thermostat error when using remote sensor

I have a White-Rodgers Comfort-Set III (1F97-371) thermostat and a White-Rodgers Indoor Remote Sensor (F145-1328). I have installed the two pieces of hardware as specified in the instruction manuals.
When I cut the specified jumper on the back of the thermostat (jumper W922) and set the configuration to use the remote sensor, I get an error "Check Stat" on the display and the temperature display shows 07 degrees F.

I have tried a new remote sensor, a new thermostat, and a short (3 foot) wire to rule out a problem in my wiring.

Does anyone have any additional troubleshooting tips or any ideas what this problem could be?

Thanks.
Scott

 

To answer your question, the only thought that I have is that 24VAC power is not continuous to your thermostat.

It is possible that when the furnace flames are not operating, that the 24VAC power to your thermostat quits. This would be normal if there is no wire connecting between the C terminal of the furnace and a C terminal on the thermostat.

I looked at a copy that I have for the White Rodgers terminal diagram setup for your 1F97-371 thermostat. The diagram that I have does not show a C terminal on your thermostat.

If this is true, (no C terminal), then I would think that you do not have a wire that runs between the C terminal of your furnace, (if there is one), and the thermostat. If so, then probably the sensor is not getting power all the time.

For what it is worth, at your furnace, put an AC voltmeter between the C terminal, (may not be one), and the R when the flames are on. You should get a voltage reading between 20 and 30 volts. Also do the same when the flames stop. In both cases, there should be the same voltage.

The reality is that The thermostat is a switching device that only needs to connect between the R and the W terminals for the flames to operate. This is why older thermostats needed only two wires, the R and the W, hence Red and White coloured wires.

Your problem however is that the remote sensor needs power all the time to remain calbrated. I wondered what WR had in mind when they put the remote sensor terminals on, but did not appear to allow for power all the time.
I think that you will find that when the flames are off, there is still 24VAC on the R at the furnace and hence to the thermostat, but due to the design of the thermostat circuit board, if you have no C wire/terminal to/at the thermostat, then the remote sensor does not get power when the flames are off. (I have seen sidenotes about running power to the thermostat, but have not understood what or how that is done - also I believe that the sensor takes DC power so powering the sensor direct from the AC is the wrong answer.)

Continous power matters as the sensor needs power all the time due to some sort of internal need to stay powered all the time in order to provide a correct reading when the flames do come on again.

As to the solution, that is not easy. First you need the C terminal on your furnace, then a wire for the C itself and then a different thermostat, one with the C terminal on it.

As best I can determine, the C terminal/wire is what in DC we would call the return or Common, and in AC, the neutral. The R is sort of the hot wire, but without a return, it means nothing.

Let me know if there is a C terminal present on your thermostat.

I would also be interested in other thoughts on the matter. The reason is that I have the 1F95-1277 thermostat and am also having problems - different ones, but still problems getting corrrect readings from my exterior temp sensor - and I do have the C connected. For what it is worth, I get a reading of about 38VDC between the + and - terminals of the little remote box that connects to the end of the sensor wire. I do not know if that makes sense or not as the furnace input voltage to the thermostat is 28VAC, as it should be.

 

After a couple calls to WR tech support, I finally talked to someone who was very helpful. He told me that the installation manual for the remote temperature sensor is incorrect. They have a new installation manual that shows how to solve this problem. He e-mailed a pdf copy of it to me.
To make a long story short, you have to put a relay switch "between" the thermostat and the furnace. The switch allows approximately 5 volts to go across it without switching on the furnace. This allows the "power stealing" functionality that is built into the thermostat to work and it can power the remote sensor. In my opinion, this sounds like a big mistake in their thermostat design that was only caught after they sold one to someone who actually tried to use their remote sensor. Why they didn't add a C line is beyond me.
The relay that I ended up using is a White Rodgers 90-290Q.

The thermostat and remote sensor are working great now that I have the relay.

I will think about your exterior sensor a little bit and see if I can come up with an idea for an explanation.

Thanks for the help.