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HTINews Article

Communicating thermostats: A comparison of control schemes X-10 vs Hardwire vs RS232 communications ... which one is for you?
by Jerry Tartaglino, Enerzone Systems Corp.

How things have changed. Only a few years ago, you had in reality no good options to integrate your thermostat into a home automation system. No more ... Now you have several very different approaches to controlling the thermostat-and many factors to consider.

The Choices?

• First, a thermostat that listens and responds directly to X-10 signals, where different unit codes get you different preprogrammed setpoints. For instance, Unit 5 OFF gets you 64 degrees, and unit 5 ON gets you 80 degrees.

• The next choice uses contact closure outputs from any source, including an X-10 universal module, to trigger setpoints that have been preprogrammed into the thermostat. As an example, a relay output from a Home automation system when open sets the thermostat to 72F, when closed, sets the temperature back to 65F.

• The third choice is true RS232/485 communications, where you now directly link to the thermostat, can see an image of the thermostat on screen, and can make and observe any changes from the PC that can be seen or made at the thermostat. Almost all major suppliers of automation systems have this type of thermostat as a part of their system.

With the three choices in mind, think about what a thermostat is expected to do, and how people tend to use a thermostat. A thermostat should control an HVAC unit around a setpoint that the user has programmed in. Failure to do so at best means annoyance, and at worst means frozen pipes, water damage, hotel bills, and big problems. So reliable control is important.

What does the average user need from the thermostat in terms of different setpoints? While that is an individual preference, consider what most conventional programmable stats offer people: 4 different setpoints per day. This allows as an example, 72F in the morning while getting ready for work, 62F during the day while the house is unoccupied, 70F in the evening after the User return home, and 68F at night while the user sleeps. This example is considered in the thermostat industry to be an average or typical user. However, the reality of the 90's is home offices, erratic schedules, full time parents, and other occupancy schedules that can make the standard programmable thermostat with its rigid scheduling program useless.

Enter the communicating thermostat. First, the communicating thermostat can be approached and adjusted with up-down arrows, just like any electronic thermostat. The communicating thermostat allows the user to call ahead on a cellular phone to request a temperature. It can tie into a light switch, security system, motion detector, or almost any other device. It can look at other control events and decide, based on its programming, what temperature level to maintain. The end result can be no more wasted dollars heating or cooling an empty house, and no more coming home to an uncomfortable house.

Now the trick is deciding what communication strategy is the best for your home and budget. To some extent, your decision may be based on what type of home control system you will be using.

First, X-10 users can use either the single or multiple contact closure thermostats, or a direct X-10 thermostat. With the contact closure models, such as Enerzone's StatNet(tm), you can connect the thermostat to X-10 universal module(s) to select between preprogrammed setpoints. Using binary logic, 1 contact closure gets 2 setpoints, and 2 contact closure's get 4 setpoints. Major advantages: Looks, Cost, (at least for the basic model taking one contact closure), simple installation, and protection from X-10 glitches -line noise, your neighbors controller, or any other type of X-10 glitch at worst will only send the contact closure thermostat to one of your pre-approved setpoints-avoiding potential disasters. Another advantage is that X-10 is not required ... any source of relay outputs or contact closures can be used, meaning you can control from hard wired relay outputs, security systems, motion detectors, and other non X-10 protocol devices. Disadvantage: Remotely, you can only select from your preprogrammed setpoints; you can not actually select any temperature you want. Street Price, $90 to $200, depending on model and options.

On the other hand, an X-10 direct thermostat actually listens for X-10 signals, just like a lamp or appliance module. By selecting a unit code and either an on or off command, you can remotely select almost any temperature you want, in two degree increments. You can also turn the thermostat on or off remotely. As an example, selecting Unit Code 16 OFF gets you a 75F setpoint. Unit Code 16 ON gets you 91F. Unit code 10 OFF gets a 69F setpoint. So grab an X-10 controller, sit back, and remotely change setpoints to your hearts desire. Advantage: Great functionality for the die-hard X-10 user, since so many remote setpoints are possible and will retrofit with existing thermostat wires in many applications. Disadvantage: Price, Cumbersome installation (4 separate parts), and could inadvertently be turned OFF by your neighbor, line noise, or some other X-10 Glitch. As with any X-10 device, the capabilities must be tempered against the worst case scenarios. Street Price, about $200.

Interestingly, some popular X-10 control systems like a JDS can use either its relay outputs for direct, reliable hard wire control of a contact closure type thermostat, or its X-10 outputs for an X-10 direct thermostat, or an RS232 output for serial control. So, questions that must be addressed are: can you run hard wire, how many remote selectable setpoints do you really need, what level of reliability must you have, and what is your budget? Questions only you can answer.

The third type of control is true communications ... RS232, 485 or the like. Linked with a computer or software driven system of some type, the capabilities are easy to describe: If you can see it or do it at the thermostat, you can see it or do it remotely from your computer or touch pad. This is the thermostat communication strategy of choice for upper end automation systems like AMX, Crestron, Phast, Vantage, Savoy, Home Automated Living, Lutron, Lite Touch And others. Here, the user can see an image of the thermostat on screen or touch pad, and can adjust and manipulate the thermostat just as if he or she were standing in front of it. But it gets better ... harnessing the power of the system software allows many control features above and beyond any stand alone thermostat, conventional, contact closure, or X-10 direct. Simplified scheduling, event based control, historical and energy use reports, alarm modes to alert of system malfunctions, and many other features become possible. Enerzone's StatNet, for instance, offers a simple ASCII command set that enable the thermostat to be easily written into almost any software based system. On interest, StatNet can communicate via RS232/485, and accept contact closure inputs at the same time. Advantages: The ultimate in remote control, true on-line communications, with capabilities only limited by software. Disadvantages: Depends on you having or buying a PC or other type of software-driven home control system. Cost: About $180 per thermostat.

So, if you want to interface you thermostat into a security system, home automation system relay output, motion detector, or some non X-10 relay output device, the contact closure thermostat is the way to go. If you want to have thermostat control from your X-10 based system, you have your choice between an X-10 direct thermostat, or a contact closure type thermostat, subject to the pro's and con's of both. If you have a computer based home control system with the right capabilities, you will want true RS232/485 "on line" communicating thermostats. The choice is yours, and all offer capabilities unavailable just a few short years ago.

Enerzone Systems Corp 4103 Pecan Orchard Parker, Texas 75002 (214)424-9808 Fax (214)424-8055