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Tech Tip #5 Two separate loads in heat pumps usually must be controlled: The compressor and the backup (auxiliary) heat. Generally, the compressor is controlled by breaking the low-voltage signal from the thermostat, usually the Yellow (Y) wire. The backup heat is controlled by either breaking the low-voltage signals from the thermostat, usually the White (W) wire and the defrost relay, or by controlling the individual 5 KW elements inside the air handler unit. Once you have completed several jobs, connecting heat pumps to demand controllers is usually simple and straightforward. Take your time on the first job or two to read the directions. Call the factory for assistance if necessary. An improper hookup will cause many callbacks and a loss of your time, money and customer satisfaction. These directions are only a general guide to the proper hookup of heat pumps. Because there are so many brands in the field, each installation may be slightly different. Please take time to read the manufacturer's diagrams carefully. A good reference guide is the Honeywell Multistage thermostat Manual, P/N 60-2116-3. for basic background information on heat pumps, refer to the Honeywell Heat Pump Controls Manual, P/N 71-97735.
Hook-up (Compressor) 1. Refer to the schematic on the compressor unit. Locate the low-voltage wire (Yellow wire or "Y" terminal) from the thermostat that signals the compressor relay to operate. 2. Run a 2-conductor, 18-gauge wire from the control relay to the point where the thermostat wire ties the signal to the compressor contactor. This point can usually be found in the air handler unit. If not, you will find it in the outdoor condensing (compressor) unit. 3. Interrupt the signal found in Step 2 with the 2-conductor wire. Connect the other end of the wire to the control relay. 4. Be sure to program the relay with a minimum OFF-TIME of at least 5 minutes to prevent short cycling of the compressor. Be sure to program a minimum ON-TIME of 6 to 9 minutes to maintain desired comfort. If the compressor contains its own internal short-cycle protection, the minimum Off Time may be set to zero (0). Caution In extremely cold areas, compressor control is discouraged because of the inefficiency that results from frequent starting and stopping the compressor.
Hookup Auxiliary Heat (Low Voltage) 1. Refer to the schematic for the auxiliary heat (usually found in the air handler unit) and locate the wire(s) that signal the auxiliary heat to come on. Auxiliary heat usually gets its signal from the thermostat Stage 2 (W2) and/or from the defrost relay. 2. If the thermostat signal and the defrost signal tie together at the air handler, run a 2-conductor, 18-gauge wire from the control unit to the air handler unit. If they don't tie together, run a 4-conductor wire to the control unit from the air handler. (These wires may be combined with the compressor wires used in the previous "Hook-up Compressor" section. Generally, you will be safe if you run a 6-conductor cable from the air handler to the controller to interrupt the compressor and the auxiliary heat.) 3. Interrupt the signal with the new wires at a point where the thermostat and defrost tie together. If they don't tie together, interrupt each signal separately by using four conductors. See diagrams above. 4. Connect the other end of the wires to the appropriate relay. See priority lists that follow. Note: This type of control is usually done in moderate climate regions where there are usually no more than two 5KW heat strips in the home. For extremely cold climates or for maximum control, refer to the following section on "Auxiliary Heat, Individual Elements."
Hookup Auxiliary Heat (Individual Elements) This should be done by a licensed electrician. Not recommended for homeowners. 1. Turn heat element breaker(s) OFF. This is generally more than one breaker. 2. Mount a Remote Relay Unit on or near the air handler unit, if required. (Tech Tip #10 explains Remote Relay Units.) 3. Using 10- or 12-gauge wire as required by breaker size and/or load size, rewire one side of each element through a relay in the Remote Relay Unit. Wire one element per relay, if possible, leaving the second, unused relay pole as a spare. If there are five or more elements, wire two elements per relay. (Also see the instructions that come with the Remote Relay Unit.) 4. Refer to the Remote Relay Unit instructions to wire the Remote Relay Unit to the control unit. 5. When installation is complete, turn ON breaker(s) to air handler or electric furnace. Note This type of control is usually done in extremely cold regions, or when individual staging of elements is desired. Control of Heat Pump:
Caution In extremely cold areas, compressor control is discouraged because of the excessive stress that results from frequent cold starting.
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