Central heating usually doesn't get noticed until you start shivering. Effective troubleshooting of central heating systems should begin by getting to know your system and inspecting it for those things most likely to cause problems. Among the aspects of a central heating system you should know are the ducts, dampers, filters and supply registers.
Check the air filter if you notice the heat isn't as warm as usual. The filter catches particles in the air to keep them from getting into the system. The problem is that after the particles are caught by the filter, air does not flow as easily, and the central heater has to work harder to accomplish the same thing. Remove the filter, and if it's dirty, change it. This may result in an immediate and noticeable change in the air temperature.
Inspect the blowers on a regular basis. Belt-drive blowers won't work well unless they have precise tension. Check the belt to see if it is frayed, cracked or worn, as this can affect the tension and cause the blower to work less efficiently. Some blow motors have oiling ports at the end of the shaft. Check to see if the blower needs more oil.
Check the thermostat, fuse or circuit breakers if the system is not producing any heat. The cause may just be that the thermostat is set too low or that the switch or main breaker is open. Double check the thermostat to make sure it is set high enough and flip the circuits to determine whether the system is getting power.
Listen closely to see if the central heater is cycling on and off too often. The most likely cause for repeated cycling of the central heater is that the filter is clogged or the blower is causing the unit to overheat. If replacing the filter doesn't solve the problem, try adjusting the blower's belt so that it is looser, but make sure it doesn't slip.
Check the thermostat if air is coming out, but it's not warm enough. If thermostat adjustment doesn't work and the filter is not dirty, then it is time to check your ducts, as they may be clogged. A vacuum with an adjustment that allows you to get into the duct can usually solve this problem. Turn off the blower before attempting this.
A final cause may be a blown fuse. If you replace the fuse and turning on the unit cause the breaker to trip again, you'll probably have to call in a trained repair person.