Hello,

I am trying to replace an old honeywell mercury thermostat with a new rht8500 honeywell thermostat.

I only have heating at home with 2 wires (Black and White) coming from the furnace to the old thermostat. There are no labelling what-so-ever on the old thermostat.

Can anybody help as to how to connect the 2 wires to the new one.

On the furnace side, the white cable is connected to the TH and the black cable is connected to the TH-TR

BTW, does anyone have a chart as to what the RC,R,W,Y,G,C,Y2,W2 stand for or mean in honeywell thermostat, same for TH TH-TR

I searched the net but found nothing.

Thank You

All you need to use is the R and the w terminals not being there I`d say the white wire would go to R on the tstat and the black wire would go to the w terminal on the tstat

You will probably need to add a wire for strait 24 volts to the C terminal. With a two wire system you do not have a constant 24 volts to C terminal.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-0925.pdf

Hello Thanks for your replies.
Unfortunately it is still not clear to me. Why do I need power to the C terminal if the tstat runs on Battery.

Any help is appreciated


I have the following tstat
Honeywell - 7-Day Programmable Heat/Cool Touch Screen Thermostat - YRTH8500D1040 - Home Depot Canada (http://www.homedepot.ca/webapp/wcs/stores/servlet/CatalogSearchResultView?D=944721&Ntt=944721&catalogId=10051&langId=-15&storeId=10051&Dx=mode+matchallpartial&Ntx=mode+matchall&recN=0&N=0&Ntk=P_PartNumber)

((Why do I need power to the C terminal if the tstat runs on Battery))

The batterys are only for backup not for full time running.

Do you understand now?

White wire goes from TH (Furnace) to R (tstat)
Black wire goes from TH-TR (Furnace) to W (tstat)

A 24V wire from (?? ) to C(tstat)

Where can I get the 24V from the furnace?
Thanks

Go to this page
https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313-2.html#post269190

Go down to fan center

There is a C terminal on the fan center transformer. That is where you get power.

To Safely get your answer I would ask Hoenywell. They have solved my problem so smooth it is ridiculous. Besides the model of thermostat that you are replacing is for hi end heating.
Ask Honeywell technical support I am sure they will give you the best soultion.
Regards

Very old post again