The HVAC and Zoning system needs 3 types of hardware:
  • Temperature Sensors
  • Air volume controllers
  • Relays to control the HVAC equipment

Temperature Sensors

An easy and inexpensive temperature sensor network is built with a Maxim 1-Wire serial adapter, DS9097U and multiple Maxim 1-Wire digital thermometers, DS18B20. The serial adapter runs about $28 and each temperature sensor is about $5. Despite their name, 1-Wire sensors require at least two wires, one for ground and the other is shared for power and data. Adding a third wire for +5v power increases the speed of the temperature sensors. While only three wires are used, most people run CAT5 cable to each temperature sensor. Maxim recommends wiring the temperature sensors in a daisy chain (that can extend for hundreds of feet). I failed to read the manual and wired my sensors in a star pattern. I get bad reads every so often, but the system is reliable enough for regular use.

I've mounted my DS18B20s in surface mount phone jacks.


Air Volume Controllers

Using servos to control balancing dampers or vents is an inexpensive way to control the volume of air delivered to a zone. A more expensive and less versitle option is to use commercial 24v two position dampers (about $60 - $100).

When my home was built, the HVAC contractor installed balancing dampers in the ducts. For about $13 each, I have attached a Hitec HS-325HB servo to control the position of each damper in my system. I use the Lynxmotion SSC-32 Servo Controller via RS232 from the computer to control the positions of the servos. A single SSC-32 is $40 and can control up to 32 servos.

Here's a picture of a servo mounted to a balancing damper


Relays to control the HVAC equipment

Thermostats control the HVAC system by shorting wires together. For example, to turn on the heat, a thermostat connects the white wire to the red wire. You can often hear the thermostat's relay click when it calls for heat or cool. (For more information about thermostat wiring, see Jeff Fisher's HVAC Control Tutorial.)

I purchased relays and soldered together a relay board. This cost me about $7. Other options are to use a pre-assembled relay board, one option is the PH Anderson board that contains both relays and a 1-Wire interface with serial port for communication with a computer. (The use of this board could elminate the need for a Maxim 1-Wire serial interface DS9097U.)

Here's a picture of my SSC-32 and my home-built relay board.


In addition to controlling servos, the SSC-32 can toggle any of its 32 output ports logic Hi or logic Low. I use this feature to energize/de-energize my relays.

Last edited Dec 30, 2008 at 4:48 PM by KevinJStricklin, version 5


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