Sizing Fans and Ducting for Solar Air Heating Collectors

This page goes over picking the correct size fan to move air through a solar air heating collector.



<should also mention the temperature rise method>

<maybe offer a quick and dirty method for screen (or other low resistance collectors) with descent duct systems that are not too long.

I this case, pick a fan that gives 3 cfm per sqft of collector flow rate with a 0.2 inches of water pressure drop.



To pick the right fan for your solar air heating collector, you need to know three basic things:

  1. The flow rate through the collector(s).
  2. The pressure drop through the collector and duct system.
  3. The noise level you can live with for the fan.

The steps below go through figureing out these three things.

1. Estimating Required Flow for the Collector

A flow rate around 3 cfm per sqft of collector area is often used. So, if you have a 4 by 10 ft colector (40 sqft), then the recommended flow rate is about:

Flow Rate = (40sqft)(3cfm/sqft) = 120 cfm

cfm stands for cubic feet per minute

Why 3 cfm per sqft?

Produces about the right temperature rise:

A flow rate of 3 cfm per sqft of collector through a well designed collector in descent sun conditions produces about 50 to 60F temperature rise from the inlet to the outlet. So, if room air enters at 65 F, it leaves at about 120F.
This is a good exit temperature for most space heating conditions -- if the air is much cooler, it tends not to feel warm to people. The 60F rise under good sun conditions also leaves some margin for not so good sun conditions.'

Good compromise efficiency level:

The 3 cfm per sqft is a pretty good compromise between efficiency, noise, fan power and temperature rise.
Flow rates greater than 3 cfm/sqft will give higher collector efficiencies because the collector runs at a lower average temperature and loses less heat out the glazing. But, high flow rates require a larger fan that uses more power, generates more noise, and costs more. And, while higher flow rates would increase collector efficiency and heat output, they reduce the temperature rise through the collector, and for the reasons discussed above, its good to keep the temperature rise up to 50F or more.

So, 3 cfm per sqft is usually a good compromise flow rate, but feel free to pick a higher or lower flow rate if it works well for your situation. For example, if you are trying to take the chill off a cold shop, you might choose to increase the flow rate to get more collector efficiency -- even 80F air is going to feel pretty warm in a 50F shop.

2. Estimating Pressure Drop:

To pick the proper fan, we will need to know what the pressure difference that is needed to force the airflow through the duct and collector.


Collector Pressure Drop:
The collector pressure drop depends on the internal construction of the collector, and its difficult to calculate, so the best guide is go by what has been measured on a collector that is similar to what you are building. There is not a lot out there on this, but here are some numbers to start with:


Flow Rate
Pres Drop
Screen absorber
2 layers black screen -- flow through
4 by 8 ft
3 cfm/sqft
0.1 inches water
Vented Alum Soffit Absorber
Vented alum soffit -- flow through
4 by 8 ft
3 cfm/sqft
about 0.1 inches of water
Backpass Collector
Solid alum sheet with all flow on back side.
4 by 8 ft
2.5 cfm/sqft
0.42 inches of water


There are descriptons of all of these collectors above and the tests done on them on this page...

The pressure drop through the collector increases as flow rate increases.

Larger or smaller collectors of about the same proportions should have similar pressure drops.

Collectors that are longer and narrower (say a 2 by 16 ft collector) will have greater pressure drops due to the longer flow path and higher velocity.


Duct Pressure Drop:





The fan you chose must provide the flow rate you calculated above when pushing air through the duct system and the collector. Fans typically produce more flow when they are operating in free air then when they are faced with pushing air through the resistance of the duct system and collector, so its important to know how to account for this resistance.

The resistance is expressed as the air pressure the fan needs to produce to force the required amount of air through the duct and collector, or as the pressure drop through the duct and collector. This is normally expressed in inches of water, where 1 inch of water is thre pressure at the bottom of a column of water that is 1 inch high -- this is equal to 0.036 psi.

A typical range of pressure drops through real collectors and ducts might range from about 0.1 inches of water to as much as 0.5 inches of water or more.






Chosing the Fan

Now that we know the flow rate through the collector, the pressure drop through the ducts and collector, and the noise level we want, we can chose an actual fan.

Basically we just want to pick a fant hat delivers the required flow rate through the duct system and that has an acceptable noise level.

Good fan manufacturers publish a fan curve for their fans that give the flow rate that the fan can produce vs the pressure drop. Here are a couple examples: