To make the motorized registers, I'm using standard units purchased at a local hardware superstore, and the small servo motors were purchased from a local hobby shop that deals in Remote Control cars, planes and boats. All that's needed is a simple circuit to control them.

Automating HVAC Registers

Chuck Muziani

Home Toys Article - Automating HVAC Registers

muziani-01.gif (14859 bytes)
by Chuck Muziani

To make the motorized registers, I'm using standard units purchased at a local hardware superstore, and the small servo motors were purchased from a local hobby shop that deals in Remote Control cars, planes and boats. All that's needed is a simple circuit to control them.


For many months now I've been wanting to fully automate our HVAC system by motorizing the air registers in each room of our house. After finding some free time here and there, and tinkering with model RC cars and boats I found at hobby stores and Radio Shacks, I finally came up with a simple solution to automate those registers.

Commercial versions of motorized registers and motorized dampers are available, but at prices of around $160 and $120 respectively, they are too expensive for my taste. Add up the number of registers or dampers you'd need for a typical house installation, and you'd find the total cost a bit staggering!

It's no too difficult to make a motorized register at a fraction of the cost of commercial units with the same capabilities, but it does take some hands-on creativity and skill.

The commercially available register I investigated works by applying power (24VAC) to a set of contacts. To close the unit, you short one pair of contacts. To open, you short a different pair. If you remove the power, or open a set of contacts at some point during the opening or closing cycles, you can leave the register in a half-open state, or some approximation thereof. That's the basic function.

To make the motorized registers, I'm using standard units purchased at a local hardware superstore, and the small servo motors were purchased from a local hobby shop that deals in Remote Control cars, planes and boats. All that's needed is a simple circuit to control them.

About The Servos

Servo motors are great devices. Simply, they are motors that can be precisely positioned via a pulse signal. A servo assembly contains gears to increase the overall torque, and an internal circuit to control the position of the splined output shaft. Three connections feed power, ground and a control signal.

The output shaft can be positioned anywhere from 0 degrees through 90 degrees typically, with special purpose servos allowing greater rotations. Torque varies with each model servo, and can range from 16 ounce-inches (1 pound) for small servos, to more than 320 ounce-inches (20 pounds!) for heavy duty units.

The internal circuit controls the position of the splined output shaft via an internal feedback loop. We do not need to know details of the internal circuit for our purpose, but we do need to know about the control pulse we must supply to the servo in order to open and close the register vanes.

In remote control cars, a servo motor positions steering tires to precise positions via a varying pulse width signal. A control pulse width of 1.5 milliseconds will position the output shaft midway through its rotation. A pulse width of 1.0 milliseconds will position the shaft to full left of its total rotation, while a pulse width of 2.0 milliseconds will position it full right. Any pulse width between those extremes will position the shaft accordingly.

For standard servo use in Remote Control cars, the pulse must occur every 20 to 30 milliseconds in order for the servo to hold its position under a load. Since the register and forced air does not present significant load to the servo it will not need a repetitive "holding" pulse. It only needs to receive a single pulse relevant to the desired position of the register vanes. If for some reason your HVAC system air velocity does present such a load so as to move the vanes from the desired position, then your control circuit must supply a repetitive pulse when the HVAC fan is operating.

When we want to close the register, we simply send a 1.0 millisecond pulse to the servo. That signal will move the shaft full left, and in turn it it will close the vanes. If we send a 2.0 millisecond pulse, the vanes will open. A 1.5 millisecond pulse will open them midway.

Now, attaching the servo to the register takes some creativity and skill. Since there is no bracket to mount the servo assembly to the register, one must be made. The first thing to take into account is the area inside the air duct. In most cases there will be more than enough room to allow for the servo attachment. Measure the depth into the duct if it angles sharply just before the opening. Depending on the servo and bracket dimensions, most installations require no more than three inches depth.

Another important factor to take into account is the force needed to open and close the register vanes. The vanes are comparable to venetian blinds or mini-blinds. They open in unison, connected by a control lever. Now, most new registers will be quite difficult to open and close due to the way they are manufactured. The reason they are manufactured "tight" is so they won't open or close via the air pressure from the HVAC fan or from their own weight. The first thing you'll need to do to that new register is loosen the control lever connection points. DO NOT remove the control lever! All you want to do is loosen the connections to each of the vanes, but still leave the control lever attached. The end result is to have a register that _almost_ opens or closes under the weight of the vanes alone. By loosening the control lever, we allow for a lesser torque requirement, and therefore a less expensive servo motor. This takes some tricky work, but it's easier than it looks.

Attaching the servo output shaft to the inside control lever is more difficult. Servo kits come with various levers that can be attached to the output shaft. The levers come with holes drilled along their lengths in order to facilitate range of motion required, and also to increase torque. The further the hole from the output shaft, the more range of motion but you have, but with less torque. The opposite is true with holes closer to the shaft. I made a a connecting rod assembly with springs to attach the control levers. The springs absorb end-of-travel loads.

Now, here's a little more information on the servo motors. The number of RC servo motors available is staggering to say the least.

The Futaba model S3003 is considered a "standard" servo in the RC trade.

Futaba model S3003 specs:
Torque: 42.0 oz/in (about 2.5 lbs of pull!)
Speed: 0.22sec/60 deg.
Weight: 1.5 oz.
Bearing: Plastic
Dimensions: 0.87 x 1.85 x 1.05 in.
Cost: ~$15

Notice the small size of this unit! That's typical, since they are used in "model" cars and boats.

Before deciding on a servo, you should check the amount of force needed to operate your particular register. A strain guage can be used, but may be hard to find. A "hanging fish scale" can be used for this. Tie a piece of string to the bottom of the fish scale. Next, tie the other end to the INSIDE portion of the register control lever (where the servo motor will be attached). Now, align the register so that the contol can be operated by pulling on the opposite end of the fish scale. Watch the scale as you do this and make note at which weight the control begins to move. This will be your guide for servo selection. If the control begins to move at three pounds, you'll need a minimum spec of 48 oz pull. (16 oz to a pound, 48 oz divided by 16 = 3)

This servo would be a good choice:

Hitec model HS225BB "Mighty Mini" specs:
Torque: 55.0 oz/in
Speed: 0.14 sec/60 deg.
Weight: 1.0 oz.
Bearing: Top Ball Bearing
Dimensions: 1.3 x 0.6 x 1.3 in.
Cost: ~ $30

A typical wall register costs $12, and a 220 oz/in servo can cost $60.00. Even still, the overall cost of all the components that make up a project such as this still costs less than the $160 commercial version.

One very important note: Many "micro" servos cost more than "standard" and "mini" servos. High-torque models cost more than lower torque models. Be careful in choosing servos. In general, the physical size of a standard servo is small enough to be incorporated into most register systems so you need not purchase a "micro" version servo where a standard or larger one can be used. Shop for torque requirement, then for price. A similar torque specification for one brand model may cost much less in another brand. A search on the web for servos will reveal many sources.

That's it for now. This has been a simple overview of this project and I hope it has given you some ideas to incorporate into your HVAC system. In the next installment I will have photos with more detailed how-to info, and a schematic and explanation for a small control circuit.


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