Can I manually run the blower on my Lennox G12Q3 furnace?
With a old school thermostat that you just adjust the temp with normally there are not any additional wires to run the fan. It can be done but will require more wires and an additional switch or new thermostat. A few houses back I had a gas forced air system like yours I added a fan over ride switch at the furnace , this turned on the blower and was helpful at cooling the house off in the summer.