Cost of Dirty Air Conditioning:

Reduced Airflow and air conditioning hygiene.

An article on the cost of dirty air conditioning, efficiency, and posable payback for cleaning.

Cost of Dirty Air Conditioning:

Reduced Airflow and air conditioning hygiene.

Rather than being another 'energy savings guru' quoting a quote from another website, a parrot once searched. I'll use some HVAC&R know-how to estimate the energy saved by cleaning the air conditioning indoor coil.

Soiling by dust and other contaminants in the air conditioning restricts airflow. These nutrients lodge in the cool damp air conditioning coil  supporting the colonisation of mould and bacteria. This exacerbates airflow and heat transfer problems reducing the air conditioners ability to maintain design conditions and compromises the quality of indoor air.

Though the 30% to 50% energy savings sometimes advertised may be plausible on some poorly maintained equipment, it's not realistic in most circumstances. What is more likely though is that cleaning your indoor coil and good air conditioning hygiene will restore indoor design conditions and save an estimated 18% on energy. For Sydney siders, this will amount to about $169 when running the air conditioning regularly between October and through to April the following year.


This estimate is based on moderate soiling of residential air conditioning with 15kW sensible cooling capacity and ran on days that the outdoor temperature exceeds 26˚C. For residential systems, a payback period for professional coil cleaning services would be under two years. When considering commercial applications, similarly sized equipment used for cooling and heating, payback would be achieved within 12-months.

Payback periods are based on professional cleaning. A typical clean involves the dismantling of the air conditioning Fan Coil Unit, HEPA vacuuming, wiping down all components, and cold-water-pressure clean of the coil. This assessment does not include soiling and cleaning of the outdoor coil as this is a relatively simple task and should be covered under routine service. Further value can be achieved when using licensed and qualified HVAC&R contractors, specialising in air conditioning hygiene.

Soiling and early stages of mould colonisation are typically evident after the first year of operation. For this reason, air conditioning cleaning should be justified based on good hygiene practice rather than payback periods.

What is Air Conditioning?

Air conditioning is a process to control a room or buildings temperature and humidity. Indoor conditions can be maintained by drawing air, adding or removing heat and moisture, and then returning the conditioned air to the space at a designed rate of air changes per hour.

Air conditioning equipment processes a designed quantity of air changes per hour to control indoor air temperature and humidity.

A ducted air conditioning unit conveys airflow to and from the conditioned space via a network of duct elements which branch from the indoor Fan Coil Unit [FCU]. The FCU contains the blower fan and indoor coil. It is the indoor coil that acts as the heat exchanger between the conditioned airflow and the refrigeration component of the air conditioner. The refrigerant compressor and heat-rejecting process are sometimes referred to as the outdoor or condenser unit.

Measurements and Sensibilities.

Éclairs and the Système International d'Unités [SI], are examples of the French contribution to global commerce, science, and mathematics (yes, French pastries with a decent coffee have sustained business and great minds all over the world).

What's a Watt? A watt is the SI unit used to define the transfer of power or heat flow. Its base units are a kg•m2•s-3. But it's easier to unpack the Watt as being a Joule per Second, i.e. energy or work done every second. Due to the size of a watt, we usually pack it in boxes of a thousand and call it a Kilowatt [kW]. For heat transfer, we use the term kilowatts refrigerant [kWr] or kW cooling.

Most air conditioning is powered by electricity. For estimating purposes, a 'near enough' assumption is that 0.31kW produces one-kilowatt refrigerated cooling (direct expansion, air cooling process). Electricity is metered in units of kilowatt-hours [kWh]. A kWh is defined as the quantity in a thousand Joule per-second per-hour so that, 0.31kWh is the power used by 0.31kW equipment, running at full load, for one hour.

Sensible heat relates the energy needed to change the ‘felt’ air temperature.

Sensible heat is defined as kilojoules per kilogram Kelvin. Low occupancy and residential air conditioning loads can be treated as a sensible heat load for estimating purposes. (For simplicity, sensible heat has been used for this estimate to avoid a discussion about psychometrics and latent heat). Furthermore, most air-conditioning will cycle on based on the sensible temperature of the conditioned space. A high school physics equation, as promised in the prelude, can be used to define the sensible heat load absorbed by an air conditioner.


Reduced Airflow: Why you should clean your air conditioning.

The refrigeration capacity of air conditioning equipment is wasted when airflow is restricted below its nominal airflow rate.

Air conditioning components need to be sized and balanced if the whole system is to run efficiently. A soiled and restricted indoor coil will reduce the coils free cross-sectional area available for heat exchanging and limit the amount of airflow. Even with capacity control, a restricted indoor coil will cause the air-cooling process to be unbalanced with the refrigeration process. This mismatch will result in reduced operating efficiency, excessive wear, and the early failure of the refrigeration compressor.

A ducted air conditioning system has an inherent resistance to the induced airflow. The index run describes the maximum pressure drop associated with the airflow resistance through the ductwork and fittings. The static-pressure-drop of the index run and the duty of the air conditioning fan will dictate the volume of air passing through the air conditioner when the system is clean. Any soiling of the air conditioning equipment will further decrease airflow due to an increase in the static-pressure-drop


A dirty coil and filter will further increase the static pressure loss and reduce airflow.

What about air filters?

Dander from people shedding dead skin cells, dust from fibres in furnishings and paper, damp areas, as well as outdoor air all affect the air conditioning filtration and indoor coil with contaminates.

For every hour of air conditioning operation, a small amount of dirt bypasses the filters and collect on the air conditioning coil.

Air conditioning filtration is never a hundred per cent efficient. Conditioning units less than 1000l/s air delivery, are mostly equipped with G2 grade filtration or lower. G2 grade filters have 65% to 85% Average Arrestance for particles greater than 10 microns (AS1324.1) These filters will catch some of the dust mite faeces, and pollens. But with some spores as small as 2 microns, there is little impact on mould bypassing the filter and colonising the surface of the indoor coil.

Research on filtration and residential air conditioning suggests that fouling time, where the pressure drop across the coil doubles at fixed airflow, occurs within four to seven years (Krafthefter B., 1987) It is important to note that these estimations consider soiling from dust particles and do not include mould colonisation.

Calculations and Charts

An 18% penalty for moderate soiling considers a reduced sensible heat capacity from limited airflow and a further loss by the refrigerant compressor heat of compression.

Energy savings are based on a room temperature of 25˚C with one-degree rise along the return air path to the air conditioning FCU. The indoor coil air-on temperature of 26˚C with 50% relative humidity has been considered. Cooling loads have been used only and are indicative of the air conditioning operation for Sydney October to April.

Outdoor infiltration and conduction, and solar loading on the conditioned space are considered while internal loads from people and equipment are ignored. Assumptions are conservative with 90%, 50%, and 19% of air conditioning capacity applied when the outdoor temperature exceeds 31˚C, 28˚C, and 26˚C, respectively.

Hourly outdoor temperatures are used to define the load are based on averaged data from the Australia Bureau of Meteorology, 1958 to 1967. Hourly temperatures indicate the daily operation of air conditioner irrespective of how the equipment is run. The load will be ether average over a full day's operation or reflect the 'pull-down' period on the stored thermal energy in the mass of the building.

Refrigerant pressure-temperature, airflow, sensible cooling, and indoor coil characteristics are modelled from OEM data for the clean indoor coil. Fan inefficiencies and posable soiling of the outdoor coil have been ignored.

In assessing power consumption of the air conditioning plant, 0.31kW power for each kilowatt of total refrigeration effect, a standard rule of thumb for estimation purposes, has been used.

The static pressure drop along the duct index run (external pressure) is 180Pa with a clean coil and filter. Reading from the original equipment manufacturers [OEM] table. This equipment is rated at 14.9kWr sensible cooling with 1000 litters per second airflow.

Moderate soiling of the indoor coil can be modelled by applying an 18% increase in static-pressure-drop across the coil, (FCU's assumed internal pressure drop of 175Pa at 1000l/s). Reading from the OEM fan curve chart, a 32Pa increase represents a 14% reduction in airflow, so that our soiled air conditioning unit now delivers a reduced airflow of 860l/s, 212Pa (180Pa external plus additional 32Pa internal coil pressure drop).

The heat transfer equation can be used for a simplified model of the sensible cooling process. The total volume airflow rate is multiplied by the specific volume of air, 0.862m3/kg (moist air at 26˚C with 50%RH) and the sensible heat factor of 1.025 kJ per Kg of moist air. 14.9 kWr sensible cooling is achieved with a temperature difference across the coil of 26˚C air-on and 13.5˚C air-off.


A 32Pa static pressure drop across the indoor coil will reduce the conditioned airflow to 860l/s at 212Pa (Figure 3, Fan Curve). Temperatures for the air-on and air-off the coil are selected after considering OEM data and psychrometric tables. Using the Heat Transfer Equation to evaluate the amount of sensible cooling available with moderate soiling reduces sensible cooling from 14.9kWr to 13.7kWr.


It is expected that the soiled air conditioner will need to run 8.7% longer to achieve the same sensible cooling capacity as a clean unit.


A dirty air conditioning coil will restrict airflow as well as limit the total surface area available for heat transfer. The coil will operate with a lower refrigerant pressure-temperate and reduced air contact time due to increased velocity. Increased air turbulence and lower coil air-off temperature come with an energy penalty for reduced airflow.

When the coil is blocked, the free surface area available for heat exchanging and airflow also reduces.

Heat of Compression loss: Reduced compressor suction.

Heat is energy that 'moves' from the higher state to the lower 'colder' state seeking equilibrium. To move energy uphill will always take a bit of work.

A major component of air conditioning's refrigeration system is the compressor. The compressor pumps low-pressure-low-temperature refrigerant from the indoor coil to the outdoor, high-pressure-high-temperature condensing coil. It is in the outdoor condensing coil that the absorbed heat from the conditioned space is transferred to the ambient outdoor air.


Charts call Pressure Enthalpy Diagrams [Ph Chart] are used in the air conditioning industry to assess and design the refrigeration process. The properties of a refrigerant at different pressures and temperatures are laid out in such a way that a refrigeration cycle can be plotted and studied (Figure 4: R410A Ph Chart). The Theoretical Power of the compressor defines the energy needed to move the refrigerant from low-pressure to high-pressure and equates to the Heat of Compression (PH Chart: constant entropy lines).

A reduction in airflow and reduced coil surface area available for heat exchange resulting in the low-pressure refrigerant being denser and lower in temperature and saturated pressure. A 9.1% efficiency loss is seen by comparing a simplified refrigeration process for a clean (blue) versus a soiled indoor coil (red).


The Sum of the Losses: The cost of soiled air conditioning indoor coil.

An 18% energy penalty can be incurred when operating a moderately soiled air conditioning unit.

The energy penalty considers cooling loads only, namely a residential load for Sydney between the months October to April. The clean air conditioning unit is assumed to use 5.89kW of power to produce 15kW sensible cooling and 19kWr total refrigeration. In contrast, the soiled air conditioning unit has a correction factor of 1.179, using 7.25kW of power to achieve the same workload. Based on a metered rate of 27.5c per a kWh, this would represent $180 additional cost per year for cooling.


It is true that air conditioning hygiene and cleaning of the indoor coil is necessary and will have an economic benefit when completed at regular intervals. But, opportunities of up to 30% to 50% energy savings from air conditioning cleaning offered by some contractors may be a little bit of an overstatement. Though plausible is some isolated cases, it's not realistic for moderately soiled air conditioning that's been in service for four to seven years.


This theoretical analysis uses a simplified refrigeration and air conditioning process. OEM tables were used to establish the operating parameters of the clean air conditioning unit. Psychrometric tables, bypass factors, and the logarithmic mean temperature difference (LMTD) were used in establishing pressure and air-off temperatures. These results confirmed the experience of equipment in service. The author wishes to emphasise that the 18% penalty for moderate soiling is a conservative guide only. Air conditioning equipment operates with diverse variables that make each installation unique. Auditing and verification in reference to standards such as AS/NZ 3598 are required to capture true energy-saving potential for each installation.



ActronAir, SRD175C-0100 / SRM175E-0100 P3-090219

ActronAir, SRD175C-0100 / SRM175E-0100-P2-080219

AIRAH, 2010. Best Practice Guidelines: HVAC Hygiene

Australian Standards, 2001. AS 1324.1

Krafthefter, 1987. Air-Conditioning and Heat Pump Operating Cost. ASHRAE Transactions, Vol. 93, pp. 1458-1473




The author, Daniel, is a mechanical tradesperson with over fifteen years of experience in the air conditioning and refrigeration industry. His role at Coil Clean Services is to create customer value by delivering site-specific and performance-driven maintenance for refrigeration and air conditioning coils and heat exchangers. Coil Clean Services is an air conditioning, and refrigeration contractor based in Sydney Hills District specialising in air conditioning and refrigeration coil cleaning. They are fully licensed for all HVAC&R work and can provide additional maintenance, and repairs on the full extent of heating, ventilation, air conditioning and refrigeration systems.