Installing Lennox 51M34 thermostat on a 80MGF furnace?


  #1  
Old 05-12-15, 02:55 PM
G
Member
Thread Starter
Join Date: May 2015
Posts: 4
Received 0 Votes on 0 Posts
Installing Lennox 51M34 thermostat on a 80MGF furnace?

My White Rogers thermostat does not always cut off when the set AC temperature is reached. The temperature drops until it reaches the setting but no further. If you tog up the setting the thermostat cuts off.

If the AC is off and you tog the setting down below the current temperature, the AC turns on.

When on, temperatures increase or decrease to the setting but may or may not turn off.

The temperature readings are high.

I would like to replace the White Rogers thermostat with a new Lennox 51M34 which I have. However There are more terminal settings on the Lennox.

I have 4 wires set on Red, Green, White and Yellow on both the W R thermostat and furnace control board. There is a jumper on the thermostat from RH to RC. There are two unused wires blue and tan. The furnace board has a fifth terminal C. There are two wires running from the AC compressor to the furnace board. A red wire connects to C and a white wire connects to Y (along with the Y from the thermostat).

The 51M34 has 6 terminals R-24VAC, Y1-first stage cooling, W1-first state heating, G-fan control, L-Service indicator, C-24VAC common. Can I use this thermostat? I assume I connect R, Y1, W1, and C as before. How do I connect to L and C? I can use the spare wire to connect C at both ends? What is a Service indicator and what do I connect it to?

So three questions? Do i need a new thermostat? Can I use the 51M34? If so , how do I connect it?

Thanks for your help.
 
  #2  
Old 05-12-15, 04:38 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,843
Received 1,477 Votes on 1,365 Posts
The 51M34 thermostat is suitable for non−heat pump, single-stage heat/single-stage cool applications with either gas or electric furnace..... so it will work fine for your application.

Red ---> R
Green ---> G
Yellow ---> Y1
White ---> W1
Blue ---> C at thermostat and at air handler end. It is ok to have two wires on C terminal.
L is not used.
 
  #3  
Old 05-14-15, 09:33 PM
G
Member
Thread Starter
Join Date: May 2015
Posts: 4
Received 0 Votes on 0 Posts
Follow up on Lennox 51M34

PJMax

Thanks for the help. I wired the unit as suggested and the AC works. However there are a couple of problems.

The 51M34 reads the temperature 2 to 4 degrees high. Is there a way to calibrate the thermostat temperature reading?

There still seems to be inconsistencies when I turn the thermostat on with settings and cut off. I punch the cool button and the temperature reading goes up. (I have not set up the program yet and have focused on just the hold mode.) I punch the setting up or down and turn the AC on. I see the fan rotating and the cool symbols. The thermostat eventually cuts off but does not turn back on. The temperature does not change after cut off. If I adjust the set temperature lower after the cut off, the unit cuts on.

If I leave the thermostat off, the temperature reading settles 2 degrees above the actual in the same area.

Any suggestions? Thanks.
 
  #4  
Old 05-14-15, 11:07 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,843
Received 1,477 Votes on 1,365 Posts
Most thermostats are not adjustable. How do you know yours is not correct ?

I'm not following you on your t'stat problem.
 
  #5  
Old 05-15-15, 12:09 PM
G
Member
Thread Starter
Join Date: May 2015
Posts: 4
Received 0 Votes on 0 Posts
PJMax

I am using a multimeter with a temperature probel I hold the meter up with the probe next to the thermostat.

I see three toggle switches that control temperature one to three plus or minus degrees. Is this a temperature digital reading calibration or how much the temperature is adjusted before the AC is cut on + or cut off -?

I haven't mounted the thermostat yet. It hangs by the wires against the wall. Could this explain why the temperature reading seems to change in the wrong direction? I have to touch the up or down arrow for the setting to appear.

Thanks again for your help.
 
  #6  
Old 05-15-15, 12:51 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,843
Received 1,477 Votes on 1,365 Posts
In case you don't have the manual. It starts on page 19 in the PDF link.
http://www.lennox.com/pdfs/manuals/L...ats_Manual.pdf

I can't find any mention of a way to recalibrate the thermostat for accuracy.

On edit.... it would be normal for the thermostat to read a degree or two higher if it was on an outside wall or the wall it's on is warmer than ambient air.
 

Last edited by PJmax; 05-15-15 at 01:34 PM.
  #7  
Old 05-16-15, 08:32 AM
G
Member
Thread Starter
Join Date: May 2015
Posts: 4
Received 0 Votes on 0 Posts
PJmax

I appreciate your sticking with me on this.

I have both the installation and Operation manual. The installation manual refers to three temperature control toggle switches. Are these offsetting switches controlling a difference between the setting temperature and the thermostat temperature reading for cut on and off or adjustments to the temperature reading on the thermostat? There doesn't appear to be any difference when I set them to negative 3 degrees on the thermometer reading when I plug the unit back in.

Another thing I have noticed is that does not seem to be an immediate impact on any setting changes on the thermostat. For example, the AC was on and the setting was below the current temperature. I brought the setting up to the current temperature reading and nothing happened. I brought the setting 1 degree above and nothing happened but the temperature reading went up one degree shortly after. I brought the setting up several degrees and the AC cut off.



I have figured out another part of my problem. I get substantially different temperature readings using my miltimeter/temperature probe depending on where I put the end of the probe wire. If I put the end at the top of the thermostat, the thermostat temperature is reading 2 degrees higher. if I let the probe wire hang down to waste level a foot out from the wall, the multimeter temperature reading is 4 degrees lower than the thermostat. When the AC is on, the difference can be 6 degrees but varies.

I am operating on the premise that the multimeter is reading correctly because the AC is still running when the room is cold and the multimeter reading in the room is a couple of degrees below the temperature shown on the thermostat which is at or above the set temperature.

If I understood how this thermostat works in terms of the toggle switches and cut on and off cycles and timing cycles based on the set and reading temperatures I might be able to figure this out. Given all this, the thermostat may be working correctly except the temperature reading is a couple of degrees higher than the room.

Any suggestions? Thanks again for your help.
 
  #8  
Old 05-16-15, 08:08 PM
Piddler's Avatar
Member
Join Date: Feb 2014
Location: US
Posts: 109
Received 0 Votes on 0 Posts
As near as I can figure, the setting you were looking at with the 1,2 or 3 degrees is the swing temperature. That's how far the temperature can go above or below the set temperature on the thermostat before the unit will turn on or off.

As far as measuring room temperature, when 2 devices disagree, I usually bring in a third to settle the argument. I'd try a different thermometer. I'd guess the thermostat is probably pretty close to being correct.

Good luck and let us know how it works out.
 
 

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