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Heating & Cooling

Changing a mercury thermostat to a digital

Can an amateur like me change a mercury thermostat to a digital by myself or should I get someone that is licensed 
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Posted 2017-12-23T09:25:35+0000  by Bragga Bragga

Hello Bragga.  Welcome to the Community!

Do-It-Yourself “amateurs" replace thermostats all the time.  The process is not a matter of special skills, but will require patience and a little learning.

There are 2 things for you to consider.  First is whether attaching a new mounting plate to the wall to replace the old one will be a problem for you.  Second is about the wiring.  There is a color code for thermostat wires, but often, especially in older homes this is not followed.  Keeping track of what wire originally went where is very important when installing the new thermostat.

Mercury thermostats are really “old school” and most have already been replaced with either newer mechanical models or digital ones.  My home still has its old mercury thermostat, running a hot water system with no central air conditioning.  As such it has only two wires going to it.  One is the “R” wire that supplies 24 volt AC power from the furnace.  It is supposed to be red by convention, and it is.  The other is the “W” wire that sends that power back to the furnace when heat is required.  It is supposed to be white, but mine is black for whatever reason.  The terminals on a thermostat are labeled with a letter code.  It is that code that matters, not the color of the wire going to it.  So the first lesson, and most important one, is to write down or mark the original wires with their terminal code before you disconnect them and lose track of where they went.

If you have a forced air system with central air conditioning, then more wires will be found.  Either an extra usually red wire will connect to the “RC” terminal, or there will be a jumper wire in the thermostat connecting “R” to “RC”.  In addition there should be a wire to tell the AC unit to run.  This will be a “Y” compressor wire and is supposed to be yellow.  There may also be a “G” wire which independently controls the blower fan motor.  You may suspect by now that it is supposed to be green, but maybe not…

Mercury thermostats use a glass capsule partially filled with mercury attached to a temperature sensitive spring.  The spring tilts the capsule as room temperature changes, moving the mercury from one end of the capsule to the other.  Inside the capsule the “R” wire runs along the capsule bottom and is always bathed in mercury.  At one end the “W” wire circuit enters the capsule.  When it gets too cold in the room the spring tilts the “W” end down and the mercury completes the furnace circuit asking for heat.  More modern mercury thermostats for forced air systems with A/C may also have a “Y” circuit that connects to the capsule at the other end.  Mercury can then connect that circuit when the home gets too hot by tilting the capsule in the other direction.  There will be a manual heat/cool switch on these units so they cannot both run at the same time.  Since you have a mercury thermostat now, you know that there needs to be only 2 or 3 wires coming to it from your furnace.

OK, so now the question becomes what you want your new digital thermostat to do.  If all you want is to be able to program the heat and maybe cooling then this should be fairly easy.  A simple, battery operated digital thermostat should work with no problem.  You will find lots of options at Home Depot.  In general, more money gets you either a bigger display and/or more days of independent programming.  No additional wires should be needed to make this conversion.

If you are looking at WiFi, networked or home automation type thermostats then more wiring will likely be required.  I will need a lot more information about your home in order to help you make that happen.

Thank you for your patience in reading through all this.  Let me know if I can be of further help.



Posted 2017-12-26T16:06:42+0000  by Chris_HD_CHI
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