Replace Condo's 2 Wire Mechanical Thermostat With Digital Thermostat - HVAC - DIY Chatroom Home Improvement Forum

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Old 05-22-2008, 11:39 AM   #1
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Replace Condo's 2 wire mechanical thermostat with digital thermostat

I decided for efficiency sake I want to replace the old mechanical white rogers thermostat with a new Honeywell digital programmable one. The digital programmable thermostat I have bought contains connection points for Rh, Rc, W,Y, and G.

The problems that I've run into are:

1: The condo has shared central heating and cooling such that in the Summer (May-October) you can only get cool air (A/C) and in the Winter you can only get warm air.

2: There are only two black copper wires to the unit and again depending on the time of year it either does A/C or Heat. The mechanical thermostat took care of this problem by connecting both the W and Y wire to one single wire.

Should I just get copper wires to split the 2 wire's from the unit into 4 (so that Rc, Rh, W, Y are all connected ) ? Is this even possible or a good idea?

Thank you for your help.


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Old 05-23-2008, 05:40 AM   #2
oops, did I do that!
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Rh- Power (24v) heating
Rc- Power(24v) cooling
W- heating
Y- cooling
G- fan
C- Power (24v) common

Problem- A 2 wire stat is very much like an automatic light switch. One wire is power, when the normaly open switch closes, it sends that power to a valve or relay, and the light(ex) would come on.

The problem is that they are not normaly connected to W and Y. One normaly goes to Rh or Rc, the other goes W or Y. If you have a 2 wire stat (existing) it wouldnt have a Y on it. Are you positive ther wasnt a 3rd wire, and that the connections you described are accurate?

So there is no confusion, on a 2 wire stat, you would have only R and W.

It would be very unusual for a 2 wire stat to operate both heating and cooling. The heating "switch closes as it get colder, the cooling "switch closes as it gets warmer, a 2 wire stat, cannot do both.

If you are describing that R,W and Y are all connected together, you have no "control", and your thermostat is nothing more than a wall decoration.


Last edited by 8 Ball; 05-23-2008 at 05:57 AM.
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Old 05-23-2008, 06:17 AM   #3
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Could it be that since it's a shared system, you do have control over the fan but nothing else. Does the t-stat actually do anything? When you turn it down can you hear a click and the air come on? Also if it is a shared system... how do you get charged for that?
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Old 05-23-2008, 06:53 AM   #4
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In the summer the condominium physical turns a switch on their master control switching from heat to A/C.

In the summer I can make it as cool as I want but if there's a cool day I can not turn on the heat. In the winter I can make it as warm as I want but for a warm day I can not turn on the air. So again in the summer I can only control air and in winter I can only control heat.

So technically this is just a 2 wire system, where I would connect to R & W but I wanted to verify that connecting to those points only would be able to handle this physical switch to heat when winter rolls around made in the master unit by the condo complex.

A/C and Heat are rolled into my condo fee.
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Old 06-02-2008, 03:44 PM   #5
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Please HELP

I had the same problem and here is how I fixed it:

For Heat
R - Blue wire
W - Yellow Wire

For A/C
Rc - Blue wire
Y - Yellow wire

Last edited by PDRR; 06-02-2008 at 08:28 PM.
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2 wire , digital thermostat , mechanical thermostat

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