While I wait for the insulated A/C exhaust hose sleeves I’ve ordered to turn up, I decided to carry out an experiment. For Science!
I had a suitable length of 125mm aluminium flexible ducting left over from an old VNC installation and my brain suggested that it would tend to reflect heat internally and radiate less heat back into the room I’m trying to cool than the super thin PVC ducting that came with the portable A/C unit does right now.
According to my IR thermometer (yes, I have a measuring instrument tism) the exterior surface of the aluminium ducting is 10-11°C cooler than the PVC does after an hour of operation, but it doesn’t actually feel much cooler to the touch.
My question is:- Does the aluminium ducting measure cooler, but feel about the same because it a far more effective thermal conductor than PVC?
1 Like
Yes aluminium as a heat sink will feel cooler except where it exits the AC unit. I have my rigid PVC wrapped in multifoil insulation that is barely warm to the touch.
1 Like
I think one needs to consider the corrugations of the pipes that creates their flexibility. If the corrugations of the aluminium pipe are deeper, and / or closer together, then for a given length of pipe there will be a greater surface area than for the PVC.
Assuming for the purpose of comparison that the amount of heat being sent into the pipe from the AC unit is the same, then that amount of heat may be being dispersed over a larger surface area and so would measure cooler.
If your hand were to be able to change area in the same ratio as the aluminium to PVC, then you would probably be able to feel the difference by touch as well.
Of course comparing by touch is an incredibly imprecise method as the amount of heat sensed through the skin will vary tremendously with the pressure of your hand against the pipe.
1 Like
To a point yes, greater surface area the greater the cooling but far more to do with thermal conductivity. A pvc pipe and an aluminum pipe in winter the aluminum one will always feel cooler as it conducts heat away from the skin quicker. Corregations in the pipe do reduce airflow due to turbulence created.
1 Like
Science Update and Public Safety Announcement:
When I went to move the A/C unit into the bedroom late last night, I found that the aluminium ducting didn’t support its own weight once fully it got thermally soaked after 6 hours use and was sagging to the point that it was restricting exhaust flow.
Which is surprising as I see to remember the original packaging saying it was good for 100°C for use with tumble dryers.
So use the PVC ducting which comes with you A/C unit folks!
Nope, use PVC waste pipe, smoother inside = greater air flow and you want that hot air out by the quickest shortest route. Wrapping in insulation so its doesnt become a radiator.
2 Likes
That would indeed be the most efficient solution if I wasn’t moving my unit between 2 locations that needed the ducting to be at different heights.
I guess I could make up 2 x 125mm PVC pipes, 1 for each location…
Would you believe I have the same issue and 2 pipes to suit. My new 9000 btu out performs my much older unit hence it gets moved to where I need it most. 
1 Like