View Single Post
Old 02-09-16, 03:29 AM   #1
SDMCF
Apprentice EcoRenovator
 
Join Date: Jul 2014
Location: Finland
Posts: 125
Thanks: 5
Thanked 35 Times in 34 Posts
Default Vertical hot air tube

We all know that the air near the ceiling in a room is warmer than the air near the floor and that blowing the warmer air downwards should make the room feel warmer at "people level". That could be done either with a ceiling fan or a tube with a fan in it. I was wondering how effective the fan-in-tube could be and couldn't find any data on it so decided to run my own test.

The tube I used was a plastic pipe with 10cm (4 inch) internal diameter.

I put a fan at the top and a fan at the bottom of this pipe. The fans were (identical) 12cm computer case fans running off 12 volt DC. I have no other specification for the fans.

I put the tube in a 4.3m x 5.9m room with 3m ceilings. (Call it 14ft x 18ft 6in with 10ft ceilings.) The tube was at the edge of the room, 1.4 metres from an internal corner, next to the main door of the room and away from the external wall which has 2 windows each with a radiator beneath.

To measure the temperatures I put 4 temperature sensors on a pole 2.4m into the room from the tube. I put it there because it was easy to fix to a ceiling fixture, no other reason.

I logged the temperatures at floor level and heights of 1m, 2m and 3m (ceiling level) for 96 consecutive hours.
24 hours without the fans running, just to get a baseline.
24 hours with the bottom fan running.
24 hours with both fans running.
24 hours with the top fan running.

The temperature in that room was far from constant. During the test period the temperature at floor level ranged from 13.2 to 19.9 Celsius (56 to 68 Farenheit) and at ceiling level from 16.0 to 23.9 Celsius (61 to 75 Farenheit). It is important to understand that the tube/fans neither heat nor cool the room overall; the aim is to change the distribution of warm air within the room.

Here are some graphs to show my results.









What can we conclude? Your comments are welcome.

In a standard room (the first graph, without the fans running) I find it surprising that there is so little difference in temperatures once you get above 1m off the ground, with a big drop between 1m and floor level. Is this a normal temperature gradient or is a lot of heat being lost through the floor? I don't know.
Anyway, the fact that there is so little temperature difference between 1m height and the ceiling suggests to me that there is little scope for improvement by blowing warm air down from the ceiling.

The top fan on its own seems to do nothing. This confirms the impression I had during the test because very little air seemed to come out of the bottom of the tube with just the top fan running.

The temperatures with the bottom fan running are interesting because the graph has 3 distinct regions. Consider the middle portion, from 6 to 14 hours. The upper levels were cooling very slightly and the floor level was fairly static or maybe rising slightly. I interpret this to be the effect of running the fan with a fairly constant room temperature and it seems the fan effect is small.
The other portions of the graph show the room temperature rising & falling and I don't see any effect of the tube/fan.

The most interesting part of the "both fans" graph is between 15 and 18 hours. At 15 hours the floor level cooled rapidly (I think a door to an adjacent unheated room was open for a few minutes). After that the floor level was fairly constant with the other levels steadily cooling. Why?

Overall I am not sure that there was any significant gain from running the fan(s). It is clear that the top fan was useless, at least on its own. The bottom fan may have had an impact, but not too significant. To get a real benefit would more tubes/fans be needed? Should I test further in a smaller room? Or is the whole thing a waste of effort?

My own impression is that the tube made no appreciable difference but SWMBO did point out one weak point in my test. This air flow may well have warmed the floor itself, which I did not measure. SWMBO Claimed the floor felt warmer and it seems logical that it would do, but I didn't notice any difference.

Attached Images
    
SDMCF is offline   Reply With Quote
The Following User Says Thank You to SDMCF For This Useful Post:
Daox (02-10-16)