Where should the expansion tank be installed on a boiler​?

If your boiler system is experiencing pressure issues, knocking sounds or leaking valves, the problem could be a wrongly installed or failing expansion tank.

The expansion tank plays a vital role in absorbing excess pressure when water is heated. But for it to work correctly, it needs to be installed in the right location. Whether you're upgrading your heating system, diagnosing a problem or installing a new boiler from scratch, correct placement of the expansion tank is essential.

In this guide, we’ll explain exactly where the expansion tank should be installed, why it matters and how to avoid common mistakes that could cost you time, money and comfort.

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Signs of a Poorly Placed Expansion Tank on a Boiler

A badly installed expansion tank can cause serious issues in your heating system. If the tank is in the wrong location, not secured properly or installed at the wrong angle, it may fail to control pressure effectively. Here are the key warning signs to look out for:

1. Sudden Banging or Knocking Sounds

This is often caused by hydraulic shock when water pressure builds up and has nowhere to go. It usually happens when you turn off a tap or the heating switches off.

2. Leaking Around the Tank or Pipework

Leaks after installation can mean the tank is on the wrong side of the system, has poor connections or is failing due to pressure imbalances.

3. Pressure Spikes on the Gauge

If your boiler pressure climbs quickly after heating starts, it may be a sign that the expansion tank isn’t absorbing excess volume properly.

4. Water Surging from Taps

An over-pressurised system can cause an initial burst of water when you open a tap, especially in systems that heat water for both heating and domestic use.

5. Faulty Relief or Bleed Valves

If bleed or pressure relief valves are constantly releasing water or not working as expected, it could mean the expansion tank isn’t regulating pressure correctly.

6. Dull Thud When Tapped

A healthy expansion tank should sound hollow when tapped. A full, waterlogged tank will sound dull and solid, which means it may be failing.

7. Popping or Cracking in Pipes

Unusual noises, especially when the boiler shuts off, may indicate that excess pressure is stressing the pipework due to a malfunctioning expansion tank.

Which side of the boiler is right for the expansion tank?

The expansion tank should always be installed on the supply side of the boiler system. This is the part of the pipework where heated water leaves the boiler and flows into the central heating system.

Installing the tank on the return side, where cooler water comes back into the boiler, can prevent it from functioning properly. The tank needs to respond to pressure increases caused by heat, so placing it on the hot side ensures it can absorb expanding water effectively.

For best results, the expansion tank should also be installed:

• Between the boiler and the circulator pump, if one is present on the supply side
  
  
• Above the pressure relief valve
  
  
• At least 18 inches away from the boiler's vent connector
  
  
• In an upright position, unless otherwise specified by the manufacturer
  
  

Getting the placement right helps maintain stable pressure, reduces strain on components and extends the life of your boiler system.

Can I install an expansion tank on its side?

Most expansion tanks are designed to be installed vertically, with the air valve facing down. This position helps maintain proper pressure control and allows the internal diaphragm or bladder to function as intended.

Some models can be installed on their side, but only if the manufacturer clearly states that horizontal installation is supported. If it's not designed for this orientation, placing it sideways can cause the diaphragm to shift, wear out unevenly or trap water in the wrong part of the tank.

Before installing a tank horizontally, always check:

• The installation instructions provided by the manufacturer
  
  
• Whether the model is rated for multi-position mounting
  
  
• That it is fully supported and not left hanging from pipework
  
  

Unless you are using a tank made for side installation, it’s always safest to mount it upright with proper support brackets.

Different types of expansion tanks

Expansion tanks come in two main types: steel expansion tanks and bladder expansion tanks. Both serve the same purpose of managing pressure in your heating system, but they operate in different ways and have different maintenance needs.

Steel Expansion Tanks

Steel expansion tanks are the older and more traditional option. They do not have a diaphragm or internal bladder. Instead, the air and water share the same chamber.

Key features:

• Simple construction with no moving parts
  
  
• Air cushions the expanding water directly
  
  
• Air and water are in contact, which can lead to corrosion over time
  
  
• Typically require regular draining to maintain proper air levels
  
  
• Often found in older heating systems
  
  

Installation note:
Steel tanks are usually installed vertically above the boiler on the cold water line, using a tee fitting and proper support.

Bladder Expansion Tanks

Bladder or diaphragm expansion tanks are more modern and commonly used in today’s systems. These have an internal rubber diaphragm that separates the water from a sealed air chamber.

Key features:

• Prevents air and water from mixing
  
  
• Lower risk of corrosion
  
  
• Requires less maintenance than steel tanks
  
  
• Typically lasts longer
  
  
• Pre-charged with air to match system pressure
  
  

Installation note:
Bladder tanks are usually installed vertically and can be fitted with a valve between connectors for easy removal and replacement in the future.

Assessing the tank

Before installing or replacing an expansion tank, it’s important to check the condition of the tank and ensure it’s suitable for your system.

What to Look For:

• Visible damage such as dents, rust, cracks or pitting
  
  
• Signs of leakage around the fittings or seams
  
  
• Weight check — if the tank feels heavy when it shouldn’t, it may be full of water and no longer functional
  
  
• Pressure rating — the tank’s pre-charge should match the system fill pressure, usually around 12 to 15 psi
  
  

Pre-Charging the Tank:

Most new expansion tanks come pre-charged, but you should always:

• Use a pressure gauge to confirm the pre-charge matches your boiler’s cold fill pressure
  
  
• Adjust if needed using a standard air pump
  
  
• Test again to ensure the reading is accurate before installation
  
  

Skipping these checks can lead to poor performance or system damage after installation.

Installing the tank

Once you’ve assessed and pre-charged the expansion tank, you’re ready to fit it into the system. Follow these steps carefully:

Step-by-Step:

1. Shut off power and let the boiler cool
   
   
2. Turn off the water supply to the system
   
   
3. Drain the system using the boiler’s drain valve and a hose
   
   
4. Remove the old tank, supporting it as you disconnect the fittings
   
   
5. Fit the new tank in the same location, ensuring:
   
   
   
   • It’s on the supply side of the boiler
     
     
   • It’s between the boiler and the circulator (if present)
     
     
   • It’s installed upright and properly supported
     
     
   • All fittings are sealed tightly with thread sealant or tape
     
     
6. Refill the system by opening the water supply valve
   
   
7. Bleed air from the system using the pressure relief valve
   
   
8. Check the pressure gauge to make sure the system is within the recommended range
   
   
9. Inspect for leaks at all joints and connections
   
   
10. Restore power to the boiler and monitor its performance
    
    

If you notice any discharge, pressure spikes or irregular sounds after installation, shut down the system and recheck the tank placement and charge.

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