OPINION: I was on Facebook the other day, and something caught my eye: someone was using the fireplace to vent their portable AC out through the chimney. Genius, was my first reaction.
I live in a Victorian terraced house, which can get very hot in the summertime, and trying to get a portable AC unit near a window can be a real faff. So, using an open vent that’s there all of the time seems like a brilliant idea, and the unsightly hose can be tucked out of the way.
Only, the more I thought about it, the less this idea made sense. Despite finding people online who say that they’ve done this and it’s worked, there are definite issues that mean I would not do this.
It’s not just hot air
Before I get into the details, it’s worth going over a few details (you can read in more detail how a portable air conditioner works if you like, too). The very short version is that portable AC units don’t cool the room by blowing air, they extract hot air from your room and then output through the hose and out of the window (the cold air coming out the front is just the air in your room with the heat removed).
Not only that, but all modern portable AC units (or any worth their salt) also dehumidify the room. To avoid using a tank, the water extracted from the air is also sent out through the same hose (the small tank inside and the drainage port are only needed in extreme humidity).
So, you have hot, moist air going out through the hose, and cool, dry air coming out of the front vent.
Condensation will drip back down
Hot wet air is the main concern here. Chimneys are designed to deal with very hot, dry air. Putting the AC pipe in means that the hot air will come into contact with the cooler chimney surface, so the moisture will condense and run back down your chimney.
As well as creating a damp environment, the run-down will be messy, mixing with years of soot. This is not a good idea.
If you’ve got a stove and a lined chimney, this still isn’t a good idea. You’d have to seal the front of the stove to get the AC pipe in, but the hot air will condense in the liner, and you’ll get water in your stove.
You often don’t know how good a chimney is
Not all fireplaces are working ones, particularly in old homes. It’s possible that the chimney has been capped or is blocked. Either way, there could be issues to stop hot air from escaping.
Distance is a real issue
If you somehow manage to wedge the AC pipe into the flue, the warm air may not rise all of the way out of the chimney, and some can flow back into your home.
Ideally, you’d want to seal the chimney completely, say with insulation board and/or a sheet of ply, with insulation tape around the edges. Then, with a hole in your insulation, the AC hose can be fed into the chimney space.
But there’s a secondary problem: distance. Using a chimney, the hot air must travel a long way. A typical Victorian home will have 10-foot or 12-foot ceilings. From the ground floor, that’s at least 20 feet to the loft (just over 6m), and then you’ve got even more to get above the roof line. So, you’re looking at a good 9m or more.
Portable AC units have a maximum hose run of 1.5m. Longer than this, and you can get back pressure, which can stress the compressor. At best, this reduces efficiency; at worst, you could damage the unit. In simple terms, portable AC units are not powerful enough to push the output the distances you’d require in a fireplace.
Get a proper window seal
For all these reasons, I would not use a fireplace, working or not, for a portable AC unit’s exhaust. A window is your best option. With sash windows, many units ship with a sliding bracket that can go in the bottom, but it’s easy to find window kits for casement windows that seal the window while allowing room for the hose. If you really don’t want anything on display and want something much neater, you need a properly installed split AC unit instead.
