Radiator Not Heating Up? Steam and Hot Water Fixes

A radiator that will not heat up has a different cause depending on your system. On a hot water system, a cold top means trapped air (bleed it), a cold bottom means sludge, and a fully cold radiator can be a stuck valve or low boiler pressure. On a steam system, the usual culprit is a stuck or clogged air vent. Identify your system first.

A lot of online advice about radiators comes from the UK, where hot water radiators are standard. In the US, many older homes have steam heat, and the fix for a steam radiator can be the opposite of the fix for a hot water one. This guide covers both.

First: do you have steam or hot water?

If you cannot tell, check the boiler gauges or take a clear photo of the radiator and its valves to show a technician.

If you have a hot water radiator

Cold at the top, warm at the bottom: trapped air

Air rises and collects in the top of the radiator, blocking hot water. The fix is to bleed it. Our guide on step-by-step radiator bleeding shows the steps, and you will need a radiator key and a cloth.

Cold at the bottom, hot at the top: sludge

Rust and debris settle at the bottom and block circulation. A flush clears it, and this is usually a job for a heating professional.

Cold all over: valve, pressure or pump

• A stuck valve. Valves can seize if they sit closed all summer. Check that the valve and the thermostatic valve (TRV) are open and turned to a warm setting. If the pin is stuck, a plumber can free it.
• Low boiler pressure. Check the pressure gauge on the boiler. Many residential systems sit at a fairly low pressure when cold, often in the neighborhood of 12 to 15 psi, but check your manual. If it is well below that, the system may be leaking or may need to be topped up by a professional.
• A failed circulator pump or zone valve. If a whole zone is cold and others work, a pump or zone valve may have failed.
• No heat from the boiler. If every radiator is cold, check the boiler first. See finding a heating technician.

If you have a steam radiator

Steam heating works differently. The boiler makes steam that rises through pipes into the radiators, and air must escape through a vent so steam can fill the radiator.

Cold, or only warm at one end: a stuck or clogged air vent

This is the most common cause. If the air vent is clogged, the radiator cannot fill with steam. Signs are a radiator that stays cold or heats only partway. The fix is to replace the vent, which screws on and off. They are available at hardware stores, and the right one depends on your radiator. Let the system cool and use gloves.

The supply valve is partly closed

On a one-pipe steam system, the radiator valve should be fully open or fully closed. Partly closed valves cause hissing, spitting and uneven heat. Open it fully.

The radiator is not tilted correctly

A one-pipe steam radiator should tilt slightly toward the supply valve so that condensed water runs back out. If it tilts the wrong way, water collects and the radiator can bang and heat poorly. Many people add a small shim under the legs at the end farthest from the valve. A professional can check the pitch.

Water hammer

Banging and clanking mean water is trapped in the pipes. Pitch problems and open valves are common reasons.

The boiler is low on water or off

A steam boiler needs the right water level. If it is low, there is not enough steam. If you are comfortable, check the sight glass. Otherwise call a technician.

What the symptom tells you

How to fix it

1. Confirm the boiler is on and the thermostat is set above the room temperature.
2. Identify your system using the table above.
3. Check the radiator valve. Make sure it is fully open.
4. For hot water, bleed the radiator with the heating off, and watch the boiler pressure after.
5. For steam, check the air vent, and replace it if it is clogged. Do not bleed.
6. Check the pitch of a steam radiator.
7. Look at the gauges on the boiler. Low pressure or low water means a technician.
8. Call a professional for sludge, pump or zone problems, and anything involving the boiler.

Is it worth repairing or upgrading?

Bleeding a radiator and swapping an air vent are inexpensive. A flush, a new pump or a zone valve is a moderate cost. A boiler repair or replacement is a major one. If your boiler is 20 years old or more, and it needs repair, ask a heating professional about a replacement, and about service plans. An annual boiler service can catch problems before a cold snap, and many technicians are busy in winter, so it is wise to book early.

When to call a professional

Call a licensed heating technician if:

• Every radiator is cold, or the boiler will not fire.
• The boiler pressure or water level is wrong.
• You smell gas or burning, or hear loud banging from the boiler. Leave and call your gas utility if you smell gas.
• Sludge is blocking several radiators.
• A radiator leaks.
• You are not sure whether you have steam or hot water.

Cold pipes can also freeze, so if your radiators go cold in a cold snap, see protecting pipes in a cold snap.

What not to do

• Do not bleed a steam radiator.
• Do not open a boiler's panels or touch gas parts.
• Do not leave a leaking radiator running.
• Do not close radiator valves partway on a steam system.

Bottom line

First work out whether you have steam or hot water. A cold top on hot water is air, so bleed it. A cold radiator on steam is usually a stuck air vent, so replace it. If every radiator is cold, the problem is the boiler, not the radiator. Drafty rooms also feel cold, so sealing drafts around windows and doors can help.