Heater won't turn on with new thermostat


  #1  
Old 09-16-14, 08:51 AM
M
Member
Thread Starter
Join Date: Sep 2014
Posts: 6
Received 0 Votes on 0 Posts
Heater won't turn on with new thermostat

I have been attempting to use several different types of digital thermostats to replace my old mercury thermostat, however none of them are able to control the furnace properly. All of them are able to control the air conditioning unit properly.

Only the mercury thermostat has been able to control the furnace properly. All of the digital thermostats will turn on the thermostat for about 5 minutes and then it will turn on, but the display for each thermostat will indicate the heater is still on.

When I do engage the heater function, it sounds like a flame is coming on, then for about 2 minutes nothing happens, but then after that the fan starts blowing. With the mercury thermostat the air eventually gets warm. With the digital thermostats, the system turns off after about 5 minutes and the air never gets warm.

I have 6 wires coming from my wall:
Blue, White, Red, Green, Brown, and Black.

From the wiring of my old mercury thermostat the wires were as follows:
Red-power
White-connected to white in the old system, so i connected it to white here
Blue-connected to yellow in the old system, so I connected it to yellow here.
Green -Fan
Brown-Unknown
Black-Unknown

I measured the voltage between the red and black wire, because I thought the black wire was a common, but when I connect all of the wires to the digital thermostat, the voltage between the wires changes. Between black and red disconnected, I get 26 V, once connected the voltage will drop to 0 volts, so this leads me to suspect the black wire isn't ground.

I have included a picture of my old thermostat wiring. I checked the compresser and there is a red and wire connected to it. I haven't checked the furnace because it is in the attic which is hard to access and I'm concerned about asbestos (the house was built in 1920). It was updated sometime in the past to have central heating and air.
Name:  2014-08-13 21.25.21.jpg
Views: 1183
Size:  22.6 KB
 

Last edited by morpheus8100; 09-16-14 at 10:14 AM.
  #2  
Old 09-16-14, 10:12 AM
firedawgsatx's Avatar
Member
Join Date: Jul 2013
Location: USA
Posts: 1,726
Received 2 Votes on 2 Posts
A couple of those wire colors are not traditional but they will work. Cooling is usually yellow and heat is usually white, It would really be helpful to know what your connections are at the furnace. The two wires going outside to the a/c should be the wire from the thermostat Y terminal and the common wire from your transformer.

One thought: when you install the digital thermostats are they the programmable type? Thermostats of that type need to be set-up to control the type of system you have and are often times instructions are not included in the owner's manual but in a separate installer's manual. The reason they need to be set-up is because they are capable of controlling many different systems such as gas furnaces, heat pumps, conventional, dual transformer systems and etc.
 
  #3  
Old 09-16-14, 10:17 AM
M
Member
Thread Starter
Join Date: Sep 2014
Posts: 6
Received 0 Votes on 0 Posts
Sorry, i edited my original post. The blue wire controls the air conditioner. I thought it was the white wire because that is what is hooked up to the compressor outside, but when I short the blue with the red, the compressor comes on, so
Blue-compressor
white-furnace.

I can't really get into the attic.

I do have a multimeter, so I could measure any voltages between wires to determine stuff.

All of the digital thermostats I have tried are programmable, yes. I set them to forced air with gas.
 
  #4  
Old 09-16-14, 11:34 AM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
You MUST ascertain what colors are on what terminals at the airhandler/furnace in the attic. Best to go with the standard color code to preclude these kind of errors in the future.
 
  #5  
Old 09-16-14, 11:41 AM
M
Member
Thread Starter
Join Date: Sep 2014
Posts: 6
Received 0 Votes on 0 Posts
I switched the mode on the nest from GAS to ELECTRIC and the system is staying on now. I'm giving it about 20 minutes to see if the heat stays on.

Any idea what that would do?

I just basically need the power wire connected to the furnace wire when i want heat. That is how my old mercury thermostat did it.
 
  #6  
Old 09-16-14, 11:57 AM
firedawgsatx's Avatar
Member
Join Date: Jul 2013
Location: USA
Posts: 1,726
Received 2 Votes on 2 Posts
It is difficult to tell from the photo of your old thermostat, but do you have separate red wires, one going to RC and one going to RH or does it have just one red wire going to RH first and a short jumper to RC?
 
  #7  
Old 09-16-14, 12:03 PM
M
Member
Thread Starter
Join Date: Sep 2014
Posts: 6
Received 0 Votes on 0 Posts
Just one red wire. With a jumper between RC and RH.
 
  #8  
Old 09-16-14, 01:18 PM
firedawgsatx's Avatar
Member
Join Date: Jul 2013
Location: USA
Posts: 1,726
Received 2 Votes on 2 Posts
When you wire it to the Nest thermostat how are you connecting the red wire? Verify you don't have a jumper between RC and RH on the Nest thermostat as it is not supposed to have a jumper.
 
  #9  
Old 09-16-14, 01:25 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 62,166
Received 1,525 Votes on 1,408 Posts
Gas= furnace turns blower on when heated.
Electric = thermostat turns on blower and heater (furnace in your case) at the same time
 
  #10  
Old 09-16-14, 03:38 PM
M
Member
Thread Starter
Join Date: Sep 2014
Posts: 6
Received 0 Votes on 0 Posts
When I wire it to the nest, I have red connected to RH (I don't have a jumper because I know it doesn't this internally). I have the white connected to W1, blue connected to Y1, Green connected to G.

I also tried connected the Red to RC with the rest of the wires left the same and it didn't work. I haven't tried connecting Rc and Rh using my own jumper with the nest.
 
  #11  
Old 09-16-14, 03:56 PM
firedawgsatx's Avatar
Member
Join Date: Jul 2013
Location: USA
Posts: 1,726
Received 2 Votes on 2 Posts
You aren't supposed to use a jumper between RC and RH on a Nest thermostat. Many of the Nest thermostats experience problems when a common wire is not connected to the C terminal. Here is a link to some information that might assist you: http://support-assets.nest.com/image...ller-Guide.pdf

Page 29 and 30 might provide some insight into the problem you are experiencing because voltage drops when the Nest attempts to recharge the battery.

The C wire allows the Nest to charge its battery without stealing power from the W wire. The power stealing that the Nest does when there is no C wire can actually cause enough of a voltage drop to cause your furnace to act erratically.
 

Last edited by firedawgsatx; 09-16-14 at 04:17 PM.
  #12  
Old 09-18-14, 11:07 AM
M
Member
Thread Starter
Join Date: Sep 2014
Posts: 6
Received 0 Votes on 0 Posts
So I tried hooking up an external 24 V transformer that supplies up to 300ma to the nest and I still couldn't get it to work.

I tried the following configurations:

Transformer to Rc and C
Transformer to Rc and C, Red coming from wall to Rh
Transformer to C and Red coming from wall to Rh
Transformer to Rh and C

The only configuration I didn't try was manually shorting Rh and one of the transformer wires and connecting that to the nest, then also connecting the C wire. I was afraid I would mess something up because I measured the voltage on the transformer wires, and it was around 27V, but the transformer on the furnace must not be referenced to ground. I got 5 V or 8 V depending on which wire from the transformer I referenced with the red wire from the wall.
 
 

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