The old thermostat RET-24 has one contact that activates a relay for the heat. The other two contacts are just for the 24 volt power supply only.
Old thermostat wiring and what it did.
The 24 volt line side as in + goes to the B terminal only (RED WIRE)
The 24 volt - side goes to the A or neutral side
AND the neutral activation leg of the heat relay should already be wired elsewhere. (BLACK WIRE)
The 24 volt + relay activation (HEAT) leg comes off the number 3 terminal of the thermostat. (GREEN WIRE)
See inclosed link below. Page 2 RET-24
http://na.heating.danfoss.com/PCMPDF/RET24-DS.pdf
Now the Honeywell will use. USE CONVENTIONAL WIRING SIDE ONLY. NO HEAT PUMP TERMINALS USED.
RED wire to terminal RH-RC (+24 volts) Hot
BLACK wire to the C terminal. (-24 volts) Common
Green wire to the W terminal (Heat activation) + 24 Volts when activated by the thermostat for heat relay.
The color code cannot be followed but the change over from Danfoss to Honeywell can and that is what I did.
Read the Danfoss directions and transpose the wires like I did and you should see how it works. I could not find a Honeywell DTH8500 so I used the Honeywell RTH8500. I can find no such model as was postad by the original poster but all the 8500 series are the same. NOTE: You will have to run the setup for the thermostat and pick the system (heat only) and the type of heat (electric). Now I have no idea if the thermostat has a setting for in floor heat so you might have to find one of the other settings besides Electric heat that will work better that you can experiment around with. Good luck.