Fan clutch failure does not change heater output, but if clutch is old, then thermostat may be old too. If thermostat fails in open position, engine does not run hot enough. To diagnose, idle car for 10 minutes to warm up. Test temperature of radiator hose at top of engine. You shouldn't be able to hold your hand on it for more than 5 seconds. If it feels hot, but not "can't keep my hand on it hot," then thermostat needs replacement. To confirm that problem is "Running open thermostat," put pieces of cardboard infront of radiator and see if temperature gauge rises, and radiator hose gets hotter. The purpose of cardboard is to stop airflow over radiator, thus forcing temps to rise. You don't want to overheat engine, so maybe block off half the radiator and see what happens.
Poor heating can also occur if there is not enough coolant in the system, or trapped air. Buy service manual for your car and learn where the bleeder valves are. Bleeder only needs to be opened a small amount when cool engine is started. Do not attempt on hot engine.
On many cars there is a valve that opens and closes to regulate flow of hot water into heater core. Look on firewall. On some large american engines, small bits of sealant can break off inside the cooling system and actually block the valve. Probably have to remove the valve to find this. On a very old car that has not had proper service, the heater core can become blocked. Search YouTube for videos on how to unclog heater core. Messy DIY job, but very expensive for shop to replace a core.
Good luck,
Booger Vance
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