Gravity Furnace wiring question, looking to replace thermostat


  #1  
Old 03-13-18, 08:54 PM
J
Member
Thread Starter
Join Date: Mar 2018
Location: USA
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Question Gravity Furnace wiring question, looking to replace thermostat

My existing thermostat is a two wire (R and W) cable to the gravity furnace. I've traced the wiring and have come with the attached diagram.

The transformer is an old Honeywell AT72A1K2. Secondary output is 25v/40va.
There's also an old Honeywell PilotStat, part #C418A1 and the Gas Valve is a White Rodgers 25G01.

I'm yet to start anything, just wanted to note the way it is wired now because it seems odd to me that the W wire is going straight to the transformer while the R wire goes to the transformer side of the gas valve, out on the thermostat side of the gas valve, in/out of the PilotStat and then on to the Transformer. But I'm not an HVAC guy so maybe that's the correct way.

In any case, in order to install an ecobee I need a C wire and I'm trying to determine the best way to do it. I intend to run a new wire from the ecobee to the furnace, just not sure what would be the proper way to connect it. Any help is appreciated.
 
Attached Images  
  #2  
Old 03-13-18, 09:22 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 65,616
Received 4,083 Upvotes on 3,666 Posts
Welcome to the forums.

With a standard two wire thermostat.... it just completes the circuit so there is no attention paid to the terminal designations. Now that you are installing a thermostat that requires a C connection the wiring becomes important. The diagram below illustrates what needs to be done.

Name:  FurnaceWiring.jpg
Views: 4128
Size:  17.0 KB

You could swap the pilot stat for the gas valve. You could reverse the wiring on the gas valve or the pilot stat. What's important is those two devices are powered by the W and C connections.
 
  #3  
Old 03-13-18, 09:44 PM
J
Member
Thread Starter
Join Date: Mar 2018
Location: USA
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks Pete,

One more question if I may. There's nothing on the transformer to tell me with terminal is the common vs the hot. I assume with the rewiring work that becomes important. How can I determine which is which, assuming I get a multimeter?
 
  #4  
Old 03-13-18, 10:16 PM
ray2047's Avatar
Member
Join Date: Mar 2006
Location: USA
Posts: 29,711
Upvotes: 0
Received 15 Upvotes on 13 Posts
Both are actually "hot" but it is optional which hot you use as common.
 
  #5  
Old 03-13-18, 10:24 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 65,616
Received 4,083 Upvotes on 3,666 Posts
Since this is AC..... there is no polarity.
The red goes on one terminal and the common(s) on the other one.
 
  #6  
Old 03-14-18, 04:17 PM
J
Member
Thread Starter
Join Date: Mar 2018
Location: USA
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thank you guys. Everything worked on the first try.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: