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BobB59

06:22AM | 10/30/08
Member Since: 10/29/08
1 lifetime posts
Bvelectrical
I'm looking to replace a Pneumatic thermostat with a Digital Model. How difficult is this?

Techgromit

10:27AM | 12/09/08
Member Since: 12/27/06
14 lifetime posts
Generally replacing Thermostat is pretty easy if you don't mix the wires up. First things first, open up the old Thermostat and write down what colors are connected to what before you touch anything. If they are not colorized, then label them with pieces of paper and tape. Nothing more embarrassing that having to pay an HVAC contractor to come in, re-wire your thermostat and charge you $100 for your stupidity.

You shouldn't have to turn off the power, the voltage is too low to be harmful, but it may be a good idea to do so. Some very old thermostats use 120 volts, which can kill you, if the wires are any thicker than phone or stereo wire, you might want to test them to see if they are high voltage. (120v) Chances are your thermostat used low voltage, generally you can follow the diagram that comes with the new thermostat. If worse comes to worse, and you can't get it to work right, you can put the old one back on. Just make sure it both the heat and air works before tossing the old thermostat.

A special side note, Heat pumps require special thermostats, generally the generic thermostats you get from a big box supply store isn't going to work for you.

TimBonham

11:57PM | 12/11/08
Member Since: 01/09/07
197 lifetime posts
"If they are not colorized, then label them with pieces of paper and tape."

How you do that is look at the terminals where the wire is connected. You should see a notation near there (sometimes you may have to look under the wire to see it).

What you should see are colors, or codes for colors. Normal ones are:

C-common (usually black wire)

R-Red

W-White

Y-Yellow

G-Green

You might also see more than one, especially with White & Yellow. These are then normally marked Y1, Y2, etc. (But Y2 probably isn't a yellow wire, but some other color wire that is in the cable.)

On newer systems you may also see additional wires:

O-Orange

T-Tan

B-Blue

You might also see other colors, like Pink, Brown, Gray, but those are normally marked by various function codes rather than color. And they will likely vary by manufacturer.

One quick guide is to count the number of wires in the cable between the furnace & the thermostat. Really old furnaces will have only 3 wires (Black, Red, White). Newer ones, with Air Conditioning (or plans to add it) will have 5-wire cable, with Yellow & Green added. Additional wires indicate a newer system with multi-zone systems, heat pumps, or multi-stage furnaces.

And one thing to watch for is that old-time contractors sometimes used whatever piece of cable they happened to have around, ignoring the color code. Not uncommon to see old phone cable (red, green, black, yellow) used with red & black as normal, either yellow or green used for the white wire, and the other just unused.

norespect

12:45PM | 12/12/08
Member Since: 12/30/02
46 lifetime posts
If this is truly a pneumatic t-stat then there should'nt be any wires to mix up.You should just pull off the air line and attach it to the same place on the new t-stat
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