Correct Wiring for Heat Pump Thermostat


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Old 04-23-08, 08:09 AM
T
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Correct Wiring for Heat Pump Thermostat

I am installing a Honeywell programable thermostat RTH7400. Outside A/C unit is Carrier model 38BYC03 and inside air handler unit is Carrier model FC4BNF036; both 3 ton units. Old thermostat is Honeywell model CT 3611 which was installed by contractor when new system was installed in 2004. New thermostat is Honeywell RTH7400. Not sure if wired correctly and have followed instructions with new thermostat.

The old CT3611 thermostat was wired as follows;

B: Nothing
L: Nothing
C: Blue Wire
R: Red Wire
W2: White Wire With Jumper To E
Y: Yellow Wire With Jumper To W1
O: Orange Wire
G: Green Wire

I Wired the new RTH7400 thermostat as follows:

(There are two notes with instructions for the RTH 7400 that states;
1. If old thermostat has Y1, W1 and W2 wires, stop now and contact a qualified contractor for help.
2. If E and Aux terminals do not each have a wire connected, use a small piece of wire to connect them to each other.)

RC:
R: Red wire with metal Jumper to RC
O/B: Orange wire
Y: Yellow
G: Green
C: Blue
L: Nothing
E:
Aux: White wire with copper jumper to E

The CT3611 had the jumper from Y treminal to W1 terminal. Why doesn't the RTH 7400 have that jumper?


Thank You for all the help!
 
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Old 04-23-08, 08:24 PM
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Originally Posted by tmart
The CT3611 had the jumper from Y treminal to W1 terminal. Why doesn't the RTH 7400 have that jumper?

Thank You for all the help!
In your CT3611 the W1 is a terminal that is utilized in the event of your outdoor unit having a two speed compressor. In which case the jumper is removed so that during the summer the t-stat's Y-terminal juices up the compressor on high speed, while in the winter W1 juices up the compressor's low speed while W2 the Auxiliary Heat Relay.
Though I must add that a much better way in such case would be to have thermostat with 3-stages of heating (w1= compressor low speed; w2= compressor high speed; w3= auxiliary heat)

Your outdoor unit likely has a single speed compressor and since it must run on a call from the thermostat (whether a call for heating or cooling), it is the reason why on the CT3611 you saw Y and W1 jumpered. Y juices-up the compressor on a call for cooling (summer), W1 on a call from heating (winter). Without the jumper the compressor would only operate in one of these two modes but not both.

The RTH7400 was NOT made to operate multi-stage compressors. The Y-terminal controls the compressor in summer AND winter, so there's no need for a W1-terminal. Your Y is both a Y and a W1 at once in one single terminal.

my humble 2-cents
 
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Old 04-26-08, 11:56 AM
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Thank You! I do appreciate the detailed explanation.
 
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Old 04-29-08, 03:41 PM
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How do I determine if I have a single speed compressor?

Thank You!
 
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Old 05-01-08, 06:14 PM
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As per manual, single speed
 
 

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