20 year old Rheem 90 plus 120k btu furnace blower runs contantly. Furnace and AC work fine (start and stop according to thermostat setting). What is problem? How fix?

You will need to jump out the tstat to determine that the tstat is sending the signal to the furnace. Sounds like a thermostat problem.

Go to your thermostat and access the fan mode. Change the fan mode to Automatic, AU. This will stop the AC fan from blowing when the furnace is turned on. If the fan mode is on Hi or Lo and the furnace is turned on it will turn on the AC blower. Bad design...

Gas valve does not open or gas supply is shut off.

bad gas valve or control board

I had this same problem with my 6300B thermostat. I did not see this when I installed the tstat and did setup for it the first time. If you go to page 16 of the install guide, there are instructions for setting up your tstat for your furnace and AC. The setting I had wrong was on page 20 of the guide. It defaults to a setting for a furnace/ac that is less than 90% efficient. When I changed the setting to a furnace that is more than 90% efficient, the fan not turning on or off problem went away. The setting on page 20 should be set to "3" if you have a more than 90% efficient furnace/ac. Hope this helps your problem

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.

NO BECAUSE THE BOARD IS TELLING THE FANS TO STARTARE THE CONTACTS FUSED TOGETHER OR STUCK IN THE CLOSED POSISTION CHECK THIS FIRST

You may have a dirty flame sensor. There are a number of other possible causes as well. Rheem/Ruud/Weatherking furnaces also have problems with the connection on the blower deck.
The flame sensor sometimes must be bypassed around the MOLEX CONNECTOR in the deck due to humidity.
Hope this helps.
Matt
[email protected]

If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve.
Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.

Make shure the 24 volts is hooked up to power that is only powered up when the blower is running. Some furnaces have a HUM terrminal on control (make shure the HUM is 24 volts some are 120 volts then you must use the transformer supplied with the AP 600). You can use a RP # 50 relay that senses current only when the furnace motor is running on the heating speed. Hope this helps