Electronics > Projects, Designs, and Technical Stuff

Project: ISO Example Home HVAC Thermostat Control Schematics and Design Info

(1/3) > >>

thakidd:
I am currently interested in building a project to replace my home thermostat. How does a home thermostat control 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 if I do not know where to start. Winter is coming and I would not want to replace my heater...plus the circuits I have run into do not take care of all possible operations that a normal thermostat provides.

The question I seek an answer to: How do you build a complete home thermostat? A device that protects and is built to ensure no damage is ever 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 detail in 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)

One note to point out...Both heater and air condition systems runs off of electric.

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 right now but do not have another thermostat to put in its place until I get paid.

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

PetrosA:
Generally speaking all a thermostat does is act as a switch on a low voltage AC line (12V-24V). If you have a heat/ac t-stat, the basic setup is one power in, two out - one turns on the heat, the other the ac. More advanced will have a + and - to power the t-stat or to run emergency backup heat of some sort. How it does those things (especially regarding heat) may differ from one system to another. In some, it will turn on the main control which starts the heater (forced air systems) in others, it may control a zone valve or zone relay. Either way, it's all low voltage AC power unless you have electric baseboard heat, which it doesn't sound like you do.

Also, check from page 15 as it seems to give a pretty good description of what the different setups do.

JohnS_AZ:
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.

thakidd:
Excellent...thank you. Just what I was looking for...I also found this link on the web: http://www.hometech.com/kb/questions.php?questionid=53#nonhp

It pretty much describes the same thing and between your extra advice in your posts and that manual, I got it now. I think I was just having a horrible Google Search moment or something.

So question for you guys. Suppose the power fails or the unit fails. Is there any type of circuit I could include which would turn off/protect the HVAC unit if all else fails?

Also I have a fifth wire for thermostat power. I am guessing C is +24V or whatever and R/RH is GND?

thakidd:
I grabbed my trusty DMM and confirmed that W, Y, G, and C all measure 30VAC+ when tested against RH 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 and subsequent start-up.

If anyone can think of any additional circuit protection outside of what you all 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 default-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 (i.e. detect power failure, disable/shut off all controls).

Navigation

[0] Message Index

[#] Next page