Radiator Not Heating

Common Symptoms

• Radiator is completely cold — no heat at all
• Radiator is warm at the bottom but cold at the top half
• Steam radiator hisses continuously from the air vent
• Steam radiator makes loud knocking or banging sounds
• One radiator cold while others on the same system heat normally
• Thermostatic radiator valve (TRV) head feels stiff and won't turn
• Hot water radiator lukewarm — never reaches set temperature
• Air vent on steam radiator drips water continuously

Most Likely Causes

1. 1

   Trapped Air in Hot Water Radiator — Bleed Valve Procedure

   In a hot water (hydronic) heating system, air becomes trapped in the top of radiators and sections of piping as the system is filled or when new water is added to make up for pressure losses. Air is less dense than water and rises — it collects at the highest points of the system (typically the tops of radiators and the far ends of long baseboard runs). An air pocket prevents hot water from circulating through that section, leaving the radiator cold even when the rest of the system is warm. The bleed procedure: locate the bleed valve on the radiator — it is typically a small brass screw or slotted cap at the top corner of the radiator body, or on the top of a panel radiator. You will need a radiator bleed key (a flat-key tool, usually $3–$5 at hardware stores) or a flathead screwdriver for slotted types. Close the system — do not add water while bleeding. Place a small towel or cup under the bleed valve to catch drips. Insert the key and turn counter-clockwise 1/4 to 1/2 turn — do not remove the screw completely. You will hear a hiss as air escapes. When the hissing stops and a steady stream of water flows without air bubbles, close the valve immediately. Check the boiler pressure gauge after bleeding all radiators — the pressure will have dropped slightly. Restore to 15 PSI via the fill valve if below 12 PSI. If the same radiator repeatedly accumulates air, the system has an ongoing air introduction source: a failing auto air vent on the boiler, a micro-leak pulling in air, or dissolved oxygen from untreated system water. Add Fernox F1 corrosion inhibitor to the fill water.

2. 2

   Thermostatic Radiator Valve (TRV) Stuck Closed — Danfoss RA2000 and Honeywell T104A

   Thermostatic radiator valves (TRVs) are motorized or wax-cartridge valves mounted on the radiator inlet pipe that modulate flow based on room temperature. They are extremely common on European-style systems and are increasingly retrofit onto older North American hydronic systems. TRVs fail in a specific way: the valve pin under the TRV head seizes in the closed position from mineral deposits or extended disuse (over summer shutdown). The symptom is a radiator that stays cold even with the TRV head turned to the maximum position. Test and fix: first, remove the TRV head — it usually unscrews or snaps off with a 1/4 turn. Under the head, you will see a round pin sticking up from the valve body. On a working valve, this pin should move freely up and down. If the pin is stuck down and won't spring back, the valve seat is seized. Apply a small amount of WD-40 around the pin base. Wait 5 minutes. Press the pin down several times manually to work the lubricant in. If it frees up and springs back, reinstall the head. If the pin remains stuck, the valve body must be replaced (drain the section first). Common TRV models: Danfoss RA2000 series (1/2-inch and 3/4-inch), Honeywell T104A (direct-acting wax type), Myson TRV heads, and Caleffi TRV bodies. Replacement heads that fit without draining: Danfoss RA2000 head only ($15–$35). The valve body requires draining the radiator circuit to replace ($30–$80 body + installation).

3. 3

   Zone Valve Closed — No Flow to Radiator Zone

   Hydronic systems with multiple heating zones use motorized zone valves (Taco 571-2, Honeywell V8043F) to direct hot water to each zone. If the zone valve serving a set of radiators fails to open, all radiators in that zone will be cold even though the boiler fires normally and other zones heat fine. Diagnosis: locate the zone valve for the cold zone — it is typically in the mechanical room on the supply or return pipe. Find the manual open lever on the zone valve actuator body. Push it to the OPEN position. If the radiators warm up when the valve is manually opened, the actuator motor has failed. Electrical test: with the zone thermostat calling for heat, use a multimeter to verify 24VAC at the zone valve motor terminals. Correct voltage + valve won't open = dead actuator ($25–$45 to replace, no drain required). No voltage at terminals = thermostat wiring fault or blown fuse on the transformer board. Check the 3-amp fuse on the boiler control board and verify continuity of the thermostat wire between the thermostat and boiler.

4. 4

   Steam Air Vent Clogged — Gorton #1 or #5 Replacement

   On a one-pipe steam system, each radiator has an air vent — a small valve that allows air to escape from the radiator as steam enters, but closes automatically when steam contacts the vent's bimetallic element. When the air vent clogs (with mineral deposits or rust), it cannot open to allow air to escape — the air locks in the radiator and steam cannot enter, leaving the radiator cold. A clogged vent may also spit water or fail to close (hissing steam = vent stuck open, wasting energy). Test: remove the air vent (screws onto the side of the radiator near the top, typically with a 1-inch wrench or by hand), and immerse it in boiling water for 30 seconds — the vent should close (no air flow through it). Then hold it under cold water — it should open (air flows through freely). Failure in either direction means replace the vent. Standard replacement vents: Gorton #1 (slow venting, for radiators near the boiler or in warm rooms, ~$12–$18) and Gorton #5 (fast venting, for large radiators or rooms far from the boiler, ~$14–$20). A Gorton D or #2 main vent is used on the steam mains (the large supply pipes) — undersized main vents cause slow heating and water hammer. Installing faster vents (Gorton #2 or Hoffman 75) on the main vents is one of the most effective upgrades for slow-heating steam systems.

5. 5

   Steam Trap Failure — Stuck Open (Hissing) or Stuck Closed (No Heat)

   Two-pipe steam systems use steam traps at the outlet of each radiator to allow condensate to drain back to the boiler while blocking live steam from escaping into the return lines. When a steam trap fails in the open position, steam blows through continuously — you will hear a constant hissing or high-pitched sound at the trap, and the return pipes will be very hot (should be cold — they carry condensate, not steam). This wastes significant fuel and also causes water hammer when live steam meets condensate in the return lines. When a trap fails in the closed position, condensate cannot drain and backs up into the radiator — the radiator fills with water, knocks and bangs, and eventually provides no heat. Trap test: on a two-pipe system, use an infrared thermometer or place your hand on the pipe downstream of the trap. The inlet side (before the trap) should be hot steam temperature (212°F). The outlet side (after the trap) should be warm but significantly cooler (120–150°F at most). If both sides are equally hot, the trap is stuck open. If the inlet side is hot and the outlet side is cold and the radiator isn't heating, the trap is stuck closed. Replacement traps: Hoffman 1B float-and-thermostatic trap (~$25–$50) or a thermostatic trap from Watts or Spirax Sarco. Draining the radiator circuit is required for trap replacement.

6. 6

   Water Hammer — Slope Problem and Condensate Drainage

   Steam radiators that knock and bang loudly when heat comes on are experiencing water hammer — steam and condensate colliding in the supply pipe. Water hammer in steam systems almost always has one of two causes: (1) Incorrect slope — radiators on a one-pipe steam system must pitch toward the steam supply side at a minimum of 1/4 inch per foot. If a radiator has settled level or pitches away from the supply, condensate pools in the radiator and at the pipe connection. When steam rushes in, it hits the pooled water and causes a violent knock. Fix: place shims under the far legs of the radiator to restore the correct pitch toward the supply valve. Verify slope with a level — the supply valve end should be 1/4 to 1/2 inch lower than the far end of a typical 3-foot radiator. (2) Steam trap failure — see above. Water hammer can also occur on two-pipe systems when a failed steam trap allows live steam into the condensate return line, where it meets cooler liquid. On hot water systems, banging is usually caused by a partially open zone valve restricting flow velocity, or a pump running too fast — not by the condensate drainage issue that affects steam systems.

7. 7

   Balancing Hot Water Systems — Lockshield Valve and Delta-T

   In a multi-radiator hot water system, radiators near the boiler receive more flow than radiators at the far end of the circuit — this is called hydraulic imbalance. The symptom is that radiators near the boiler are very hot while distant radiators are lukewarm or cold, even though the system pressure and pump are working correctly. Balancing corrects this by partially throttling the flow to near-boiler radiators, forcing more flow to distant ones. Each radiator has two valves: a hand valve (the supply valve you open and close) and a lockshield valve (a capped valve on the return side of the radiator, usually the opposite end from the TRV or hand valve). The lockshield is called 'lockshield' because it is normally covered with a plastic cap to prevent casual adjustment. Balancing procedure: start with all lockshields fully open. Start the system and allow 30 minutes to reach steady state. Use an infrared thermometer to measure supply and return temperatures at each radiator. A correctly balanced radiator has a delta-T (supply minus return temperature) of 11–17°F (6–9°C) for a standard 180°F boiler system. Radiators near the boiler with delta-T below 5°F are receiving too much flow — gradually close their lockshield 1/4 turn at a time and recheck after 15 minutes. Radiators at the far end with delta-T above 20°F are flow-starved — open their lockshields fully. The process takes 1–2 hours for a typical 8–12 radiator system. You can also use Caleffi or Danfoss pre-settable lockshield valves for more precise balancing.

Quick DIY Checks

1. 1Identify your system type (hot water or steam): look at the boiler pressure gauge. Hot water (hydronic): gauge reads 12–15 PSI cold. Steam: gauge reads 0–2 PSI. Also look at the radiators — steam radiators have only one pipe connection at the bottom (one-pipe steam) or two connections (two-pipe), and have an air vent on the side near the top. Hot water radiators have two pipe connections (supply and return) and may have a TRV on the inlet side. This determines the entire repair path.
2. 2Hot water system: bleed the cold radiator — locate the bleed valve at the top corner of the radiator (requires a bleed key or flathead screwdriver). Place a rag under the valve. Open counter-clockwise 1/4 to 1/2 turn. Listen for hissing air. When steady water flows without bubbles, close the valve. Check boiler pressure after bleeding — restore to 15 PSI if below 12 PSI. If the radiator is warm at the bottom but cold at the top, this confirms trapped air as the cause.
3. 3Hot water system: test the TRV pin — remove the TRV head (unscrew or quarter-turn snap-off). Look for the pin protruding from the valve body. Press it down with your finger and release — it should spring back freely. If stuck down, apply WD-40, wait 5 minutes, and press repeatedly. A TRV with a freely moving pin that still doesn't heat the radiator = the valve seat may be scaled closed; open the TRV head to maximum setting and verify the pin is being depressed by the head's actuator. Danfoss RA2000 head twist-on: align the arrows, push and turn clockwise. Honeywell T104A: press head down until it clicks.

1. 4Steam system: test the air vent — unscrew the vent from the radiator (1-inch wrench, counterclockwise). Immerse in a cup of boiling water for 30 seconds. The vent should close (you should not be able to blow air through it). Then hold under cold running water for 30 seconds — the vent should open (you can blow air through freely). A vent that doesn't pass both tests is faulty. Replace with a Gorton #1 for close-to-boiler radiators or Gorton #5 for large or distant radiators ($12–$20). Apply PTFE tape to the threads and tighten snugly — do not overtighten.
2. 5Steam system: check radiator slope — place a level on the top of the cold steam radiator. The supply valve end (where the one pipe connects) should pitch slightly downward — 1/4 inch drop per foot toward the supply. If the radiator is level or pitched away from the supply, condensate is pooling and causing water hammer and poor heating. Shim the far legs of the radiator to restore the correct slope. Use metal furniture leg shims or flat washers stacked under the radiator feet.
3. 6Two-pipe steam: test the steam trap — with the system running and steam in the mains (20–30 minutes after boiler starts), use an infrared thermometer or careful touch to compare temperatures on both sides of the trap. Inlet (supply side): should be hot (near 212°F). Outlet (return side): should be warm but significantly cooler — around 130–160°F if the trap is working. Both sides equally hot: trap stuck open (replace). Inlet hot, outlet cold, no heat in radiator: trap stuck closed (replace). Hoffman 1B trap: $25–$50.

Repair vs Replace

Recommended Tools & Parts

• Gorton #1 Steam Radiator Air Vent (Slow Vent)

  Replacement air vent for one-pipe steam radiators. Gorton #1 vents slowly — best for radiators near the boiler or in rooms that overheat. Fits standard 1/8-inch NPT radiator vent port. Bimetallic element closes on steam contact.

  $12–$18

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• Gorton #5 Steam Radiator Air Vent (Fast Vent)

  Fast-venting replacement air vent for large or distant radiators on one-pipe steam systems. Allows air to escape quickly so steam fills radiator faster. Gorton #5 or equivalent for radiators slow to heat.

  $14–$22

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• Danfoss RA2000 TRV Replacement Head

  Drop-in replacement TRV head for Danfoss RA2000 body valves. No draining required — head-only swap. Resolves seized pin that holds radiator stuck closed. Fits 1/2-inch and 3/4-inch RA2000 bodies.

  $15–$35

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• Radiator Bleed Key

  Essential tool for bleeding air from hot water radiators. Fits standard square bleed valve nipples on European and UK-style panel radiators as well as most North American hydronic radiators.

  $3–$8

  Buy on Amazon →

• Hoffman 1B Steam Trap

  Float-and-thermostatic steam trap for two-pipe steam radiator return connections. Allows condensate to drain while blocking live steam. Replaces failed traps stuck open (hissing) or closed (no heat). Size: 1/2-inch.

  $25–$50

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• Infrared Thermometer

  For testing steam trap inlet vs. outlet temperatures (trap open/closed diagnosis), checking delta-T across radiators for balancing hot water systems, and verifying heat distribution across fin sections.

  $15–$30

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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