Types of Fans

A wide variety of fans are available on the market.  The main two categories are axial and centrifugal.   You can also get various configurations such as with the motor in the duct or outside the duct.  Fans can be direct drive or belt driven.  They can have a single speed or variable speed.  Fixed pitch or adjustable, or even “variable pitch in flight”…  You can Google to get lots of general info on fans, or even more specifically for residential HVAC, so we won’t get into all of that.   Here we specifically interested in driving an Earth Air Heat Exchanger (Earth tube) system, so we are expecting lower speed flows where back pressure is the primary concern.

Example Fan Curve Chart

This is roughly what you would expect from a backward facing centrifugal fan curve chart. Note the power peaks at about 80% of max flow and the max fan pressure is still good in this range. A forward facing fan curve chart would have a positive power slope all the way to 100% and the pressure curve would fall off much more quickly.

Fans are often rated on pressure and flow rate.  These parameters are not independent because the flow rate is dependent on the system pressure (the pressure the fan must overcome to move the air).   This system pressure is the sum of all the pressure losses including dynamic and friction losses as discussed on the Earth Tube Design page, plus losses due to air filters, coils (heating or cooling), dampers, grills, etc.   1 Pa/m is a typical loss in a straight section of duct.  An air filter could add 25 to 50 Pa of loss.

The energy required to run fans is a significant portion of the cost of running an HVAC system.  Careful selection of the fans to size them correctly to the flow requirements and back pressure of the system is important if you want your home to be as economical and as green as possible.  If you look at a fan curve (pressure, flow rate, power and efficiency chart that should be provided by the manufacturer), you will find that they are only efficient within a very small range.  Since you are probably on a budget, you may not have much control over the speed or power.  However, if you have a good estimate the total system pressure (plus some safety factor) and your required volume flow rate, you can buy the fan designed for that combination.  If you get an overly oversized fan, even at a great price, your efficiency will be much lower and you will pay more over the long run.  And of course, if your fan is undersized…

It is also important to note that pressure drop is proportional to the square of the velocity of the flow in the system.  Earth tube systems can be more efficient if the airflow is slower.  Since they are typically needed only for ventilation, they can be run at a small fraction of the speed needed by typical HVAC systems.  One way to reduce the speed is to increase the area of the pipe.  If you do so, make sure to transition gradually.  If you are using any coils (heat transfer) in your flow, keep in mind that constrictions in the flow increase both speed and dynamic losses.  Look for “low face velocity” coils for reduced losses.  These cost more upfront (paying for the advanced design), but have lower lifetime operating costs.


Axial Fans

I am really only including this section to tell you why you should not use Axial fans ;^)

Axial fans are the “propeller” type fans that push the air along their axis of rotation.  Of course, human beings are very creative, so there are a number of different types of axial fans, you can google for more information.  The little fan that cools your computer is an axial fan, and so is the propeller on the front of a WWII bomber.  The designs can be improved in many ways, such as by reducing the tip gap (if it is ducted) to increase the pressure (increasing precision increases cost) or by adding vanes to straighten out the flow and increase efficiency.   In general, axial fans are more efficient than centrifugal fans for their design range because they don’t have dynamic losses associated with changing flow direction and they are often directly connected to their motors, avoiding transfer losses.  However, their design range is low pressure flows.   You can also pick up a basic axial fan for much less money than a centrifugal fan.

This link to the Lowes home hardware site shows a six inch booster fan for only $26.77.   The fans are “inline” which makes installation easy and compact.  They are sold as booster fans for furnace systems and proudly state 250CFM on the box… That is 250 cubic feet per minute for less than $30!!!  But a closer inspection of the literature will explain that this is the maximum they will “allow”… These “booster” fans are meant to be used in a furnace system with a centrifugal fan as the primary blower, at this maximum flow (generated by the main blower), the booster fan actually starts inhibiting the flow.  These axial fans can help increase flow thru parts of the system that had low CFM, but really can’t be used to drive the system.  In this case, the fan labeled “250 CFM” could actually move “free air” at 160 CFM with no duct resistance.  Any resistance at all would quickly reduce its ability to move air.   These are quite unsuitable for Earth Tube systems.


An axial fan (in this case without vanes) can move the air efficiently when there is low back pressure, but high back pressure causes the majority of the flow to circulate near the blades.

If you look at a fan perfomance curve, you can see that the ability of axial fans to move air efficiently quickly drops off as back pressure increases.  If your earth tubes are more than 16 ft long (5 m), (count ever turn as an additional 5 ft) your axial fan won’t be able to overcome the frictional and dynamic losses.  This is why they should never used for ducted flow situations.  Basically, as the pressure rises, the air will take the path of least resistance, which is usually to flow back up thru the tip gap (between the blade tips and the duct wall).  You can increase the speed/power, but you will just end up furiously circulating the air near the fan with very little flow moving thru the duct.

If the back pressure is low, an axial fan will get you more flow per unit of power.  But earth tube systems generally have back pressure that reduces axial fan efficiency to near zero, so don’t use these no mater how cheap they look.  (See, I am so serious, I put it in a red box this time ;^)


Ducted axial fans are ideal for situations where the duct is very short, such as just passing thru a wall.

Centrifugal Fans

Centrigfugal fans produce more pressure for a given air volume, which makes them better suited to longer duct runs.  Rather than “screw” their way thru the air, these fans use centrifugal force to throw the air out at a right angle to the intake.  Because of their geometry and mode of operation, they require more torque to get started (larger motor), but the higher number of blades can raise their pitch out of our range and they tend to be quieter also.

These fans can be a little more awkward to integrate into the duct system in that they are usually quite a bit larger than the duct.   These are the “squirrel cage” blowers that you find in your furnace or auto HVAC.  Axial versions are also available, but at double the price and with lower efficiency due to the dynamic losses of straightening the flow again  With careful design, you may be able to create a layout that can take advantage of the 90 degree turn thru the cheaper squirrel cage fans.

Centrifugal fans also tend to be quite a bit more expensive;  Fantech makes some “inline” centrifugal fans that are easier to add to duct assemblies.  A 6 inch fan costs roughly $260, or about 10 times the cost of the axial fan of the same diameter.

You can get backward curved blades (usually airfoil shaped), radial blades or forward curved blades.  Each configuration produces a different fan performance curve.   The backward curved airfoil shaped fans are the most expensive because they are more difficult to manufacture, but they are also the most efficient.  You can also get backward curved fans made of single thickness metal or airfoil shaped plastic that are still very efficient and ideal for higher pressure applications.   Radial blades are not at all appropriate for HVAC and forward curved blades are better for high volume at low back pressure.

Centrifugal Fan Types

Backwards curving centrifugal fans are the best choice for earth tube systems, but they also tend to cost the most…


Fancy Controls

You could leave your earth tube fans running at full blast all the time, or your could build some smarts into the system so they come one only when needed.

VSD Fans

Variable speed drive fans cost more, but can save money if you know you don’t plan to run them full speed all the time.  This is because they can adjust the fan curve as needed to improve efficiency.  For instance, you could set the fan speed to reduce when no one is home.  Since the power is proportional to the flow rate cubed, reducing the the flow to 50% with a VSD fan can actually reduce the power required down to 50%^3=12.5% of the maximum power consumption.

Optimal Start-Stop

Earth sheltered homes have a lot of thermal mass which means that if you want things warm at 6:00 AM, you may need to start quite a while before that.   Modern thermostat systems often include technology that learns how long a building takes to heat up or cool down based on variables including outdoor temperature, indoor solar gain, etc.  These systems can drive fans to provide ventilating or heated/cooled air in an intelligent way.   These systems used to be expensive, but this sort of technology is getting easier and easier to come by.

The 2nd generation of the Nest learning thermostat is available for under $250 and can be installed by the home owner.  It includes temperature, humidity  motion and light sensors.  It doesn’t quite feature Optimal Start-Stop yet, but it is on its way there.  Other systems are also on the market.  


In addition to the standard temperature sensors included , humidity (20$) and even CO2 sensors (250$) are available.  They can be used for monitoring your home or connected to control systems that operate the fans, dehumidifiers, etc. in order to get the most comfort most efficiently.   They can also be connected to dampers that would allow you to exhaust stale air based on high CO2 levels or when the weather is pleasant outside, or even shut down the earth tube fans when CO2 levels are low or the intake air is too humid.  Sensors could also be used to adjust the ventilation mix between fresh air and return air


Upstream and Downstream

Ducted fan efficiency is very much affected by the ductwork up and down stream of the fans.   The air near the fan is the most energetic and therefore dynamic losses are greatest.   Therefore, if you need to turn the duct or change in cross section area, near the fan is the worst place for those changes (although you could use your centrifugal fan to make the turn).   If you must, use turning vanes or keep the slope small (15°).  This includes avoiding situations where fans are open to a room, either as an inlet or an outlet.  Your fan’s efficiency will be improved if you can put it further along the duct rather than sitting at the opening.

(more to come)

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