Trapped air
The most common cause. Air collects at the top and blocks hot water or steam from filling the radiator. On hot-water systems, a bleed key opens the valve to let the air out until water appears.
A stuck or failed valve
On steam systems, a clogged vent or a stuck inlet valve keeps the radiator from heating. On hot water, a stuck thermostatic valve can do the same. Valves wear out and are usually inexpensive to replace.
Sludge and scale
Years of sediment can settle in the lowest part of a radiator and in the pipes, leaving the bottom cold while the top is warm. A flush or professional cleaning restores flow.
Low water or a circulation problem
If several radiators are cold, check the boiler pressure gauge. Low pressure, a failed circulator pump or a zone valve that is not opening can stop heat from reaching a whole zone.
Safe things to try first
Make sure the thermostat is calling for heat and the boiler is on, open the radiator valve fully, and bleed hot-water radiators with the system off and cool. Do not force stuck valves. If a radiator leaks or the boiler shows an error, call for service.
Need a hand?
If it is more than a simple fix, we are a call away in Clifton and nearby towns.