Instructions for Drayton central heating controls


Mains Pressure Hot Water Systems explained and compared!

Things you should know about the hot water systems avaibale to you

Index Page > Mains Pressure Hot Water Systems Explained

This simple guide is intended to help you understand how mains pressure hot water systems work, and in doing so help you to compare various products and system types from different manufacturers.


Mains Pressure Hot water Systems - Explained

You may be considering installing or changing a system to provide mains pressure hot water in your home. Generally speaking, mains pressure hot water systems out-perform the more traditional gravity systems by providing higher pressure and flow rates, but there is a simple rule of thumb...


A mains pressure system is only as good as the

mains supply (Pressure and Flow Rate) feeding it.


A mains pressure system may be an ideal choice for you, but before you make your decision it is prudent to fully understand how these systems work and consider all the facts.



The first and most important thing you must understand is a mains pressure system is only as good as the supply entering the property. This is to say, the pressure you get at a tap for example is the product of the water pressure entering the property from the water main.

The components and the products attached to a mains pressure system do not in any way enhance the pressure or flow rate that can be achieved within the system or at an outlet. For the avoidance of confusion, if your incoming mains water pressure is poor then the pressure at all outlets on that system will be poor.

Example: The product specification of a piece of mains pressure equipment may for example boast 35 litres per minute flow rate at 3 bar pressure. But if the water main and pipe work to the property can only supply 15 litres at 1 bar then this is all you can expect to receive.

 Although your mains pressure equipment may be able to handle higher performance it is not capable of increasing the supply pressure or flow. Figures quoted against products are indicative of their potential based on an appropriate supply. They are not necessarily a definitive statement of what you will get.


How do mains pressure hot water systems work?

In a property with a mains pressure hot water system, all taps and water outlets attached to that system will have water at mains pressure - be they hot or cold.

It probably isn't difficult to understand that all cold taps or cold outlets are connected directly to the same incoming cold water supply. Understanding where your hot water comes from takes a little more technical understanding as it is still fed directly from the incoming main but water must pass through equipment designed specifically for heating it at pressure.

There are different methods of doing this and many interpretations on product design. Hopefully having read this guide you will possess a better understanding of the various processes and be in a much better position to make an informed choice (negating the risk of dependency on a pushy plumber or the need to trust whatever half-baked  nonsense you might be fed!).


Selecting your hot water system

Devices that heat mains pressure water differ greatly from more or less all other types of hot water heater. You should never try using a device designed to heat low pressure water (a gravity copper cylinder for example) in a mains pressure system. A low pressure copper cylinder which is typically fed by a cistern in a loft operates under 'gravity' pressure (hence the term) which is in effect the weight of water created by the higher position of the storage tank/cistern. The pressures in a gravity system are typically much lower than those in a mains pressure system of a typical house. Components designed for use on a gravity system are often manufactured in a manner and from materials incompatible for use with high pressure.

Warning: If you attempt to heat mains pressure water using a cylinder designed for a low pressure system the risk of explosion exists.

The purpose of this article is to describe how water may be heated directly from the main. To be correct one must therefore acknowledge Combination Boilers as these too connect directly to the main heating water instantaneously. As such, they are regarded as mains pressure devices. We shall not however dwell on this type of unit as they do not deliver the flow rate of a true mains pressure system.

In the field of domestic mains pressure hot water the highest performing systems require a cylinder. It must be noted however that the function and purpose of the cylinder may differ according to which system you prefer.

There are essentially two types of mains pressure hot water system -

  • Unvented Mains Pressure Systems (using a cylindrical pressure vessel)

  • Vented Mains Pressure Systems (utilising a Thermal Store)

The term vented and unvented refers to whether the system is partially vented to atmosphere or completely pressurised and therefore unvented. These terms might seem confusing and just a little contradictory so we will try to explain more fully.


Unvented Mains Pressure Systems

In an unvented system, a vessel is filled with cold water directly from the incoming water main. The vessel is kept under pressure from the incoming water supply and heated either by immersion heaters (a direct model) or by a heat source located outside the vessel such as a boiler or a wet solar system (an indirect model).

The 'vessel' to which we refer normally takes the shape of a cylinder. In an unvented mains pressure hot water system the cylinder should be regarded as a pressure vessel. This type of cylinder is referred to as 'an unvented cylinder' because it is not vented to atmosphere in the same manner as a normal gravity copper cylinder would be. Unvented means exactly that - no open vents.

The unvented cylinder will have outlet pipe work connected to it that will carry the heated contents around the property to various taps and outlets. As soon as a tap or appliance is opened the pressure of the incoming cold water to the cylinder forces (displaces) the contents of the cylinder (hot water) towards the open outlet. Hence, hot water is delivered to the tap or outlet at a pressure relative to the force of the incoming cold water main to the cylinder.... Hence, mains pressure hot water.

Examples of unvented cylinders include: Stainless Lite PLUS by Gledhill, or the Megaflo Heatrae Sadia. There are of course many others manufacturers and brands.

Unvented hot water cylinders have to withstand considerable internal forces. Stainless steel is a popular material for unvented water cylinders because of the high tensile strength that can be achieved from a relatively thin sheet. Although a heavy gauge copper may be used (and is generally regarded as a better water storage material) the comparative cost of manufacture often makes this choice of material prohibitive.

Before using a cylinder to store mains pressure water you should ensure it was designed to do so. Failure to follow this simple advice could result in explosion.

NOTE: Unvented cylinders do represent a risk. Consider... you have a large volume of very hot water in a cylinder under pressure. If the contents of the cylinder were for example to get too hot (because of some system malfunction) the risk exists that the contents of the cylinder could boil, turn to steam and the internal pressure cause structural failure. We are talking about explosion! To combat this issue all unvented cylinders must be fitted with pressure/temperature relief equipment that will vent the cylinder in case of such a problem.

To ensure that unvented cylinders are fitted correctly, safely and by a competent installer the Government introduced Building Regulations (G3). These Regs cover any vessel that contains more than 15 litres of hot water under pressure.

NEVER attempt to install an unvented cylinder unless you are trained to do so. The consequences could be catastrophic.

If you have any doubts take a look at these:


Makes you think... eh?!!


Vented Mains Pressure Systems - Thermal Stores

Having read about unvented hot water systems it might seem somewhat of a contradiction that mains pressure water can be achieved from a vented cylinder - but it can.

They say that the best ideas are the simplest. Well, that could be argued about what's known as a 'Thermal Store'.

To the untrained eye a thermal store and an unvented cylinder look very similar. They are typically cylindrical in shape and the are both full of hot water. The difference is how they produce domestic mains pressure hot water.

Having covered the unvented cylinder in the section above we shall now focus on the thermal store.

At the outset it must be said that one of the most attractive properties of a thermal store is the fact that it is vented. This is to say that the body (where the great volume of water is stored) is at atmospheric pressure. It is therefore inherently safe as dangerous pressure build up is impossible.

A thermal store is heated in exactly the same way as any other cylinder (directly or indirectly). But unlike nearly all others, the water in the cylinder is not destined to appear at any taps or outlets - water is used solely to store a large quantity of thermal energy (a little like the chemicals in a battery store electrical energy). We therefore view the cylinder as a thermal battery or thermal store.

The way a thermal store achieves mains pressure hot water is by imparting the stored energy (from the hot water stored at atmospheric pressure) into incoming cold water at mains pressure. This is done by a process known as heat exchange and is achieved by using.... (surprise surprise!)... a heat exchanger.

There are different types of heat exchange device/method available with thermal stores, but essentially they all to the same job - they take heat from the open vented source and transfer it to mains pressure water as it passes through. In this way vented and unvented can exist in proximity to each other but never mix.

Different heat exchange methods claim different performance characteristics. Some are simple (immersed coil heat exchangers for example), some may require more complex control (plate heat exchangers for example).

Whichever method is used they all share they all share they same benefit - mains pressure hot water from a vented cylinder that is inherently safe.

Thermal stores, apart from being inherently safe, offer several other advantages that are beyond the capability of an unvented cylinder.

  1. Thermal stores run much hotter than a normal or unvented cylinder (each of which must be set at the temperature you want water to arrive at taps). So, litre for litre a thermal store holds more energy. To prevent any risk of scalding, the temperature of the mains pressure water leaving a thermal store is governed. The method by which this is achieved varies according to the method of heat exchange . Some utilise a thermostatic mixing valve (a mechanical blending valve that prevents water from exiting the cylinder above a set temperature), whereas the 'plate heat exchanger type' will regulate the ratio of heat given up to the mains water as it passes through the heat exchanger. The latter will generally produce much higher flow potential as the heat transfer process is more capable.

  2. A thermal stores can accept multiple heat sources simultaneously. Although this can also be said for vented and unvented cylinders, more importantly thermal stores can accept heat from a wider range including 'uncontrollable' heat sources such as wood burning stoves and Aga's that cannot be turned up or down as required. They are also ideal for appliances that operate by virtue of gravity (unpumped / thermo siphon) circulation.  (It is worth emphasising that an unvented cylinder should never be connected to an uncontrollable heat source. To do so is highly dangerous!)

  3. As a thermal store is, as its name suggests, a place where thermal energy is stored, it can be configured at the 'centre' of your household hot water system, taking all sorts of heat inputs (including wood burner, AGA, solar, ground source and air recovery) then give back energy not only for domestic hot water, but also central heating, under floor heating etc. This is achieved by taking additional feeds directly off the thermal store. So, for example, a wood burner heats the store, then hot water from the store is pumped off around radiators - the wood burner now contributes to your central heating! Everything that heats the store contributes towards its function. You may begin to appreciate the versatility of this type of system.

  4. You do not need to notify Building Control when you install a thermal store. G3 Regulations only apply to a vessel containing more than 15 litres of hot water under pressure. The body of a thermal store is vented and there is less that 15 litres of hot water in the heat exchanger. Thermal Stores are not governed by these regulations.

  5. Because there is no inherent danger with a thermal store they may be fitted by a competent person without specialist training.

  6. As a thermal store is open vented and an internal build up of pressure is impossible, no pressure relief equipment or pipework is necessary. As such, no annual inspection of safety equipment is necessary saving the householder associated ownership costs.

Thermal stores are particularly useful (and in some situations essential) in conjunction with renewable energies fuels and are available in various models including heat pump and solar. You might like to take a look at some of the models:


Torrent ECO


What to Consider - Mains pressure hot water systems

Unvented and vented systems both provide hot water at mains pressure. They achieve this by slightly different methods. There are however certain considerations when choosing the system that suits you best.

Unvented Mains Pressure Systems

  • Unvented cylinders provide mains pressure hot water.

  • In an unvented system you will be storing a large volume of hot water under pressure. For reasons of safety these systems must be installed by qualified technicians with relevant experience, CITB training and G3 certification.

  • As a precaution, pressure-relief pipework and valve/s must be installed to protect against unsafe pressure build-up within the vessel which could result in explosion. Pressure relief pipework must vent outside the building.

  • Your Local Authority (Building Control Dept) will need to be advised of your intention to install an unvented system.

  • For reasons of safety, your system will require an annual maintenance check to ensure safety equipment is functioning correctly (BS2870).

  • An unvented system must be commissioned and certified by the installer. 


Vented Mains Pressure Systems

  • Thermal stores provide mains pressure hot water

  • By their vented nature, this type of system does not store a large volume of hot water under pressure. The Thermal Store, whilst containing hot water, remains at atmospheric pressure.

  • There are no hazardous issues with a vented system. Pressure relief valves and pipework are not therefore required although a small F&E cistern is necessary for operation. This may be remote or attached to the thermal store depending on installation/operational necessities.

  • Vented systems such as these do not require Building Control approval.

  • Because of the simplicity and safety of such systems, installers do not need certification or specialist training.

  • Vented systems do not need certification.

  • There are no special annual maintenance requirements or associated costs of ownership.


Other drawbacks and considerations of a mains pressure hot water system

Do not enter the realm of mains pressure hot water systems (vented or unvented) unless you are aware of the potential pitfalls.

Firstly, a mains pressure water system is only as good as the supply to the property.

Sounds obvious? If your pressure or flow rate (both are very important) are questionable do not proceed. Once you install your system you have what you have and there's little you can do about it. Be sure you check what pressure your Water Authority is prepared to guarantee you. Some unvented systems misbehave if the pressure is too low, but then again, why bother if this is the case. A pumped gravity system with all its faults might be better than a poor mains pressure system!

Improving performance of a mains pressure system by adding pumps.

Under Water Bye-Laws you may not add a pump directly to your mains fed system to increase pressure or flow rate. If the supply in your house is poor there will be no simple way to improve it.*

(*Recently, a provision to fit a special small booster pump has been allowed. This is a very small pump that will typically raise a low flow/pressure to 1bar with a max flow of 12 litres/min. Often used in conjunction with Combi Boilers, the wisdom of installing a full mains pressure system if your supply is that poor should be seriously considered.)

Additional performance usually entails bringing in a new, larger supply pipe to the property. Even then, you should ensure the new improved supply will be adequate as some districts suffer from frustratingly low pressure - something a bigger pipe won't fix.

Always remember - your system can never be better than the supply to it.

Mains pressure systems are driven by the supply which is at the mercy of fluctuations. This may not be an issue in the majority of cases but should be considered before investment.

Mains pressure systems can give 'power shower' performance, providing supply flow and pressure is good. If you like a vigorous showering experience and your supply pressure is poor, remember - you cannot install a shower booster pump to improve matters. Pumps may ONLY be added to a gravity system (one with a tank of stored cold water).

You may need to change your shower equipment

Furthermore, mains pressure systems may require you to change your shower valve/s. Because of the likelihood of pressure (and/or temperature) fluctuations in the system you will need to install either 'pressure balancing' or 'thermostatic' shower valves. Simple 'manual' valves can misbehave terribly manifesting huge temperature swings - a very nasty experience!

If you previously had a power shower (one with a booster pump either remote or all-in-one) these will have to come out. Remember, no pumps allowed with a mains pressure system. Apart from contravening Bye Laws, shower booster pumps are designed to be used with a low pressure gravity feed. The high pressure nature of a direct mains feed will more than likely blow the seals out of your pump and invalidate any warranty you may have.



Compared - The Good       ....and the Not So Good  

Unvented mains pressure system

(Unvented hot water cylinder)

Vented mains pressure system

(Thermal Stores)

Mains pressure hot water to all your taps

Mains pressure hot water to all your taps

No loft tanks - space saving

No loft tanks - space saving

(remember an F&E cistern is required)

High Pressure storage cylinder

Vented cylinder

External pressure relief pipework required

No risk of pressure build-up

Specialist installation requirements

No special requirements

Annual maintenance required

No special requirements

Notification to Building Control required

No special requirements


And Finally........

Make sure you check out the comparable -

Cost of purchase




Flow rate potential

General running costs





Mains Pressure

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Electric Fires

Hot Water Cylinders

Thermal Stores

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