Condensing boilers are water heaters fueled by gas or oil.
They achieve high efficiency by using waste heat in flue gases to pre-heat cold water
entering the boiler.
Water vapour produced during combustion is condensed into liquid form, which leaves the
system via a drain.
In many countries, the use of condensing boilers is compulsory or encouraged with financial
incentives.
Principles of work In a conventional boiler, fuel is burned and
the hot gases produced pass 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, 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 of vaporization.
A typical increase of efficiency can be as much as 10-12%.
While the effectiveness of the condensing process varies depending on the temperature
of the water returning to the boiler, it is always at least as efficient as a non-condensing
boiler.
The condensate produced is slightly acidic, so suitable materials must be used in areas
where liquid is present.
Aluminium alloys and stainless steel are most commonly used at high temperatures.
In low temperature areas, plastics are most cost effective.
The production of condensate also requires the installation of a heat exchanger condensate
drainage system.
In a typical installation, this is the only difference between a condensing and non-condensing
boiler.
To economically manufacture a condensing boiler's heat exchanger, the smallest practical size
for its output is preferred.
This approach has resulted in heat exchangers with high combustion side resistance, often
requiring the use of a combustion fan to move the products through narrow passageways.
This has also had the benefit of providing the energy for the flue system as the expelled
combustion gases are usually below 100°C and as such, have a density close to air,
with little buoyancy.
The combustion fan helps to pump exhaust gas to the outside.
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 the first country to adopt them broadly.
In Europe, their installation is strongly advocated by pressure groups and government
bodies concerned with reducing energy use.
In the United Kingdom, for example, all new gas central-heating boilers fitted in England
and Wales since 1995 must be high-efficiency condensing boilers unless there are exceptional
circumstances; the same regulations apply to oil-fired boilers from April 2007.
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 resulted in increased retrofitting
of existing boiler installations with condensing equipment.
Efficiency Condensing boiler manufacturers claim that
up to 98% thermal efficiency can be achieved, compared to 70%-80% with conventional designs.
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 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 are critical.
Matching the radiation to the Btu/Hr 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 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 space and water heating systems.
Some boilers can be switched between two flow temperatures such as 63°C and 84°C, only
the former being "fully condensing."
However, boilers are normally installed with higher flow temperature by default because
a domestic hot water cylinder is generally heated to 60°C, 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.
For this reason, most condensing boiler heat-exchangers are made from stainless steel or aluminum/silicon
alloy.
External stainless steel economizers can be retrofitted to non-condensing boilers to allow
them to achieve condensing efficiencies.
Temperature control valves are used to blend hot supply water into the return to avoid
thermal shock or condensation inside of the boiler.
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 new domestic condensing boilers produce a basic "fit all" control
system that results in the boiler running in condensing mode only on initial heat-up,
after which the efficiency drops off.
This approach should still exceed that of older models.
Control The control of the domestic condensing boiler
is crucial to ensuring that it operates in the most economic and fuel efficient way.
Almost all have modulating burners.
The burners are usually controlled by an embedded system with built-in logic to control the
output of the burner to match the load and give best performance.
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 who may not understand their operation.
Claims of unreliability have been contradicted by research carried out by the UK-based Building
Research Establishment 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 liquid with anti-corrosion and buffering agents reduces
corrosion of the aluminum heat exchanger.
Some professionals believe that the condensate produced on the combustion side of the heat
exchanger may corrode an aluminum heat exchanger and shorten boiler life.
Statistical evidence is not yet available since condensing boilers with aluminum heat
exchangers have not been in use long enough.
Myths The Building Research Establishment, which
is the UK's major research body for the building industry, has produced guidance on domestic
condensing boilers.
This was originally published in 2003 as General Information Leaflet 74, entitled "Domestic
Condensing Boilers: the benefits and the myths".
The publication is based upon the BRE's long experience with installed condensing boilers
since the 1980s, and is now also published by the Energy Saving Trust as document CE52.
It seeks to highlight the many myths and misconceptions about condensing boilers, and to explain the
known benefits.
This guidance can be downloaded from the Norfolk.gov.uk web site: http:www.west-norfolk.gov.ukCE52.pdf
In particular, the BRE's research has found the following benefits:
modern condensing boilers are just as reliable as standard boilers
condensing boilers are no more difficult to service, nor do they require more frequent
servicing servicing is not expensive; the only additional
task is to check the correct function of the condensate drain
condensing boilers are not difficult to install under all operating conditions, condensing
boilers are always more efficient than standard boilers
Exhaust The condensate expelled from a condensing
boiler is acidic, with a pH between 3 and 4.
Condensing boilers require a drainpipe for the condensate produced during operation.
This consists of a short length of polymer pipe with a vapour trap to prevent exhaust
gases from being expelled into the building.
The acidic nature of the condensate may be corrosive to cast iron plumbing, waste pipes
and concrete floors but poses no health risk to occupants.
A neutralizer, typically consisting of a plastic container filled with marble or limestone
aggregate or "chips" can be installed to raise the pH to acceptable levels.
If a gravity drain is not available, then a small condensate pump must also be installed
to lift it to a proper drain.
The primary and secondary heat exchangers are constructed of materials that will withstand
this acidity, typically aluminum or stainless steel.
Since the final exhaust from a condensing boiler has a lower temperature than the exhaust
from an atmospheric boiler 38°C vs. 204°C a mechanical fan is always required to expel
it, with the additional benefit of allowing the use of low-temperature exhaust piping
without insulation or conventional chimney requirements.
Indeed, the use of conventional masonry chimney, or metal flue is specifically prohibited due
to the corrosive nature of the flue products, with the notable exception of specially rated
stainless steel and aluminum in certain models.
The preferred/common vent material for most the condensing boilers available in North
America is PVC, followed by ABS and CPVC.
Polymer venting allows for the added benefit of flexibility of installation location including
sidewall venting saving unnecessary penetrations of the roof.
Cost Condensing boilers are up to 50% more expensive
to buy and install than conventional types in the UK and the US.
However, as of 2006, at UK prices the extra cost of installing a condensing instead of
conventional boiler should be recovered in around 2–5 years through lower fuel use,
and 2–5 years at US prices.
Exact figures will depend on the efficiency of the original boiler installation, boiler
utilisation patterns, the costs associated with the new boiler installation, and how
frequently the system is used.
The cost of these boilers is dropping as the mass takeup enforced by government takes effect
and the manufacturers withdraw older, less efficient models, but production cost is higher
than older types as condensing boilers are more complex.
The increased complexity of condensing boilers is as follows:
increased size of heat exchanger, or the addition of a second heat exchanger
The necessity of a fan-assisted flue.
Many non-condensing boilers also have this feature, however
as the cooler flue gases produce condensate, this needs to be drained, and so the boilers
are plumbed into a waste or drain With respect to modern boilers, there are
no other differences between condensing and non-condensing boilers.
Reliability, as well as initial cost and efficiency, affects total cost of ownership.
One major independent UK firm of plumbers stated sarcastically in 2005 that it had made
thousands of call-outs to mend condensing boilers, and that the greenhouse gas emissions
from its vans were probably greater than the savings made by the shift to eco-conscious
boilers.
However, the same article points out that the Heating and Hotwater Information Council,
together with some installers, have found that modern condensing boilers are just as
reliable as standard boilers.
Gallery
See also Energy efficiency in British housing
OpenTherm References
External links European Patent Office Patent for drain free
elimination of condensate.
Condensing boilers page on the website of the UK National Energy Foundation
Comparisons of different boiler systems available in North America on the Natural Resources
Canada website Exploded diagram of condensing boiler
On the design of residential condensing gas boilers