What should I do if one radiator is not heating?

What this symptom usually points to

When only one radiator is not heating, the boiler itself is often still producing heat. The problem is usually at that radiator or in the short section of piping that feeds it: air trapped inside, a valve not opening, poor flow because the system is out of balance, or blockage from sludge. That is different from a whole-house no-heat problem, where the boiler, burner, circulator, or main controls are more likely to be at fault.

What you can tell from the way the radiator feels

A radiator that is cold at the top and warmer at the bottom often has air trapped inside. A radiator that is hot at one end and cold at the other more often suggests weak flow, balancing issues, or internal blockage. If the entire radiator stays cold while others are hot, a stuck TRV pin, closed lockshield, or failed flow path to that radiator is more likely than a boiler failure.

If the issue happens only in one room or zone, pay attention to patterns. One cold radiator in an otherwise warm zone points toward that specific radiator or its valves. Several cold radiators on the same branch suggest a zone valve, circulator, or control problem reducing water movement to that section of the system.

Clues from airflow, noise, and visible water

With radiators, 'normal' or 'weak airflow' usually is not the deciding clue unless the unit has a built-in convector or fan. More useful signs are hissing, gurgling, ticking, or a metallic ping as water tries to move past air or a partly closed valve. If you see water around the bleed screw, valve body, or pipe union, or if part of the radiator is icy-cold while the rest is warm, the service call shifts from simple air removal toward valve repair, leak correction, or circulation diagnosis.

What a technician usually does on the visit

A technician will confirm that the boiler is actually supplying hot water, then compare the temperature of the pipe entering and leaving the cold radiator. They typically inspect the TRV or manual valve, check whether the lockshield has been shut too far, look for trapped air, and assess whether sludge is restricting the radiator or branch line. If one room or loop is affected, they may also test the zone valve, circulator operation, and controls serving that section.

What affects repair cost

Cost depends mostly on whether the fix is local to that radiator or involves the distribution system. Typical 2025-2026 US pricing is often about $150-$350 for a basic diagnostic and minor valve or bleeding work, roughly $250-$600 if a TRV or shutoff valve needs replacement, and about $400-$1,200+ if sludge removal, power flushing, branch piping work, or zone-valve/circulator repair is needed. Older radiators with seized fittings, hidden piping, or repeated sludge problems usually increase labor time.