Is the Furnace Filter Filthy?
This is the most common cause of heating (and cooling) troubles. Change the filter and the problem usually disappears.
Are the Radiators Clear?
Whether you have electric or hydronic baseboard units or old-fashioned radiators, they won't throw maximum heat unless air can flow through them. If you move the bed against a baseboard unit or toss a blanket across a radiator, the room might get chilly.
Is the Radiator Air-Locked?
If you have a hot-water radiator that's not heating, the cause is usually trapped air. Getting rid of it is simple. Use a radiator key, 1/4-in. 12-point socket or a flat screw- driver (depending on your valve type) and slowly turn the valve counterclockwise until water drips out. This will release trapped air and let hot water flow. While you're at it, repeat the process on your other radiators. Bleeding the radiators will lower the pressure in your system, so you might have to slowly add water to increase the pressure. Do this by opening, then closing, the valve on the water pipe above the boiler.
If you're unfamiliar with your system, call a pro. How much pressure you need depends on how high the water has to rise. The basic rule is 1 lb. of pressure for every 2 ft. of rise. Your gauge may read in pounds, feet, or both. A basic two-story house, with the boiler and expansion tank in the basement, needs 12 to 15 lbs., or 25 to 30 ft., of pressure.