Can I easily integrate it with my existing hot water & central heating system?
Properties in the UK have a variety of different heating and hot water systems. Many of us may not even be sure exactly how the heating and hot water systems in our houses work.
We thought a quick rundown of the common systems in use may be helpful. Below we will cover the following:
- Gas or oil boiler systems, including vented/un-pressurized and unvented/pressurized
- Combination (combi) boiler systems, where water is instantly heated when needed
- Thermal store
- Direct system
Gas or oil-fired central heating and hot water systems
For houses across the UK, these are the most commonly installed systems. Typically, they provide central heating through wall mounted radiators, along with hot water stored in a hot water cylinder – often sited in an airing cupboard.
The following separate items are likely to be part of such a system:
- Boiler
- Hot water cylinder
- Programmer
- Room or radiator thermostats
- Hot water cylinder thermostat
- Radiators
- Diverter valve
- Pump
Your boiler may be powered by oil from an oil tank outside or by mains gas. Mains gas remains the most cost-effective for the majority of the population who can now be supplied in this way. Some systems may be run from propane gas although this is less usual.
New boilers fitted now must be of a condensing type – a condensing boiler – more efficient, i.e. less fuel is needed to produce the same heat and hot water.
Regardless of boiler type, the boiler heats water in two closed circuits: one for the central heating and one for the hot water. The central heating water is warmed and pushed by the pump around the radiators in your house. The hot water system is like another radiator circuit – with a ‘closed’ volume of hot water pushed to the hot water cylinder where a heat exchange coil heats the water in the cylinder.
‘Closed’ simply means that the hot water you use in the bath is not directly heated by the boiler. Instead, the same water is heated again and again – being used to then transfer heat either to radiators, or via the heat exchange coil to the water in your cylinder. This water contains corrosion inhibitor, to slow internal corrosion of the system. This water may be drained and replaced every now and again by a heating engineer.
So what is the heat exchange coil? The hot water cylinder has a long coil of pipe inside, through which the hot boiler water flows. The boiler water is kept separate from the cylinder water, and heat is transferred through the wall of the coiled pipe into the water in the cylinder.
As a backup to the boiler hot water, your hot water cylinder will also have an electric immersion heater installed. This is effectively an electric element within the cylinder that can be turned on to heat the water in addition or instead of the boiler coil.
Control of the system is provided by a number of controls:
- The ‘diverter valve’ is an electrically operated valve which switches the hot water flowing from the boiler to the radiators or hot water cylinder depending on which thermostat is calling for heat. If both are calling for heat at the same time the valve will assume a ‘mid-position’, sharing the flow between cylinder and radiators. The diverter valve is sometimes called a ‘mid-position valve’ for this reason. The control of this valve is automatic.
- The ‘programmer’ is just a clock with at least two separate switches or channels: one for heating and one for hot water. Each of these channels is either on or off. In modern systems, there may be 2 or more central heating channels –for instance, one for upstairs and one for downstairs. When any channel is ‘on’, the boiler is then activated by the thermostats within the system.
- ‘Thermostats’ control the operation of both the central heating and the hot water circuit. For central heating, a room or zone thermostat may be sited on a wall of the house. When the temperature drops below the set temperature (assuming the heating channel is ‘on’ at that time), the boiler will operate.
Similarly, a thermostat on the side of the hot water cylinder drives the operation of the boiler.
Some houses do not have room/zone thermostats and rely on individual thermostatic controls on many of the radiators. Although such individual radiator controls can be beneficial, all systems should have an overall room/zone thermostat to function most efficiently.
- Most boilers will also have a changeable temperature control. This sets the temperature that water is heated to. This sets the temperature of the radiators, and the maximum temperature for hot water. It should be higher than the 55-60deg C setting on the cylinder thermostat.
Some older systems may rely on convection (hot water rising) to drive the hot water circuit and feed to the cylinder. In most modern systems, both circuits (hot water and central heating) are pump driven.
Vented or Unvented
Most older systems of this type will be vented, or unpressurized. Cold water is fed from a header tank usually in the loft, and the whole hot water system is unpressurized. Hot water tap pressure and flow will typically be less than the cold water flow. Such as system may also have a central heating expansion tank in the loft.
More common for modern installations are pressurized or unvented systems. Here the water feed to the cylinder is direct from mains water. The hot water tap will run with similar pressure to the cold water taps. To allow for expansion of water when heated, the hot water cylinder will have a metal sealed ‘expansion vessel’ sited near to the cylinder (often in the airing cupboard).
Combination (Combi) boiler system
These systems are most prevalent in smaller properties, where demand for hot water is less and there is typically only one bathroom. The hot water, for central heating and the hot water system, is heated on demand. Characteristics of this type of system are as follows:
- Hot water heated on demand, with the hot water system having priority over central heating
- Flow rate of a hot water tap will be low, with only one hot tap operating at a time.
- There is no hot water storage and no hot water cylinder in such a system. The lack of cylinder makes this system less appropriate for the addition of a solar water heating system.
Thermal Store
These systems operate very differently to standard systems described above.
A very well insulated cylinder is kept hot at all times, and the hot water and central heating circuits are provided via mains-fed heat exchange coils within the cylinder. So the hot liquid in the cylinder warms the water within the two heat exchange coils which is then pumped to hot water taps and radiators.
The heat to the cylinder is supplied by a normal gas or oil boiler – although it can often be undersized since the demands on it are more constant to maintain the heat in the thermal store and less ‘peaky’ to supply instant central heating. Thermal stores can work well with solar thermal systems.
This type of system is evolving at the moment, with new ‘liquids’ or mediums likely to be available that provide ever better heat storage characteristics.
Direct system
A direct hot water system is one where a boiler is not present at the property, and the hot water cylinder is heated solely with the electric immersion heater.
With such a system, there is no central heating system at all, with heating typically provided by storage or panel heaters – powered by electricity.