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Mains Pressure Hot Water System Explained

Mains Pressure Hot Water System

If you currently do not have a mains pressure hot water system in place in your home you may be thinking of changing your current water system or installing a new system replacing it to a mains pressure hot water system. Generally speaking, a mains pressure hot water system has a much improved flow rates than the older style gravity system as it provides a lot more pressure. However, a mains pressure system is dependant on the pressure and flow of the main feed, meaning if the mains pressure is poor the system will be poor, if the mains pressure is high the system will function correctly.

You may be thinking that a mains pressure system is the ideal choice for you, but just before you make that final decision it is sensible to look into fully understanding the mains hot water system, how they work to obtain all the do’s don’ts and facts.

In General

The most important factor to remember and understand regarding a mains pressure system is that it will only be as good as the supply, which enters the property. Meaning, the pressure received at the tap is the water pressure that is entering the property from the mains supply. Appliances, products or components attached to the mains pressure will not improve the flow rate pressure whatsoever. An incoming poor mains pressure will result in poor pressure leading to all outlets in the system.

Here is an example of what we mean

An appliance or product like a boiler may say that it can deliver a flow rate of 35 litres per minute at 3 bars of pressure. But in truth the existing incoming water mains pressure only has a flow rate of 15 litres per minute at 1 bar of pressure then this is exactly what you will receive to each outlet. Even though your mains pressure appliances are built to handle much higher volumes of pressure unfortunately its abilities do not include being able to increase the flow rates of the mains pressure. Numbers quoted on appliances are indicative to their possible performance based on a sufficient mains water pressure flow rate supply. They are not a definitive statement of what you will actually receive.

Mains Pressure Hot Water System

How does a mains pressure hot water system work?

When a property has been installed with a mains pressure hot water system it provide all outlets, such as taps, with mains water pressure being either hot or cold. It is easy to understand that all cold outlets and cold taps are connected directly to the external water main feed but to understand where the hot water comes from is another matter, In simple terms it is still fed by the external main but it needs to run through specialised appliances designed for heating water at pressure. There are a numbers of way this can be done and a number of designs on the market. With luck, once you have read and understood these notes it will provide you with a better knowledge base to the process so you will be able to pick a much better choice for a design system that suits your needs best.


Selecting your hot water system

Equipment used to heat mains water pressure is very different from most other types of water heaters. It is not recommended to use equipment specifically designed to heat water at low pressure, like gravity fed copper cylinder, in a mains pressure system. Low-pressure copper cylinders are fed using gravity from a cistern tank situated in a location higher than the cylinder usually in the loft. It fills the cylinder because of the weight of water created due to the cistern tank being positioned higher than the cylinder. As the cistern tank is depleted it is filled using a ball valve connected to the mains high pressure. The pressure in a gravity fed system is significantly lower than what you receive from a main pressure system. Equipment designed to be used on gravity systems are made from materials not really compatible to use with a high-pressure system.

Please feel free to contact us for help or advice. If you are interested in booking a survey for any of our services or just a gas safety check, please contact Acorn Plumbing & Heating (APH) and a member of our team will be happy to help or advise you. CALL – Acorn Plumbing & Heating (APH) now on 01752 201077


A system commonly used to heat main water is a Combination Boiler. There are a number of different size combination & condensing boilers on the market. They are connected directly to the mains water supply and flash up when an outlet, such as a tap is turned on, and heat the water instantaneously so they are regarded as mains pressure appliances. However, they do not deliver the maximum flow rate possible for a mains pressure system. To get the very best main pressure hot water system a cylinder is required. The correct cylinder must be chosen as they differ depending to the system you require or prefer.

Main pressure hot water systems designs:

  • Vented Mains Pressure System, using a thermal store.
  • Unvented Mains Pressure System, using cylindrical pressure.

The term unvented and vented refers to the system being either completely pressurised being unvented, or open to the atmosphere being vented.

Unvented Mains Pressure System:

An unvented system is a cylinder, or pressure vessel, filled and connected directly to the external water main. It is pressurised by the incoming water supply and heated by either a direct model being an immersion heater or by an indirect model being a boiler or solar energy.

The pressure vessel or cylinder is known as an unvented cylinder because it is not vented to the atmosphere like a normal gravity fed copper cylinder is. Unvented means exactly what it says on the tin. Not vented.

An unvented cylinder has pipework plumbed into it to enable hot water to be carried around the property to outlet taps. When an outlet tap is cracked open the pressure created by the incoming external water pressure feeding the cylinder displaces and forces the hot water inside the cylinder out and races to the open tap. So you receive hot water from the tap at the same pressure as the incoming cold-water pressure feeding the cylinder the result being you get mains pressure hot water.

Unvented hot water cylinders need to endue significant internal forces so the preferred material used in their manufacture is stainless steel as this provides high tensile strength, required by the cylinder, from a relatively thin based sheet metal. A heavy thick gauge copper based material can be used as copper is commonly regarded as a much better water storage material but the price to manufacture it not cost effective whatsoever.


Whenever a cylinder is used to store mains pressure water you should always ensure it is fit for purpose meaning its the correct cylinder for the task in hand. If you fail to do this and use the wrong cylinder you risk an explosion.

Unvented cylinders are pressurised vessels and consequently carry a risk factor. Consider this. You have a large cylinder full of hot water under extreme pressure. If the water were to get too hot and begin to boil, due to a malfunction, the boiling water would turn into steam leading to an further increase in pressure resulting in structural failure and BOOM! Explosion.

To safeguard this issue an unvented cylinder must be fitted with some form of temperature and pressure relief valves so in the event of such a malfunction happening the excess pressure can be vented safely. To safeguard that all unvented cylinder are installed safely, correctly and by a trained competent person the Government introduced Building regulation (G3) for vessels that contain more than 15 litres of hot water under pressure.

Vented Mains Pressure System:

Once reading and understanding about unvented systems it may seem impossible that mains pressure water can be sort from a vented cylinder, however, it can.

Have you ever heard the saying that "the best ideas are usually the simplest ideas." You could say this about the idea of a Thermal Store. An unvented cylinder and a thermal store are very similar to look at being full of hot water and cylindrical in shape. However, it’s how they produce the mains hot water is what makes them different.


The most beneficial factor regarding a thermal store is that it is vented meaning that the volume of water it is storing inside the tank is at atmospheric pressure, therefore safe from dangerous build up of pressure as it can never happen.

A thermal store can be heated directly or indirectly like any other cylinder, but unlike other cylinders the water is not provided for taps or outlets. Instead the water is used to store large quantities of thermal energy a bit like the way a battery stores electrical energy. So you could say the cylinder is a thermal battery or better known as a thermal store.

A thermal store achieves mains hot water pressure by imparting its stored energy, or the hot water stored at vented atmospheric pressure, into the cold water incoming from the external mains pressure. A process known as heat exchange performs this and the equipment used is called a heat exchanger.

There are a number of different heat exchangers available on the market using different methods all with thermal stores and all essentially all do the same thing which is taking heat from an open vented source and transferring that heat to mains pressure water as it passes through. It is possible to have unvented and vented together existing in proximity to one another but you can never mix them.


Different methods of heat exchangers have different characteristics in performance. Some like plate heat exchangers need a complex control while other designs like immersed heat exchangers are a lot simpler. Whatever device and method used will all provide the same result being hot mains water from an intrinsically safe vented cylinder.

Advantages of Thermal Stores

Apart from essentially being very safe, thermal stores provide several further advantages, which are beyond the abilities of an unvented system cylinder.

These advantages are:

  • Thermal stores do run a lot hotter than ordinary cylinders or unvented cylinders that are set at a specific temperature desired to arrive at the outlet taps with this in mind a thermal store holds more energy litre for litre. The temperature of the mains pressure water leaving the thermal store is regulated to prevent the risk of scalding. The way it does this varies depending on the heat exchanger being used but the majority use a thermostatic mixing valve that prevents water from being released over a certain set temperature.
  • A thermal store is able to accept many heat sources all at the same time. This can be said the same for unvented cylinders and vented cylinders but what makes a thermal store different is the fact that it can accept is a heat source that is uncontrollable such as Aga’s or wood burning stove that cannot be controlled by turning them up or down. They are also perfect for appliances, which use gravity circulation such as thermosiphon reboilers or unpumped systems.
  • A thermal store can be placed as the centre hub of your hot water system and be, configured to take all different forms of heat source energy’s such as air recovery, solar and ground source, then return that energy to give you hot water, domestic central heating and even under floor heating all by taking separate feeds directly from the thermal store. Example of its versatility – a wood-burning stove heats the store. Hot water is drawn from the store and pumped round a radiator system; the wood-burning stove has just contributed to the central heating system. Everything connected to the store will contribute towards its function. It really is a versatile system to be appreciated.
  • When installing a thermal store there it is not a requirement to notify Building Control. G3 Regulations only apply to vessels that can hold more than 15 litres of hot water under pressure. There is less than 15 litres of hot water in an heat exchanger and the main body of the thermal store is vented to the atmosphere.
  • A competent person can install a thermal store as it is classified as having no significant danger related with it so specialist training is not required for installation. Thermal stores in some situations are essential as they can be used in conjunction with renewable energy fuels such as solar and heat pumps.

What to consider with mains pressure hot water

Vented and unvented systems do both offer and provide hot water from the mains pressure. They do achieve this in different ways. There are certain important deliberations to consider when choosing a system that is going to best suit your requirements.

Unvented Mains Pressure System:

  • Unvented cylinders can provide hot mains pressure water.
  • An unvented system stores a large volume of hot water that is constantly under pressure. For safety reasons such systems needs to be installed by a person trained and qualified to do so having CITB training and hold a G3 certification to comply with current building regulations.
  • Pressure relief valves need to be installed to external pipework and vented externally to protect the system from unsafe build up of pressure inside the cylinder that could possible result in an explosion.
  • Local Building Control will require to be notified of any intension to install an unvented system.
  • An unvented system will require an annual maintenance service to ensure all safety equipment is working correctly. (BS2870)
  • Once an unvented system has been installed and commissioned it must be certified by the engineer who installed it.

Vented Mains Pressure System:

  • Thermal stores provide hot mains pressure water.
  • Vented systems do not store large volumes of hot water under considerable pressure, but rather the thermal store holds hot water but is vented so stays at atmospheric pressure.
  • There is no dangerous issues whatsoever with vented systems, so pressure relief valves in external pipework are not necessary although some cisterns may need them for operation. This could be attached to the thermal store or remote depending on the operational and installation requirements.
  • Building Control notification and approval is not required as a necessity.
  • Installers just need to be a competent person, due to the simplicity and safety a certified trained installer is not necessary.
  • Vented systems will not require a certification and there is no need for an annual service maintenance check requirements.

Other drawbacks to consider with a mains pressure hot water system

Before deciding to go down the road of installing a mains pressure hot water system whether is a vented or unvented you should make sure you do your homework first so as you are fully up to date with all the pitfalls that can occur.

"mains pressure water system is only as good as the supply fed to it". Remember its exactly we said at the beginning of these notes, as it’s a very important starting block for being able to have mains pressure hot water. I know it sound obvious by it is often overlooked until its too late and the system has been installed and not functioning how it should due to poor pressure or flow rate. Both of these are are important and if questioned don’t proceed. Once its installed you will have to live with it being was it is. Check the pressure your local Water Authorities to supply you. If the pressure is too low some unvented systems will not like it and start playing up. If this is the case its not the end of the world. Why not use a pump gravity system, its does have faults and issues but it still will be a better option than an unvented system with poor mains pressure.


Under water Bye Laws states you are not permitted to connect a pump directly to your mains fed system to increase its flow rate or pressure. If the mains pressure or flow rate supply is poor there is no easy method to improve it. To achieve a higher flow rate and pressure entails the installation of a new larger supply pipe to the property by the water board at great expenses. Even then you need to check and double check that the new supply will be adequately sufficient to run an unvented system as some districts in the UK do frustratingly suffer from low pressure and flow rates even when installing a bigger pipe.


Mains pressure systems are themselves driven by a supply that can fluctuate. It’s not always an issue and with a lot of cases it wont be noticed but it still should be considered before you invest in this type of system.

Mains pressure systems can offer a power shower performance offering a good pressure and flow rate supply. If you enjoy an energetic showering experience but your pressure supply is poor, you cannot install a shower pump to boost the performance.

Also remember that having a mains pressure system might require you to change the valves to your shower because risk of pressure or temperature changes in the system. A thermostatic shower valve or a pressure-balancing valve will require being installed. Manual valves can play up and deliver huge temperature swings providing a very unpleasant experience.

If current in your existing system have a form of booster shower pump powered by all in one or remote, unfortunately it will have to be removed.

Please feel free to contact us for help or advice

Renewable Energy

If you are interested in booking a survey for any of our services or just a gas safety check, please contact Acorn Plumbing & Heating (APH) and a member of our team will be happy to help or advise you.

CALL – Acorn Plumbing & Heating (APH) now on 01752 201077 or 07779 777965

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