Condensing Boilers Reliability
Condensing boilers are claimed to have a reputation for being less reliable, requiring professional installation and regular service, and may also suffer if worked on by installers and plumbers that may not understand their operation.[5] Claims of unreliability have been contradicted by research carried out by the UK-based Building Research Establishment (see "Myths" below.)
Heat exchangers in condensing boilers are primarily manufactured using stainless steel and aluminum. Initial testing and annual monitoring of the heat transfer fluid in condensing boilers with aluminum or stainless steel heat exchangers is highly recommended. Maintenance of a slightly alkaline (pH 8 to 9) liquid with anti-corrosion and buffering agents reduces corrosion of the aluminum heat exchanger. There is a feeling among some professionals in the field that the condensate produced on the combustion side of the heat exchanger may corrode an aluminum heat exchanger and shorten the boiler's life. Statistical evidence is not yet available since condensing boilers with aluminum heat exchangers have not been in use long enough.
To ensure reliability, condensing boilers must be properly installed and maintained.
Thursday, October 27, 2011
Condensing Boilers Efficiency
Condensing Boilers Efficiency
Condensing boiler manufacturers claim that up to 98% thermal efficiency can be achieved,compared to 70%-80% with conventional designs (based on the higher heating value of fuels). Typical models offer efficiencies around 90%, which brings most brands of condensing gas boiler in to the highest available categories for energy efficiency. In the UK, this is a SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) Band A efficiency rating, while in North America they typically receive an Eco Logo and/or Energy Star Certification.
Boiler performance is based on the efficiency of heat transfer and highly dependent on boiler size/output and emitter size/output. System design and installation is critical. Matching the radiation to the Btu/Caloric output of the boiler and consideration of the emitter/radiator design temperatures determines the overall efficiency of the space and domestic water heating system.
One reason for an efficiency drop is because the design and/or implementation of the heating system gives return water (heat transfer fluid) temperatures at the boiler of over 55°C, which prevents significant condensation in the heat exchanger.[ Better education of both installers and owners could be expected to raise efficiency towards the reported laboratory values. Natural Resources Canada also suggests ways to make better use of these boilers, such as combining the space and water heating systems. Some boilers (e.g. Potterton) can be switched between two flow temperatures such as 63 degrees Celsius and 84 degrees Celsius, only the former being "fully condensing". However, the boilers are normally installed with the higher flow temperature by default because a domestic hot water cylinder is generally heated to 60 degrees Celsius, and this takes too long to achieve with a flow temperature only three degrees higher. Nevertheless, even partial condensing is more efficient than a traditional boiler.
Most non-condensing boilers could be forced to condense through simple control changes. Doing so would reduce fuel consumption considerably, but would quickly destroy any mild steel or cast-iron components of a conventional high-temperature boiler due to the corrosive nature of the condensate, and is the reason why most condensing boiler heat-exchangers are made from stainless steel or aluminum/silicon alloy.
The lower the return temperature to the boiler the more likely it will be in condensing mode. If the return temperature is kept below approximately 55°C the boiler should still be in condensing mode making low temperature applications such as radiant floors and even old cast iron radiators a good match for the technology.
Most manufacturers of the new domestic condensing boilers produce a very basic "fits all" in-built control system that ends up with the boiler running in condensing mode only on initial heat-up, after which the efficiency drops off, although it should still exceed that of older models (see the following three documents published by the Building Research Establishment: Information Papers 10-88 and 19-94; General Information Leaflet 74; Digest 339. See also Application Manual AM3 1989: Condensing Boilers by Chartered Institute of Building Services Engineers).
In the United States, all residential boilers (of foreign or domestic origin) are tested and rated by the US Department of Energy (D.O.E) to an Annual Fuel Utilization Efficiency (AFUE) rating. All residential condensing boilers currently available have an AFUE of 90% or more. The vast majority are now rated at 95%+. All condensing boilers also qualify and are usually listed as EPA "Energy Star" appliances, qualify for power company rebates and federal tax credits. All condensing boilers in the U.S. are fitted with microprocessors to modulate output, and capable of operating on outdoor reset.
Condensing boiler manufacturers claim that up to 98% thermal efficiency can be achieved,compared to 70%-80% with conventional designs (based on the higher heating value of fuels). Typical models offer efficiencies around 90%, which brings most brands of condensing gas boiler in to the highest available categories for energy efficiency. In the UK, this is a SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) Band A efficiency rating, while in North America they typically receive an Eco Logo and/or Energy Star Certification.
Boiler performance is based on the efficiency of heat transfer and highly dependent on boiler size/output and emitter size/output. System design and installation is critical. Matching the radiation to the Btu/Caloric output of the boiler and consideration of the emitter/radiator design temperatures determines the overall efficiency of the space and domestic water heating system.
One reason for an efficiency drop is because the design and/or implementation of the heating system gives return water (heat transfer fluid) temperatures at the boiler of over 55°C, which prevents significant condensation in the heat exchanger.[ Better education of both installers and owners could be expected to raise efficiency towards the reported laboratory values. Natural Resources Canada also suggests ways to make better use of these boilers, such as combining the space and water heating systems. Some boilers (e.g. Potterton) can be switched between two flow temperatures such as 63 degrees Celsius and 84 degrees Celsius, only the former being "fully condensing". However, the boilers are normally installed with the higher flow temperature by default because a domestic hot water cylinder is generally heated to 60 degrees Celsius, and this takes too long to achieve with a flow temperature only three degrees higher. Nevertheless, even partial condensing is more efficient than a traditional boiler.
Most non-condensing boilers could be forced to condense through simple control changes. Doing so would reduce fuel consumption considerably, but would quickly destroy any mild steel or cast-iron components of a conventional high-temperature boiler due to the corrosive nature of the condensate, and is the reason why most condensing boiler heat-exchangers are made from stainless steel or aluminum/silicon alloy.
The lower the return temperature to the boiler the more likely it will be in condensing mode. If the return temperature is kept below approximately 55°C the boiler should still be in condensing mode making low temperature applications such as radiant floors and even old cast iron radiators a good match for the technology.
Most manufacturers of the new domestic condensing boilers produce a very basic "fits all" in-built control system that ends up with the boiler running in condensing mode only on initial heat-up, after which the efficiency drops off, although it should still exceed that of older models (see the following three documents published by the Building Research Establishment: Information Papers 10-88 and 19-94; General Information Leaflet 74; Digest 339. See also Application Manual AM3 1989: Condensing Boilers by Chartered Institute of Building Services Engineers).
In the United States, all residential boilers (of foreign or domestic origin) are tested and rated by the US Department of Energy (D.O.E) to an Annual Fuel Utilization Efficiency (AFUE) rating. All residential condensing boilers currently available have an AFUE of 90% or more. The vast majority are now rated at 95%+. All condensing boilers also qualify and are usually listed as EPA "Energy Star" appliances, qualify for power company rebates and federal tax credits. All condensing boilers in the U.S. are fitted with microprocessors to modulate output, and capable of operating on outdoor reset.
Condensing Boiler Usage
Condensing Boiler Usage
Condensing boilers are now largely replacing earlier, conventional designs in powering domestic central heating systems in Europe and, to a lesser degree, in North America. The Netherlands was probably the first country to take them up in a large way[citation needed]. In Europe, their installation is strongly advocated by pressure groups and government bodies concerned with reducing energy use. In the United Kingdom, for example, since 2005 all new gas central-heating boilers fitted in England and Wales must be high-efficiency condensing boilers unless there are exceptional circumstances, and the same applies to oil-fired boilers from 1 April 2007 (warm air central heating systems are exempt from these regulations). In the United States, there is a Federal tax credit for the installation of condensing boilers and additional rebates from power companies in some states. In Western Canada, energy suppliers now offer energy rebates when these systems are installed in multi-unit dwellings. The decrease in natural gas prices in North America has not hindered the retrofit of existing boiler installations with condensing equipment.
Condensing boilers are now largely replacing earlier, conventional designs in powering domestic central heating systems in Europe and, to a lesser degree, in North America. The Netherlands was probably the first country to take them up in a large way[citation needed]. In Europe, their installation is strongly advocated by pressure groups and government bodies concerned with reducing energy use. In the United Kingdom, for example, since 2005 all new gas central-heating boilers fitted in England and Wales must be high-efficiency condensing boilers unless there are exceptional circumstances, and the same applies to oil-fired boilers from 1 April 2007 (warm air central heating systems are exempt from these regulations). In the United States, there is a Federal tax credit for the installation of condensing boilers and additional rebates from power companies in some states. In Western Canada, energy suppliers now offer energy rebates when these systems are installed in multi-unit dwellings. The decrease in natural gas prices in North America has not hindered the retrofit of existing boiler installations with condensing equipment.
Principles of work- Condensing Boilers
Principles of work-Condensing Boilers
In a conventional boiler, fuel is burned and the hot gases produced are passed through a heat exchanger where much of their heat is transferred to water, thus raising the water's temperature.
One of the hot gases produced in the combustion process is water vapour (steam), which arises from burning the hydrogen content of the fuel. A condensing boiler extracts additional heat from the waste gases by condensing this water vapour to liquid water, thus recovering its latent heat. A typical increase of efficiency can be as much as 10-12%. The effectiveness of this condensing process varies, it depends upon the temperature of the water returning to the boiler, but for the same conditions, it is always at least as efficient as a non-condensing boiler.
The condensate produced is slightly acidic, 3-5 pH, so the choice of materials used in the wetted areas have to be suitable. At high temperature most commonly used are aluminium alloys and stainless steel, in the low temperature areas plastics are most cost effective, for example uPVC and polypropylene. The production of condensate also requires the installation of a heat exchanger condensate drainage system. For a basic installation this is the only difference required compared to a non-condensing boiler.
For the heat exchanger within a condensing boiler to be economic to manufacture, and for the appliance to be manageable at installation, the smallest practical size for its output is preferred. This has produced heat exchangers with very high combustion side resistance and so the use of a combustion fan to move the products through narrow passageways has been adopted. This also has had the benefit of providing the energy for the flue system as the expelled combustion gases are usually below 100C and no longer have much natural buoyancy.
In a conventional boiler, fuel is burned and the hot gases produced are passed through a heat exchanger where much of their heat is transferred to water, thus raising the water's temperature.
One of the hot gases produced in the combustion process is water vapour (steam), which arises from burning the hydrogen content of the fuel. A condensing boiler extracts additional heat from the waste gases by condensing this water vapour to liquid water, thus recovering its latent heat. A typical increase of efficiency can be as much as 10-12%. The effectiveness of this condensing process varies, it depends upon the temperature of the water returning to the boiler, but for the same conditions, it is always at least as efficient as a non-condensing boiler.
The condensate produced is slightly acidic, 3-5 pH, so the choice of materials used in the wetted areas have to be suitable. At high temperature most commonly used are aluminium alloys and stainless steel, in the low temperature areas plastics are most cost effective, for example uPVC and polypropylene. The production of condensate also requires the installation of a heat exchanger condensate drainage system. For a basic installation this is the only difference required compared to a non-condensing boiler.
For the heat exchanger within a condensing boiler to be economic to manufacture, and for the appliance to be manageable at installation, the smallest practical size for its output is preferred. This has produced heat exchangers with very high combustion side resistance and so the use of a combustion fan to move the products through narrow passageways has been adopted. This also has had the benefit of providing the energy for the flue system as the expelled combustion gases are usually below 100C and no longer have much natural buoyancy.
Condensing Boiler
Condensing Boiler
This is a design of boiler which can have an increased efficiency over the more traditional boiler. The efficiency of a typical non-condensing boiler is around 75%, whereas with condensing boilers it can be over 87%. This increased efficiency is due to the extraction of heat from the otherwise wasted flue gases. Most boilers have a single combustion chamber enclosed by the waterways of the heat exchanger through which the hot gases can pass. These gases are eventually expelled through the flue, located at the top of the boiler, at a temperature of around 180°C.
Condensing boilers, on the other hand, are designed first to allow the heat to rise upwards through the primary heat exchanger; when at the top the gases are rerouted and diverted over a secondary heat exchanger. These can reduce the flue gas temperature to about 55°C. This reduction of temperature causes the water vapour (formed during the combustion process) to condense and, as the droplets of water form, fall by gravity to collect at the base of the flue manifold. The remaining gases are expelled to the outside environment through a fan-assisted balanced flue. The condensation produced within the appliance should be drained as necessary into the waste discharge pipework or externally into a purpose-made soakaway.
It is only possible for a condensing boiler to work to these very high efficiencies if the flow and return pipework is also kept below 55°C. The flow & return temperatures need to be maintained for the heat transference to occur from the flue to the water (i.e. heat transference goes from hotter to cooler materials).
This is a design of boiler which can have an increased efficiency over the more traditional boiler. The efficiency of a typical non-condensing boiler is around 75%, whereas with condensing boilers it can be over 87%. This increased efficiency is due to the extraction of heat from the otherwise wasted flue gases. Most boilers have a single combustion chamber enclosed by the waterways of the heat exchanger through which the hot gases can pass. These gases are eventually expelled through the flue, located at the top of the boiler, at a temperature of around 180°C.
Condensing boilers, on the other hand, are designed first to allow the heat to rise upwards through the primary heat exchanger; when at the top the gases are rerouted and diverted over a secondary heat exchanger. These can reduce the flue gas temperature to about 55°C. This reduction of temperature causes the water vapour (formed during the combustion process) to condense and, as the droplets of water form, fall by gravity to collect at the base of the flue manifold. The remaining gases are expelled to the outside environment through a fan-assisted balanced flue. The condensation produced within the appliance should be drained as necessary into the waste discharge pipework or externally into a purpose-made soakaway.
It is only possible for a condensing boiler to work to these very high efficiencies if the flow and return pipework is also kept below 55°C. The flow & return temperatures need to be maintained for the heat transference to occur from the flue to the water (i.e. heat transference goes from hotter to cooler materials).
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