I live in the UK (where air conditioning units, and thought for where they'd go, is very uncommon in homes), and where the temperature in summer is increasing every year. I also live in an apartment where I'm not allowed to alter the exterior of the building.

I'm trying to figure out the best way to install A/C in my bedroom (~40㎥) for our impending summer.

TL;DR: Am I being dangerous/risky if I use my ventilation system's ceiling vents as a portable A/C's exhaust vent?

I've thought through a couple of possible approaches, and not had much luck.

  • Professionally installed split A/C units are unachievably complex/expensive

    The quote I (and others in my building) have received is £20k/$25k and upwards for installation (beyond the unit). This is broadly because the only place for the external part of the split unit is the balcony, which is two full rooms away (along an external wall), and trapped on the other side of a floor-to-ceiling window and a load-bearing concrete pillar.

  • Window vents seem problematically complex

    The two windows, next to each other, are large (1.5m x 1.5m each) and swing open from the middle either 10cm or a full 90º (when the child lock is off). They're a few mm off flush with the ceiling above them and the window sill below. I've experimented with making a seal above/below and have been unsuccessful so far (there's no room to work, or fix Velcro or similar). Besides, London can get noisy at night, so this isn't ideal either.

  • Venting into other rooms might work, but seems inefficient

    The flat is incredibly well insulated, even internally. I might have some luck installing a portable A/C unit venting into a neighbouring room, but I'd definitely get heat leaking back in, so this would be inefficient (and would leave me with a hot flat to work in the next day)

  • Using the ventilation system's piping as an exhaust vent?

    I do have a ventilation system set up in my apartment. The bedroom has an ensuite with a ceiling port for extracting damp air from the shower; I believe I could 3D print a connector for a portable A/C unit's exhaust that would allow me to vent directly outside via the ventilation system. The exchange unit has a summer bypass mode (which will stop it from capturing the extracted heat), and the ventilation inflow is slow enough that I don't believe it'll heat the room enough to cause serious inefficiency.

This last option is the interesting one. Am I setting myself up for any serious problems if I buy a portable A/C unit and get it working?

  • My only consideration is whether I'd be causing damage to the ventilation system's outflow fan, in the case where it runs slower than the A/C unit's exhaust (I'd be building up pressure in between the two, which would make the other bathroom's inlet an accidental exhaust for my A/C unit!
    – JP.
    Apr 26, 2023 at 9:25
  • 1
    Portable ACs - ie one pipe solutions, will not push through that existing air system. You'll just overload it. They're designed to go out of a window; that's it. They don't even push well if you extend the external pipe (& it will void the guarantee too)
    – Tetsujin
    Apr 26, 2023 at 18:57

1 Answer 1


These window AC units have huge airflow. The usual diameter and length of ventilation duct can't manage this airflow: the pressure drop would be too high.

So if you use the ventilation duct as an outlet the portable AC unit will probably not get the airflow it needs to work properly.

If you use a one-box AC it looks like you're going to have to engineer a way to connect the ducts to the window properly. I'm saying ducts because dual duct unit have much higher efficiency than single duct units: dual duct AC sucks outside air using one duct, and blows it back out using the other (with added heat). Single duct units blow hot air out, but they suck the air from inside the room, which is then replaced by hot outside air... so they don't cool anything really.

If you want something cleaner you can find whoever manufactured your windows and have them make a replacement part... now I have a vocabulary issue, I mean the whole part that swings open including glass, frame and hinges... with two extra holes, either in the glass or have the lower part of the glass replaced with plastic with the holes in them. Then you can put this one on in summer, and put the normal window back in winter.

If you can get an authorization to drill through the wall, you can make two holes for either a fixed or mobile dual duct AC. The fixed ones can also be used as heating in the winter. If you put grilles on the outside to cover the holes, painted the same color as the wall, it can be very hard to notice.

If you are allowed to hang stuff from the window like drying clothes, you can also use a mobile split unit, these have flexible hoses, so you can mount it yourself.


Your ventilation unit is dual flow with heat recovery so it has outside air intake and exhaust. So you could replace it with a similar unit with heat pump, which provides AC and heating. These are quite expensive (although much less than the 20k for the split AC) and usually not very powerful, due to the long ducts restricting airflow. But it's an option to consider.

  • Thank you so much! Your point on the pressure drop is an excellent one; though the ventilation unit also has a fan, it looks to move ~30㎥/h at max, compared to a portable A/C unit's ~330㎥/h (so very much not viable!) The windows aren't replacable without a crane (there's an 11 storey drop immediately outside them, and being 1.5m x 1.5m of glass they weigh a non-trivial amount too). Given I've been denied permission to drill to outside I think it's either attempting to build a suitable seal for a dual duct unit, or give up 😅 Thank you so much for your help!
    – JP.
    Apr 27, 2023 at 8:47
  • You're welcome! Oh I had forgotten that English windows open towards the outside, yeah in this case there's no way. I'm French, ours open towards the inside ;)
    – bobflux
    Apr 27, 2023 at 11:09
  • I think with the climate hysteria, governments are likely to make it easier to replace old heating systems with heat pumps, so you may get a change in legislation in the coming years that allows you to install one with less headaches. Hopefully.
    – bobflux
    Apr 27, 2023 at 11:11
  • 1
    @bobflux - maybe so, eventually. At the moment there is a definite drive toward heat pumps… but this being the UK, we're allowed to heat in winter - then just sweat in summer;) There's no provision for full 'air-con' in the current legislation.
    – Tetsujin
    Apr 27, 2023 at 13:07
  • 1
    Yes, here people buy "heating only" heat pumps to get a good energy score, because a full reversible heat pump would result in a worse energy score. But these are actually reversible, because manufacturers don't bother to make several models, the aircon mode is simply locked out by a jumper on the motherboard. So people just hack them to enable the air conditioning mode. Bureaucracy FTW.
    – bobflux
    Apr 27, 2023 at 15:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.