Cold Radiators? How to Bleed and Balance a Hot-Water Heating System

Quick answer: A radiator that's warm at the bottom but cold at the top has trapped air. Turn the heat off, let the system cool, then open the radiator's bleed valve with a radiator key until water appears, and close it. Check the boiler pressure afterward. If radiators need bleeding often, rooms stay cold even with hot radiators, or the pressure keeps dropping, a technician should check for leaks, a failing air separator, circulator or zone valve.

Radiators and baseboard heat give some of the most comfortable heat there is, until one room stays chilly. It's a common winter complaint in older homes across Metuchen, Highland Park, Matawan, Palmyra and countless other New Jersey towns. Most of the time, the fix is simple.

This guide is for hot-water systems. Steam radiators work differently; see steam vs. hot water heat to check which you have.

Why air gets into the system

Water naturally contains dissolved air, which comes out of solution as the water heats. Air also enters during repairs, when the system is drained and refilled, or through small leaks. Most systems have an air separator or automatic air vent near the boiler to remove it, but some air still collects at high points, usually the tops of upstairs radiators.

Signs of trapped air

• Radiator warm at the bottom, cold at the top
• Gurgling or trickling sounds in pipes or radiators
• Upstairs radiators colder than downstairs
• Baseboard convectors that heat only partway along

How to bleed a radiator

You'll need: a radiator key (or a flathead screwdriver for some valves), a cloth and a small cup or container.

1. Turn the heat off at the thermostat and let the system cool for at least an hour.
2. Start with the radiator closest to the boiler on the lowest floor, then work outward and upward.
3. Find the bleed valve, a small square or slotted valve at the top corner of the radiator.
4. Hold the cloth and cup under the valve.
5. Turn the key slowly counterclockwise about a quarter turn. You'll hear air hiss out.
6. When water starts to come out steadily, close the valve. Don't overtighten.
7. Repeat for each radiator that needs it.
8. Check the boiler pressure gauge. Bleeding lowers pressure. If it's well below your normal reading, the system may need water added. Many systems have an automatic fill valve; if yours doesn't, or the pressure won't recover, call a technician.
9. Turn the heat back on and check the radiators as they warm up.

Baseboard systems usually have bleed valves (or small "coin vents") at the ends of runs or high points. Some don't have accessible vents at all and need to be purged at the boiler by a technician.

Balancing heat between rooms

If radiators are fully hot but some rooms are still too cold (or too warm), the system may need balancing:

• Radiator valves: partially closing valves on radiators in rooms that run hot can send more water to cold rooms. Make small adjustments and wait a day to see the effect.
• Zone adjustments: in multi-zone homes, check that each zone's thermostat is set and working.
• Radiator size: a room with large windows or poor insulation may simply need more radiation than it has.
• Outdoor reset: on modern boilers, adjusting the water temperature curve can even out comfort.

When it's not air

Call a technician if:

• Pressure is near zero or drops repeatedly: there may be a leak
• Pressure climbs near the relief valve rating or the relief valve drips: likely an expansion tank problem
• The boiler runs but radiators stay cold everywhere: possibly a failed circulator pump
• One zone is cold and bleeding doesn't help: a stuck zone valve or faulty thermostat
• You bleed constantly: air is getting in somewhere, or the air separator has failed

See boiler not heating? 9 checks for more troubleshooting.

Hot-water vs. steam radiators: different fixes

It's important to know which kind of system you have, because the fixes are different.

Hot-water radiators are full of water. Air trapped at the top keeps them from heating fully, so bleeding the radiator through its small bleed valve usually helps. These systems also have a pressure gauge on the boiler; after bleeding, the pressure may need to be topped up by a professional.

Steam radiators don't get bled with a key. They have an air vent on the side that lets air out as steam rises. A vent that's clogged or painted over can keep a radiator cold, while a vent that spits water or hisses constantly may need replacing. One-pipe steam radiators should also be pitched slightly back toward the supply pipe so condensed water can drain.

If you're unsure which you have, look at the boiler: a sight glass with a water level points to steam, while a pressure and temperature gauge with a circulator pump points to hot water. See steam vs. hot-water heat.

When to call a technician

Call for service if bleeding doesn't help, if boiler pressure keeps dropping (a sign of a leak), if radiators bang loudly, or if several radiators stay cold. A technician can balance the system, check the circulator, expansion tank and vents, and find leaks before they cause damage.

Keep it running smoothly

A yearly boiler tune-up checks pressure, the expansion tank, circulators, air removal and safety controls, and purges air from the system. See the boiler maintenance checklist. Learn more about our boiler services, including boiler repair in Metuchen and Highland Park.