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Old 02-09-16, 04:29 AM   #1
SDMCF
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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.

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Old 02-09-16, 08:10 AM   #2
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Very dang interesting! I would also point out that you aren't moving a lot of air. The more you move the better the results to a degree I would think.

This interest me so much because my main living space has 16feet vaulted ceilings. And I burn firewood, so I know the temp at the very top is where all my heat is going. I have poured concrete totally uninsulated floors. I plan to dig down to the footing and at least insulate the side of the slab, but my main concern is that by blowing the air onto my floor I would loose heat because it is such a thermal load.

I do run the ceiling fans sometimes, but they seem to move so much air even on the lowest setting that the air hitting you seems too cold.

I believe you should keep running the air tube and get use to how it makes the room feel. Then stop using it and see if you can tell a difference.

Thanks for posting!
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Old 02-09-16, 08:14 AM   #3
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VERY nice data!

For me, running fans in the winter, make me feel colder. This likely the result of enhancing skin evaporation with more dry air and loss of the boundary layer. Yes, you may circulate a couple degrees of "warmer" air from the ceiling, but I have long felt that it is NOT worth it.

Summer seems quite different. Despite the fan now adding hotter air, the air movement cooling effect is far more effective.

For me, I do not use my ceiling fans in winter and do use them in summer.


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Old 02-09-16, 08:36 AM   #4
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Steve, I agree, my fans make me feel colder too. But with this tube design you could maybe place it in a corner and the air could come out right at feet level. The air hitting you directly would be much less, but you would still get the temp rise. For me I just measured my air temp at the top of the room (78.0F Dry and 58.8F Wet bulb) And at the floor (71.7F Dry and 54.3F Wet bulb)
What if we could turn the fans on for about 3-5 minutes and then off for awhile? Only feel the cooler air long enough to make a temp exchange.
My floor is always cold, if I could heat it with the extra heat from the wood and solar heater that would make this house much more comfy I think.
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Old 02-09-16, 11:37 AM   #5
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I run my ceiling fan when I know the heater is running and no one is under it to mix the air before it escapes to the upstairs. Maybe reverse the fan to suck cold air up and displace the hot air above. Another thought, as the air travels through the tube it will conduct temp with the tube and air around the tube. By the time it travels to the end of the tube the temp might be equalized.
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Old 02-10-16, 06:20 AM   #6
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@MEMPHIS91: You are right, this arrangement probably doesn't move a great deal of air although I have no way of measuring how much air it does move. A ceiling fan should move a lot more. The advantage of the tube is lower cost and a more discreet installation - but of course that doesn't matter if it doesn't work. Perhaps it needs multiple tubes to move enough air.
I imagine a tube would work better with your 16ft ceilings. Why not try it?


@stevehull: I agree that the fans could make you feel colder. Certainly the air coming out of the bottom of the tube felt cool even though it cannot have been cooler than the air already at floor level. However, compared to a fan, a tube should make it easier to locate the air flow where you don't feel it directly.


@gtojohn: I did consider reversing the air flow through the tube but could see no reason why it might be better. I could test that I suppose, if you can convince me it might help.
Your point about the warmer air cooling as it goes down the tube is a good one, and something I had not considered. I should have had another temperature reading at the bottom of the tube. It could explain why the tube has little effect on the floor-level temperature.


Overall though I am disappointed with the results and - looking at the "no fans" graph - there seems to be less potential in this than I had imagined.
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Old 02-10-16, 07:39 AM   #7
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I ran some test of my own last night, I turned the fans on low, air blowing down. And found the same 7-9 temp difference from the floor to the ceiling. So I reversed them, same temps. I had to turn the fans on high and air blowing down to see equal temps. That's A lot of air!
So today I'm going to grab some 6" pipe and hack me a tube for cheap, I already have a 6" 240 cfm fan left over from the solar air project.
I'll post result as soon as I'm done.
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Old 02-10-16, 08:56 AM   #8
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Yes, do let us know the results!

I have just had another thought about my results. What I am trying to do is to reduce (ideally eliminate) the temperature difference between floor and ceiling, so that is what I should measure. With that in mind I took figures from the "no fans" and "both fans" data, and calculated that floor-to-ceiling temperature difference. I considered only ceiling temperature between 16 and 20 Celsius because that is where I have data for both series. Plotting the results and adding trend lines gives this graph:



This is a lot more promising. It shows that with both fans running the difference between floor and ceiling temperatures is consistently over 1 degree less than with no fans running. It was also nice to see that the 2 trend lines are parallel, which gives me a bit more confidence in the result.
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Old 02-10-16, 10:12 AM   #9
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Great info collection guys! I thought I tested this quite a while back in my office which is roughly 14'x14'x8'. I'll see if I can't dig up that info. I tested with my ceiling fan though.
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Old 02-10-16, 10:19 AM   #10
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Here is some testing from build it solar:

Thermal Stratification With High Ceilings -- A Test

I can't seem to find a thread on the test I did. I just found something that said I had a 4-5 degree temp difference in my 10ft ceiling kitchen from the ceiling to the counter height.

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