Radiator Not Heating? What to Check on Hot-Water and Steam Systems

A radiator that won't heat usually has air trapped in it, a valve that's closed or turned down, or, on a hot-water system, pressure that has dropped too low; on steam heat, a clogged air vent is a common cause. Weil-McLain's boiler manual lists air in the system and low system pressure among the reasons some radiators don't heat. The fixes depend on whether your boiler makes hot water or steam, so check that first: you bleed a hot-water radiator, but never a steam one.

For the bleeding itself, see how to bleed radiators. If only one room is cold and you have forced air rather than radiators, see one room colder than the rest.

First: Hot Water or Steam?

Our bleeding guide covers the differences in more detail under confirm it's hot water, not steam.

What the Pattern Tells You

What You Can Fix Yourself

1. Make sure something is calling for heat. Turn the thermostat up. Weil-McLain's list for a burner that won't fire starts with a service switch turned off, a tripped breaker or blown fuse, a thermostat set below room temperature, a closed fuel valve, and an oil tank running low. See thermostat not working for the thermostat itself.
2. Open the valves. On hot water, Caleffi's checklist asks whether the radiator's inlet and outlet valves are fully open and whether a thermostatic radiator valve is turned all the way up. On a fin-tube baseboard, Caleffi says to check that the dampers on the cover are fully open. On steam, Hoffman says the radiator's supply valve works only fully on or fully off; it can't be left partly open. If a valve is stiff or weeps at the stem, leave it for a technician.
3. Clear the radiator. Caleffi asks whether furniture, boxes, drapes, quilts, or shelves are blocking airflow through it, and the DOE says to keep radiators clear of furniture, carpeting, and drapes.
4. Bleed a hot-water radiator that's cold at the top. Weil-McLain's fix for air is to bleed it through the radiator vents. Our step-by-step guide covers cooling the system first and catching the water.
5. Read the boiler gauge. On hot water, Weil-McLain gives 12 psi as the normal cold fill pressure for a house, and Caleffi says at least 12 psi for buildings of three stories or less. It rises as the water heats: Slant/Fin expects about 12 to 15 psi up to about 100°F and 15 to 25 psi up to 240°F. Weil-McLain says a reading over 24 psi may point to an expansion tank problem; that's a call to your technician.
6. If the pressure is low, follow your boiler's manual. Weil-McLain's owner's manual says to fill to the correct pressure and check for leaks; Slant/Fin says to call your service organization if pressures are higher or lower than normal. Either way, pressure that keeps dropping means a leak, and Weil-McLain says to have a technician check the boiler and piping at once. Never add water while the boiler is hot.

On a Steam System

• Check the water level in the glass. Weil-McLain puts the normal waterline halfway up the glass; follow the mark on your own boiler. Peerless notes the level can drop out of sight just after the boiler shuts off, until the condensate returns from the radiators. If the low-water cutoff has shut the boiler down, turn off the gas and call (Peerless).
• Leave the pressure alone. Hoffman's steam newsletter says not to raise the pressure control thinking it will solve a heating problem; set too high, the radiator vents close and stay closed, so air can't get out. Peerless says one-pipe systems should run at around 2 psi at the boiler.
• Look at the vents, but don't open them. Hoffman says a little vapor from a vent is normal, steam passing through continuously means it has failed, and vents should be replaced as needed or every 10 years. A technician should replace a failed one.

What Not to Do

• Don't bleed a steam radiator. Opening its vent can release steam and scalding water.
• Don't add cold water to a hot boiler. Weil-McLain warns the thermal shock can crack the heat exchanger.
• Don't turn up the steam pressure to push heat to a cold radiator (Hoffman).
• Don't open the boiler or its controls. Slant/Fin's manual says there are no user-serviceable parts and not to service or repair the boiler yourself.

When to Call a Technician

• The relief valve lets out water, or a hot-water gauge reads over 24 psi (Weil-McLain, Peerless).
• The pressure keeps dropping, or the system needs water often; that means a leak (Weil-McLain).
• A steam boiler's low-water cutoff shuts it down, or you can't see water in the glass with the boiler cool (Peerless).
• A radiator stays cold after bleeding, with its valves open and the pressure right.
• A whole zone stays cold. Caleffi says a zone valve with a thermal motor can take up to three minutes to open; if it never opens, the actuator likely needs replacing.
• The upstairs radiators lag the downstairs ones. Caleffi says missing or badly set balancing valves are often why some branches don't get enough flow.
• A radiator is cold at the bottom and you suspect sludge; Worcester Bosch says the system needs flushing by an installer. Caleffi says sediment is especially common in older systems with iron piping and cast-iron radiators.
• Anything about a steam system's vents, pressure control, or piping, and the boiler's yearly service.

While you wait, keep the heat no lower than 55°F if you'll be away, the American Red Cross minimum, and watch for frozen pipes. A portable electric space heater can keep one room warm: keep it 3 feet from anything that can burn, plug it straight into a wall outlet, and turn it off and unplug it when you leave the room or go to sleep (CPSC, NFPA).

Frequently Asked Questions

Why is my radiator cold at the top?

On a hot-water system, it's almost always air, and bleeding the radiator lets it out (Weil-McLain, Worcester Bosch). On steam, it can mean the air vent is clogged with scale or failing, Hoffman says, and the vent needs replacing. Never bleed a steam radiator.

Why is my radiator cold at the bottom but warm at the top?

Worcester Bosch says that pattern can mean a buildup of sludge or debris, which needs the system flushed by an installer. Caleffi says sediment is especially common in older systems with steel or iron piping and cast-iron radiators.

Why is my radiator still cold after I bled it?

Bleeding lets water out with the air, so check the boiler gauge: Weil-McLain's fix for low pressure is to fill to the correct pressure and check for leaks. If water came out and the radiator is still cold, the problem isn't air; check that its valves are fully open, then call a technician about the zone valve or circulator.

What should my boiler pressure gauge read?

On hot water, about 12 psi when cold in most houses (Weil-McLain), rising as it heats; Slant/Fin expects up to 25 psi at 240°F. Weil-McLain says to call if it goes over 24 psi. On steam, zero when cold and usually 2 to 3 psi when running (Hoffman), never over 15 psi (Weil-McLain).

Why are my upstairs radiators cold?

On hot water, check the pressure first: Caleffi says a building of three stories or less needs at least 12 psi. After that, it's often balancing, a job for a technician. On steam, main vents that don't work let the nearest radiators get steam first, Peerless says.

Should I turn up the steam pressure?

No. Hoffman says raising the pressure control won't solve a heating problem, and setting it too high makes the radiator vents close and stay closed, so the air can't get out.

The Bottom Line

Find out whether you have hot water or steam. On hot water, open the valves, clear the radiator, bleed a cold top, and check that the gauge reads about 12 psi cold. On steam, open the supply valve fully, check the water in the glass, and leave the pressure and vents to a pro. Never bleed a steam radiator or add cold water to a hot boiler, and call a technician for leaks, a relief valve letting out water, a low-water shutdown, sludge, or a zone that won't heat.

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