Expansion Vessel Flushing and Legionella Risks

A guide to Expansion Vessels and Legionella Risks

What is an expansion vessel?


Expansion vessels are important components in most modern pressurized domestic hot water heating systems. They are designed to protect the components of the system from a build-up of pressure which could cause damage to the installation.

In most hot water systems, expansion vessels are filled with one-half water and one-half air. When the pressure builds up in the water system, that air becomes compressed to absorb any excess pressure which helps to protect the water system including the cylinder and pipework. When operating correctly, an expansion vessel will partially fill and empty depending on the demand and use of the hot water.

In simple terms, this means that when hot water heats up in the hot water cylinder, it expands, and therefore needs somewhere to head for more space. The expansion vessel provides this space with a bladder which acts like a balloon to expand and decrease in size depending on the use and heat of the hot water.

Expansion vessels can normally be found in any hot water system where the water is pressurized. This means when the water supply is supply by a water system that is pressured such as mains water supply or boosted water supply. This shows us that expansion vessels can be installed almost anywhere from dentists to hotels, schools or care homes.


Expansion vessels and the risks from Legionella


Due to the installation and design of an expansion vessel, the Health and Safety Executive have identified expansion vessels as a potential Legionella risk. This is mainly because they often have low water flow or stagnation problems, especially where system pressures and temperatures remain steady and do not increase/decrease. Often acting as a dead leg, no water passes through the expansion vessel, or away from the vessel, leading to stagnation and a good environment for bacterial growth.

If you have a water system within the UK and an expansion vessel forms part of your water system, it is your legal duty to be aware of the potential risks from Legionella bacteria and other waterborne pathogens and take appropriate action to minimise the risk. This process normally starts by undertaking a legionella risk assessment.

Why does Legionella grow inside expansion vessels?


As with any water system, asset or component, if the conditions are suitable for Legionella growth, then bacteria may multiple. An Expansion vessel often has the following conditions that will encourage Legionella growth:


  • Stagnation and poor flow
  • Nutrients such as scale, rust and sediment
  • Temperatures that encourage Legionella growth, between 20 and 50°C

How to reduce the risk from Legionella in expansion vessels?


Whether you have an existing expansion vessel or going to install a new expansion vessel, there are a few things you can do to minimise the risk of Legionella growth within your expansion vessel. “To minimise the risk of microbial growth, expansion vessels should be installed:


  • in cool areas on cold flowing pipes
  • mouthed as close to the incoming water system as possible
  • mounted vertically on pipework to minimise any trapping of debris
  • with an isolation valve and drain valve to aid flushing and sampling
  • to minimise the volume retained within them
  • designed to stimulate flow within the vessel”

HSG 274 Part 2 p.238

What is included in an expansion vessel flushing service?

Isolation and flush through

Sample upon Request


Re-open & Record Keeping

Service process

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L8 Risk Assessment FAQs

When and how often does my central heating system need a risk assessment?

Under the Health and Safety at Work Act 1974 and in compliance with the Approved Code of Practice (ACoP L8, HSG 274), duty holders, including employers and those in control of premises, must ensure the health and safety of their employees or others who may be affected by their undertaking, this includes the risk of legionella. Legionnaires Disease is a form of pneumonia, which can be fatal and the main route of infection is through inhalation, by inhaling airborne water droplets that contain Legionella. This also applies if you are a private Landlord and have rental properties which are occupied.

This includes taking suitable precautions to prevent, manage and control the risk of exposure to legionella. You can do this by undertaking the following: an expansion tanks risk assessment, actioning all identified risks within the initial report with remedial works, sampling the expansion tanks water quality to confirm bacterial presence with water testing, and carrying out ongoing record keeping also known as monitoring.

Does my Central Heating System need testing, a vessel risk assessment or both?

Legionella testing, water testing or sampling is not to be confused with a legionella risk assessment. You are required by law to undertake a legionella risk assessment under the Health and Safety at Work Act 1974, COSHH and a number of other regulations where you can show you are working to comply with the HSE’s HSG 274 legionella guidelines. Testing or sampling whilst not a legal requirement on it’s own, it plays an important role in determining how efficient a water system is managed. Your risk assessment may identify that sampling is required due to existing risks or as a control and management precaution. In this instance sampling then becomes a requirement from your assessment. Without an L8 risk assessment you will not know where and how many samples are required. And without water testing/sampling you cannot determine how affective your control and management is. These two actions therefore should be undertaken together, not necessarily at the same time but unison to effectively management your risk from legionella.

Water testing services include Legionella, e-Coli, total viable count (TVC), Pseudomonas, hard water, chlorine free, pH levels and more. Learn more about legionella testing. We only use UKAS approved laboratories giving you peace of mind that the test will be of the highest quality possible.

How much does an L8 risk assessment cost?

All assessments vary in prices depending on the number of water outlets, cold water storage tanks, cylinders, combi boiler, expansion tank and other water assets that may be present. The number of assets within your expansion vessel pressurised heating systems will determine the time spent on site and how long it will take to produce your report.

So how much does it cost? Across the industry, prices vary enormously often depending on the size of the company you use and their processes, the speed of works and professionalism. There are two types of assessments, the first is commercial and the second domestic, also referred to as residential rental properties or landlord risk assessments.

A commercial expansion vessel domestic heating systems risk assessments normally start from £250 to in excess of £750, however our prices start from £99 depending on the size of the water system and location.

Domestic expansion vessel pressurised water heating systems risk assessments can range tremendously with a number of plumbers, gas engineers and other tradesmen offering these assessments without the specialist training, it’s important to check that your assessor or engineer is properly qualified. Our prices start from £55 for multiple landlord properties to £95 for single landlord properties. Our prices for residential rental properties are based on the property being no more than 5 bedrooms.

How long will the central heating system risk assessment take?

On average, the time our qualified assessor spends on site is 1 hour for every 10 outlets (sinks, showers, baths, etc). So an assessment for expansion vessel pressurized heating system can take anywhere between 30 minutes to 6 or 7 hours. It entirely depends on the size of your for expansion vessel pressurized heating system and the number of water assets you have such as cold water storage tanks, cylinders or combination boilers.

Our engineers will work closely with you to ensure minimal disruption to your business is caused. Clear and easy access to rooms and buildings will play a part in time spent on site at your premises.

When will I receive your findings?

You will receive your LRA report between 2-10 working days from the date the for expansion vessel pressurized heating system assessment was carried out. From time to time, not often, our team will need a little more time, so please expect to have your report no later than 30 days. Need your assessment sooner? Get your report, guaranteed next day delivery via email for just £50.

What happens after I receive the report?

Within your report document, you will have an action plan offering guidance which lists all identified risks, with photos, that need remedying in order of importance. By following our advice and undertaking remedial works you will reduce the risk of exposure to Legionnaires’ disease.  Once all the remedial works are complete, you will then need to focus on implementing your monitoring (periodical checks and maintenance) with a logbook for recording keeping all legal duties and monitoring for expansion vessel pressurized heating system requirements.

Are uRisk qualified?

Yes, all of our assessors are fully qualified and accredited by City & Guilds and can help assist you in meeting compliance today for HSE (ACoP L8). We are also Safe Contractor and Construction Online approved so you can rest assured that you are working with a trusted water hygiene service provider who put health and safety first.

All our engineers are DBS Checked to ensure suitable people are entering your premises and to safeguard vulnerable people. We are committed to maintaining the highest standards.

Do uRisk service my area?

Yes, our services are nationwide. Whether you have one or multiple water systems and one or multiple sites, we provide services across the country, with qualified engineers scattered throughout the UK and offices based in Hertfordshire which is a 30-minute train journey to London King’s Cross.

Why choose uRisk?

25 Years Experience

Over 25 years of water hygiene and legionella control experience.

Customer Service Excellence

Friendly and simple advice by phone, email & online at your convenience.

City & Guilds Qualified

All staff are trained water hygiene consultants and engineers.

Nationwide Coverage

With engineers located all across the UK, we're never too far away.

UKAS Accredited Testing

Our labs are UKAS approved providing you with quality assurance.

Full Compliance Services

Your one stop shop for Legionella control, no need to go elsewhere.

A fabulous company, excellent customer service, they try to accommodate appoitments, and do there utmost in emergencies, been with them a few years now,  honestly cant fault them. Coming out of Covid 19, having our building urgently disanitised,  big thank-you to Luke and Lee for staying so late to get the job done!! Thank you Devon, for juggling the diary to fit us in so soon. A Team 🙂
Jo Maskell
Jo Maskell
18:02 15 Jun 20
Extremely happy with our legionella risk assessment carried out by uRisk, easy to understand and great customer service. Thanks again
Nickayla Robertson
Nickayla Robertson
21:22 15 Jan 19
Brilliant service! Thanks a lot for the hard graft
Leon Macauley
Leon Macauley
21:12 15 Jan 19
Brilliant Service, efficient, professional, punctual and friendly.
Alex Windsor
Alex Windsor
18:21 15 Jan 19
uRisk have been helping us with our Legionella Control for a number of years now. We've recently had a CQC audit and they were very impressed with our legionella compliance! Thank you for all your help.
Mike Port
Mike Port
20:41 07 Aug 18
Fantastic training courses and we've found the legionella risk assessment very easy to read and far more detailed than any we have previously had. 5 star service!
Gemma Pond
Gemma Pond
20:37 07 Aug 18
Incredibly detailed and professional team. Have been beyond helpful with with our CQC legionella compliance for our Safety. They’ve found solutions to fit around how we work and our budgets. We are always advised with reference to HSE which has been brilliant.
Amy Thorne
Amy Thorne
20:29 07 Aug 18
We're a small GP practice and started using uRisk for legionella risk assessments. We opted to use their remedial and monitoring services and they've been absolutely great. We cannot recommend them enough! Thank you so much!
Amanda Pates
Amanda Pates
18:30 07 Aug 18
Brilliant company, customer service was great! And engineers were fantastic. Thanks again uRisk! Will definitely be using them for all future work.
Nikesh Gohil
Nikesh Gohil
12:49 12 Mar 18

We want to help you comply!