thermostat will not turn on furnace, bypas will

[batt]

I have a suburban gas furnace which works great( after thoroughly cleaning it), however the thermostat will not turn it on. I purchased a new thermostat thinking it was the problem, but still it does not turn it on. If I use a jumper and connect the two wires it works, just not the through the thermostat. Am I missing something basic here?

[xo1rider]

Just a shot in the dark but have you done a continuity check on the wires from the thermostat to the furnace?

[Friday]

... and a quick check to see if the thermostat is making contact as well ... are you touching the wires together at the thermostat? If you are and you're getting furnace action, you've found your problem... if not, you've got a break somewhere in between...

[Minno]

You could have a bad thermostat out of the box. It happens.

Chris

[gettysburg]

We had a similar problem. We had a bare spot on our furnace wire rubbing on the corner of furnace metal. So, check your wire near your furnace

[batt]

I am touching the wires at the thermostat. If I contact the wires at any location, ie at the wall where the thermostat is located or at the furnace it will fire. It will not fire through the thermostat. I replaced the old one because of this thinking it was the thermostat , purchased a new one from vintage trailer and have the same issue. Any further suggestions?

[batt]

Originally Posted by friday

... And a quick check to see if the thermostat is making contact as well ... Are you touching the wires together at the thermostat? If you are and you're getting furnace action, you've found your problem... If not, you've got a break somewhere in between...

i get the furnace to switch when i contact the wire using a jumper at the thermostat or at the furnace . It will not work using the thermostat as a switch. I purchase this at vintage trailer listed as 24v to work on a suburban furnace. Odd thing is the same issue with the original when i reinstalled it. Are you saying its the thermostat, since i can make it work by the jumper? Just need to verify.

[TG Twinkie]

How many screws are on the back of the thermostat? Many have more than 2. If you know how to use a VOM, you should identify which 2 screws give you continuity when you turn the Tstat up.
Is it a mechanical thermostat with a mercury bulb? If so, follow the wires attached to the bulb to locate the correct terminals to use
If it is the bi-metal strip type there will be a set of contacts, where each side of the contact is connected to a screw. Connect the furnace wires to these 2 terminals.

[batt]

Thanks for the reply. This thermostat only has tow screws, to connect to and is not a mercury type, so no screw up there. The old original was a coil type as well so I just figured since I could get the furnace to operate by connecting the two wires together then it had to be the thermostat, only even with a new one I have the same issue. How do you check out the thermostat?

[azflycaster]

Attach an Ohm Meter where the wires connect to the thermostat and move the setting back and forth. When the heater should be on you should see zero Ohms which is a short. When the furnace should be off you should see infinite Ohms which is open.

Make sure that the contacts in the thermostat are closing (touching) when the furnace should be on.

[Antique Pedaler]

The thermostat is nothing more than a simple on/off switch. Touch the two wires to it together. If the furnace lights, the problem is the thermostat. If it doesn't, the problem is along the wire line.

[richinny]

just for kicks reverse the wire positions on the thermostat. it is unlikely that it is the problem on a mechanical thermostat but it is an easy check to do :-)

[batt]

Hey everyone just to let you know I finally figured it out. I used a multi-tester and determined that the stat worked to the central thermo coil , but did not carry to the screw wire connection. Looking closer at the thermostat I noticed that it had a dial with settings on it and a metal pointer, that pointer had to be on one of the settings to make contact and follow to the screw and make the switch complete.

I now need to find the setting its supposed to be on for a Suburban furnace. Have to try and find that out now.

Thanks for your help!

[TG Twinkie]

That is the differential setting. In other words you can adjust it to turn on and off at what temp you choose the differential. Let's say you have it set at 70 degrees, by adjusting the differential you can get it to turn off at 72 and on at 68. Which would be a 4 degree differential. Or +or- 2 degrees.
This is just an example. You don't want the furnace turning on and off in short cycles.
Does that make sense to you?