I am replacing a Danfoss RET-24 thermostat with a Honeywell RTH8500.

There were three connections on the old unt:
Black wire (connected to slot A: RC/24V)
Red wire (connected to slot B: RH/24V)
Green (connected to slot 3: ON/W)

There is 24VAC between the black and red wires.


I've tried a few wiring combinations and have tried playing with different settings on the new thermostat as well, but haven't had any luck yet.

Any advice would be appreciated.

The 24VAC difference is also present from Red (RH) to Green (W).

Checking the old thermostat, there is no resistance between the Black (RC) and Green (W) regardless of the temperature setting.

SO I am under the impression the old thermostat turns on by connecting the Red (RH) to both the Black (RC) and Green (W)

In the thermostat there is a contact closure between Rh and W when there is a call for heat.

In the thermostat there is a contact closure between G and Rh that turns on the fan.

For a thermostat that is configured for Electric heat, the thermostat controls the fan.

For a thermostat configured for Fossel fuel heat the furnace controls the fan.

Heat pump tstat and duel fuel stat operation left out.

Rc will not be used, but Rc can be connected to Rh and that wire would be considered R. Separate Rc and Rh allows a different transformer for heat and cool.

The Danfoss-24 is a line voltage stat. http://randall.danfoss.com/xxTypex/296337_MNU17433277_SIT313.html

I hate to brake the news to you, but you probably let out the magic smoke. They don't appear to be compatible. The wires are probably very thick coming out of the wall.

There are no cooling or fan units, and all three wires are required on the old thermostat to get things working...
I have a feeling the label of RC on the old thermostat for the black wire must be bogus.

The wires are quite (18-20). As far as I can tell, the new thermostat works (it is battery powered, and the touch screen works fine).

There are no cooling or fan units, and all three wires are required on the old thermostat to get things working...
I have a feeling the label of RC on the old thermostat for the black wire must be bogus.
(I've read black can commonly be used for G)

The wires are quite thin (18-20). As far as I can tell, the new thermostat works (it is battery powered, and the touch screen works fine).

I looked closer at the stat. Apparently there are 24V and line voltage versions.

The reason why there are three wires is one would be (C) or common. Between (R) and (C) there will be 24 VAC all the time.

(C) is not required for battery powered stats, but tape it. Otherwise, contact with (R) may blow the transformer or low voltage fuse.

As it turns out, there is in fact about 3-5 Ohms between the black RC and green W terminals on the old thermostat (probbaly for the LED status indiciator) which would would seem to confirm the theory that the black RC wire is in fact acting as a common C wire...

Only thing is, the new Honeywell thermostat doesn't work with that wiring combination either (Red RH to RH, Green W to W, and either the black RC connected to C or taped off).

Is there anyway to test if the thermostat is working properly? e.g. when it activates, should there only be little resistance between the RC and W terminals?

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.