Effect of Reducing Vent Size on Thermosyphon Solar Air Heating Collector

This is a look at whether the inlet and outlet vent sizes can be reduced in area on our shop heating thermosyphon collector.

This question came from Paul House of Choice Energy  who has built several of these collectors on Bozeman area buildings, and will fix you up with one for about $2000 -- probably the best payback commercial collector out there by a wide margin.

Paul would like to be able to use a six inch hole saw to do the vents.  This would be faster and cleaner and improve the interior appearance.  Going from the current 4 by 18 inch rectangular vents down to the 6 inch round vents would reduce the vent area by a bit more than 50%, so the question is what is the effect on the collector performance for these smaller vents?

Bottom line is that the smaller vents do quite well -- the flow rate is down for the smaller vents, but the temperature rise is up enough to compensate.


Testing the Smaller Vent Performance

Since my collector has 5 identical 4 ft wide bays, I set up the small vents on one of the bays, and compared the temperature rise and air velocity of that bay the adjacent unmodified bay.  Heat output from the collector is proportional to (airflow)*(temperature rise), so, a lower heat output will show up as a drop in the product of these two numbers.


thermosyphon solar air heating collector
The collector from the outside. 

This is the 8 ft high by 20 ft wide thermosyphon collector.   Its a very inexpensive yet quite efficient way to heat a large area.  Because it thermosyphons air through the collector, no fans and no controls are needed.  The only moving part is a simple poly film backflow preventer on each upper vent.

About the only disadvantage of the collector is that it uses a large number of fairly large vents.  This is a bit of a hassle during the build, and the large vents might get in the way a bit inside the space. 

This shows most of the collector from the inside.
There is a 0.5 sqft (4 by 18 inch) vent every 2 ft across the top and across the bottom.


Cutting Vent Size to 6 inch

Paul's idea is to use 6 inch diameter ducts to penetrate the wall for the vents, and my first thought was to do exactly that by installing 6 inch ducts in the center of each of the 4 vents in one 4 by 8 collector bay, but the construction of my collector does not allow the center part of the vent to be cut out to 6 inches. 

So, I just masked in the existing 4 by 18 inch vents down to 4 inches high by 7.5 inches wide.  This is 30 sq inches --just a bit more area than a 6 inch round duct would give.

small vents
Upper vents masked down to 4 by 7.5 inches.
The duct tape looks almost transparent in this picture, but it is airtight and well stuck together.

I measured the flow velocities with the Dwyer Vanemeter and a Kestrel wind turbine meter.

The temperatures were measured using one thermistor for the two lower vents, and a separate thermistor for each of the two upper vents.  Temperatures were logged on an Onset Computer U12 logger.

thermistor temperature logging
Fast acting thermistor measures air temperature.

Dwyer Vanemeter to measure flow velocity.
A Kestrel wind turbine meter was also used.

Logging the temperatures.



I did not get to this until after 3 pm, and the building is pointed about 20 degrees east of south, so we are about 4 hours past peak sun on this nearly shortest day of the year.

I'll try to remember to repeat this closer to midday just to make sure the results hold up when the collector is cooking harder.


The log of vent temperatures.

The inlet vents are picking up air near the floor at about 57F.

The 6 inch vent exit temp is 102.4F
The 4 by 18 vent exit temp is 86.5 F

Vent configuration Flow Velocity (fpm) Flow Rate (cfm) Inlet temp (F) Outlet Temp (F) Temp Rise (F) Heat output per bay (BTU/hr) Change in Heat Output
4 by 18 vent 70 70 cfm per pay 56.1 86.5 30.4 2154 Base
4 by 7.5 vent (sim 6 inch round) 113 47.4 cfm/bay 56.1 102.4 46.3 2202 About the same


So, within the experimental error, there appears to be no drop in heat output for going down to the smaller vents.  The flow rate drops significantly for the smaller vents, but the temperature rise goes up by enough to make the heat output the same.  That is, the air is flowing through the collector more slowly which lowers heat output, but it has more time in the collector to warm up more, which raises heat output.  Normally you would expect some drop in efficiency as the temperature rise through the collector increases because the collector internals are running hotter and lose more heat out the glazing, but this is apparently not that significant in this case.









December 20, 2011