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I have a strange question/theory that I'd like some advice on.

I currently have a garden shed that is being used as a home office. On sunny days, the inside of the shed gets very hot - too hot for me to be comfortable - about 30C. Outside temperature would be around 22-23C during these times.

My understanding is that the inside of the shed is getting heated by radiation - both from the small perspex window as well as the wooden walls/roof (the walls and roof are insulated with thin PIR insulation and plasterboard. This only has limited insulation capability as the frame of the shed is only 20mm thick so I couldn't fit any thicker celotex.)

Now, I have installed a single-hose portable air conditioning unit inside the shed, with its hose fed outside through a hole that's been drilled. This unit does work, but at 30C inside (with the unit set to 25C), it does struggle to bring the temp down.

Due to the single hose nature of the unit, this creates a negative pressure differential causing outside air to come inside. Normally, this is why single-hose units are deemed "bad" and inefficient. However in my case, as the outside air is significantly cooler than inside the shed, and I'm only setting the unit to 25C, does opening the door actually help with efficiency in my case? From a very brief and quick non-scientific experiment, the shed does seem to cool down faster....

Am I on to something here? I'm running the unit on solar power to efficiency does matter.

If the answer is "yes", then my next question is: Is is more efficient than a simple fan extractor installed at the opposite end of the door (instead of the air conditioning unit)?

Thanks for your help

user42931
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    Is there a reason you don’t just use a fan? – Eric S Sep 05 '23 at 13:43
  • If you're asking about a simple desk fan pointed to my face, I do indeed use this as well. But it's not enough to keep comfortable when it gets close to 30C. – user42931 Sep 05 '23 at 14:16
  • no, a box fan in a window – Tiger Guy Sep 05 '23 at 14:32
  • @user42931 are you asking if bringing in 22-23C air from the outside cools off the shed? – jsotola Sep 05 '23 at 15:32
  • An attic fan would help a lot. – Eric S Sep 05 '23 at 16:06
  • @jsotola that is indeed part of my question. That, and how does having the door open with my AC unit compare to door closed. – user42931 Sep 06 '23 at 14:06
  • I'm just trying to understand why you are asking this question ... is it not your experience that when you leave a door open, then heat moves through the door, lowering the temperature on one side and raising the temperature on the other side? ... think about leaving a refrigerator door open ... or opening your front door in winter, or summer ... I am probably not understanding your question – jsotola Sep 06 '23 at 15:38
  • Leaving the door open, on it's own, does not reduce the temperature inside the shed fast enough for comfort. – user42931 Sep 08 '23 at 10:59
  • Is this AC unit expending energy first chilling the outside air to dehumidify it, then heating it back up again to 25°C (i.e. hotter than it was to start with)? Unless you actually have a specific need for dehumidification, that strikes me as the first place to look for avoidable wastage of energy. –  May 23 '24 at 09:49

3 Answers3

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Based on what you're telling us, venting the space is likely a better option than this one-tube air conditioner. It will use much less power and make the space cooler. ACs do have an advantage in that they also dehumidify the air, which can increase the comfort level dramatically.

Why not cut a box shaped hole in the side and install a window AC? That should cool the space adequately. It doesn't draw in outside air the one-tube AC.

You may also be able to reduce the radiant heating of the exterior by shading it, perhaps with a tarp or other covering stood off by a meter or so.

Drew
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Wouldn't comfort be cooling the office down below the outdoor temperature? If you are ventilating the office then it doesn't make sense to condition the air. If one conditions air wouldn't one prefer to keep that air and the comfort it provides within the office?

Michael King
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If the outside air temperature is only $22^o C$ and inside in $30^o C$ this can only be due to solar radiation. Just as on a warm, sunny day the inside of your car can be hot enough to bake a cake.
There are a few things you can do:

  1. To decrease solar heating, increase the albedo of your office. Paint the outside white, and put light reflecting roofing on it. Put your window on the opposite side from the sun.
  2. Increase the amount of solar panels on your roof. This will increase your available energy and simultaneously decrease the amount of light/heat striking your roof.
  3. Have continual air circulation with the installation of an exhaust fan near the roof. This can be controlled by a thermostat near the ceiling. There are fans available that take very little energy.
  4. If your roof is high enough, especially if you have a peaked roof, put in a ceiling with a space above it so that this fan can keep that space cool, with vents at both ends for air ingress and exhaust. This separation will also decrease the amount of hot air moving down as your air conditioner circulates air in the room.
  5. There can be significant stratification of the air in your office. Mount your air conditioner as high as possible with the cool air blowing down. Then it will be using the hotter air as a medium of heat exchange and be preferentially exhausting this hotter air, rather than the air it has already cooled at a lower level.
Rich
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