Boilers which heat hot water for space heating (radiators) and also for domestic hot water direct to the hot water draw off points are known as combination boilers.Unlike a conventional central heating and domestic hot water system , a combi boiler does not store domestic hot water in a separate large domestic hot water cylinder.It heats cold water directly from the rising main.
A combination boiler or “combi” is a boiler which has a pump, heat exchanger, diverter valve (usually), expansion vessel and controls all inside one unit. Consider a combination boiler to be two appliances housed within one case i.e. a boiler for central heating and an instantaneous water heater for domestic hot water.
It is unlike a conventional heating and hot water system. There is no hot water cylinder , no feed and expansion cistern or cold water storage cistern in the loft and therefore no roof space pipe work and associated insulation.Water for domestic hot water is supplied from the cold mains supply and will exit the hot water draw off points at mains pressure.
A combi saves space and reduces hot water costs, supplying hot water at mains pressure.It provides central heating and domestic hot water.Usually the heat output of combi boilers is governed by the hot water requirements of the property.As this is often more than space heating requirements, modern combi boilers are designed with modulating burners which reduce output to meet the lower space heating demand.Some combi boilers will modulate whilst in domestic hot water mode.
Condensing Boiler Condensing combi boilers are always more efficient than traditional combi boilers because they extract more heat from the combustion gases.Traditional combi boilers produce a high flue gas temperature between 150 C -250 C resulting in heat to waste to the atmosphere..The heat within the flue gases is referred to as “latent heat” and much of this is captured within condensing combi boilers by using either a larger heat exchanger or in some instances a secondary heat exchanger.Collecting this otherwise wasted heat increases the efficiency of the condensing combi boiler from around 80% to over 90%.
- Instantaneous Hot Water
- Constant Hot water
- Water Heated on Demand
- Mains Pressure Hot Water
- No Contamination
- Potable Drinking Water
- Less System Noise
- “Dry” Roof Space
- Space Saving
- No Hot Water Storage
- Reduced Heat Loss
- Less Controls
- Installation Cost Reduced
- Reduced Corrosion Potential
UNVENTED HOT WATER SYSTEM
You may be considering installing new, or changing an existing system to provide mains pressure hot water at your taps. This may be an ideal choice for you, but before you make your decision understand how these systems work and consider all the facts. Boiler Engineer 4U Ltd. has registered and fully qualified engineers to handle the installation/repair of unvented hot water systems
Any system providing a mains pressure supply is only as good as the supply entering the property. A product may boast for example – 35 litres per minute flow rate at 3 bar pressure – but if the supply to your property is inadequate and can only muster say 15 litres at 1 bar, this is all you will achieve. Figures quoted against products are indicative of their potential based on an adequate supply.
HOW DOES IT WORK
In general, a mains pressure system comprises a means of heating water taken directly from the incoming water main prior to delivery to your tap. There are several ways to do this and many different manufacturers interpretations on product design.
There are two main types of domestic Mains Pressure Hot Water Systems.Unvented Mains Pressure Systems Vented Mains Pressure Systems
The term vented refers to whether the system is partially vented to atmosphere or completely pressurised and therefore unvented.
UNVENTED MAINS PRESSURE SYSTEMS
In an unvented system, incoming cold mains water typically enters a pressure vessel (for example: a Megaflo made by Heatrae Sadia, or a ‘Vantage’ made by Vaillant – there are of course others) where it is heated either Directly by means of electrical heater/s, or Indirectly by means of a primary heat exchanger within the vessel being supplied by your central heating boiler. The hot water stored within the vessel is forced out by the incoming cold water when you open a tap hence – Mains Pressure Hot Water.
VENTED MAINS PRESSURE SYSTEMS
In a vented system, heat is imparted to the incoming mains water by means of a secondary heat exchanger within the vessel. Water in the vessel is used rather like a battery for heat – a Thermal Store. The incoming mains water passes through it’s heat exchanger within the vessel drawing heat from the store. The Thermal Store temperature is maintained in the usual way – either Directly with electrical heaters, or Indirectly by your boiler.
Because the water within the Thermal Store does not need to be under pressure (as with an unvented system) it is vented to atmosphere. The body of water within the Thermal Store is used purely as a medium for storing and imparting heat to the secondary exchanger.
Controls for domestic Central Heating Systems – Definitions
1. TIME SWITCH
A switch operated by a clock to control either space heating or hot water ,but not both.The user chooses one or more “on ” periods , usually in a daily or weekly cycle
Two switches operated by a clock to control both space heating and hot water .The user chooses one or more “on ” periods,usually in a daily or weekly cycle.A mini-programmer allows space heating and hot water to be on together ,or hot water alone.A standard programmer uses the same time settings for space heating and hot water.A full programmer allows the time settings for space heating and hot water to be fully independent.
3. ROOM THERMOSTAT
A sensing device to measure the air temperature within the building and switch on and off the space heating.A single target temperature may be set by the user.
4. NIGHT SETBACK
A feature of a room thermostat that allows a lower temperature to be maintained outside the period during which the normal room temperature is required.
5. PROGRAMMABLE ROOM THERMOSTAT
A combined time switch and room thermostat which allows the user to set different periods with different target temperatures for space heating , usually in a daily or weekly cycle.
6. DELAYED START
A device, or feature within a device, to delay the chosen starting time for space heating according to the temperature measured inside or outside the building.
7. Start OPTIMUM START
A device, or a feature within a device, to adjust the starting time for space heating according to the temperature measured inside or outside the building, aiming to heat the building to the required temperature by chosen time.
1. WHAT ENERGY EFFICIENCY MEANS
The energy efficiency of a dwelling depends upon how well it is insulated and how well the heating is controlled, as well the efficiency with which its heating and hot water systems can convert fuel to heat.
The fabric of the dwelling has an important influence on the amount of energy required to keep it comfortable.If the building is badly insulated, even the most efficient heating system will require a great deal of energy to keep it warm.Overall, an energy efficient dwelling is one that is well insulated, has an efficient boiler and good heating controls.Although it may not always be possible to improve building fabric insulation,the heating installer should be aware of opportunities for improved insulation and bring them to the client’s attention.Better insulation will generally improve comfort and client satisfaction and may lead to opportunities for more competitive quotation.
Hot water systems should be insulated to minimise heat loss from storage cylinders and primary circuits; heat output from them may contribute to space heating requirements in the winter but in summer it is wasted and may make the house uncomfortably warm.
2. ENVIROMENTAL IMPACT
The burning of fossil fuels, such as gas,oil and coal, is responsible for large proportion of all carbon dioxide(CO2) emissions to the atmosphere. The concentration of CO2 in the global atmosphere has risen by about 30% since the start of the industrial revolution.Over the past 15 Years, climatologists and others have formed a consensus view that the “greenhouse effect” arising from CO2 and other man-made gases in the atmosphere is likely to cause global warming and consequent changes to climates around the world.This has led to agreements reached under the auspices of the United Nations Organisation to limit further emissions of greenhouse gases. By 2010, the United Kingdom is committed to reducing its greenhouse gas emissions to 12.5% below the 1990 level.The household sector currently accounts for about a quarter of all UK CO2 emissions and energy efficiency measured applied in the sector are expected to contribute a similar proportion of the necessary reductions.Reductions will be achieved by better insulation in new and existing houses, more efficient and better controlled heating systems, and improvements to electrical appliances.
SEDBUK is an acronym for “Seasonal Efficiency of a Domestic Boiler in the UK” . The method used in SEDBUK was developed under the UK Government’s Energy Efficiency Best Practice Programme with the co-operation of boiler manufacturers, and provides a basis for fair comparison of different models.
SEDBUK is the average annual efficiency achieved in typical domestic conditions, making reasonable assumptions about pattern of usage,climate,control, and other influences.IT is calculated from the results f standard laboratory tests together with other important factors such as boiler type, ignition arrangement, internal store size, fuel used, and knowledge of the UK climate and typical domestic usage patterns.For estimating annual fuel costs SEDBUK is a better guide than laboratory test results alone. It can be applied to most gas and oil domestic boilers for which data is available from test conducted to the relevant European standards. The SEDBUK method is used in SAP.
As a simple guide to efficiency, a scheme has been created with SEDBUK efficiency bands assigned to boilers on an “A” to “G” scale. The band is shown in the database and may be used on product literature and labels, though there is no requirement for manufacturers to do so.The scheme is temporary as it will be withdrawn when European directive on boiler energy labelling is introduced.
A number of factors need to be considered when choosing which type of hot water system to install and it is essential the client understands the characteristics of the various options.The main requirements to be considered are:
1. The volume of hot water required
2. The flow-rate and dynamic pressure at which it needs to be delivered.
Both of these requirements are related to the number of people expected to live in the house and the number of bath or showers that may be used at the same time. The choice of system must also take account of the space available in the house for hot water storage and whether or not it is important to have a dry loft, containing no cisterns or water pipes.For all systems that supply hot water directly at mains pressure, including combination boilers and thermal stores, it is particularly important to ensure that the incoming water supply to the dwelling has adequate dynamic pressure and flow at timesof maximum demand. Note that the performance of mains pressure water heaters is specified in terms of temperature rise above the temperature of the incoming water supply.
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