How to Fix a Leaking Radiator (Hot Water Heating Systems)

A leaking radiator is more than a nuisance — water damage to floors and walls is expensive to repair, and a slow leak that goes unaddressed can lead to corrosion that shortens the life of the entire heating system.

A leaking radiator is more than a nuisance — water damage to floors and walls is expensive to repair, and a slow leak that goes unaddressed can lead to corrosion that shortens the life of the entire heating system. The good news is that most radiator leaks in residential hot water heating systems trace back to a handful of well-known weak points, and most repairs are straightforward once you identify the source.

This guide focuses on hot water (hydronic) radiator systems, which are common in homes built before 1980 in cooler climates, as well as modern radiant heating installations.

What You Need

Depending on the repair, you’ll need some or all of these:

• Radiator key / bleed key — essential for bleeding air and operating bleed valves
• Replacement radiator bleed valve — for a failed bleed valve that leaks at the tip
• Packing string / valve packing — for repacking a supply or return valve stem that seeps
• Union radiator fitting and gasket — for a leaking union connection
• Pipe thread sealant / PTFE tape — for sealing threaded connections
• Adjustable wrench and basin wrench — for valve packing nuts and union fittings

Understand Your System Before You Begin

Hot water heating systems are closed loops: a boiler heats water, which circulates through pipes to radiators and back. The system operates at a specific pressure (typically 12–15 PSI cold) and temperature (160–180°F at the boiler). Unlike steam systems, hot water systems don’t have the high-pressure steam that makes steam radiator repairs more hazardous.

Two-pipe vs. one-pipe systems. Two-pipe systems have a separate supply pipe and return pipe connecting to each radiator — they’re easier to work on because you can isolate individual radiators by closing both valves. One-pipe systems have a single main loop that passes through each radiator with supply and return connections on the same pipe run — isolating individual radiators may not be possible.

Before any repair: Know how to shut off the boiler and how to turn off the system’s water supply (usually through a pressure-reducing valve near the boiler). Know the location of the system’s pressure relief valve. Have towels and a bucket available — even draining “just” a radiator releases a meaningful amount of water.

Locate the Leak Source

Before starting any repair, pinpoint exactly where the water is coming from. This matters because different sources require different fixes.

Dry the surrounding area thoroughly with towels. Run the heating system until it reaches operating temperature. Watch closely at all potential leak points:

• The tip of the bleed valve (small valve near the top of the radiator)
• The packing nut area of the supply and return valves at the floor or wall connection
• The union fittings (round nuts threaded onto pipe connections)
• Joints where the pipe connects to the radiator body
• The radiator body itself (look for rust staining or mineral deposits that indicate a past seep)

Use dry toilet paper to pinpoint small leaks — it’s more sensitive to moisture than a dry finger.

Fix 1: Leaking Bleed Valve

The bleed valve (also called the air vent) is a small valve typically located at the top of each radiator, used to bleed trapped air. It’s one of the most common leak sources because its packing or seat degrades over time.

Water dripping from the valve tip (the pointed or slotted end): The valve seat or packing has failed. First, try tightening the valve stem: using a radiator key, bleed valve key, or small flathead screwdriver, turn the stem clockwise (closed) firmly. If this stops the drip, monitor for 24–48 hours. If it drips again, the valve needs replacement.

Replacing a bleed valve: Close the radiator supply and return valves (if available) or drain the radiator section. Unscrew the old bleed valve counterclockwise — it threads into the radiator body, typically at 1/8-inch NPT thread. Apply PTFE tape to the threads of the new valve (wrap clockwise, 3–4 layers). Thread in the new valve and tighten — snug plus a quarter turn. Open valves and refill the system. Bleed air from the new valve before closing it fully.

Fix 2: Leaking Supply or Return Valve (Packing Nut)

The valves where the radiator connects to the supply and return pipes have a packing nut — a nut that compresses packing material around the valve stem to prevent leakage. When packing deteriorates, water seeps out around the stem while the valve is in any position.

The quick fix: tighten the packing nut. With the system at operating temperature, use an adjustable wrench to turn the packing nut clockwise — just a quarter to half turn at a time. Check whether the seeping stops. This works if the packing hasn’t fully failed, only compressed.

If tightening doesn’t work: The packing needs replacement.

Shut off the boiler and let the system cool. Close any isolating valves to the affected radiator. If no isolating valves are present, drain the system to below the level of the affected radiator.

Unscrew the packing nut fully (counterclockwise). Pull it off the valve stem. Use a pick or small screwdriver to remove the old packing material from the packing chamber (the recess in the valve body around the stem).

Wind new packing string clockwise around the valve stem 4–6 times. Push the packing string into the packing chamber with a screwdriver. Thread the packing nut back on and tighten until snug — you’re compressing the new packing around the stem. Restore water to the system, bring it to temperature, and check for seeping.

Fix 3: Leaking Union Fitting

Union fittings are the round, three-piece connectors that allow a radiator to be disconnected from the piping without cutting the pipe. They consist of two threaded ends and a union nut that draws them together, compressing a soft metal or fiber gasket at the connection face.

Decades of heating and cooling cycles harden the gasket, and eventually water seeps from the union face.

Before attempting a union repair: Drain the system fully. The union is a wet connection — water will drain from both the radiator and the connected pipe when disassembled.

Open the union nut. Use two wrenches: one to hold the pipe (or the radiator tail) still, and one to turn the union nut counterclockwise. The nut may be corroded and require significant force — penetrating oil applied the day before helps.

Inspect the gasket. Remove the union nut and pull apart the two union halves. The gasket (usually a soft copper, fiber, or rubber washer) will either be stuck to one face or will fall out. Replace with an identically sized new gasket — bring the old one to the hardware store to match.

Reassemble. Clean the mating faces of any mineral deposits. Insert the new gasket. Thread the union nut on by hand, then tighten with wrenches — snug plus a half turn. Avoid overtightening, which can crack cast iron radiator tails. Refill the system, restore pressure, and check for leaks at operating temperature.

Fix 4: Air in the System — How to Bleed Radiators

Air pockets in a hot water system cause cold spots in radiators and can mask or worsen leak symptoms (air pockets change pressure distribution). Bleeding the system is routine maintenance that should be done at the start of each heating season.

Bleed from the lowest floor up. Air rises, so you’re chasing it upward through the system. Start with ground-floor radiators, then move to upper floors.

Bleeding procedure for each radiator:

1. Run the system until at operating temperature (radiators should be warming).
2. Turn off the circulator pump (the heating system’s water pump) or lower the thermostat to stop circulation — you want still water while bleeding.
3. Hold a small container under the bleed valve. Using a radiator key, turn the valve counterclockwise just slightly — you’ll hear hissing as air escapes.
4. When water begins to flow steadily (without sputtering), close the valve.
5. Move to the next radiator.

After bleeding all radiators, check the boiler pressure gauge. Bleeding releases water along with air and drops system pressure. Use the fill valve (usually a small valve on the feed line near the boiler) to restore pressure to 12–15 PSI. Run the system and recheck all bled radiators — some will need a second bleeding session if the first released only part of the trapped air.

Fix 5: Balancing Radiator Flow

If some rooms are too hot while others stay cold even with the system running at proper pressure, the system is unbalanced — flow is not distributed evenly between radiators. This isn’t a leak issue but often gets conflated with it because people are investigating the system anyway.

Each radiator typically has two valves: a control valve (the one you can open and close by hand for temperature control) and a lockshield valve (covered by a cap, requiring a wrench to adjust). The lockshield controls flow rate through the radiator and is used for balancing.

Basic balancing procedure: Open all lockshield valves fully. Run the system for a full heating cycle. Identify which radiators are hottest (overfed) and which are coolest (underfed). Partially close the lockshield valves on the hottest radiators — start with a quarter turn closed. Allow one to two more heating cycles to stabilize. Repeat adjustments until temperatures are even across rooms. Full balancing of a large system is iterative and may take several days of adjustment.

When to Call a Heating Professional

Call a licensed HVAC technician or plumber experienced with hydronic systems if:

• The radiator body itself is cracked or has a pinhole leak — cast iron radiator bodies are repairable by specialists but the repair involves specialized compounds or section replacement
• The boiler pressure relief valve is actively dripping — this indicates a system pressure problem that requires a professional evaluation of the expansion tank and pressure reducing valve
• Multiple radiators in multiple zones are leaking — widespread leaking suggests water chemistry problems (low pH causing corrosion) that require water treatment, not just mechanical repair
• You have a steam radiator system (identifiable by a single pipe connection and an air vent on the side of each radiator) — steam systems operate differently and require different repair procedures

Related Reading

• How to Fix a Baseboard Heater
• How to Fix a Furnace Not Heating
• How to Fix Noisy Pipes