In recent years the practice for integration was to allow both worlds, automation and HVAC, to be integrated only by a ‘breakout box’. This magical box was put together as a ‘truce’ between the two parties, giving each the control and separation wanted yet performing the task of integration for the customer. The box simply switched the equipment control from the HVAC contractor’s thermostats to the automation contractor’s thermostats. In this manner, if anything went ‘wrong’ with the automation gear or the HVAC equipment, it was easy to flip to the other guy’s system and determine fault. What an unpleasant way to go through a project, not to mention tripling the number of wires needed!

One of the important features added to the best of these thermostats is the basic separation of the two worlds, automation and HVAC. Figure 1 shows a typical communicating thermostat. Note the standard HVAC terminals on the right side of the thermostat, and the communications terminals on the left. This ‘separation’ will allow both installation experts to do their jobs with minimal concern for the other, though coordination is always important.

It is important to befriend your HVAC contractor on the project from the very beginning. Unless he has worked with you before, you will find it much easier sailing to have a short meeting at the beginning of the project along with the homeowner and builder. Here’s a simple step-by-step task list to use as a guide.

• Initial Meeting including Automation Contractor, HVAC Contractor, Builder, and Homeowner
• Discuss the task. As an automation contractor, you’re typically not only there to get the house wired and get your equipment installed properly, but also to help coordinate with others, as you integrate with nearly every other contractor on the project.
• Give the thermostat manufacturer’s specification sheets to the HVAC contractor. Get him involved by having him select the proper thermostat model (number of heat/cool stages, conventional or heat pump, etc.). This will help all parties feel more comfortable. If there is are any questions regarding compatibility with the equipment, call the thermostat manufacturer. Most will have a model that covers 90 percent of the equipment in the market.
• Show a sample of the communicating thermostat to be installed. Touching is believing. Show how the standard thermostat connections are still there, and that the thermostat will operate ‘stand-alone’ if there is ever a communication problem.
• Point out the 24VAC ‘common’ terminal. Make sure the HVAC contractor understands an additional wire will be needed in his normal brown ‘thermostat’ wire. If he normally pulls a 4-wire cable for a single stage heat/cool unit, he must now pull a 5-wire cable.
• Ask the HVAC contractor to install the thermostat. He has to install a thermostat anyway, so it might as well be the right one. This gives him the opportunity to verify the equipment is working with the thermostat.
• Coordinate Wiring with the HVAC contractor and builder
• Allow the HVAC contractor to select the location of the thermostat(s), as is normal. After he has pulled his brown thermostat wire, you’ll know where your communications cable needs to go.
• Use a single gang box or mud ring for the thermostat. Don’t allow the thermostat to be mounted to a stud.
• Pull the thermostat communications cable. As always, make your cable pulls clean and neat.
• Make sure wire location ‘change orders’ are noted. If the thermostat location moves for any reasons, make sure it is noted and the communications cable is moved as well.
• Verify all wiring before and after sheet rock installation. ‘Tone out’ wires immediately after sheet rock completion. If there is any doubt about the cable integrity at this point, get an ohmmeter. Check the resistance by twisting a pair together and measuring the other end. Make sure it is within the cable specifications for typical resistance.
• Coordinate Installation with the HVAC contractor
• Give the HVAC contractor the thermostat(s) for installation. Be on site the same day if possible to answer any questions.
• Ask the HVAC contractor to verify proper operation. This will eliminate many potential questions in the future.
• Install the communications cable after operation with the HVAC equipment is verified.
• Check out the operation of the communications link. Most communicating thermostat manufacturers have a method to verify communications is operational. Using a laptop computer with a free terminal emulation program is common. Consult the manufacturer for the best method and their particular techniques.
• Integrate the thermostats with the automation system. With communications verified, plug the thermostat interface into the home automation system, and have fun.

Now that the basic project flow is in place, let’s look at more of the wiring details before discussing the thermostat systems. All of the wiring techniques apply to nearly any residential HVAC control system in the market. When wiring for communicating thermostats, the same care must be taken as with all other low voltage cabling. Keep in mind that any technology based system is only as good as its weakest link. Take the same care with the thermostat communications cable that you would a good line audio or composite video distribution cable. It is recommend that you use a single-hub or multiple-hub configuration when wiring the communications network. See Figure 2 for examples. You may have also heard the single-hub method of wiring called a "star" or "home-run" configuration. With this approach, the thermostat network interface (adapter) is the center of the hub from which you run a cable to each thermostat location. Using this technique, you will be ready for communications using RS-232, RS-485 and custom drive levels. This configuration is ideal for most home systems, as they will have less than 32 thermostats. In commercial systems, there is one technology that uses a combination of a simple and robust protocol in conjunction with RS-232 and RS-485. Using the same wiring technique, this gives the expansive capabilities of over 8,000 thermostats controlled from a single RS-232 port on a PC. This technology is very applicable to both residential and commercial structures. Apartment complexes, hotels, commercial buildings, and schools are examples of projects larger than 32 units. Using a multiple-hub configuration in this application is the most efficient use of cable. This method allows shorter ‘home runs’ to several areas, which are then linked together with a single run of cable.

4) Pull the communications cable as specified above from your central location to each of the thermostat locations. With existing construction, it may be necessary to use a mixed configuration wiring backbone, as shown in Figure 2.

5) If a 24VAC (common) wire is not available in the existing thermostat cable, make sure you have an available wire pair in your communications to power the thermostats remotely.

There are three classes of communicating thermostat solutions: Power-line carrier (X-10), communicating thermostats and true sub-system networks, also referred to as cluster controllers.

The advantage of using an X-10 approach is straightforward, no additional wires. The X-10 thermostats use the existing 4-conductor thermostat wire as the communications cable. This is typically a two-piece solution for each HVAC unit controlled. The user interface looks like a normal thermostat and is mounted to the wall in the space to be controlled. The second piece is a ‘black box’ that is mounted at the HVAC unit. The black box contains the relays for controlling the equipment and the X-10 communications circuitry. Due to the number of house codes and addresses employed by the X-10 protocol, the most common use for X-10 based thermostats is in residences with four or fewer HVAC units. Due to its two-piece solution, X-10 is very appropriate for retrofit applications when pulling a communications cable is difficult.

The X-10 thermostats can be controlled from many popular home automation controllers and from several security systems. Friendliness of the user interfaces varies widely, as does the amount of time needed to set up the environment. Take time to examine the potential automation controller software and flexibility if you plan to use it as your user interface.

Communicating Thermostats

Most communicating thermostat adapters will allow up to 32 thermostats to be connected to the host automation system. One of the most popular of these allows an open ASCII protocol to be used from any RS-232 port. This flexibility allows many home automation controllers to connect to the thermostat network very easily, and writing software for the interface is simple. Using XCI’s Serial-Stat™ protocol as an example, in order to Read the Indoor Temperature of thermostat at address 1, the command RIT1 can be sent from the serial port. Full control and feedback of all modes of operation is available with all the thermostats. Standard communicating thermostat adapters are ‘protocol converters’ that change the information as noted above to digital information for the thermostats. The conversion is necessary in order to drive the commands over long distances and to verify the validity of the command being sent to the thermostat(s). XCI’s communications adapters allow any Serial-Stat to be up to 1,000 feet away, and up to 5,000 feet of CAT5 wire to be connected. In practice, this is enough cabling to connect a 50,000 square foot building with just one pair in the CAT5 cable. Using an automation controller makes sense in many cases. Additional user interfaces can be placed around the home as needed, scheduling can be performed for lighting and HVAC, and macros can be written to automate complex and multi-function tasks. Be careful not to overload the controller though. It is very easy to plan a significant amount of automation in a residence, only to find that the full complement of tasks desired really need to be controlled by separate sub-systems. Scheduling is one of those tasks that seem to be shortchanged even in some of the more expensive controllers.

Stand-Alone HVAC Subsystems

Taking the next step is simple. For a really robust and flexible HVAC control environment, use a subsystem. The difference between a complete HVAC subsystem and a communicating thermostat network is that the subsystem goes the extra mile. With a subsystem, all scheduling tasks can be easily updated by the homeowner from windows-based PC software or the automation controller. This allows the subsystem to operate independently of the automation controller, freeing up valuable resources from the controller and also gives the automation controller access for updates and feedback whenever requested. For an example of a true HVAC subsystem, see Figure 3. The XCI network subsystem has two RS-232 ports, one RS-485 port and one Serial-Stat communications port. With these ports, the system is flexible enough to allow the homeowner to be connected to the network via PC as well as the home automation controller. The real-time clock/calendar allows all HVAC related scheduling to be created and modified from either the software or the home automation system and stored in the subsystem controller. This allows the scheduled temperature environments to take place from the HVAC subsystem, freeing the home automation controller from yet another scheduling task.

A final thought regarding successful HVAC automation. Design a system to be consistent with the desires of your customer. The modern HVAC control technologies presented here can make your life as an automation specialist much easier. Follow the guidelines above, listen to your customers needs, be pro-active regarding coordination with others, and implement a system that minimizes after-installation issues.