Thermostats 101 and 102
#9
A question shows up every now and then so.......

     Boiled down to its simplest a thermostat is a switch, it takes power from a source and sends it to various other places to do things when required.  Its a thermostat because it responds to temperature.  The switch(s) can be mechanically operated or electronically operated.  Electronic stats can be powered by the equipment power ( power robbing, they rob power from the equipment power supply to run themselves) or batteries.  A battery operated stat that has dead batteries will not run your equipment.  Pick a day, any day that you will remember, and annually change the batteries in your thermostat and smoke/CO detectors. AFAIK, at this time, all internet connectable stats are power robbing.  More on that later.

    What about all those different coloured wires and letters and all the different kinds of equipment that might or might not be connected.  Unlax, there's an industry standard that covers 90 some percent of the stuff out there. Gas and LP, oil and electric furnaces,heat pumps and air conditioners mostly follow this standard.  Note that I am not covering electric baseboard or radiant heating systems or some of the more complex hydronic, solar or other systems.  The standard is generally called series 20 wiring, it is a nominal 24 volt AC system and it is based on letter codes, not wire colours.

R - power to the switch(s) from the equipment control circuit.  On the furnace it is power out, on the stat its power in.  R on one connects to R on the other. RC and RH are split inputs, they allow a single thermostat to control a separate heating system, RH, and a separate cooling system, RC.  If only one piece of equipment is providing control power, RH and RC will have a jumper wire across them.

C - the 24v neutral.  It is required on all power robbing thermostats.  Connecting C on thew furnace to any other terminal on the stat will result in a dead short when that terminal's function is activated.  Don't do that, the magic smoke will escape.

W - and any numbered variants, is a heating call from the stat, back to the equipment. W1, W2 are stages of heating for multistage equipment, don't mix them up, it won't work.

Y - is the same as W except its the cooling function.

G - is a manual call for the circulating fan on equipment that has that capability.  On older equipment it activates the fan on cooling speed, on newer it may activate a slower circulate speed.

O and B are heat pump signals to energize the compressor, fan and diverting valve on that type of equipment.

If the thermostat has additional terminals, check the wiring instructions, some have additional functions that may be connected such as humidity control and outdoor temperature display.

Programming the functions on electronic stats and enabling all the functions some are capable of is wizardry, if you don't have a wizard handy, consult the book or on line destructions, they are all different.
Blackhat

Bad experiences come from poor decisions. So do good stories. 


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#10
Thank-you! that is perfect. I'm adding it to my favorites; in the Building Codes folder. Can that also be my "Learn something new every day."?
 (Be a damned poor day if you didn't.)
Sign at N.E. Vocational School Cabinetmaking Shop 1976, "Free knowledge given daily... Bring your own container"
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#11
Thanks muchly!

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#12
Very useful, thanks a lot. Now if I could figure out how to file this away for future reference.....
I started with absolutely nothing. Now, thanks to years of hard work, careful planning, and perseverance, I find I still have most of it left.
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#13
"O and B are heat pump signals to energize the compressor, fan and diverting valve on that type of equipment."

  O or B are usually specific to heat or cool operation via the reversing valve. On what equipment does O or B energize the contactor or board that controls the compressor and fan?  Was it Ruud/Rheem who did that with their 3 or 4 wire setup?
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#14
I just installed a new Honeywell stat---on the back was a row of stab terminals(with set screws) labeled with the wire colors. Plus the directions had a list similar to yours.
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#15
(07-13-2017, 05:49 AM)fredhargis Wrote: Very useful, thanks a lot. Now if I could figure out how to file this away for future reference.....

Try right clicking on the thread banner ("Thermostats 101 and 102" just above Blackhat's photo), choose add to favorites or create a shortcut.
Sign at N.E. Vocational School Cabinetmaking Shop 1976, "Free knowledge given daily... Bring your own container"
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#16
(07-13-2017, 05:49 AM)fredhargis Wrote: Very useful, thanks a lot. Now if I could figure out how to file this away for future reference.....


I use Bullzip, and print the page to a PDF file (not paper) using the Bullzip printer function.  There are other's out there, like Cute PDF, etc.
I tried not believing.  That did not work, so now I just believe
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