit is well insulated, and you’re using outside air to cool the hot side. Unfortunately, that last bit doesn’t quite work out with a single-duct portable air conditioner.

In a single-duct portable air conditioner, there only is one single duct, and it throws the hot exhaust from the hot side outside the room. But where does the air come from, you ask? Well, it’s using the air inside the room, which it just spent so much energy cooling down. Also, because it’s throwing cold air outside the room, it creates negative pressure inside the room, and this vaccuum gets replaced by warm air rushing into the room from every single crack and crevice that allows air to get into the room.

It’s so catastrophic, single-duct portable air conditioners should just be illegal, especially when the solution is so damn simple: dual ducts.

So I set about converting this unit into a dual-duct unit - one duct to take in fresh outside air, run it through the hot side of the unit, and then the second duct to throw the hot exhaust out.

And this Comfee unit makes it really easy. The hot side has only one air intake, a 26cm square grille right at the bottom of the unit. So I bought another ducting pipe from Amazon, fashioned an airbox out of some cardboard and, well, duct tape, and insulated the inside with some cotton wool padding covered with aluminium foil:

The Cardboard Airbox

Then I used more duct tape, along with some foam padding that came with the unit to tape the airbox to the hot side intake grille, and ran the pipe to the outside using the same window:

The Airbox Attached to the Unit

As you know, hot air rises, so I’ve kept the intake pipe on the bottom, pointing slightly downwards. Also the intake pipe is just plastic and not insulated, so it really won’t be nice if it manages to take back any of the hot air that’s going out. The exhaust pipe, the one that I bought additionally isn’t exactly insulated, but it is double layer, with a reflective aluminium inner layer covered by a PVC outer layer, so it shouldn’t get too hot and radiate the heat back into the room.

The Unit, Positioned Correctly

The Ducts

And that’s it. Now I can stay cool, knowing that I’m not damaging the environment nearly as much as I would have been if I didn’t make the dual-duct conversion.

Until next time!