Home Improvement

Boiler Leaking Water (Most Common Reasons)

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A leaking boiler, be it a combi or system, is never good. Still, that doesn’t mean all hope is lost and the appliance is no longer usable. It all comes down to the size of the leak. If it’s small, chances are you can fix it yourself. So, if you

  • Have an old boiler which has sprung a leak, but still manages to work perfectly
  • Wonder whether the unit can still be used or not
  • Feel stuck in a dilemma between repairing the old boiler and buying a new one

Then read along, as in this post, we will discuss the most common boiler problems…

Table of Contents:

Pressure valve leaking

Usually a boiler’s pressure levels should be around 18 or 21 PSI (pounds per square inch). If the pressure exceeds this level, it might result in a leak from the relief valve. That’s a good thing as it guards the boiler from rupturing.

There is another instance where the valve might leak and that is when it’s opened for a first time in many years. Open and close it a few times, so it would seat itself. If there is still a leak after a day or two, you have two options:

Replace the pressure valve

  1. Turn off the gas supply to the water heater.
  2. Close the cold water cut-off valve (the one which goes into the water heater).
  3. Open both the pressure relief valve and the valve on the bottom. This will drain a little water out of the tank to relieve the pressure.
  4. There is an overflow pipe from the pressure relief valve, remove it. If it’s been glued on, cut if off.
  5. Unscrew the old pressure relief valve.
  6. Wrap the threads of the new valve with teflon tape. Screw it in whilst orienting the overflow pipe away from the tank.
  7. Reattach the valve drain pipe. Use Teflon tape on its threads.
  8. Open the cold water cut-off. Now check for any leaks around the relief valve.

Bleed the radiator

  • Close the filling loop tap, if it’s been opened
  • Find a radiator key
  • Use this key to loosen the nut at the end of your radiator
  • Put a bucket underneath the nut, to catch the water
  • Tighten the nut back

You may need to do this on every radiator on your property, if the pressure level doesn’t drop.

If the pressure is still too high, then you might have a limescale buildup inside the boiler. That will require professional assistance in order to be removed.

Temperature valve leaking

If you’ve noticed water leaking from this valve, that means the temperature is too high. The possible reason is a faulty temperature probe which needs to be replaced.

Leaking due to corrosion

Over time, corrosion can build up on all types of metal machines and appliances, and your boiler is no different. Corroded pipes inside and outside the boiler body will result in leaks. Corrosion builds up inside the tank, as well.

If only one component has become rusty, a professional will easily be able to replace it, and end the leak problem. Some parts, like the heat exchanger, are more expensive than others. So, the repairs will ultimately be too costly and not worth doing. If the damage is too great, or the spare part is too expensive, then it’s better to replace  the boiler altogether.

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Leaking from cracks

Besides corrosion, your boiler can develop cracks, as well. All that warming and cooling of water causes the metal to expand and contract. In time, splits or cracks can form. Much like rust, cracks are easily visible. Sadly, there is no magic tape to patch up this problem.

Another kind of leak directly resulting from contraction and expansions of the boiler is loose joints. Fortunately, all you need to do in this case is check the fittings and pipes and tubes, leading in and out of the system. If you find anything loose, tighten it up.

Leaking from seals in boiler pump

Another thing, which can break down in time, are the rubber seals. They can be destroyed by elements, which move through the heating system. Or they can become hard to the touch and no longer serve their purpose. This usually happens to old boilers, but new ones can also get problematic if the system has been running under pressure for too long.

Also, sometimes, the boiler pump can loosen itself, which would spawn a leak. This means that it needs to be put back in its place.

Problem with expansion tank

This part of the boiler can be either internal or external depending on the model. If it is external, you will be able to see the problem right away. Leaks usually occur when the boiler burner has been working for a long time.

Leaks around pipe fittings

This issue is very common with new boilers and it’s not the unit’s fault, but rather the technician’s, who has sealed poorly the unit during the installation.

Most common questions on leaking boilers

Is a leaking boiler an emergency?

The leak will only get worse and pose a threat not only to the unit itself, but also to your home. A constant leak can cause corrosion to your boiler and even a short circuit. It can also cause mould in the room, where the unit is kept. That’s why you should take care of the leak or call a professional as soon as possible. 

Can I use my boiler if it’s leaking?

Depending on where the leak is, you might be able to use the unit. However, more often than not, the pressure will drop drastically, due to the leak, making the unit unusable for the time being.

Why is water leaking from my boiler when it rains?

Water can get inside the boiler through a poorly installed flue. In this case, you will see leaks only during heavy rainfalls. Still, it’s a problem you shouldn’t ignore.

The boiler stopped leaking on its own, why?

If you’ve stopped the heating, then that is likely the reason why. When you do that, the pressure inside the system drops and the leakining stops. When you turn the heating up again, you can be sure the leak will return, as well.

Another reason could be a badly fitted flue, like mentioned in the previous paragraph.

Should I turn off a leaking boiler?

Yes, hopefully the leak will stop, as well, until help arrives. In the meantime you should place a bucket or another kind of plastic container below the leak, so the water wouldn’t drop on the ground.

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Hire a professional

If you’re still not sure where the leak is coming from and how it can be fixed, then it’s better to leave the repair job to a professional. Fantastic Services works with certified and fully equipped London tradesmen, who can tackle any boiler issue and have your unit back on track in no time.

You don’t need to waste your time on trying to figure out what’s wrong with the appliance when, quite frankly, you should call a certified engineer to see to the problem anyway. You can also consider the idea of getting a boiler cover care plan and save yourself the stress of dealing with unexpected breakdown.


  • Boiler leaks don’t make the unit unusable, but they need to be addressed as soon as possible.
  • Correct pressure levels if you can!
  • Place a bucket under the leak, until the problem is resolved!
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