I have a Carrier single stage heat pump model 25HCB3/25HCC5 and it has an aftermarket Lux TX900TS thermostat wired as described at the bottom of page 11 in the manual.
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I have no B connected but all of the others are connected as in the diagram, including the optional C.
The problem is the jumper between W and Y. It must be there in order for the compressor to come on for heating, otherwise only the heat strips work. It must not be there in order for the compressor to come on for cooling. I can't figure out how to wire it so that the compressor comes on for both heating and cooling.