Boiler Leaking Water: Find the Pipe, Valve or Boiler Leak

By Ray Bennett · Updated 17 October 2026 · 7 min read · Checked against the sources at the end

Boiler Leaking Water: Find the Pipe, Valve or Boiler Leak

Short answer

Start with the wet part, not the puddle. A relief pipe, drain valve, pump joint and condensate drain need different checks. Lochinvar's Knight manual says boiler and pipe leaks need prompt repair; its relief valve may weep when the expansion tank is not working properly.

Boiler leaking water can mean a wet pipe connection, a valve letting water out, or a leak inside the boiler. Before deciding what needs replacing, look for the first wet part you can see without opening the case.

Lochinvar’s Knight manual says leaks in the boiler or piping need repair at once. This guide helps you describe that leak and understand the repair, without taking apart a hot, pressurized system.

Before you start

What to carry: a torch and your phone. From a dry place, photograph the wet part, the model label and the gauge together if possible. Leave tools for loosening fittings upstairs.

Gas, oil or electric? The examples here are gas hot water boilers, including condensing models. Do not use their gas controls or pressure instructions for an electric boiler or a steam system.

Which appliance? Check the model label and its manual once before following a shutdown procedure. Water from a separate hot water cylinder belongs in water heater relief valve leaks; a wet radiator belongs in radiator leaking.

Renting? Tell the landlord or building manager where the water is coming from and whether the heating has stopped. Ask them to arrange the repair.

Read this before switching anything off. Overheating is the exception. Lochinvar’s Knight manual says that if the boiler overheats or its gas supply fails to shut off, leave the circulator’s electrical supply on and shut the gas off outside the appliance. Arrange urgent service; do not reach through water to a control.

Smell gas? Lochinvar says not to light an appliance, operate an electrical switch or use a phone in the building. Call the gas supplier immediately from outside and follow their instructions. If you cannot reach them, call the fire department.

For an ordinary water leak, stop and arrange repair. Weil-McLain’s WM97+ startup checks say to shut the boiler down and repair leaking system pipes immediately. Use your boiler’s own normal shutdown instructions, only from a dry, safe position.

Keep clear of hot water. Lochinvar warns that relief valve discharge can scald and requires a pipe to a safe place. Leave that outlet unobstructed. Its Knight manual also requires electrical isolation and cooling before maintenance. Lochinvar says not to use the boiler if any part has been under water; it must be replaced.

Find where the water starts

Use the torch from outside the case. Describe the location as closely as you can: inside a pipe outlet, around a fitting, at a pump connection, or emerging from the cabinet. If the source is hidden, tell the repair company that rather than guessing.

A location guide, not an installation drawing. A to D identify visible parts; the arrangement on your boiler may differ.

From the top or bottom? Include that in the description, but also say which part is wet.

The six places to check

Check 1A service visit to check the valve and system pressure

Water from the relief valve pipe

Look at the outlet without touching it. Lochinvar’s Knight owner checks include the relief valve and its discharge pipe. Its manual says frequent weeping may mean the expansion tank is not working properly and needs a technician’s inspection.

That makes the boiler pressure reading useful information to send with the photograph. The separate expansion tank guide explains that part of the service check.

A valve that does not reseat. Lochinvar also tells owners to seek service if its Knight valve continues weeping after the manual’s lever test and cleaning attempt. You do not need to perform that test to identify an active leak.

Check 2A valve repair or replacement after isolation and draining

Water at the drain valve

Identify the drain before handling it. Weil-McLain’s WM97+ Series 2 manual identifies a boiler drain valve and uses it to empty the boiler. Its final position depends on the hot water piping arrangement, so do not assume every low pipe is a drain.

Tell the technician whether water is leaving the valve outlet or appearing around its connection. Leave it closed while you wait. Opening a drain to investigate changes the amount of water in the system; Weil-McLain warns that rapid water flow when releasing boiler pressure can cause injury.

This is a different part from the relief valve in check 1. Do not try to stop a relief discharge by treating it like a dripping tap.

Check 3A service visit, with the joint and its seal checked

Water around the pump flange

The bolted connection matters. On its flanged 007e stainless steel circulator, Taco requires the supplied O ring gaskets. It warns that using flat rubber gaskets instead can cause leaks.

If that joint is wet, describe it as the connection between the pump and the pipe. It gives the technician a specific place to inspect before deciding whether the pump itself needs replacing.

Leave the bolts alone. Taco’s service procedure requires power off, both isolating valves closed and the pump allowed to cool; without shutoff devices, the system is drained. Those are preparation steps for service, not instructions to undo a running pump.

Check 4A technician to repair the connection or replace a leaking vent

Water at a pipe joint or air vent

Follow the visible piping with your eyes. Weil-McLain tells the technician to check the WM97+ piping inside and outside the boiler and repair any leaks. Keep your inspection to exposed parts; leave the cabinet closed.

For the WM97+ automatic air vents, Weil-McLain’s service instructions specify replacing any leaking vents. A wet vent above the boiler therefore deserves its own mention when booking the repair.

Check 5Inspection of the drain, fittings and trap

Water at the condensate drain

A condensing boiler has another water path. Weil-McLain’s WM97+ manual identifies a flue gas condensate drain and a separate trap assembly. Lochinvar’s Knight owner checks cover the condensate drain line, PVC fittings and trap.

TWO SEPARATE WATER PATHSCONDENSINGBOILERATRAPBARelief dischargeFollow the pipe from the valve.Keep its outlet clear.BCondensate drainInspect the line and trap.Knight: keep the trap water filled.Simplified layout. The actual pipe positions depend on the boiler.

On a condensing boiler, the relief discharge and condensate drain are separate paths. Identify which one is wet before describing the fault.

Water carried away inside that drain is different from water escaping around it. Report a wet fitting, detached line or water beneath the trap as precisely as you can.

Do not bypass the trap. Lochinvar says its Knight trap must remain filled with water during operation to prevent flue gas escaping through the condensate drain. A drain repair must leave that protection working.

Check 6Internal diagnosis, with repair options after the source is confirmed

Water coming from inside the boiler

Ask for the source to be shown. Weil-McLain’s WM97+ manual lists heat exchanger cracks from hard water deposits and freezing damage among system problems to investigate. Its inspection also includes internal water piping, so water beneath the case alone does not identify a failed heat exchanger.

The useful question is which component is leaking and what the repair includes. Ask the technician to distinguish an internal connection or condensate problem from damage to the heat exchanger before you approve replacement work.

Why a small leak still needs fixing

Lochinvar’s Knight manual explains that continually adding fresh water brings minerals and oxygen into the system. It links mineral buildup to reduced heat transfer and heat exchanger failure, and oxygen to internal corrosion.

That is why repeated topping up is not a repair. Lochinvar also forbids petroleum based sealing compounds in this system because they can damage gaskets and seals. Give the technician the leak history, including any recent refill or service work.

After the repair, Weil-McLain’s WM97+ startup procedure checks for leaking pipes before proceeding. Ask the technician to confirm the repaired part stays dry and the boiler is ready to return to service.

What each one takes

Where the water starts	What the work involves
Relief valve discharge	Valve and pressure investigation, including the expansion tank
Drain valve	Isolation and a valve or connection repair
Pump flange	Safe isolation and inspection of the joint and gasket
Pipe joint or air vent	Connection repair or replacement vent
Condensate drain	Drain and trap inspection, with the gas seal restored
Inside the cabinet	Diagnosis before choosing parts or boiler replacement

Questions people ask

What should I say about a boiler leaking from the bottom?

Say whether you can see a wet valve, pipe or pump connection, or whether the water comes from inside the case. Send the wider photograph from Before you start so the repair company can see the layout.

What if the boiler leaks from the top?

Weil-McLain’s WM97+ service checks cover pipe leaks and leaking automatic air vents. The technician needs to find which part is leaking before choosing the repair.

Is the overflow pipe the same as the condensate pipe?

Do not use that name alone when describing the leak. Identify where the pipe connects: the relief valve in check 1 or the condensate assembly in check 5. The drawing shows the distinction.

Does this guide cover a steam boiler?

No. The worked examples are hot water boilers. Use the steam section of the boiler pressure guide for that system, and give the service company the steam boiler’s exact model.

Will replacing the boiler fix a leaking external pipe?

Ask for that pipe repair to be included explicitly. Weil-McLain’s WM97+ manual tells the installer to correct system problems before installation, including leaks that can damage the new boiler.

The other boiler guides: pressure, expansion tanks, radiator leaks and a boiler not working.

Sources: Lochinvar Knight 80 to 285 User's Information Manual KB-USER-07 and Service Manual KB-SER-08; Weil-McLain WM97+ Series 2 Boiler Manual 550-142-204/0119; Taco 007e stainless steel instruction sheet 102-563.