You may find some of these descriptions useful :

Open Vented Heating System

An Open Vented Heating System is one where there is a Feed and Expansion Tank (F&E Tank) typically in the loft and this is connected to the heating pipework. As the system heats up, the water expands into the open tank and then contracts back into the system as it cools down.  If the water in the tank falls below the required level it is topped up from the cold mains via a ballcock valve.

Pressurised Heating System

Unlike and Open Vented Heating System, there is no Feed & Expansion tank.  Instead an Expansion vessel is connected to the system.  This is a sealed metal container with a neoprene bubble inside into which the heated system water expands and contracts.  Because it is a sealed system, not open to the air, it is possible to expand too much and to break pipework, possibly dangerously – therefore a safety device called a Pressure Relief Valve is also fitted so that if the system pressure exceeds the design pressure (typically 3 Bar), water is released from the system.

As the system is sealed and does not have an automatic ball valve to allow mains water into the system a filling loop is used which fills the system to a pressure shown on a pressure gauge. This filling loop can take several forms and can be a part of the boiler itself but in many cases is simply a metal-braided pipe connecting a cold mains pipe to the heating system with a valve at each end and a one-way check valve to prevent heating water going back into the mains drinking water.


Open Vented Hot Water System

This is where a large tank of cold water is sited at the top of a house and is filled from the cold mains drinking water using a ball valve to maintain a level.  One or more pipes lead from the tank to most of the cold water taps in the building.  The kitchen tap is always supplied from the cold mains directly and sometimes other cold water outlets as well. 

Another pipe from the cold tank leads to the bottom of the hot water cylinder which is generally made of copper and sited in an airing cupboard below the cold tank.  A pipe from the top of the hot water cylinder goes both to all hot water outlets in the building and back up and over the top of the cold water tank so that if the water in the cylinder expands it can overflow into the cold tank which is open to the atmosphere (albeit it should be covered with a dust, light and insect-proof lid).

The pressure of the hot water at the taps is determined by how high the cold tank in the loft is above the tap.  Water from a tank in the loft of a typical two storey house is approximately 5 meters above the ground floor.  The pressure produced is 0.1 bar per meter.  Therefore a pressure of 0.5 Bar is produced.  As we generally want better pressure in our showers than this system can provide, supplementary shower pumps are often used to pressurise the hot and cold water to shower outlets.

The pressure of water from the cold mains is generally between 1.5 Bar and 3 Bar.    Because of this, it is always greater than the pressure of the hot water and therefore it is generally bad practice to have cold mains water supplying a shower or bath where it is mixed with hot water from an open vented system as the cold water will always overpower the hot and make it very difficult to mix the two together to achieve a satisfactory temperature of water.

Because this system relies on a tank of cold water in the loft at atmospheric pressure and with old tanks this may well be uncovered and uninsulated, there is a risk of Legionella as water can stagnate, heat up partially and feed the hot and cold outlets.

 Hot water cylinders should be fitted with an immersion heater so that if the boiler fails, water can be heated using electricity.

Pressurised (Unvented) Hot Water System

Unlike an Open Vented Hot Water System, there is no cold tank in the loft supplying taps/outlets or the hot water cylinder.  Instead, all cold water outlets are connect to the cold mains drinking water. The copper hot water cylinder is replaced with a steel cylinder able to withstand much greater pressures (typical design pressure 10 Bar) and this is fed directly from the cold water mains.  This is heated either  by an electric immersion heater or by a boiler so that hot water to all hot water taps is therefore at the same pressure as the cold mains. 

This means that there is a much reduced risk of Legionella or other impurities getting into the tap water and that a much better pressure and flow is available at all outlets making the use of shower pumps unnecessary.  in fact, you cannot and must not pump mains water in such systems.

Because hot water expands and this is already a pressurised system, further expansion must be allowed for and and expansion vessel or similar device is necessary to safely allow for this plus safety valves – Temperature and Pressure Relief Valves – to release water from the system should the pressure get too high.

Hot water cylinders should be fitted with an immersion heater so that if the boiler fails, water can be heated using electricity.

Heat Only Boiler

A Heat Only boiler heats water for the central heating system and hot water cylinder.  It is the simplest of the boilers as there are fewer pipework connections to be made and fewer components to go wrong.

Hot water from the boiler flows through either the radiator circuit or through a coil in the hot water cylinder to heat the water in the hot water cylinder,  The water heated by the boiler does not itself go into the hot water cylinder and out of the taps, it flows through a coil in the hot water cylinder,  heating up the cold water in the cylinder indirectly.  

Other components of the system – pumps, valves and expansion vessels if required are external to the boiler itself.


System Boiler

Like the Heat Only boiler, this heats water for the central heating system and hot water cylinder.  The difference however is that it is used for a pressurised heating system and contains a number of system parts that would otherwise be external to a Heat Only boiler, for example the pump, expansion vessel and pressure relief valve.

Where major changes are being made to a system it is often cheaper and easier to install a system boiler rather than the separate component parts however it also means that when components fail, they must be replaced by the correct parts from the manufacturer and not generic items.

Combi Boiler

With a Combi Boiler, the boiler heats the radiator circuit just like Heat Only and System boilers,  The heating system is pressurised with the expansion vessel being sited inside the boiler itself. The hot water for all taps and showers is produced by water from the boiler passing through a plate heat exchanger.  Cold water from the mains runs through the other side of the plate heat exchanger and is heated instantaneously.

All cold taps in the premises are connected directly to the cold mains drinking water with the benefits of increased pressure and safety from Legionella and impurities.  The hot water, coming also directly from the cold mains is at the same pressure as the cold mains meaning that there is much less Legionella risk and of course it is easy to produce good pressurised showers without the use of shower pumps. 

As there is no cold tank in the loft and no Feed and Expansion tank and as there is no hot water cylinder, significant storage space can be gained, particularly in the airing cupboard.  One significant disadvantage of Combi boilers are that if anything goes wrong with the boiler there is no backup of stored hot water in a cylinder.

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