Address the problem. The fan is not the problem.

First off, is it the engine, the cabin or both that is not warming up?

I'd agree with others that it is most likely the thermostat if the engine is not warming up. If just the cabin is cold, then suspect a plugged heater core or a malfunctioning heater control valve. You could even check the functioning of the damper doors - but that's pretty unlikely.

For the engine, be sure to purchase the proper thermostat. There is a difference. Best to purchase from a place that is more familiar with older Volvos than your corner auto parts store.

Keep in mind that these cars were designed with tough Swedish winters in mind. They will function perfectly well and produce a hell of an amount of heat with the factory equipment as long as it is working properly.