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I am currently interested in determining how a home thermostat controls a home HVAC unit. I have run into plenty of schematics that can control a HVAC window unit, walk in freezer, water temperature, etc. However, using a circuit like those on a unit as expensive as a HVAC unit is just nuts as far as I am concerned...plus it doesn't take care of all possible operations.

The question I seek an answer for: How do you build a complete home thermostat. A device that protects and is built to ensure no damage is done to the HVAC unit. I am planning on buying a cheap $10-20 thermostat just to tear it apart and study the board in detailin a few weeks if I cannot find any other answers.

My current home HVAC unit uses a 4 Wire Heat/Cool setup which is referenced on page 17 (10 on adobe reader) of this thermostat installation manual: http://www.ritetemp-thermostats.com/60XX/images/6022_installation_guide.pdf

Connections are as follows:
W - Heater Control Line
Y - Cooler Compressor
RH - Heater/Cooler Power
G - Fan Control
C - Optional Thermostat Power (which I have)

What kind of control does this device use? Is it 1-wire? Where is the GND connection? In addition to this wire diagram, the unit also provides C which powers the thermostat itself ensuring no batteries are required. I wish I could tear this unit apart but do not have another thermostat to put in its place until I get paid.

If you can help me/point me in the right direction; I would be much appreciative.

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  • \$\begingroup\$ One note to point out...Both the heater and air condition system runs off of electric. \$\endgroup\$ Commented Oct 15, 2010 at 5:23

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After reading a few posts and doing a little testing with a DMM, I believe I have solved my questions concerning how a home thermostat works. First the posts/info that led to the conclusion:

RiteTemp Installation Guide (pg 17 - 4 Wire Heat/Cool No Pump)

JohnSAZ from EEVBlog:

Of course your thermostat may be different but every one that I've ever looked at worked the same (and your color codes pretty much indicate an industry standard setup).

Think of the thermostat as nothing more than three switches. The "RH" is your common. If you short "RH" to "G", your blower fan will turn on. If you short "RH" to "Y", the air conditioning compressor will turn on. If you short "RH" to "W", the heater system will turn on. This could be a gas burner solenoid, or relays to energize resistive electric heaters. If the unit is a heat pump, it will turn on the air conditioning compressor, but it will also energize a reversing valve that will make the freon run the other way around (kinda).

Obviously you need to ensure that the "G" and "Y" leads are NEVER turned on at the same time. You also need to make sure that the eater system can not be turned on with the blower off. Also, you need to make sure that when the compressor turns off, it is not allowed to turn back on for some time (10 minutes? don't know the time limit for sure). Also, higher quality thermostats keep the blower running for some time after the compressor (or heater) has turned off to suck every last Btu out of the system before shutting down.

I grabbed my trusty DMM and confirmed that W, Y, G, and C all measure 30VAC+ when tested against RH (or R) - GND. When the system is running, G drops to 0.05 for both heat and cold (meaning the fan is running). W drops to 0.05 if the heater is running. Y drops to 0.05 if the Cooler is running.

I still need to research the appropriate timing for cooler/heater shutdown etc.

If anyone can think of any additional circuit protection outside of what JohnSAZ noted, please let me know. I am interested in adding a fail safe to the circuit design in case the unit fails. Not sure if relays in off positions during power outage would take care of business or not. Either way, I do plan on providing battery backup to the project just in case (IE. detect power failure, disable all controls).

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    \$\begingroup\$ Why does it say no G and Y at the same time? I would think you don't want W and Y at the same time (heating and cooling simultaneously) \$\endgroup\$
    – ajs410
    Commented Oct 15, 2010 at 16:12
  • \$\begingroup\$ Good eye. I think that was a mistype. In fact, I can now prove that the thermostat does run G and Y or G and W at the same time. Y and W however is a bad combination. \$\endgroup\$ Commented Oct 15, 2010 at 17:36
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    \$\begingroup\$ On systems I'm familiar with, the thermostat is not responsible for controlling the blower in heat mode. When heat is called for, the "W" lead is energized, turning on the burner(etc.). When the temperature at the heat source reaches a specific point, the furnace turns the blower on via an internal thermostat. This prevents the cold draft that would otherwise result if the blower is switched on immediately. Similarly, the blower will continue to run after the heat is turned off, while the system cools down. I don't know if all-electric systems work this way. \$\endgroup\$
    – TomG
    Commented Oct 16, 2010 at 18:59
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    \$\begingroup\$ That is a good observation and makes since. My HVAC unit was installed in 92. I noticed that when the current (home depot) thermostat turns on it blows cold air for a few minutes before it heats up (when on heat). Not sure if the thermostat is responsible for this or if the internal temp sensor as you noted does not work properly. I think I may grab an Arduino and setup a logging circuit for a week or so and take a look at the results before I start playing with everything. This will also give me a good idea on the timing used in an existing product to ensure the HVAC unit doesn't die. \$\endgroup\$ Commented Oct 16, 2010 at 21:34
  • \$\begingroup\$ I'd be interested in your circuit setup for logging - I'm interested in the same thing. See my question electronics.stackexchange.com/questions/4606/… \$\endgroup\$
    – TomG
    Commented Oct 25, 2010 at 2:28

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