Furnace fan going on/off every few seconds after changing to programmable tstat

Hi

I have a really old thermostat with a s8600 control board. Bought an Aube relay based programmable thermostat recently. With the old manual thermostat it's all fine. Now when the heat is supposed to be off, the furnace fan turns on and off very few seconds. Heat on (temp set is higher than ambient) does properly have the furnace firing up.

The thermostat is connected just through a 2 wire setup. After removing the thermostat, if the 2 wires are completely disconnected (just hanging there), the furnace is completely off. Furnace fan does not cycle. If the 2 wires are crossed, the fan turns on and the furnace eventually is fully on. I tried just connecting a multimeter, and even that gets the furnace fan to go on/off every few seconds.

Is this a case of where both my multimeter has low impedance, and also the thermostat having low impedance in the heat off condition? If so, I'm not too sure how to get this remedied either. Please advice. Thanks!

 

Welcome to the forums.

Pretty old system there. Looks like the thermostat interrupts the 24v from the transformer to the ignition module. If that is a relay based thermostat it should work if you connect to the W and R/Rh terminals.

That should be a battery operated thermostat. If it's not it will try to steal power from the furnace to operate.

Can you post the thermostat model number.

 

Hi,

Thanks for the response.

It is a Aube by Honeywell TH141HC-28-B/U. And yes, it is battery operated.

The weird thing is even a multimeter in Vac mode trips it into this on/off toggling behaviour.

Thanks!

 

I took a Digital Multimeter to the Thermostat. ~27Vac (confirmed 60Hz) at about .57A coming in. I also checked resistance of the Thermostat. When not heating, it's essential infinite ohms, when heating it's in the single digit ohms.

Now I wire the thermostat and the digital multimeter so I can get a current reading in the heat off state. It seems there's about 0.3 mA AC flowing through constantly. But that would imply 90k ohm in the heat off state for the thermostat! yet when measured with no voltage applied, the resistance is supposingly over the M ohm range...

Very very confused. Help much needed. Thanks!

 

You can't measure the resistance of the circuit since the circuit isn't powered.

Either that thermostat does not use a relay for switching or it has some type of heat anticipation that presents a load to the line.

Here's something you could try. Connect the furnace wires to Rh and G. Set the thermostat to electric heat. When the thermostat calls for heat the Rh to G connection gets closed in the electric heat mode. You may need a jumper in the Rc-Rh postion.