I have a new heat pump with 2-stage cooling and 3-stage heating (plus emergency). The thermostat leads from the system are C, R, Y1, Y2, O, G, W and L.

I purchased a Honeywell RTH 7500 thermostat, but am not sure how to wire it. I can connect the C to C, G to G, Y1 to Y, O to O/B, R to R, W to Aux, and L to L, but which terminal on the RTH7500 is the Y2 terminal?

The sequence of events for the heating on my unit would be...
Heating, 1st stage (Y1,G) 75% fan and 1st stage compressor
Heating, 2nd stage (Y1, Y2, G) 100% fan and second stage compressor
Heating, 3rd stage (Y1, Y2, W, G) 100% fan, compressor and backup electric heat
Emergency Heat (W,G) Fan at 110% and backup electric heat.

For the cooling mode first stage is Y1, O, G and second is Y1, Y2, O and G.

Any help? Perhaps I need to buy a different thermostat. If I do it won't be a Honeywell.

Thanks in advance for any responses.

Is the new stat a 2 cool, 3 heat stat?

Is the new stat a 2 cool, 3 heat stat?

I thought it was, as the literature said that it would work for multi-stage heat pumps, but after looking at this further I believe it is a 2-stage heat stat. At any rate, I'm about to take it back. :(

I have a new heat pump with 2-stage cooling and 3-stage heating (plus emergency). The thermostat leads from the system are C, R, Y1, Y2, O, G, W and L.

The above make No sense. The leads indicate 2 stage cool (# of y terminals) and single stage heat (# of W terminals).

Aux and Emergency heat gets confusing at times. Emergency heat usually means disabling the compressor on purpose or just using AUX resistance heating without any assist from the heat pump.

The resistance heating USUALLY is counted as a stage. Having too many stages is fine than not having enough. More stages cost more $ because of the extra relay involved per stage.