Energy Saving in Buildings
Better AHU Pre Filters can bring
substantial energy savings
From: Camﬁl Farr Ltd
Date: June 2010
Camfil Farr – Better AHU Pre‐filters Can Bring Substantial
It is generally accepted that using Low Energy Air Filters will minimise energy use for a given filter
particulate efficiency and air flow rate. This is achieved by the low pressure drop across the filter.
Another way the air filters can help save energy and maintain plant efficiency is by keeping coils and
other heat exchanger surfaces clean. The coils will then work to their full rated capacity.
The comparison of pressure drop reduction energy saving and energy changes involved in coil
efficiency reduction in this paper indicate that they are of a similar order and both significant. These
potential savings can be realised by using low energy air filters and to some extent by improving
heating and cooling controls. Other savings and benefits on maintenance costs and Air Quality can
also be made.
This White Paper will show a way to assess the potential energy saving benefits of a maintained coil
and heat exchanger heat transfer efficiency, when fitting F7 class Low Energy Air filters as Air
Handling Unit pre‐filters. There has been limited testing and research into the energy saving benefits
of clean coils and the linked gain of using higher grade pre‐filters. Reduced variations in pressure
drop and coil heat transfer efficiency offer the prospect of tighter HVAC system control and
improved energy savings.
The Legislation concerning energy usage in Air Handling Units
Current legislation for energy inspections of Air Conditioning systems with Air Handling Units is
covered by the Energy Performance Buildings Directive. This requires an inspection and report which
includes the condition and effective function of the air filters. Energy saving measures such as the
upgrade of air filter systems with Low Energy Air Filters should be considered.
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The Business Challenge
The coming period will be difficult for energy managers and plant engineers running air conditioning
plants. The current budget constraints are being felt across the economy. The Carbon Reduction
Commitment (CRC) Energy Efficiency Scheme was launched in April 2010, impacting on an estimated
20,000 UK businesses. The CRC will no doubt bring the subject of energy consumption to
boardrooms across the country, with Indoor Air Quality and low energy consumption presented as a
business challenge requiring urgent resolution.
The good news is that it is a straightforward process selecting the correct air filters to ensure Indoor
Air Quality is maintained and energy consumption reduced. An Opportunity Assessment Survey
(OAS) or Air Handling Plant Assessment (AHPA) can be carried out to evaluate the potential for
Better AHU Pre‐filters Can Bring Substantial Energy Savings
Some air filter manufacturers have for a long time advised the fitting of better pre‐filters to give an
energy efficient performance.
When using the Eurovent filter Life Cycle Costing model (LCC) Camfil advises fitting an Hiflo M7 F7
Low Energy Air Filter. For a 2‐stage filter system an Hiflo M7 F7+F7 set up is recommended.
EN13053 also states to AHU manufacturers that 'If a single stage of air filtration is used, a minimum
filter class of F7 shall be fitted'.
Traditionally' G3/G4 panels have been used as pre‐filters and indeed sometimes they are the only
AHU filters fitted. To be effective a pre‐filter stage should be positioned just upstream of the coils
and heat exchangers in the HVAC system. This gives increased protection to the AHU coils as well as
meeting recommended air quality requirements.
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The current Eurovent LCC model for air filters covers maintenance issues, which can arise due to
dust deposition; duct cleaning for example. However has consideration been given to the loss of coil
efficiency due to the deposition of fine dust layers on the heat transfer surfaces?
Studies on HVAC coil efficiency
Studies on HVAC coil efficiency and cleaning companies indicate even very thin dust layers on coils
can mean significantly more energy usage, eg. 0.3mm. depth results in a 10% increase.(see graph)
Let us now try to quantify and compare a. and b. below.
a) the energy usage and possible saving by using F7 pre‐filters to prevent coil heat transfer
efficiency loss due to dust deposition on heat transfer surfaces.
b) the energy saving that can be made on reduced air filter pressure drop using the Eurovent
LCC model for air filters.
(Note : In a. It is assumed that the HVAC control system would use energy to increase the
temperature difference of the heating hot water or reduce the temperature of the cooling chilled
water to compensate for the loss of coil efficiency, otherwise the system will not deliver the
To make a comparison we can adopt a building energy usage model. A recently published HVAC
model for air‐conditioned offices has been used for this purpose.
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The assumption has been made that all heat transfer losses have occurred through the cooling coils,
heating coils and air‐to‐air heat exchangers. In reality there will also be increased losses through the
pipe‐work system at places with poor or no insulation if temperature differentials are increased.
Although heat losses are not always lost to the building envelope leakage of heat must be regarded
as a very inefficient use of energy. Any loss of coil heat transfer efficiency must be made good by
increased input from the boiler in a heating system and the compressor in a cooling system if it is a
There is no attempt to directly fix the upper and lower levels of savings that can be made by
maintaining clean coils with the optimising of air filters but there is clearly a strong link. The
comparison made is to try and establish a reasonable estimate of the scale of the energy saving
The upper level of 30% and lower of 10% for coil efficiency energy losses seem to be generally
accepted as the level of losses commonly encountered in HVAC systems that are at the upper and
lower ends of the plant condition and maintenance spectrum.
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Although poorly maintained coils can suffer heat transfer efficiency losses in excess of 50% and
significant pressure drop energy losses due to partial blockage. Coil pressure drop losses incurred as
a result of dust loading are not quantified in this paper but can be significant.
The upper level 18% and lower of 9% reduction for fans, pumps and controls is an estimate based on
LCC energy calculations for pressure drop and some reduced pump load when coil efficiency is
maintained. Controls energy usage is usually quite low.
The resulting figures suggest that at the lower (10/9%) level the benefit of F7 class pre‐filters based
on pressure drop compared to benefit of maintenance of clean coils are broadly similar.
Ratio (1.28:1) in favour of coil efficiency benefit.
At the higher (30/18%) level the maintenance of coil efficiency indicates an increasing benefit
approaching twice as much as that of filter pressure drop.
Ratio (1.92:1) in favour of coil efficiency benefit.
Given the current problems with rising energy cost and supply this relationship deserves further
investigation. There can be little doubt of the increased energy saving benefit given by F7 class pre‐
filters when considering the longer maintained coil efficiency.
Consideration should be given as to whether the coil efficiency energy saving benefit should be
included in the filter LCC model. The UK government should seriously consider including air filtration
as an energy saving technology and include it on the energy technology list (ETL).
The analysis in this report indicates that savings of up to 14% of the Total
Building Energy costs are possible using F7 class Low Energy Air Filters as pre‐
filters to maintain clean AHU coils.
The effect of using F7 class pre‐filters in HVAC systems to reduce energy usage seems to be
underestimated by many designers and plant operators.
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There are a huge number of Air Handling Units dating from the 1980's designed with poor grade
G3/G4 pre‐filters and side withdrawal frames (SWF) but never the less have a generously sized
plenum depth (700mm. or more).
These otherwise well engineered AHU's are now ripe for air filter section refurbishment. If they are
fitted with Front Withdrawal Frame (FWF) filter mountings the typical payback time is only 3‐6
months. A refurbishment kit based on an Hiflo M7 F7 class bag filter plus the FWF mounting would
yield very significant energy savings in HVAC systems across the UK and thus should be included on
the approved Energy Related Products list. It should form an active energy saving measure
supported as part of the government Carbon Reduction Commitment (CRC) program.
The Camfil Solution
Refurbishment of existing Air Handling Units will be a necessary action to meet energy efficiency
targets over the next few decades. Camfil can offer a complete refurbishment solution for air
filtration systems in existing air handling units.Camfil Farr offer low energy air filter products that
provide the highest indoor air quality and deliver the biggest energy savings to customers.
Camfil Farr low energy air filters are manufactured using fine fibres that consistently deliver effective
filtration and low energy consumption throughout their installed lifetime. The combination of fine
fibre technology and high‐quality engineering and construction enables our filters to deliver both
Indoor Air Quality and optimised energy performance.
Use Camfil Low Energy Air Filters to ensure that buildings achieve Indoor Air Quality and deliver low
energy consumption and improved Life Cycle Costing to ensure business needs are well maintained.
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Camfil low energy air filters are the solution. They offer the lowest air filter energy cost throughout
their installed lifetime.
For more information visit the Camfil Farr website www.camfilfarr.co.uk
Or subscribe to the Low Energy Air Filter Blog at www.lowenergyairfilter.co.uk
Save money and reduce carbon ‐ Commercial HVAC Nigel Lenegan – Low Carbon Consultant
Recommendation concerning calculating the Life Cycle Cost of Air Filters Jan 1999
Air Filters and LCC for F+S October 2002 Jan Gustavsson
EN13053 Ventilation for buildings ‐ Air handling units ‐ Ratings and performance for units,
components and sections
Clean coils equals efficient coils ‐ Feb 2010 Tom Fitzsimons
Study verifies coil cleaning saves energy ‐ Ross Montgomery ASHRAE Nov2006
Glossary of Acronyms:
AC Air Conditioning
AHPA Air Handling Plant Assessment
AHU Air Handling Unit
CIBSE Chartered Institute of Building Services Engineers
CRC Carbon Reduction Commitment
CLG Communities and Local Government
CO2 Carbon Dioxide
EN European Standard
EPBD Energy Performance Buildings Directive
Etracker Energy usage monitor and recorder
EU European Union
HVAC Heating Ventilation and Air Conditioning
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IAQ Indoor Air Quality
LCC Life Cycle Costing
kW Power rating kiloWatts (for AC Cooling capacity)
MEMU Mobile Energy Monitoring Unit
NOx Emissions of Oxides and Nitrogen
OAS Opportunity Assessment Survey
PM10 Particulate matter 10 microns and below in size
PM2.5 Particulate matter 2,5 microns and below in size
SBS Sick Building Syndrome
SOx Emissions of Oxides of Sulphur
TM Technical Memo (CIBSE)
VAV Variable Air Volume
VOC Volatile Organic Compound
UK United Kingdom
Tel: 0044 (0)1706 238000
Fax: 0044 (0)1706 226736
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