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Old 04-11-2007, 02:54 PM   #1
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Thermostat replacement


I am trying to replace a mechanical thermostat - Honeywell T87F w/Q539J subbase - with a programmable one - Honeywell RTH 7400. The system has red/green/yellow/white wires that were connected to the R/G/Y/W terminals. When I connect the wires to the R/G/Y/W terminals on the new thermostat, the fan doesn't come on when on "auto," but does when fan is set to "on." The condenser does come on, just not the fan. I have a commercial heat pump that has hot water supplied by the building's boiler. Any help is greatly appreciated.
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Old 04-11-2007, 06:43 PM   #2
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Thermostat replacement


i would ask is this a digital thermostat and if so there are some internal settings that need to specify your exact type of system or it wont work correctly. there sould be instalation instructions that came with the stat , and if its not any internal settings than youve got a bad stat.
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Old 04-12-2007, 09:03 AM   #3
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Thermostat replacement


Thanks, jmosh. Apparently none of the internal settings you mention actually match my system. Thanks for your reply.
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Old 04-12-2007, 09:04 PM   #4
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Thermostat replacement


so does the fan come on in cooling mode , just not in heat pump mode?
do you have a hot water coil on your air handler for emergency backup heat?
Hold on you have a heat pump but the only wires are those 4 colors ?
that sounds like strictly air conditioning and hot water coil to me with wire colors like that .
heat pumps generaly have red, blue, yellow, orange, white, and some times brown for emergency heat and black for second stage heat/cool.


if your system is a 1 heat 1 cool nonheatpump system with a hot water coil than the fan would not be controled by the thermostat, it would be controled by a temperature sensor attached one of your hot water coil pipes at the airhandler. and would only turn on the fan upon temperature rise in the hot water coil ( in heat mode)
in this type of system (if that is what you have)
setings would be like this
0170 system type heat/cool 1
0180 heat fan control 0
0240 heat cycle rate 1

i hope that this helps
if you have a different kind of system let me know
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Old 04-13-2007, 09:36 AM   #5
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Thermostat replacement


Thanks again. I believe the auxiliary heat is electric and that it must be energized by the system and not the thermosstat. On the mechanical thermostat I'm trying to replace, there is no auxiliary heat setting. It is definitely a heat pump, but it is not located outside; it is actually in my condo unit. The building supplies hot water to the heat pump during the cold season, so it rarely uses the auxiliary heat unless I need heat after they've turned off the hot water supply. The mechanical tstat is the Honeywell round one w/a Q539J subbase. Other than the 4 wires, R/G/W/Y, that were connected to their respective terminals, it has an E/F switch set to "E" and heating cycle rate switch set to 3 cph. I just saw in the installation manual that I can set the cycle rate and I have not tried that yet, although I'm not sure it will make any difference. I have tried setting the 180 function to 1 and the condenser does not power up. I have tried setting the 170 to 4 and get fan and condenser, but it is cold air! I'm running out of options: maybe I'll just stick with the mechanical one.
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Old 04-14-2007, 09:30 PM   #6
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Thermostat replacement


without actualy seeing how this system is operating im afaraid i cant realy give you anymore ideas.sorrry
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