Why a conservatory is a hard room to cool
An air conditioner has to remove heat as fast as it arrives. In a brick room that is people, a television and a bit of sun through a window. In a conservatory, the whole roof is a window pointing at the sky. That is why the room in our model reaches 47°C in its hottest hour even with a door opened at 25°C, and why the Comfort Calendar shows the room overheating on so many ordinary afternoons, not just heatwaves.
Undersize the unit and you get the worst of both: the highest bill, and a room that is still warm at teatime.
Portable or split
The two kinds work on the same principle, but they behave very differently in a glass room.
Portable unit
- Typical cooling 2 to 3 kW, often less in practice
- Hot exhaust hose has to go out of a door or window, letting warm air back in
- Loud, because the compressor is in the room
- No fitting needed, and the cheapest to buy
Wall mounted split unit
- Typical cooling 2.5 to 3.5 kW, and it holds that
- Compressor sits outside, so the room stays quiet
- Most run in reverse as a heat pump for winter
- Needs a refrigeration engineer to fit, and a wall for the outdoor box
What it costs to run
A split unit cooling at about 3 kW draws roughly 1 kW of electricity, about 26p an hour at 26.32p a kWh. Our model room spends 1,320 waking hours a year above 25°C under polycarbonate and 317 under a panel roof. If the unit ran for each of those hours at that draw, the summer bill would look like this. It is a rough upper figure, but what matters is how far apart the two bars sit.
When it is worth it, and when it is not
- Worth it: a room used daily as an office or lounge, a split unit that heats as well as cools, and a roof that does not let much sun in.
- Borderline: a well shaded or north facing conservatory used most weekends.
- Not worth it: a portable unit in a south facing room with a clear roof. It will run, it will be noisy, and the room will still be hot by mid afternoon.
Air conditioning and a new roof together
The two are not rivals. Swap the roof first and the cooling load falls so far that a small unit copes, or you find you do not need one. The panel roof cuts the hours above 30°C from 556 to 34 a year in our model, and in winter the smaller heat loss lets the same split unit heat the room on less power. We set the numbers side by side in new roof versus air conditioning, and the prices page has the roof costs.
