If your radiators don't seem to be keeping your house toasty in the winter, there may be several causes but fret not. Because hot water radiator systems are very simple systems, the problems are likely as simple to warm things up. There are three things to get radiators up to heat: bleed the air from the system, increase the boiler heat and add reflectors behind the radiators. If your system utilises a circulating pump, the problem may lie there. The worst-case scenario could be the boiler itself.
Feel the top and bottom of the radiators. If the bottom of the radiators feels significantly warmer than the top, there is a good chance air is in the line and the radiators need to be bled of the air. Air holds the warmth produced by the boiler as well as the water. Most radiators have a bleeder valve that protrudes from the side near the top of the radiator.
Turn the thermostat off. Starting with the top floor and the radiator furthermost from the boiler, use the radiator key and turn the valve 1/4- to 1/2-turn counterclockwise. Hold a rag or towel under the valve until it begins to drip water, then close the valve.
Repeat Step 2 by proceeding to the radiator on the same floor second-furthermost from the boiler. Continue repeating this process until you've bled all the radiators from the air in the system from the top floor, then move down one floor and continue bleeding the radiators until they have all been cleared of air.
Place a bucket under the relief valve on the boiler and refill the boiler tank by turning the handle on the water line leading to the boiler. If only a little air was in the line, it shouldn't take even a minute to refill the void in the system left by the air. Most system's have an overflow tank to make certain the system remains full of water and any overflow should empty into it. If there is no overflow tank, it will empty into the bucket.
Turn the thermostat back on. If the system utilises a circulating pump, turn the thermostat to high and the pump should start and circulate the water. If there is no pump, the boiler should turn on to heat the water. If either of these things happen, set the thermostat to its normal setting and give the radiators time to warm up. If neither the pump nor boiler kick on, you've got a more serious problem and a plumber should be called to diagnose the problem. It could be the thermostat or the pump, if your system uses one, that needs to be replaced. If you notice water on the floor near the boiler, it may need to be replaced.
Place reflector screens behind the radiators to prevent radiant heat from escaping into the walls. The screens will help reflect the heat into the rooms. This should be done regardless of problems with the pump or boiler.
If the radiators still don't heat to your satisfaction, turn the heat on the boiler higher. Do this incrementally over the course of a day or two to give the system sufficient time to heat the home.
Your radiators should be bled at least once a year to make certain there is no air in the system, which diminishes efficiency.