I have manually operated mercury bulb type thermostat, make YORK(honeywell), model T834C in my home. It is two stege heat and one stage cool thermostat(multi stage). It has 6 wires , Red two, Yellow two, white two. The terminals where the wires are connectd are not marked. 3 wires(R,Y,W) are soldered to mercury bulb, one white is soldered to a dial, one Red wire and one white wire are screwed on terminals not marked.How to assign terminal codes to these 6 wires for connecting to new thermostat?

I want to replace it with a Honeywell programmable Thermostat, 5-2 or 5-1-1 or 7 day programmes, which is easy to install. What is the model suitable for replacement of T874C thermostat?
If any one has done the replacement of T874C with a programmable one, please advise.
Honeywell customer support is requested to give me the answer.

SSNORI
7037939927

Please clarify how many wires are coming out of the wall and where they were connected to the old stat. Thanks

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Originally Posted by acetc

Please clarify how many wires are coming out of the wall and where they were connected to the old stat. Thanks

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6 wires are coming out of the wall.
3 Red,yellow, White)wires are connected to the mercury bulb. One white wire is connected to a dial like item. 1 Red wire and one yellow wire are connected to two screws on the board and there are no markings near the terminals. Tell me how to mark them to connect it to new thermostat.
Thanks for your reply to my question.
ssnori

It appears that whoever wired the previous thermostat used 2 thermostat cables, rather than using one with 6 different colors. If you have no idea where the wires went before, there is no telling where they go to or what they control. In my opinion a new cable needs to be installed, replacing what is there now. That will give you the opportunity to put the correct colored wire in the proper place on both the rurnace / air conditioner and the thermostat. Sorry if that's bad news, but as it stands now you would just be guessing as to where they go and that can cause serious damage to controls or worse yet to you and your home. Unless you can install a new wire, properly connect it at both ends, you may want to hire a professional to take care of this for you.

Go to the sticky at the top of the forum and read the section on thermostat wires. Go to the furnace and identify how the terminals there are marked. If you had the manual for both the old thermostat and furnace, it would be easier. Likely a new thermostat for 2 stage heat and 1 stage cooling will come with RH, RC, W1, W2, Y, and G. Connect the wires soldered to the old switch with the red to RH and RC, white to W1, an yellow to Y. You then need to identify which wires activated the second stage heat and cooling blower. Connect them to W2 and G.

To acetc,

Answer to the question of "How many wires are actually coming out of the wall?'

Four wires only are coming out of the wall. These are conncted to four terminals marked as follows;
R-Red wire
W-White wire
Y-yellow wire
G-Green wire
Terminal markings and color code of wires match.

I have purchased Honeywell digital programmable thermostat model RTH 6300 D which has six terminals marked as follows.
Rc, R, Y, Not used, W O/B, G
How can I connect the four wires of old thermostat to the new one?
Old one is multistage heating and cooling type. New one is single stage heating and cooling. Do you recommend I get model RTH 7400 D which is multistage type?
Wait for your reply. Thanks for your help.
Sameer