Leaking Residential Boilers: Is A Waterlogged Expansion Tank To Blame?

Posted on: 18 August 2022

Residential heating boilers are generally very reliable pieces of equipment, so it can be quite alarming if your boiler suddenly starts leaking water. Leaking boilers can cause catastrophic water damage, and will increase your water usage bills as they take in more water to replace the lost liquid. If your boiler is leaking, finding and fixing the cause of the leak as quickly as possible can save you a lot of money.

Waterlogged expansion tanks are a common cause of leaking in residential boilers. If your boiler is leaking due to a problem with the expansion tank, you should call in a residential boiler repair service to repair or replace the expansion tank before any serious damage is done.

What Are Boiler Expansion Tanks?

When a boiler heats its supply of water, the volume of the water expands, raising the internal water pressure of the boiler system as a whole. If this heated water were allowed to expand unimpeded, the extra pressure would place a massive strain on the boiler's pipes and radiators, potentially causing them to burst.

Expansion tanks are designed to accommodate this extra volume of water and protect the boiler's components from pressure damage. They are filled with inert gas (usually nitrogen), which is compressed as the extra water volume flows into the expansion tank.

Older boiler models may be fitted with basic, steel expansion tanks, which allow water to come into direct contact with the inert gas. Newer boilers have diaphragm tanks, which contain a rubberized bladder that contains the inert gas.

How Do Expansion Tank Problems Cause Leaks?

If you have an older boiler model with an expansion tank that doesn't contain a diaphragm, inert gas from the tank will be absorbed into the water every time excess water volume flows into the expansion tank. This reduces the amount of gas within the tank over time, which allows more water to flow into the tank.

When one of these expansion tanks loses most or all of its gas supply, it will become filled with water. Waterlogged expansion tanks can no longer relieve excess water pressure. When a boiler with a waterlogged expansion tank reaches high-pressure levels, its pressure release valve is forced open, causing excess water to spill from the valve.

Diaphragm expansion tanks were designed to prevent this problem from occurring. However, if the tank's internal diaphragm is burst or punctured, water will mix with the tank's inert gas supply, causing similar problems with waterlogging. 

What Should You Do If A Waterlogged Expansion Tank Is Causing Boiler Leaks?

If your boiler is venting water from its pressure release valve, check the boiler's pressure gauge or digital readout — if the pressure shown is excessively high, a waterlogged expansion tank may be causing the leak. Deactivate the boiler as quickly as possible, and call in a professional residential boiler repair service to inspect your boiler's expansion tank. 

If your boiler has an old-style steel expansion tank, it will need to be repressurized and refilled with a new supply of inert gas. If the boiler model is compatible with diaphragm tanks, your repair service may recommend replacing the old steel tank with a diaphragm tank — this can be a very wise investment, as it will minimize the risk of future waterlogging problems.

If your system already has a diaphragm tank, its internal diaphragm has probably failed due to a manufacturing defect, and the tank will need to be replaced. If your boiler is still under warranty, the manufacturer may be liable to cover the cost of purchasing and fitting a new diaphragm tank.

Reach out to a residential boiler repair service to learn more.