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Boiler Leaking Water from the Bottom: How to Fix a Boiler Leaking Water

A boiler leaking water from the bottom is an alarming sight that warrants a swift fix. A boiler leaking water consistently is cause for concern because it points to a fault with one of the boiler’s components. Any number of issues can cause a boiler leak because boilers are complex machines with many water-filled components. However, proper attention and maintenance allow you to correctly diagnose, mitigate, and minimise instances of a boiler leaking from the bottom (or anywhere else).

Boiler leaking water
Boiler leaking water from the bottom

The following are five key signs of a boiler leaking water.

  • Pools of water on or around the boiler
  • Drop in water pressure
  • Mould or staining near the boiler
  • Corrosion on the pipework
  • Error codes displayed on the boiler interface

The best course of action once a boiler starts leaking water is to call a Gas Safe engineer. Gas Safe engineers have the skills and tools to inspect the boiler leak and recommend the best course of action. The inspecting engineer may suggest either repairing or replacing the leaking boiler depending on the age of the boiler and the severity of the leak. Older boiler models are less efficient and more prone to leaking water due to wear and tear. There’s a strong case to replace rather than repair an old boiler once it is no longer covered by the warranty (especially if the leak is severe).

DIY fixes may temporarily stop a boiler leak. For example, using a boiler leak sealant to close any damaged seals can stop a leak and prevent the spread of further damage, such as to the Printed Circuit Board (PCB). However, calling a qualified professional is the only true way to handle a leaky boiler, even if a boiler leak can be contained with boiler leak sealant.

A boiler leaking from the bottom isn’t necessarily a dangerous situation, but it can become hazardous if left unrepaired. Boilers operate on pressure. If the boiler leak occurs because the pressure is too high, the boiler is at risk of exploding. Likewise, if a boiler leaks gas or carbon monoxide, the danger increases with potentially fatal consequences.

Whenever a boiler is leaking water from the bottom, the correct process is to turn the boiler off at the mains and immediately call a Gas Safe engineer. Do not use a leaking boiler unless absolutely necessary.

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How do I know if my boiler is leaking water?

You know your boiler is leaking water if any of the statements below are true.

  • Water pools on or around your boiler: Seeing water on or around your boiler is the most common sign of a leaking boiler. Water usually collects under the boiler, but it’s possible for pools of water to form on top of the boiler if the boiler is leaking from the top.
  • The water pressure drops: A boiler pressure drop is one of the first signs of a boiler leak. A boiler’s water pressure remains within a steady range while the boiler is operating. The pressure drops as soon as a boiler is leaking water.
  • There’s mould or staining near the boiler: Some boiler leaks start small, so the water loss may be imperceptible. The appearance of mould or rusty staining on or around the boiler is a key indicator of a leak that doesn’t manifest as pressure loss or water pooling.
  • There’s corrosion on the pipework: A boiler leaking water results in pipework corrosion. Corroded pipes indicate a potential boiler leak or damage resulting from an old leak that has since been repaired.
  • The boiler interface displays the corresponding error code: Many modern boilers have a digital interface. Issues or faults within the boiler have unique fault codes which show when there is a problem. A boiler leaking water will show a specific fault code depending on the make and model of the boiler.

The above are telltale signs of a leaking boiler, but only a Gas Safe engineer can accurately diagnose the problem. Thus, it is imperative to call a Gas Safe engineer to know for sure whether your boiler is leaking water.

What do I do if my boiler is leaking water?

The following five steps describe what you should do when your boiler is leaking water.

  1. Turn off your boiler: You must first turn off the boiler leaking water via the mains socket. After all, water and electricity are a dangerous combination.
  2. Check the pressure valve: Pressure build-up in a boiler can result in a boiler leak from the Pressure Release Valve (PRV). Check the PRV to make sure it is not letting through water because it is stuck.
  3. Check whether the filling loop lever is closed: Opening the filling loop (via a key or lever) allows water to flow into the boiler. Leaving the filling loop open causes too much water to enter the boiler, resulting in a boiler leak.
  4. Bleed the radiators: Allow the boiler to cool, then bleed one of the radiators in the property. Bleeding radiators reduces the excess pressure in the system. However, you should only bleed radiators once you’ve identified and fixed the root cause of the boiler leak.
  5. Call a Gas Safe engineer: A leaking boiler should always be checked by a Gas Safe engineer. A boiler leaking water is likely to cause more damage to unseen parts of the boiler. Leaving these unrepaired could cause additional and more costly damage to the boiler.

How to fix a boiler leaking water?

You should call a Gas Safe professional to fix a boiler leaking water. A qualified engineer is able to assess the extent of the problem and stop your boiler leaking water.

There are times when it is possible to use a DIY fix when your boiler is leaking water. However, it is essential to understand that any form of a DIY boiler fix is only a temporary measure. For example, a boiler leak sealant such as Fernox F4 stops minor boiler leaks quickly and cheaply. However, DIY sealant fixes on a leaky boiler only address the surface-level problem of temporarily stopping the leak. Sealant does not fix the root cause of the leak. That’s why you should request repairs from a qualified engineer as soon as practicable whenever you have a boiler leaking water.

Why is my boiler leaking water from underneath?

Your boiler is leaking water from underneath because of a leak from one of the components held within the boiler’s protective casing. Any leak from inside the boiler will run downwards due to gravity. Finding water leaking from underneath your boiler gives no further insight beyond the fact that there is a leak without a proper subsequent investigation. You should contact a Gas Safe registered engineer to identify the source of your boiler leak and fix the problem.

Below are the seven core reasons your boiler is leaking water.

  • Pressure issues: Water pressure causes leaks if it exceeds a certain threshold or if the PRV is stuck.
  • Boiler heating pump leaks: The boiler pump leaks water when its seals reach the end of their service life and no longer block water flow at the connections they protect.
  • Seized boiler auto-air vent valve: The boiler auto-air vent leaks water when its valve seizes and fails to close properly, thus letting water get by.
  • Faulty boiler connections: Boiler connections allow water to leak through if they’re installed incorrectly, or the joints have degraded due to wear and tear.
  • Heat exchanger corrosion: Corroded heat exchangers develop cracks through which water can leak out of the boiler.
  • Corroded pipes and soldered joints: Corroded pipes and soldered joints often crack and thus allow water to leak out.
  • Temperature too high: Excessively high temperatures result in high water pressure, which in turn causes a leak if the PRV is letting water through.

Pressure issues

Pressure issues are a common reason for a boiler leaking water from underneath. The pressure release valve is designed to vent excess pressure. Relieving excess pressure vents any excess water that has come into the system. The pressure release valve sometimes becomes clogged due to natural sediment accrual. Sediment causes the valve to remain open and thus leak water and lose pressure even when there’s no excess pressure to trigger the mechanism.

The ideal pressure range for a boiler is 1.3 bar. However, general guidelines state to keep boiler pressure between 1-2 bar. Pressure issues can arise when the boiler is overfilled, or the filling loop is left open.

There are some instances where a boiler leaking water due to pressure issues is not a great concern. For example, the outlet pipe is a seal-less section of pipework that is there to alleviate pressure build-up inside a boiler. The outlet pipe can drip small amounts of water, but the amount is negligible and does not require any repairs.

Any boiler leaking water due to sustained high-pressure problems should be inspected and repaired by a Gas Safe engineer. Learn more about high boiler pressure in our handy guide.

Boiler heating pump leaks

Boiler heating pump leaks commonly result in a boiler leaking water for two reasons. First, boiler heating pump seals deteriorate as the boiler ages. This slow degradation can go unnoticed until the boiler starts leaking water. Second, boiler heating pump leaks can arise due to sustained high pressure blowing the seals.

For more information, learn what causes heating pumps within boilers to leak.

Seized boiler auto-air vent valve

A seized boiler auto-air vent valve is a common reason for boilers to leak water. The boiler auto-air vent valve is located on the top of the boiler. The valve opens and vents excess pressure when pressure surpasses the internal threshold level. The auto-air valve does not close properly if the vent becomes faulty or blocked by sediment over time. The vent’s inability to close causes the boiler to leak water and lose pressure.

The boiler auto-air vent fault results in a boiler leaking water from the top. In some cases, this water leaks down through pipework and pools beneath the boiler.

Faulty boiler connections

Boiler connections cause boiler leaks for two reasons. First, boiler connections weaken over time, resulting in a possible boiler leak. When water is heated, it expands, and when it cools, it shrinks. This repeated expansion and contraction wears away at the pipework joints causing them to loosen over time. Second, boiler connections cause a boiler leak on freshly installed boiler units if they’re not installed properly. There is a chance that poor workmanship during the installation could result in a leaking boiler, although such an occurrence isn’t common.

Perform the three steps below to check if a boiler is leaking water through faulty boiler connections.

  1. Dry the pipework with a clean cloth: Drying the pipes removes any dampness or water condensate.
  2. Leave the boiler for a few minutes: Allowing the boiler to operate gives any leak to present itself when you return.
  3. Tighten the connections should the pipework be wet: A simple quarter turn will suffice.

Heat exchanger corrosion

Heat exchanger corrosion is a serious problem that often leads to a boiler leaking water from the bottom. Corrosion in a heat exchanger often causes the component to crack and allow water to leak out.

Boiler leaks stemming from heat exchanger corrosion generally necessitate a complete boiler replacement. A boiler leak due to heat exchanger corrosion is costly and it is often more cost-effective to replace an older boiler that’s not covered by a warranty. You can get a fixed price for a replacement boiler from Heatable and compare it with quotes from local installers.

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Corroded pipes and soldered joints

Corroded pipes and soldered joints occur as a boiler ages, resulting in water leaking from the bottom. Boilers do not last forever, and corroded pipes and soldered joints signify a boiler is approaching the end of its lifespan. On average, a boiler warranty covers five years, while the lifespan of most boilers is ten. Therefore, once a boiler is leaking water from the bottom due to corroded pipes and soldered joints, a full replacement is often recommended.

Temperature too high

Having the temperature too high in a boiler results in two problems. Firstly, the boiler produces scalding hot water from the taps. Secondly, the pressure within the boiler increases as the water temperature increases. Increased pressure due to the water temperature being too high leads to an increased risk of a boiler leak.

Many boilers have a temperature control valve (TCV) regulating water temperature. Should the TCV be faulty, the water will continue to heat up. However, the TCV can be adjusted manually. Turning the TCV needle anticlockwise decreases the water temperature threshold.

How to prevent a boiler leakage?

The following are five ways to prevent boiler leakage.

  • Schedule an annual boiler inspection. An annual service serves a dual purpose. Firstly, an annual service prevents boiler leakage by ensuring the boiler is in full working condition. Secondly, an annual service catches any small or unnoticed boiler leakage and corrects it before it becomes a larger or costlier issue.
  • Closely watch the boiler. Monitoring boiler pressure helps prevent boiler leakage, as a spike in water pressure is a common cause of a boiler leak.
  • Chemical flush treatment: A chemical flush treatment washes through the heating system to remove any limescale or general debris build-up. A chemical flush treatment prevents boiler leakage by reducing the risk of internal pipework corrosion and prolongs the boiler’s lifespan.
  • Magnetic boiler filter: A magnetic boiler filter helps prevent boiler leakage by collecting any metallic debris that would otherwise accumulate in the boiler pipework.
  • Replace an old boiler: Old boilers are more likely to leak. Replacing a boiler once it leaves its warranty period minimises the chance of experiencing a boiler leakage. The warranty cover the repairs should a newly installed boiler leak.
  • Consider boiler cover: Boiler cover does not stop your boiler from leaking. However, having a comprehensive boiler cover policy helps you get emergency repair service (with most costs covered) if your boiler develops a leak.

Boilers that often leak (with fault codes)

Below are the six boilers that often leak, along with fault codes that point to a boiler leak.

Boiler Make Fault Codes Warranty Period
Vaillant F22, F24, F13, F73, S41, S53 Up to 10 years
Ideal F1, L1, FD 2-12 years
Worcester Bosch A1, E9, CE207, H07 5 years as standard, but a 10-year warranty is available on some models
Baxi 117, 118, 125, E78, H02-06 E78, H02-06
Viessmann F0, F4 5 years
Alpha 10 5-7 years

What are the best new boilers to replace old leaky boilers?

Below are the five best new boilers to replace a boiler that’s old and leaky.

  • Worcester Bosch Greenstar 8000: The Worcester Bosch Greenstar 8000 is the ideal new boiler for a large family home. The Greenstar 8000 is a top-of-the-range boiler that saves money and the planet since it offers a 12.3 fl flow rate and 94% efficiency.
  • Viessmann Vitodens 100-W: A new Viessmann Vitodens 100-W boiler comes with a 12-year warranty. The Viessmann Vitodens 100-W is a pricier option. However, with 30-35kW power, it is suitable for most family homes. The extended guarantee also offers peace of mind that the boiler will last a long time. When picking a new boiler to replace an old leaky boiler, longevity can be a key deciding factor.
  • Worcester Bosch Greenstar CDi Compact: The Worcester Bosch Greenstar CDi Compact boiler is a small boiler that packs a punch. Thirty centimetres smaller than the Greenstar 4000, the Greenstar CDi compact is still suitable for a 5-bedroom home. Complete with a 10-year warranty, the Greenstar CDi compact proves that size isn’t everything.
  • Worcester Bosch Greenstar 4000: A popular choice when looking for a new boiler, the Worcester Bosch Greenstar 4000 is a solid option when it comes to replacing old leaky boilers. Compatible with LPG and natural gas supplies, the Greenstar 4000 offers 25-30kw power. A new Worcester Bosch Greenstar 4000 comes with up to 10 years guarantee.
  • Alpha E-Tec: The Alpha E-Tec 33kw new boiler is the best value boiler on the market. Replacing an old leaky boiler with a new boiler can be a big expense. The Alpha E-Tec offers a quality boiler at a reasonable price. The Alpha E-Tec also comes with a 10-year warranty and offers 89% efficiency with 33kW of power.

You can get on-screen quotes for the boilers listed above by filling in an anonymous questionnaire from Heatable.

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What factors determine the best replacement boilers?

Several factors are necessary to determine the best new boilers to replace old leaky boilers. The lifespan of a boiler can be upwards of 10 years. Therefore, a like-for-like replacement is rarely a sensible option. There are 5 key considerations when shopping for new boilers to replace old leaky boilers.

  • Cost: A new boiler costs money. There is a range of boilers on the market to suit all budgets. When considering the installation cost of a boiler, however, it is important to look beyond the purchase price alone.
  • Efficiency: The more efficient a boiler, the lower its running costs. Efficient boilers heat the water quicker and expend less energy in doing so.
  • Warranty: The warranty period attached to a boiler is a key consideration because it defines how long you will be covered against any boiler leaks or other damage.
  • Flow rate: The boiler flow rate determines how fast the boiler can send water to the different taps, radiators, baths and showers. Larger properties or those with higher demand must consider a boiler with a higher flow rate.
  • Power: The more powerful a boiler is, the more radiators it can heat and the more taps it can supply with hot water. Much like flow rate, larger properties need to consider the power requirements of a new boiler.

You can use Heatable’s boiler selection tool to find the ideal replacement boiler for your home if you’re not sure how to choose one yourself.

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Are leaking boilers dangerous?

Yes, a leaking boiler is potentially dangerous. The severity and location of the leak ultimately determine the level of danger. Here are three things to consider.

  • There is a strong chance the water is leaking into the electrical components if you have a boiler leaking water from the bottom. Usually, the electrical components within a boiler are sealed. To be safe, however, switch off your boiler whenever it is leaking water and call a Gas Safe engineer.
  • Leaky boilers pose a safety risk and, when not dealt with swiftly, may incur hefty repair costs. For example, replacing a damaged printed circuit board (PCB) costs in excess of £500.
  • A leaking boiler can also cause extensive damage to other areas of the property. Depending on where the boiler is located, a serious leak could cause damage to carpets, furniture, and other family belongings. The dangers associated with this damage spread likewise depend on the size of the water leak and the items damaged.

Can a boiler explode if it’s leaking water?

Yes, a boiler can explode if it’s leaking water. However, it is a very rare occurrence and does not happen without warning or signals of disrepair. A boiler leaking water is a common complaint. Usually, the fix is quick and simple. However, when the damage is extensive, or the boiler is old, a full boiler replacement is needed. There are a few outlier instances where a boiler leaking water can explode.

The leading cause of a boiler exploding is a severe increase in water pressure. When the pressure inside a boiler rises but is not vented, the boiler will explode. The three main reasons for a pressure-based explosion in a water-leaking boiler are below.

  • Faulty expansion vessel
  • Faulty auto air vent
  • Blocked pressure outlet pipe

Can a leaking boiler make you ill?

Yes, a leaking boiler can make you ill. However, it depends on the type and severity of the leak. Below we explain the different types of boiler leaks and their impact on an individual’s health.

  • A boiler leaking water does not make you ill: When a boiler leaks water and is dealt with quickly, there is no danger posed to an individual’s health. A leaking boiler is a financial burden but otherwise poses no health risks.
  • A long-standing boiler leak creating mould outbreaks can make you ill: When a leaking boiler goes unnoticed or unrepaired, it can result in mould growth. Prolonged exposure to moulds can be very hazardous to an individual’s health.
  • A boiler leaking carbon monoxide makes you ill: Carbon monoxide poisoning is a serious problem. A boiler leaking carbon monoxide causes serious health problems or even death.
  • A boiler leaking natural gas makes you ill: A natural gas leak in a boiler not only impacts an individual’s health but could also cause an explosion. Any suspected boiler gas leaks must be dealt with immediately.

While not every instance of a leaking boiler makes you ill, they are always serious and pose a threat that needs to be dealt with. Turn the boiler off at the mains whenever you suspect a leak. Disable the water supply via the stop cock, and turn off the gas supply to the property when you suspect a gas leak. Call a Gas Safe engineer and wait for their inspection before using the boiler again.

Can I use my boiler if it’s leaking?

Yes, it is possible to use a boiler if it is leaking, but it is not a good idea. When a boiler starts leaking, it means there is a problem that needs to be addressed as soon as possible. Boilers operated based on pressure. As soon as a boiler starts leaking, the pressure drops, and the boiler begins to operate less effectively.

Something as simple as a damaged seal can cause a boiler leak. However, if left without repairs, or if the boiler continues to be used before a repair can be carried out, there is a chance that the leak could worsen and cause more extensive damage.

When a boiler is leaking, turn the boiler off at the mains, and switch off the water supply at the stop cock. Then, contact a Gas Safe engineer and wait for them to assess the extent of the damage.

What can I do when my boiler is leaking heavily?

You should shut down the boiler via the main switch and also disconnect the mains water supply via the stopcock when a boiler is leaking heavily. Some properties have more than one stop cock. When a boiler is leaking heavily, it needs to be inspected and repaired. Shutting down the system limits the extent of the damage. As the system drains, the leak should slow and stop.

The extent of the damage caused when a boiler is leaking heavily is dependent on the cause and how long the leak has been undetected. Calling a Gas Safe engineer is the only safe way to deal with a heavily leaking boiler.

Why is my boiler dripping slowly?

Your boiler is dripping slowly because it most likely has a leak. A leaky boiler does not always mean water is gushing at a fast rate. A boiler leak often begins as a slow drip. A common cause for a slow dripping boiler leak is a failure in the expansion vessel or a fault with the auto air vent.

Boilers require a specific and balanced amount of pressure to function correctly. When the pressure is too high, the expansion vessel kicks into action and absorbs some of the excess pressure. The trapped pressure is then ejected via the auto air vent. When either of these fails, the pressure remains in the boiler causing a slow dripping leak.

Why is my boiler leaking water when it rains?

Clogged rainwater pipes or a blocked drainage system commonly cause a boiler to leak when it rains. Many condensing boilers’ condensate pipes connect to the home’s rainwater pipe. During periods of heavy rainfall, drains and rain pipes are prone to becoming blocked.

Leaves and other natural detritus easily cause blockages to external pipework and drainage systems. The resultant backflow of rainwater can flood the flue and, subsequently, the boiler. You need to take action if your boiler is leaking water when it rains. A flooded boiler is a very serious problem. Often the boiler cannot be salvaged, leaving you needing a full boiler replacement.

Installing an external air break into the condensate pipework before it connects to the main drain pipe is the best way to reduce the chance of your boiler leaking water when it rains.

Why is my boiler leaking water from the top?

A faulty auto-air vent is the main cause when a boiler is leaking water from the top. The auto-air vent is located at the top of the boiler. If you fear a leak at the top of your boiler, the auto-air vent should be the first component to check.

When the pressure in your boiler is too great, the auto-air vent automatically opens. It vents air to reduce the pressure. The auto-air vent can become faulty and stop closing completely over time, causing the boiler to start leaking from the top.

Why is my boiler leaking water after service?

Faulty installation of a part is the most common reason for your boiler to be leaking water after a service. Depending on the level of work involved in the service, there is a chance some seals or other components may have been replaced due to age or general wear and tear. If the replacement parts are not correctly fitted, the boiler is liable to start leaking.

Incorrect pressure levels are another reason for a boiler leak after a service. If your boiler is repressurized too high, excess stress will be placed on the system. For example, a boiler leak could come from the pressure release valve venting to reduce the internal pressure. Or you could see a boiler leaking water because the pressure stresses the seals and joints, causing them to become damaged and leak.

A boiler is a complex unit built from many essential parts. A boiler leaking water after service could be nothing, or it could be a sign of a serious fault. Any sign of leak warrants calling a Gas Safe engineer.

Why is my boiler leaking water from the flue?

There are several reasons why your boiler is leaking water from the flue. One of the most common causes for a boiler leaking water from the flue is boiler plume gases condensing as they leave the boiler. If the angle of the flue is not correct, this condensate gathers and corrodes the flue resulting in a leak.

A boiler leaking water from the flue is a common problem for most modern boilers. Modern-day boilers are condensing boilers, which means the exhaust heat from the boiler is cycled back into the system to pre-heat the water going in. Condensing boilers are more efficient and use less energy.

The downside of a condensing boiler is that the plume gases that exit via the flue are colder. This lower temperature often results in the formation of highly acidic condensation. A leaking boiler flue comes about when the acidic condensation damages the structural integrity of the flue.

Does a boiler leaking water lose pressure?

Yes, a boiler leaking water always loses pressure. While there could be other reasons for a drop in pressure, the pressure will drop anytime a boiler starts to leak. Even a minor boiler leak causes a drop in pressure, impacting your boiler’s performance. A Gas Safe engineer should inspect any boiler with consistently low pressure or signs of a water leak. Early intervention is crucial to prevent further damage to your leaky boiler.

Why is my pressure release valve causing a boiler leak?

Your pressure release valve is causing your boiler to leak because the pressure has climbed too high. Water pressure in your boiler should be between 1.2-2.0 bar, with 1.5 considered normal. If your water pressure climbs above 2.0 for a sustained period, the pressure release valve is at risk of blowing. When this happens, the valve stops functioning correctly, resulting in a boiler leak.

Replacing the damaged pressure release valve will correct the leak. However, you will need to be mindful of your water pressure to prevent a recurrence.

Can a boiler leak from internal parts or the central heating pump?

Yes, a boiler leak can come from the internal parts as well as from the central heating pump. A cracked heat exchanger is one of the more common faults that cause a boiler leak.

Always check the central heating pump when you have a boiler leaking water because the heating pump can, over time, work itself loose, or the integrity of the seals can decline, resulting in a boiler leak.

Can I replace the corroded parts that are causing my boiler to leak?

Yes, you can replace the corroded parts that are causing the boiler leak. However, only do this when it is cost-effective to do so.

Replacing the corroded parts is a viable option for a leaky boiler less than five years old. Most boilers have a five-year guarantee, so any damage or repair work should be covered. Regardless, replacing the corroded parts of a boiler under five years old will often be the cost-effective solution.

However, it’s important to remember that repairing a boiler leak could extend beyond replacing the corroded parts that caused the leak. Having a Gas Safe engineer check your boiler ensures there is no water damage or secondary issues that need attention.

Replacing the corroded parts that caused your boiler to leak only to be hit with a £500 bill because the printed circuit board (PCB) was also damaged is a costly mistake nobody wants to make.

How do I fix leaky boiler pipework?

There are many ways to fix leaky boiler pipework depending on the underlying cause of the leak. For example, the fix is relatively straightforward if your leaking boiler is because of degradation in the soldered joints. However, should your leaky boiler be due to problems with faulty pipework, the issue becomes far more complex.

Regardless, the best way to fix a leaky boiler is to call a Gas Safe engineer. They check everything and are required to ensure all boiler pipework is up to standard. This may mean performing other adjustments indirectly related to the initial boiler leak.

Why has my leaking boiler stopped leaking?

The most common reason why your leaking boiler stopped leaking is that your heating system has been turned off. When you use your heating, the heat increases the pressure in your system, exposing the leak location. When your leaking boiler is turned off, the system cools down, pressure drops, and the leak appears to stop.

Below are four other questions to ask yourself when your leaking boiler has stopped leaking.

  • Has it been raining heavily? An incorrectly fitted flue is the likely cause, and water is leaking back down the flue and into the boiler. It’s not meant to do this!
  • Does this leak happen sporadically? The problem is likely with the pressure release valve or the auto air vent. They are designed to expand and contract to control the boiler’s pressure. If there is a fault with either of them, it can cause a leak.
  • Have you recently topped up the boiler’s pressure using the filling loop? Over time filling loops degrade like all other parts on a boiler. Check to see if there is a visible leak on the filling loop.
  • Is the water leaking from a pipe? We’ve visited properties where the pipework from the pressure release valve has been fitted incorrectly or simply not tightened enough. If this is the case, this could be the source of the leak.

How much does it cost to fix a leaky boiler?

It will cost between £300-£500 to fix a leaky boiler. There is no exact price, as the eventual cost of boiler repairs depends on a number of factors.

A simple fix, such as an external filling loop repair, will cost around £95-£125 to fix, including parts and labour. Replacing a standard external straight combi-filling loop on a leaky boiler will take around 2 hours.

The fix for a more serious issue, such as a damaged heat exchanger, runs anywhere between £300 and £500 for parts and labour. It is a more expensive leaky boiler fix. In addition, it takes approximately 8 hours to fix a damaged heat exchanger.

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  1. Kevin Molyneux says:

    The cold water pipe going to my boiler is leaking can this affect the pressure

    1. Darren coppard says:

      The boiler is dripping water from the bottom corner not a constant drip but dripping,I’ve turned it off any help would be appreciated thank you

    2. heatingforce says:

      Any leak will result in pressure loss.

      There’s a few posts on pressure/leaks here: https://heatingforce.co.uk/blog/boiler-fault-finding/

  2. Claudine Chapman says:

    Hi my boiler has developed a small ‘drip’ overnight. Heating (using Hive) was actually off. Turned it all back on, but one small drip from the underneath of the unit and it’s wet in the front left bottom corner of the boiler. Using a bucket. Have a homecare agreement but with the nightmare weather, can’t get an appointment for another week+ – my question is, it is safe to have the heating on. All seems to be working, but just worried about the water drips. Only serviced last Saturday with parts replaced. Thanks.

    1. heatingforce says:

      I can’t advise on safety over the internet for obvious reasons.

      What parts were replaced? I think I’d stress the fact that to the engineer & get them out as soon as possible.

      To stop the leak you can open the bleed valve on radiators which will release water. You’ll need towels to protect floor/carpet & something to catch the water. By releasing some water, you’ll drop the pressure, and that should be enough to stop the leak.

      Alternatively, you could drain the whole system if you know where the drain valve is.

  3. Elaine says:

    Boiler is leaking after releasing some water into it to increase pressure last night. Is it possible that we just released too much water and the leak will stop once it has balanced out?

    1. Alex Ion says:

      It could be that something has damaged inside. It might be that the leak will stop, but it’s something I’d keep an eye on.

  4. Duanne Mahon says:

    My automatic air valve is leaking water on a Glow Worm CX30. Can I replace the valve myself and if so approx cost of replacement please?

    1. Alex Ion says:

      I guess it depends how good of a DIYer you are 🙂

  5. Christine Baker says:

    Hi ,I have a valiant boiler and it’s only leaking water when the heating is switched on from the left hand bottom ,any ideas what this is ,also the pressure is fine ?

  6. Hill says:

    My Vokera combi boiler stopped working. Repair done but is still,leaking from bottom. Boiler bone dry inside , bolder working perfect , but floor around is still quite wet when boiler first comes on in the morning. Help

    1. Alex Ion says:

      Call the people who installed it to check.

  7. John says:

    Why would there be water on the floor around my boiler only sometimes? I’ll check other times and it has dried up.

    1. Alex Ion says:

      Because the network may have ups and downs and not constant water pressure.

  8. Daz says:

    I lost pressure on our combi boiler so re pressurised it. Whilst doing this I notice the valve behind the pressure gauge was crusted up. After turning it a bit I’ve noticed a drip every 30 seconds or so. Pressure and system are working fine and this is the first time I’ve had to do this,is this a problem and shall I get someone out or is there anything I can do myself? Thanks

  9. Avtar says:


    A very good article, actually gives a solution, why FERNOX F4 surely, any water resistant, leaking sealant should work, i know Fernox f4 is the best and excellent, it is only, about 5 pounds to buy, a good price.

    This might be a solution however you still have to find the leak, which would be difficult for an amateur to find, and even if he does, there might be other leaks, that he has not seen. So it can be tricky.

    My boiler was leaking from the bottom Valliant, i find three leaks, small ones coming from the heat exchanger, so i put some water proof sealant on it, two layers, it was Evo Stick Sticks, like Sht. It did not stop the leak, however did reduce it, i was lucky, i managed to find the leak area, if it was, elsewhere, and hard to get too, no chance for me. As legally you are not allowed to work on boilers, unless you are qualified, which makes sense.

    A leak fix, is a small, risk, so i had a go. It has helped a lot.

    An excellent article.

  10. Anna-Sophitia Joleigh Ashford says:

    I noticed my boiler was dripping water from the bottom, I called to have it repaired but the only appointment they have is for next Thursday. I don’t know anything about boilers, I’ve put a bucket underneath it but I don’t know if I can use my boiler in the meantime.

  11. Derek says:

    My Grant combi boiler was losing water and pressure to the extent I needed to top it up 3 times in one day. I called out a plumber but by the time he arrived the leak had stopped and the garage floor was dry. That was 3 days ago and everything is still okay.

    Any ideas as to what is happening here please? Should I just keep an eye on the situation or is there something else to do at this stage?

    1. Alex Ion says:

      Please don’t think the boiler fairy is going to protect you for the winter. It may be that you will face the same problem in the coming weeks, or months, or days. Have a knowledgeable engineer look into it and tell you what may be the cause of it.

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