Will not work unless air conditioner is on---i want to run it to circulate air not cool the house

There should be a fan switch on the thermostat that says ON or AUTOMATIC. Turn to fan for constant operation. If the switch is not there then you will need someone to install a new thermostat with the proper control.

If this is a mobile home then the issue is a totally different ball game.

There should be a fan switch on the thermostat that says ON or AUTOMATIC. Turn to fan for constant operation. If the switch is not there then you will need someone to install a new thermostat with the proper control.

If this is a mobile home then the issue is a totally different ball game.

The fan switch does not work when turned to "on" It only works when air conditioner is running

Then it is probably the fan relay that is causing the problem.

It sounds to me as though someone has done some "creative wiring" possibly because there weren't enough wires available. I would suggest having a experience tech confirm this if possible. The control voltage wiring for the fan may be wired to the coolling circuit only. The heating may be something entirely different, you may want to confirm the heat is available before winter. You may want to have a system maintenance done. This may answer some questions if you tell the tech prior to him(or her) starting.