Why You Need To Keep Adding Water to Your Boiler

If you have to keep adding water to your boiler, it is usually due to faulty air valves, a leak in the heating circuit, or a leak in the boiler itself. 

Basic steam boiler maintenance always involves regularly checking the water level and topping it up when needed. But while boilers need regular maintenance, it’s not normal for your boiler to run out of water constantly.

In this guide, we take you the reasons why your boiler keeps running out of water, and how to identify and fix each problem.

Reasons Why You Need To Keep Adding Water to Your Boiler

For steam boilers to work efficiently, there must be a minimum amount of water in the system at all times. Once the water starts dropping below this level, it needs to be topped up.

If you notice that your boiler needs a top-up more frequently than usual, it means that

  1. There are faulty air valves in the system
  2. There is a leak in the heating circuit, or
  3. There is a leak in the boiler itself.

A few years back, topping up the water in the boiler had to be done manually. Fortunately, automatic water feeders now exist.

Also read how to manually check the water level in your boiler and how to top it up here.

These water devices sense when the water level is too low and automatically lets more water in (often using a pump). 

One downside of using an automatic water feeder is that it becomes easier to miss when your heating system starts using more water than usual.

When the automatic water feeder kicks in you can usually hear it as it refills the boiler.

If the frequency of this happening increases, there may be a fault with the automatic feeding system, or it can point to any of the problems explained below.

1. Faulty Air Valves

Boilers work by heating the water in the boiler until it turns to steam.

This steam then rises and travels through a system of supply pipes to radiators at different locations around the house. 

At these radiators, the steam releases its heat, transferring it to the radiator which then transfers the heat into the room via convection.

Once it loses its heat, the steam condenses into water and travels back down into the boiler where the cycle is repeated.

During each heating cycle, air has to be released from the circuit of pipes to allow the steam to travel through to reach the radiators.

This air is released through air valves or air vents that let air out, allowing the steam to replace the vented air.

These air valves are supposed to close once the air has left so that steam can fill the radiators.

But, if these air valves have developed a fault, they don’t shut properly and allow steam to leave the heating system as well, instead of just air.

The loss of steam means water is being lost from the system during every heating cycle. As a result, you have to add water to your boiler more frequently than normal.

If your heating system has this problem, you can easily diagnose it by looking at the air valves in the radiators around your home.

Leaking valves will occasionally hiss steam or spurt out water. If you have multiple valves leaking, or a large leak at one valve your boiler will lose water even faster.


Faulty air vents can rarely be fixed and usually need to be replaced.

Sometimes, the vents may be serviceable. You can servicing these vents by removing the cover and cleaning out debris. Then wait until the next heating cycle to see if the leak has stopped.

For the air vents that are not serviceable, they will need to be replaced.

This is most common for air vents that fail due to old age. If this is your case, you may benefit from changing all the vents in the system at a go instead of waiting for them to fail one at a time.

2. Leak in the heating circuit

Most steam boiler heating systems either have one-pipe or two-pipe systems to move the steam around your home, creating a circuit with your radiators.

In one-pipe systems, the same pipes that carry steam to the radiators also take water back into the boiler after the steam has re-condensed into water. 

Two-pipe systems return the water through a different set of pipes. These return pipes are also called the condensate line

A boiler can begin to lose more water than normal if there is a leak in this circuit of pipes. This leak can be from loose connections or broken pipes.

In two-pipe systems, this leak is more likely to occur along the condensate return line. These pipes may be visible in the basement or may be hidden within walls or even under the basement floor.

To check for leaking pipes, look for signs of water damage on walls such as patches of discoloration or flaking paint on the walls, floor, and even on the ceiling. 

Check exposed pipes for wetness. Sometimes, the wetness may be due to normal condensation, not a leak. To confirm, wipe the pipe dry and place a flat plate underneath to check if the pipe is dripping.


Broken pipes need to be replaced. If you notice leaks in the heating circuit, call in a plumber to change the pipe or tighten loose connections.

3. Leaking boiler

If there are no leaks in the piping and the vents aren’t hissing steam or spurting water, then it means there is probably a leak in the boiler itself.

Boiler leaks can be from parts that have loosened or corroded over time. These problems can also occur because of poor maintenance but are mostly due to the age of the boiler.

Sometimes, you may see a visible drip from beneath the boiler. This is an indication that your boiler is leaking. 

Another sign of a leaking boiler is steam (white smoke) coming out from your chimney when the boiler is running.

If you suspect a leak in your boiler, you should call a professional as soon as possible.

Depending on the extent of damage, some parts may need repair or replacement. In extreme scenarios, the entire boiler itself may need to be replaced.

It is not advisable to remove the boiler casing or to attempt repairs on the boiler by yourself unless you are a professional.

If you are having difficulty identifying the cause of the water loss, call a professional to use their trained eye to inspect the boiler as well as the entire heating system.

Frequently Asked Questions

How Often Should a Boiler Need Water?

When working well, a boiler will typically need very little new water because water is constantly being recycled within the system. 

Adding new water to the boiler means oxygen is being added as well. This exposes the iron components to rust, reducing their lifespan.

A boiler’s lifespan can be significantly reduced when there is excessive water loss and re-filling.

Can A Boiler Run Out of Hot Water?

A boiler can run out of hot water. 

All boilers lose some water as they work. Even without a leak or fault in the boiler system. If the water level is not topped when needed, the water can run out, causing it to dry fire.

Not only will this cause your boiler not to heat your home as it should, but it could also be damaging to the boiler itself and can even cause the boiler itself to become a fire hazard.

You can prevent this from happening by installing a Low-Water Cutoff (LWCO) device on your boiler.

This simple device prevents potential hazards by monitoring your boiler’s water level and shutting off the power supply to the appliance once the water gets below a certain safety level.

How Can I Find A Leak in My Boiler?

Sometimes, finding a leak in your boiler is straightforward. You may see a puddle of water beneath the boiler or an exposed pipe. You may also see a damp patch of wall or floor.

Other times, it may require some more effort to identify a leak. 

One way to confirm that there is a leak in your heating system is to shut off the boiler, leave it off for a while and check to see if there is a pressure drop. If the pressure drops, there is most likely a leak somewhere.

To identify the leak, turn your boiler on and turn up the thermostat to the highest.

While the boiler is running, check the floor around the boiler and any exposed piping (especially at joints). Check the radiators and their air vents in each room, looking out for signs of leaks.

This should make diagnosing a leak easier. However, if you still can’t find the leak, get a plumber to take a look. 

Russell Singleton

Russell has a Bachelor of Science (Environmental and Marine Geoscience) with Class I Honors. He is currently completing his doctorate in science and is passionate about all earth processes, especially isotope geochemistry and paleohydrology.

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