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A Comprehensive Guide to Sizing Heating Expansion Vessels

A Comprehensive Guide to Sizing Heating Expansion Vessels

The guide provided is to assist with the sizing of a Heating Vessel and we would always recommend if unsure then refer to the system manufacturers guidelines to correctly size the Expansion Vessel.

Vessel KitStatic HeadSystem VolumeBTUKW
5 Litre58730,3668.9
8 Litre513945,02414.33
12 Litre520867,80421.58
18 Litre5313102,11532.5
24 Litre5435135,32143.07
35 Litre5696197,92563

This chart is to be used when sizing a Heating Expansion Vessel and not for Potable Water Applications.

Heating expansion vessels are crucial components in maintaining the safety and efficiency of your heating system. To ensure optimal performance, it’s essential to select the right size for your expansion vessel. In this guide, we’ll walk you through the process of sizing heating expansion vessels and provide helpful tips to make an informed decision.

What is a Heating Expansion Vessel?

A heating expansion vessel, also known as an expansion tank, is a crucial component of a central heating system. Its primary purpose is to accommodate the expansion of water as it heats up and prevent potential damage to the system due to increased pressure. A properly sized expansion vessel ensures the system operates safely and efficiently.

Factors to Consider When Sizing Heating Expansion Vessels

  1. System Volume – The total volume of water in your heating system is a significant factor when determining the size of your expansion vessel. To calculate the system volume, you’ll need to consider the capacity of the boiler, radiators, pipework, and any other components that may contain water. Accurate system volume measurement is essential for selecting the right expansion vessel.
  1. Maximum Operating Pressure – Each heating system has a maximum operating pressure, which is the highest pressure it can safely withstand. This value is typically provided by the boiler manufacturer. It’s crucial to ensure that the expansion vessel’s capacity matches or exceeds the maximum operating pressure to prevent system damage or failure.
  1. Initial System Pressure – The initial system pressure is the pressure of the heating system when it’s cold and not in operation. This value is typically between 1 and 1.5 bar. To ensure the expansion vessel functions correctly, it must accommodate the pressure increase as the system heats up.
  1. Expansion Ratio – The expansion ratio is the amount of expansion the water undergoes when heated. Water expands by approximately 4% when heated from 10°C to 100°C. For central heating systems operating at lower temperatures (such as 70°C), the expansion ratio will be slightly lower. Accurate calculation of the expansion ratio is crucial for selecting the right vessel size.

The Formula for Sizing Heating Expansion Vessels

To calculate the required size of your expansion vessel, you can use the following formula:

Expansion Vessel Size = (System Volume x Expansion Ratio) / (Initial System Pressure – Maximum Operating Pressure)

By applying this formula, you can determine the ideal size for your heating expansion vessel. Keep in mind that it’s always better to choose a slightly larger vessel than the calculated value to ensure adequate capacity.


Sizing a heating expansion vessel correctly is essential for the safety and efficiency of your central heating system. By considering factors like system volume, maximum operating pressure, initial system pressure, and expansion ratio, you can confidently choose the right expansion vessel for your needs. Use the provided formula to calculate the ideal size and consult with professionals if you need further guidance.

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What Is a Motorised Zone Valve?

A picture of a Motorised Zone Valve. A Zone Valve is a type of valve used in HVAC (Heating, Ventilation and Air Conditioning) systems which regulates the flow of water or steam in different zones of a building.

What Is a Motorised Zone Valve?

A Motorised Zone Valve is a type of valve used in HVAC (Heating, Ventilation and Air Conditioning) systems which regulates the flow of water or steam in different zones of a building. The valve is typically installed in the piping system and is controlled by an electric motor.

The valve has a movable disk or ball that can be rotated to control the flow of water or steam. The electric motor is connected to the valve and can be controlled by a thermostat or other temperature control system to open or close the valve as needed to maintain the desired temperature in each zone.

Motorised zone valves are commonly used in residential homes with separate heating areas for different floors, or commercial buildings with different HVAC zones for different departments or floors. They are also used in radiant heating systems (where heat is supplied directly to the floor or panels in the wall or ceiling of a building).

Please find below several frequently asked questions regarding motorised zone valves:

How Much Does a Motorised Zone Valve Cost?

If you are in the market for a motorised zone valve, we highly recommend Honeywell as the manufacturer. The zone valves we stock start from £84.97. Find out more information here.

Are Zone Valves Interchangeable?

Whether motorised zone valves are interchangeable depends on a few factors, including the manufacturer, the type of valve, and the specific model.

Some manufacturers offer interchangeable parts for their zone valves, which allows for greater flexibility in installation and repair. In these cases, you may be able to replace a faulty zone valve with a compatible model from the same manufacturer without having to make any significant modifications to your HVAC system.

However, it’s important to note that not all motorised zone valves are interchangeable. Different manufacturers use different valve designs and specifications, so you’ll need to check the compatibility of any replacement valve before making a purchase. It’s also important to ensure that the replacement valve is the same size and type as the original valve, and that it’s compatible with your HVAC system’s voltage and control systems.

If you’re not sure whether a motorised zone valve is interchangeable with your existing valve, it’s best to consult with a qualified HVAC technician. They can help you select the right replacement valve and ensure that it’s installed properly for optimal performance and safety.

Why Is My Motorised Zone Valve Buzzing?

There are several possible reasons why your motorised zone valve is buzzing:

  • Electrical issues: One of the most common reasons for a buzzing motorised zone valve is an electrical issue. This could be caused by a loose wire, a faulty transformer, or a faulty control board. If the electrical components of the valve are not functioning properly, it can cause the motor to vibrate and create a buzzing sound.
  • Dirt or debris: Over time, dirt and debris can accumulate in the valve, causing it to become clogged. This can interfere with the movement of the valve and cause it to vibrate or buzz.
  • Mechanical issues: If the valve is old or has been subjected to heavy use, it may develop mechanical problems that can cause it to buzz. This could be caused by worn or damaged gears, a faulty motor, or other mechanical issues.
  • Low voltage: If the valve is not receiving enough voltage, it may not operate properly and may emit a buzzing sound. This can be caused by a faulty transformer or other electrical issues.

If your motorised zone valve is buzzing, it’s important to have it inspected by a qualified HVAC technician. They can diagnose the problem and make the necessary repairs to ensure that your valve is functioning properly.

How To Fit a Motorised Zone Valve?

Fitting a motorised zone valve requires some knowledge of plumbing and electrical systems, so if you’re not comfortable with these tasks, it’s best to hire a qualified HVAC technician to do the job. Here are the general steps involved in fitting a motorised zone valve:

  1. Turn off the power supply to the HVAC system to prevent electrical shock.
  2. Identify the location in the piping system where the valve will be installed. This should be in the return line of the zone being controlled, downstream of the heating device and upstream of any check valve.
  3. Shut off the water supply to the zone and drain the water from the pipes.
  4. Cut into the pipe at the chosen location and install the valve according to the manufacturer’s instructions. Be sure to install the valve with the correct flow direction.
  5. Connect the wiring to the valve according to the manufacturer’s instructions. Typically, the wiring will involve connecting the valve to a transformer and control panel.
  6. Test the valve to ensure that it is functioning properly. Turn on the power supply and test the valve by operating the control system.
  7. Once you have confirmed that the valve is functioning correctly, turn the water supply back on and check for leaks.

Keep in mind that the specific steps involved in fitting a motorised zone valve can vary depending on the manufacturer and model of the valve, as well as the design of your HVAC system. Be sure to consult the manufacturer’s instructions and follow all recommended safety procedures when installing the valve. If you’re not confident in your ability to install the valve properly, it’s best to seek the assistance of a qualified HVAC technician.

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How To Bleed a Radiator

A pig on top of a radiator. Some estimates suggest that bleeding your radiators regularly could save you up to 10% on your energy bills.

How To Bleed a Radiator

Bleeding a radiator is the process of releasing any trapped air from the system, which can prevent your radiator from heating up properly. To bleed a radiator, you will need a few tools:

  1. A radiator key or a flat screwdriver (depending on the type of valve on your radiator)
  2. A towel or cloth to catch any water that may drip out

Here are the steps to bleed a radiator:

  1. Turn off your heating system to prevent any hot water from flowing through the radiator.
  2. Locate the valve at the top of the radiator. It will usually be located on one of the sides and will be a small square or hexagonal nut.
  3. Place the towel or cloth under the valve to catch any water that may drip out.
  4. Insert the radiator key or flat screwdriver into the valve and turn it anti-clockwise. You should hear a hissing sound as the air is released. Keep turning until water starts to flow out of the valve.
  5. Once water starts to flow out, quickly turn the valve clockwise to close it again. Be careful not to over-tighten the valve as this could damage it.
  6. Wipe any water that may have dripped out of the valve with the towel or cloth.
  7. Turn your heating system back on and check that the radiator is heating up properly.
  8. Repeat this process for each radiator in your home, starting with the one located furthest from the boiler and working your way back towards it. This will ensure that all of the air is removed from the system.

How Much Can Bleeding a Radiator Save?

How much bleeding a radiator can save will depend on a number of factors, including the size of your home, the number of radiators you have, and how often you bleed them. However, some estimates suggest that bleeding your radiators regularly could save you up to 10% on your energy bills.

For example, if your annual heating bill is £2,500 (a 3-bedroom home typical usage reported by British Gas), bleeding your radiators could save you up to £250 per year. While this may not seem like a significant amount, over time, the savings can add up.

In addition to reducing your energy bills, bleeding your radiators can also help to prolong the life of your heating system, as it reduces the strain on the system and prevents corrosion in the radiators. So, while bleeding your radiators may seem like a small task, it can have a big impact on your home’s energy efficiency and your overall heating costs.

Aladdin Autovent Automatic Radiator Bleeders

Aladdin Autovents automatically bleed air and improve efficiency of radiators, heating systems and underfloor heating networks.

  • No need to manually bleed radiators.
  • Keeps radiators and heating systems fully heat generating and efficient.
  • No gas = reduced corrosion = extended component life.
  • Drains air from underfloor heating networks.
  • Keeps the radiators hot to the top.
  • Save heating and energy costs by optimising radiator efficiency.
  • Avoids tenants calling out service engineers just to vent radiators.
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