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.
Solution 1: Electric Toe-Kick Heaters
These small heaters with blowers fit into the hollow space under kitchen cabinets, stair treads and vanities. This can be a good solution for a bathroom or kitchen where chilly feet are the main complaint. You can install a toe-kick heater (also called a 'kick space heater') under an existing cabinet by prying off the toe-kick. To power the heater, you'll need to run a dedicated circuit from your main electrical panel. You can control most units using a switch or a thermostat. It's worth getting a model with a temperature control (or a high/low switch). Powerful units can blow so hot on the high setting that they could overheat your feet or even soften vinyl flooring. Also, some models are noisy when the blower operates on high. Hydronic toe-kick heaters that connect to hot water heating systems are also available.
Solution 2: Duct-Booster Fans
If you have forced-air heat, you can take advantage of several types of duct booster fans that are designed to increase the flow of warm (or cool) air through your ducts into a problem room. In-line duct booster fans fit inside standard-size metal ducts. You mount the blower near the outlet end of a duct and then install a pressure switch (some models have one built in), which senses air pressure from the furnace and turns on the booster fan whenever the furnace or A/C blower turns on. Some in-line duct boosters simply plug into an available outlet, while other models are hard-wired. Cheaper units can be noisy, so it's worth buying a quality model with a powerful motor and heavier gauge housing. In-line duct booster fans retail for $30 to $150.
A 'register' booster fan is much easier to install. Depending on the model, it either sits on top of or replaces a floor or wall register grille, and plugs into an outlet. A built-in thermostat switches on when the furnace operates. Register duct boosters cost $40 to $70. Many different manufacturers make duct booster fans of both types. Search online for 'in-line duct booster fan' or 'register duct booster fan' to find manufacturers and dealers.