Please, I need help upgrading my old Mercury Thermostat


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Old 07-08-14, 02:05 PM
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Please, I need help upgrading my old Mercury Thermostat

Hi, I am new to the group and am trying to upgrade my old mercury stat with a programmable unit.

Zone 1 is controlled by a Honeywell T87F round thermostat on a Honeywell Q539A base. It has six wires R, W, Y, G, B, O.

Zone 2 has a T87F thermostat with three wires W, R, Y.

The control board is a MM2 Mastertrol Minizone-2

I wanted to replace the stats with digital programmable ones so I tried a Honeywell RTH6580WF but it requires a C wire for power. The R, W, Y, G wires are self explanatory but I have the B and O wires remaining. Other stat manufacturers mention that if there is a B and O wire present then the system is a "heat pump" setup versus a "conventional" setup. It is a Rheem propane fires furnace and Rheem central air unit that utilizes forced air.

Can someone point me in the right direction of wiring the RTH6540 or should I get a different stat although.

Any and all help would be extremely appreciated.

Thanks, Ed
 
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Old 07-08-14, 02:59 PM
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All I can find is RTH6450,could have the numbers flipped?
 
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Old 07-08-14, 03:35 PM
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It actually says rth6580wf. Sorry for the confusion.
 
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Old 07-08-14, 06:10 PM
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You need a thermostat with separate O and B terminals for your zone 1 thermostat. That Honeywell does not have separate O and B terminals.



You will need a C wire for both of those new thermostats. Do you have enough conductors to each thermostat now?

Important!
C wire is required and is the
primary power source. Without a C wire,
your thermostat will not power up.
https://customer.honeywell.com/resou...0s/69-2720.pdf
 

Last edited by Houston204; 07-08-14 at 06:26 PM.
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Old 07-08-14, 07:36 PM
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Sorry, complete novice here, what do you mean by conductors? Thx
 
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Old 07-08-14, 08:36 PM
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Wires are conductors.

Your zone 1 is using six but you will need a seventh wire for 24 volt Common (C) for most WiFi stats.
 
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Old 07-08-14, 09:21 PM
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I am not tied to the wifi unit and will glad use another model, brand or both that is recommended. I saw that Lux makes a 6/7 wire stat. Any recommendations? Thanks again.
 
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Old 07-08-14, 09:40 PM
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I prefer White Rodgers over Lux thermostats.
 
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Old 07-09-14, 06:16 AM
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Is there a particular model of White Rodgers that I should look for that will work with my application?
 
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Old 07-09-14, 08:59 PM
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I liked that UP200 White Rodgers that I posted the Amazon link to but it looks like the UP300 will not run off the batteries and only uses batteries to remember the program. The UP200 probably also suffers this feature.

We need a single stage heat pump stat with separate O and B terminals that has batteries and a "C is optional" terminal or no C terminal at all.


The Honeywell Pro TH4110D will support your system.
 

Last edited by Houston204; 07-09-14 at 09:41 PM.
 

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