Radiator Not Heating Up: Hot Water and Steam Checks

Turn it fully up and feel the whole radiator

The test. Danfoss says to turn the radiator’s thermostatic valve all the way up: the entire radiator should warm up. On a plain hand valve, open it fully. If it only partly warms, or not at all, go on to the next checks.

A thermostatic valve (TRV). Castrads explains that its head pushes on a sprung plunger in the valve body, closing the valve as the room warms and opening it as the room cools. Many have a 1 to 5 scale, and the higher the number, the warmer the room gets before it closes.

A plain cap instead of a head. Castrads says the valve is shut off completely with the decorator’s cap supplied with it, for example while decorating. A cap left on keeps that radiator off.

Bleed the air out at the top

Why. Viessmann says air in the radiators causes gurgling and radiators that stay completely or partly cold. Weil-McLain’s troubleshooting table gives the same fix for radiators or baseboard that do not heat or are noisy: bleed the air through their vents. DOE says to bleed hot water radiators at the start of each heating season.

Let the air settle. Turn the heat down, Danfoss says, and turn the circulation pump off if you can, Viessmann says. Then wait 30 to 60 minutes so the air bubbles collect in the radiators.

Get ready. Turn the radiator’s thermostatic valve to its highest setting, Viessmann says, put the rag under the bleed screw and hold the cup under it.

Open the screw slowly. Use the key, and do not open it fully: Viessmann says it usually takes half a turn or less before you hear hissing. When the hissing stops and water comes out, close it again quickly.

If no water comes out. Close the screw again. The pressure may be too low to fill the radiator. Weil-McLain lists low system pressure as the second cause of radiators that do not heat, and its fix is to fill to the correct pressure and check for leaks: boiler pressure too low or too high.

Check it worked. Turn the pump back on if you turned it off, and check the pressure on the boiler gauge. Viessmann says to run the heat for at least an hour, then check the radiator furthest from the boiler again, and that you may need to bleed more than once.

Baseboard. Slant/Fin recommends an air vent at each drop, where the pipe turns down through the floor, and says the hinged wall trim or end caps give access to it. Its other option is an automatic air eliminator at the boiler and a vent at the highest point, so not every baseboard has a vent at each drop.

Automatic air vents. Weil-McLain’s monthly check is to take the cap off and press the valve inside slightly with the tip of a screwdriver. If it works freely and does not leak, screw the cap all the way on, then loosen it one turn so the vent can work. If it does not work correctly, have it replaced. Weil-McLain puts automatic vents only on systems with a diaphragm or bladder expansion tank, and warns that with a closed-type tank, a working automatic vent lets out the air the tank needs and the tank waterlogs.

A Taco Hy-Vent is different. Taco says not to take its cap off, as dirt can get into the valve, and to open the cap two full turns from closed for automatic operation.

Free a stuck valve pin

When. Danfoss says that if bleeding has not worked, the radiator valve may be blocked. This check is for a thermostatic valve, the kind with a removable head.

Danfoss’s steps. Take the thermostat head off, with an Allen key or by turning it counterclockwise on newer models. Gently push the pin on the valve with the flat end of the Allen key. It should move in 2 to 3 mm and bounce back out.

If the pin is stuck. Danfoss says to gently alternate between tapping the side of the valve and moving the pin in and out until it frees. Check for leaks, then put the head back on.

If it still does not heat. Danfoss says to call a heating technician. Viessmann says a radiator that stays cold after bleeding may need the system balanced or the valve looked at, which is best found on site by an expert.

A whole zone cold: the zone valve and the pump

Honeywell V8043 zone valves. Honeywell says a normally closed V8043 can be opened without power by pushing its lever over the stop to MAN. OPEN. Honeywell says its end switch, which turns on the circulator, is not switched on when the valve is opened by hand. So that zone only heats while another zone is running the boiler and pump.

Taco zone valves. Taco says the manual opening lever on its 570 valves moves easily when the valve is open and meets resistance when it is closed. Pushed all the way down, it holds the valve open; Taco says to push it back up to restore automatic operation. A Taco Zone Sentry has a manual override button on top, and Taco’s reset for its actuator is to remove the power for 5 seconds and restore it.

The pump. Taco says loud water noise from a 007e with a flashing white light means air in the system: the pump may be air bound and the system should be vented. Grundfos says the system cannot be vented through its ALPHA2 pump, so the air has to come out at the radiators or vents. Taco says never to run a circulator dry.

Open the valve all the way

One-pipe steam. Castrads says the valve on a one-pipe steam radiator must be fully open or fully closed. Leaving it halfway open can cause very noisy steam hammer. Other banging: a boiler making noise. Danfoss’s instructions for its one-pipe steam radiator valve also say to make sure the hand valves are fully open. Turn an old valve with the boiler off and the radiator cold, and do not force one that will not move.

Why it matters. Castrads explains that on one-pipe steam, the water the steam turns back into leaves through the same single pipe the steam came in by, flowing the other way.

Check the air vent

How it heats. Before each cycle the radiator is full of air. Castrads says if that air cannot get out, the steam cannot get in, and the vent closes by itself once the radiator is full of steam. If a thermostatic valve sits between the radiator and the vent, Castrads says it controls how much air can escape, so turn it up first.

A vent that does not open. Danfoss lists a vent stuck closed as a cause of a room that underheats. DOE’s Building America steam report says radiator vents painted over are a common problem and no longer work, and blocked vents must be replaced. DOE says to replace any that are clogged or stuck open.

Testing or replacing one. Do it with the boiler off and the radiator cold. DOE’s steam report says to take the vent off and test it: if it does not pass air, clean it or replace it. Burnham says a one-pipe steam system needs a vent on each radiator and at the end of each main, all properly sized and in good condition, and any defective vents replaced.

Radiators far from the boiler. Gorton says larger vents go on the radiators farthest from the boiler and in the coldest rooms, and smaller ones near the boiler. Hoffman’s 1A radiator vent has six settings, from slow (1) to fast (6).

A vent that spits water. Danfoss’s list for a spitting vent is to check the radiator’s pitch, open the supply valve fully, check the riser air vent for the main system, and check for too much pressure. More in a radiator leaking.

Check the radiator drains back to its valve

The pitch. Danfoss’s one-pipe steam instructions say the system needs enough pitch for the water to drain back, with none trapped at the vent end, and that free standing radiators must be properly pitched. The water leaves through the supply valve, so the radiator must lean slightly down toward that end, never toward the vent. To fix it, the vent end is raised a little with shims under its feet, with the boiler off and the radiator cold.

A sign it is wrong. Danfoss lists the pitch among the causes of both a spitting vent and a room that underheats.

Keep the steam pressure low

Too much pressure shuts the vents. Peerless says a steam boiler set to too high a pressure can lock up the air vents, giving no heat or poor heat. Hoffman says a vent cannot open when the system pressure is above its rating, and air stays in the system. Its 1A radiator vent is rated for 1½ psig in use.

What the makers say. Peerless says its steam limit is usually set under 5 psig, and Burnham’s start-up setting for its Independence boiler is a cut-out of 1 psi. Weil-McLain says its EG limit must never be set above 10 psig, because higher pressure can damage the relief valve. For vents that will not open, Peerless’s fix is to reduce the operating pressure.

Two-pipe steam. DOE says to replace the steam traps in two-pipe systems for balanced heat. Peerless lists radiator traps that have failed open as a cause of no heat or poor heat.

The boiler itself. Burnham says wrong piping at the boiler can send wet steam into the system, one result being radiators that do not heat, and that a boiler too small for the radiators leaves those furthest from it cold. Check the water level in the glass: Weil-McLain says normal is halfway up it, and never to add cold water to a hot boiler, because the thermal shock can crack it. More in the steam boiler checks.