Radiator not heating: the pattern of cold tells you the cause
A radiator is a simple object, so a cold one narrows things down quickly. As long as you notice which part of it is cold, and whether it is the only one in the house behaving that way.
Which part of the radiator is cold?
Before anything else, run the heat for twenty minutes or so. Then go round the house with a hand on every radiator, top and bottom. Where the cold sits is the diagnosis. A radiator that is warm at the bottom and cold at the top has air trapped in it.
One that is warm at the top and cold at the bottom has sediment settled in it. One that is stone cold all over has a flow problem at its own valves. If every radiator in the house is cold, the boiler or circulation is at fault and the radiators have nothing to do with it.
| What you feel | Most likely cause | What confirms it | Who deals with it |
|---|---|---|---|
| Warm at the bottom, cold across the top | Air trapped at the high point of the radiator | Air escapes from the bleed valve before water does | Often a homeowner job, with the pressure check below |
| Warm at the top, cold along the bottom | Sludge and sediment settled in the base | The cold band sits low and stays cold however long the system runs | A technician. The system needs flushing |
| Cold all over, other radiators hot | A closed or seized valve at that radiator | Both valves are open and the pipe into the radiator is still cold | Homeowner check first, then a technician |
| Every radiator cold, boiler running | Low system pressure, a circulator pump, or a zone valve | The pressure gauge is at or near zero, or one whole zone is cold | A licensed technician |
| Every radiator cold, boiler not running | A boiler fault, a thermostat fault, or a lockout | The boiler does nothing at all on a clear call for heat | A licensed technician |
Cold at the top: why air collects there
Air finds the high point of a water-filled system, and the top of a radiator is exactly that. It is also why the radiators that give trouble first are usually the ones upstairs and furthest from the boiler. The fix is to release the air at the bleed valve, which most hot-water radiators carry at one upper corner.
Two cautions that matter more than the procedure itself. First, bleeding lets water out of a sealed system, so the pressure gauge at the boiler drops. And a system that falls too low will shut itself down or run badly. The pressure has to be checked afterwards and, if necessary, brought back into the range printed on the boiler.
Second, a system that needs bleeding every few weeks is not a bleeding problem. Air keeps arriving from somewhere, usually a small leak drawing air in or corrosion generating gas inside the system. And repeatedly topping up with fresh water accelerates the sludge problem described below. That pattern is worth a technician's attention rather than a monthly ritual.
Cold at the bottom: sludge, not air
Iron oxide and debris circulating in an older system settle where the water moves slowest, which is the bottom of the radiator. Over time that builds into a layer that water cannot flow through. So the top of the radiator heats and the bottom stays cold no matter how long the system runs. Bleeding does nothing for it, because there is no air to release.
The remedy is to clean out the system by flushing it, and typically to add a corrosion inhibitor afterwards so the process does not start again. That is a technician's job on the whole system, not on the one radiator that shows the symptom first. The sludge is everywhere, and that radiator is merely where it became visible.
One radiator stone cold while the rest are hot
If a radiator is cold from end to end while its neighbours are hot, water is not entering it, and the valves are the place to look. Hot-water radiators generally have two: a control valve at one end, which may be a manual knob or a thermostatic head. And a balancing valve at the other that sets how much flow that radiator gets relative to the rest.
A thermostatic head that has sat in the same position all summer can leave its pin stuck down. So the valve stays shut even when the head is turned fully up. Removing the head reveals the pin, and one that does not spring back when pressed is stuck: a known and common failure. And one worth naming when you call, because it is a small part rather than a big job.
Balancing is a different story. If several radiators far from the boiler are weak while the near ones are scalding, the system is out of balance. Setting it right is a measured job across the whole house, not a matter of opening one valve wider.
Steam radiators are a different animal
If your radiators are fed by a single pipe and carry a small vent on the side rather than a bleed screw at the top, you have a steam system. Most of the advice above does not apply. Never try to bleed a steam radiator: the vent is designed to release air by itself as steam arrives. And what comes out of it is steam under pressure.
A cold steam radiator most often has a blocked air vent. If the air cannot get out, the steam cannot get in. The other classic cause is a supply valve left half open. Steam valves are designed to be fully open or fully closed. And a partly open one produces both poor heating and the banging noise steam systems are famous for.
Pitch matters too, since condensate has to drain back down the same pipe it arrived through. And a radiator that has been shifted off its slope will gurgle and heat unevenly.
What not to do
- Do not keep adding water to bring the pressure back up week after week. Constantly introducing fresh water feeds the corrosion that produces sludge, and the leak that is losing it still needs finding.
- Do not repeatedly reset a boiler safety switch that keeps tripping. It is reporting something, and resetting it is not an answer to what.
- Do not attempt to bleed a steam radiator, and do not touch a steam vent while the system is running.
- If you smell gas, leave the house and call your gas utility or 911 from outside. Do the same for a carbon monoxide alarm near the boiler: leave first, and let someone qualified investigate.
When to call a licensed technician
Call when the cold band sits at the bottom of the radiator rather than the top. Call when air has to be released from the same radiator repeatedly, when the boiler's pressure keeps dropping, or when a valve pin is seized. Call too when several radiators at the far end of the house never get properly hot. And call when every radiator is cold, which is a boiler, pump or zone problem rather than a radiator one.
Frequently asked questions
How often should I need to bleed a radiator?
Occasionally, and not on a schedule. A radiator that needs air released every few weeks is telling you air keeps entering the system, usually through a small leak or through corrosion inside it, and that is worth investigating rather than repeating.
Why is the radiator furthest from the boiler always the coolest?
Either air has collected at that high or distant point, or the system is out of balance and the nearer radiators are taking more than their share of the flow. Balancing is a measured adjustment across every radiator, not a matter of opening one valve further.
Can I bleed a steam radiator the way I would a hot water one?
No. Steam radiators use an air vent that releases air by itself as steam arrives, and what would come out if you opened it is steam under pressure. A cold steam radiator usually has a blocked vent or a supply valve left half open instead of fully open.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.