How to Fix a Boiler Leaking Water – 5 Causes & Their Solutions


Leaking and faulty boilers are a common household occurrence. 

Once a boiler starts leaking, you’re in a sticky situation because the leaky boiler is going to lead to expensive water damage to the structure of your home, damage to the boiler itself. If you don’t act quickly enough – it may even need a full replacement. 

Does that sound expensive?  Spoiler alert – it is.  The most commonly used type of boiler (natural gas) will set you back anything from $3 500 – $8 000 with installation.

Let’s take a practical look at boiler leakage and what you can do about it.

Why Is My Boiler Leaking Water?

Before we look at fixes, let’s first consider causes. Here’s the 5 main reasons why boilers typically leak water:

Corrosion (AKA ‘rust’ to you and me).  If the pipes are rusted water will escape, leading to a loss of pressure and inefficient operation.

★ Faulty Pressure Valves.  Your boiler is not meant to operate at a pressure of above 21 P.S.I.  If your Pressure Valve is broken and reading the pressure at above that level, it will purposefully leak water to avoid an explosion.

★ Temperature too High.  When your boiler temperature level is too high, it may begin to leak.  A possible cause for this is a fault with the temperature sensors in the boiler.

★ Pump seals.  The pump seals in your boiler are meant to keep the water where it’s supposed to be.  However, over time these can decay and particularly if your water pressure is too high, start to leak.

★ Damaged or loose joints.  The pipes, fittings and soldered joints around the boiler will age and can corrode, leaking water.

Like any other household appliance, a boiler will wear out eventually and require replacement.  Experts reckon the life of your average, well maintained boiler at 15 years or longer.

Is A Leaking Boiler Dangerous?

A boiler leaking water IS dangerous because it can cause the electric components in the boiler to short circuit, leading to the risk of fire. 

The water will also rust piping, leading to further problems and a more expensive repair. 

Added to that, you’re looking at damage to the structure of your home, like your walls and belongings, and the eventual growth of mold and fungi from the moisture, these are bad for air quality and lead to health problems such as flu like symptoms and asthma.

Can You Repair A Leaking Boiler? 5 Causes & Solutions

The answer to that question relies on what the source of the leaking is. Once you’ve followed the list above of possible causes and done some DIY diagnostics, here’s a helpful guide that will help you sort out or at the very least diagnose your issue.

It’s important to note that your first port of call should be a licensed HVAC (Heating, Ventilation & Air Conditioning) professional as you can risk doing further damage to the boiler or experiencing injury if you are not technically skilled.  Also, if your boiler is still under warranty, this may be voided by D.I.Y.

Without further ado, here’s how fix each of the main issues covered above. (Note, some of these can only be done by a pro. Yes, really!)

Corroded/Rusted Pipes or Unit.


Yes.  This type of repair requires skilled welding and soldering.  If the rust is widespread, it is often cheaper and safer to replace the whole unit.


Contact a licensed and registered HVAC professional.

Faulty pressure Valves.


An expert is never a bad thing, but try this first:


Follow the steps below.

  1.  Turn off the boiler completely – this means turning off the breaker specifically designated for the boiler.
  2. Shut off the fill valve that supplies water to the boiler system.
  3. Wait a few hours to allow the boiler and the water to cool completely.
  4. Place a bucket under the drain valves.
  5. Open the drain valve on the boiler and drain the water out. The water should be brown or black. While draining you should hear the sound of it filling back up at some point (your fill valve at work).
  6. Once the water being drained begins to run clear, stop dumping.
  7. This helps wash out any trash that could be causing a valve from not closing, causing a leak.

The temperature of the boiler is too high, causing a leak.


Probably.  Thankfully modern boilers have inbuilt mechanisms that for the most part won’t allow a boiler to overheat to dangerous/explosion level, but 10 – 15 people in the U.S still die every year from exploding boilers, so we need to be careful.  An overheating boiler could also be caused by a range of issues, which a professional should diagnose.

As far as checking your temperature sensors or thermistors goes, that’s a good first step and may diagnose your problem without the expense of an HVAC Engineer.

The test below can let you know if the thermistor is working. To see if it’s calibrated though, you’ll need to get a temperature resistance chart for the sensor and check it with the ambient environment.


  1.  Set the multi meter to resistance mode.
  2.  Hook up the leads of the multi meter to leads on the thermistor.  It doesn’t matter which leads go to the terminals as polarity is not important here.
  3. Heat a soldering iron or get a block of ice, or something that’s hot/cold. Heat/cool the thermistor by moving your hot or cold object to it.
  4. Note the multi meter reading as you are applying this heat.  A working positive temperature coefficient thermistor will show a smooth and steady increase in the multi meter resistance reading. 
  5. Look for signs of a broken thermistor.  A steady reading that doesn’t change, a reading of zero or a reading of infinity are all signs that the thermistor needs to be replaced.  In essence, if there’s a problem, the change in reading will not be smooth, or there will be no change at all.

Ineffective/Decayed Pump Seals.


Boilers rely on a centrifugal pump system.  If you feel comfortable tackling a technical task, read on below.  If in doubt, contact a reputable HVAC Engineer.

HVAC Expert: I would strongly caution homeowners before embarking on this repair. If the seals are bad, your pump is probably old and to get a professional to repair the seals on an old pump is money wasted on just getting a new pump. A new pump might also be easier to get seals for a specific old pump.


  1.  Disconnect the pump first.  Before you start, turn off the power and disconnect the pump from the electrical system.  There must be no possibility of the pump starting whilst you are changing the seal.
  2. Using a saw, cut the plumbing tube which is connected to the centrifugal pump.  Take the pump to a larger work space.
  3. Using a wrench, unscrew the pump bolts around the housing.  Remove the motor from the wet end.  You need to put the wrench over the motor shaft.  Then open the impeller by turning anticlockwise off the shaft.
  4.   Remove the seal – take the piece of the seal from the back of the impeller.  Now slide the remaining portion of the seal off the shaft.  Keep the new seal handy.
  5.  Replace the seal – take the new seal and place it on the shaft.  Slide the seal gently along the shaft without touching the front face of it.  The seal may not work properly if the front face of the seal gets body oil from your fingers on it, so be careful to touch the front face as little as possible.
  6.  Put everything back where it was.  The task of replacing the pump seal is over.  Now you need to put everything back together.  Use the wrench to hold the shaft.  Screw the impeller onto the motor shaft of the pump.  Align the bolt holes on the motor the wet side of the pump.  Insert the bolts first.  Take the wrench and secure the pump ends together.  Place the pump back in its original position.  Place the plumbing back and secure it using glue and primer.  Let it dry well (24 hours) before using the pump.  After this time, reconnect to the electrics, and your pump is ready for use again.  

Damaged or loose joints. 


Yes and no. 

It depends on whether the fitting that is leaking is a threaded fitting versus a soldered or welded fitting.

One with a standard thread is simple to buy a replacement and switch out. However if it’s soldered or welded, you’ll definitely need an expert for this. If it is threaded though, then read on.

Let’s address damaged joints in the piping or plumbing first.  If the pipe joints themselves are leaking because of damage, they likely require complete replacement, which means an expert.  Prior to calling in a pro, follow the steps below.


  1.  Turn off the water supply.  Stop the puddle of water from growing by turning off the internal stop tap of the boiler.
  2. Switch off the heating.  If you use central heating, switch off that system as well.  As with any other appliance, disconnect from other appliances before doing anything.
  3. Drain the boiler of any remaining water.  You can do this by simply turning on your household taps, and flushing your toilets.  Once no more water is coming out, you’re all set.
  4. Mop up any water to prevent slip and fall accidents and to prevent damage to your floors and furniture.
  5. Wait for the plumber to arrive and assess the situation and repair according to HVAC standards. 
  6. In the meantime, check with your insurer – some policies will cover damage from leaking pipes but you need to check.

If your piping joints are leaking because they’re loose, get out your toolbox and follow these handyman guidelines.  You’ll need an adjustable wrench, slip point pliers, and a wrench set.  Materials used:  flexible supply tubes, Teflon pipe joint compound, and Teflon tape.


  1.  Wrap your threaded joint adapters with plumber’s/Teflon tape, wrapping around the pipe clockwise.  Three layers is usually sufficient.
  2.  Smear a little pipe joint compound on the plumber’s tape.
  3. Using two wrenches, tighten the connection over the joint compound and plumber’s tape.
  4.  Lubricate the ferrule and brass ring on compression joints with joint compound.
  5.  Thread on the valve onto the pipe joint compound on the brass ferrule, the compound creates an effective seal.
  6.  Tighten the compression fitting.  Tighten compression fittings firmly with two wrenches to crimp the ferrule onto the pipe.  Make sure that the pipe or fitting goes straight (level) into the fitting.  Misalignment will cause a leak.  If it still leaks after you turn on the water, try tightening the nut further by one quarter turn.  This usually stops any persistent leaks. 

Further Advice

Hopefully the advice given above and your own plumbing ‘know how’ have saved you some time, money, and hassle!  To add further to this article, do see below for links to some instructional videos that may be of help to you.

Pressure Valves:

Temperature Sensors/Thermistors:

Pump Seals:


A leaky boiler can be a scary thing, but thankfully many issues can be solved with a bit of DIY.

I hope this quick guide has helped you find the issue, and given you the steps you need to solve it.

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Thanks for reading, and have a great day!

I've been helping homeowners with appliance repair since 2016. Starting out as an enthusiastic amateur, I've since worked with many Appliance, HVAC, and DIY experts over the last 7+ years. My mission is to help fix your appliances and prevent future issues - saving you stress, time, and money. Visit my author page to learn more! Read more