Start with the pattern
A radiator that's cold at the top but warm at the bottom usually has trapped air. A whole zone that's cold points to a zone valve, circulator or thermostat. A single cold radiator on a steam system often has a stuck air vent.
Check whether you have steam or hot water first. The fix is different. See the comparison on the boiler FAQ.
Bleeding a hot-water radiator
- Turn the heating off and let radiators cool somewhat.
- Hold a rag and cup under the bleed screw at the top.
- Open the screw a quarter turn with a radiator key until air hisses out.
- Close it as soon as water flows steadily.
- Check the boiler pressure; top up if it dropped below the normal cold range.
Cold radiator causes
| Symptom | System | Likely cause |
|---|---|---|
| Cold at the top | Hot water | Trapped air |
| Entire zone cold | Hot water | Zone valve, zone pump or thermostat |
| One radiator cold | Steam | Stuck air vent or closed valve |
| Farthest radiators cold | Either | Balancing or insufficient venting |
| Baseboard warm at one end only | Hot water | Air lock or blocked fin-tube |
Zone valves
Many hydronic systems split the house into zones, each with a thermostat and a motorized zone valve. When the valve's motor or end switch fails, that zone stays cold while the rest of the house heats normally.
Most zone valve heads can be replaced without draining the system.
Quick checks
- The radiator valve or baseboard damper is open
- Furniture isn't blocking the radiator or baseboard fins
- The room's zone thermostat is calling for heat
- Pressure is in the normal range
- Bleeders at high points have been opened
Balancing a hydronic system
Radiators closest to the boiler heat first and can steal flow from distant ones. Balancing valves, or on steam, properly sized vents, even out the heat.
If bleeding fixes a radiator only temporarily, air is getting in somewhere, and the system needs a technician.
Radiator FAQs
How often should radiators be bled?
At the start of the heating season, and whenever one is cold at the top. Frequent air is a sign of a leak or a failing air separator.
Can I paint over radiators?
Yes, with heat-resistant paint, but avoid metallic paints, which reduce heat output, and never paint the air vent.
Next steps
Ready to get it fixed? Call (888) 555-0144 to reach a licensed provider in your area. Related guides:
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