I have an intertherm gas furnace and would like to make the fan run constantly. I currently have an old thermostat that only allows for the control of the temperature and there are no other adjustments on it. Any suggestions besides buying a new thermostat.
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Do not run contentiously during heat mode. the fan runs at a slower speed with heat and heat fires first then the fan cycles on during heat cycle then when heat satisfies the thermostat the heat will cycle off and fan will run for a couple more minutes and then cycle off. So set stat on fan cycle to auto.
The typical 'summer/winter' switch is an added switch associated with OLD heating systems back in the 50's and 60's before central air conditioning became common in homes. I doubt that your new furnace had one installed because you can accomplish the same thing by switching the fan on at the thermostat sub-base. That is, of course, providing you have a subbase with 'fan-on-auto' on it.
I checked the model #'s and they should interchange just fine. Double check your wiring. On the thermostat Red to Rc (which should also be jumped to Rh), White to W, Green to G, and Yellow to Y. Move T-stat controls from current position to oposite end and back to what you want to run ie. A/C if thats what you want. Move fan control to on and back to auto. (I find this eliminates some close but not quite right settings). This should work. If not look in furnace to verify how it is wired. Any colors that do not match the above will be your corrective action to do at the thermostat. if necessary change t-stat wires at t-stat to match what is in the furnace.
Older furnaces used to have a fan switch that read winter or summer, or auto and on. You should also have a fused switch to the furnace, wiring runs black to the switch thru the fuse and to the furnace black wire. White wire is straight to furnace from electrical panel. This is for power feed to furnace only. Now the thermostat is red to the transformer white to white. This was a very simple old furnace they had very little safety devises. The fan switch was an older high end model so you could blow the cool air from the basement thru out the whole house in the summer, and the fan ran constantly.
Some of the old heat pumps made the reversing valve in the heating mode. On your new thermostat the orange wire will need to be on the B terminal. If it was color coded? I hope you made notes of how it was.
You will likely need to install an isolation relay in the W circuit from the thermostat. Some thermostats rob a small current through the W circuit to power the thermostat. This small current can be enough to start the fan circuit timer. You can test this by disconnecting W at the furnace and allowing the timer to shut down. If the fan shuts off with the W disconnected you can install a simple relay.
The relay coil will be connected to Common on one side and the W from the thermostat to the other. Then you will run a wire from R on the furnace to the relay and from the N.O. Contacts to the W terminal on the furnace. This will kill that signal that is starting the timer and the W can steal it's power through the coil back to common.
If the thermostat is set on Fan and not Auto the fan will continue to run. If the fan control is bad or is set at too low of a setting, it will blow cold air after the furnace has shut off. From what I guess of your explanation, your furnace is not producing any heat and the room air blower runs blowing cold air. You may have two issues happening. Make sure your thermostat is calling for heat by setting it to a high setting. Inspect the thermostat to see if it is set at Auto. Go to the heating system and inspect it to see if it is heating. If you can't view any flames carefully touch the exhaust pipe and see if it is hot. If not, you have a no-heat condition. A 23 year old furnace may need some TLC so call a technician you trust. He/she may need to replace the ignitor or thermocouple to get it going. They will also check out why your fan is running when it should not.