Air Filter Alarm System to Prevent
High Energy Consumption

Devin Gable

Research Experience for Undergraduates Program

Summer 2001

Arizona State University


Air filters are an integral part of any heating, ventilating, and air conditioning (HVAC) system, but are often a neglected one. Although either changing disposable air filters every month or washing reusable air filters every other month could both save money in energy expenses and prevent damage from happening to the equipment within the system, most people do not regularly maintain their air filters. Due to this fact, the goal of this research project was to create a useful alarm which would detect when an air filter needs to be changed and alert the homeowner to do so, thereby minimizing the energy consumed by the air conditioning system through the early detection of air filter deterioration. All types of air filters were studied and many different sensors were considered in the building of the air filter alarm. It was decided that a basic cadmium sulfide photocell, or light dependent resistor (LDR), would be the most effective and cost efficient sensor for the job. The alarm was built using this sensor and calibrated to have a light turn on when an air filter needed to be changed. The difference in the energy consumption of a HVAC system when there is a clean filter being used verses when a dirty filter is being used was found to be about 3.61%. These main goals of the project were met and much more information about air filters, HVAC systems, and cost effectiveness of energy conservation was acquired as well.

1. Introduction

This project covered many aspects of HVAC systems including how they work, what types of air filters are available to homeowners, what type of maintenance procedures are required for them, and what type of things cause a HVAC system to consume more energy than it needs to. Many types of sensors were also studied and one was decided upon to be used to build the air filter alarm. The alarm was built and calibrated to go off when an air filter needs to be changed. Using a testing apparatus consisting of a small fan, some ducting, and an air filter; the amount of energy consumed when a clean filter was used was compared to the amount of energy consumed when a dirty filter was used. All of these findings are included in the following pages.

2. Background Information

2.1 The Need for an Air Filter Alarm

As stated before, an air filter is an integral part to any HVAC system. Most air filters are located inside the home in walls or ceilings where they are placed at the beginning of the ducting that pulls the warm air out of the house, also called the return duct. They are made out of various materials designed to collect airborne particles that could carry dirt directly into the evaporator coil and impair the coils heat-absorbing capacity. Air filters are very important for this reason, but if they are not replaced or cleaned regularly they become clogged and dirty and block normal air flow which reduces the systems efficiency significantly.[1]  Hence, the need for an air filter alarm. Below is a general diagram showing how HVAC systems work and where the air filter fits into the system.

Fig. 2.1 HVAC System

2.2 Types of Air Filters

In the beginning of the project the many different types of air filters were studied in order to determine what kind of materials they were made out of, how long they lasted before they needed to be changed, how they were designed to maximize air particle collection, what the health benefits were of having a clean filter, and how and why the prices of filters varied. Researching showed that there are three basic types of air filters: fiber, pleated, and electrostatic. Fiber air filters are generally made out of cotton polyester or spun glass and they are supposed to be changed every month. They range in price from $0.50 to $2.00 and only pick up large airborne particles to protect the equipment of the HVAC system, therefore they have no health benefits. Pleated air filters are generally made out of cotton polyester and need to be changed about once every three months. They range in price from about $4.00 to $17.00 and pick up large airborne particles as well as many microparticles such as dust, smoke, and viruses, therefore alleviating some allergies and respiratory irritations. Lastly, electrostatic air filters are generally made out of foam and can be reused for up to ten years, though they do need to be washed every one or two months. They range in price from about $14.00 to $25.00 and can attract up to 30 times more air particles than an ordinary fiberglass filter, therefore providing a good alternative to electronic air cleaners for allergy sufferers.[2]

Fig. 2.2 Basic Fiber Air Filter

2.3 Researching Sensors

The second major task for this research project was to find a sensor that could be used to detect when the air filter needed to be changed. This proved to be a lengthy process since there were so many types of alarms to consider. First, infrared emitters and detectors were researched to see if they could be used. This idea was discarded once it was found that the sensor could only detect when there was a definite blockage in its path, not when there was only some degree of blockage as is the case with a dirty air filter. Secondly, ultrasonic detectors were considered to measure the thickness of the clogging on the air filters, but this would be a very expensive type of sensor and not very practical for the needed application.[3] Thirdly, pressure sensors were considered to measure the pressure difference from one side of the air filter to the other. Although this could be an effective type of sensor for this application, it would be hard to simulate within a lab the actual pressure changes of a house due to things such as opening and closing of doors, therefore this idea was also discarded.[4] Finally, a cadmium sulfide photocell was considered and found to be the best sensor for the given job. A cadmium sulfide photocell is simply a light dependent resistor. When more light falls upon the photocell, the resistance decreases and when less light falls upon the photocell, the resistance increases.[5]

Fig 2.3 Photocells

3. Approach

3.1 Determining Which Air Filter To Use

Of the three basic types of air filters, the one most commonly used inside the average home is the fiber filter.[6] Although it provides no health benefits to the homeowner, it is very affordable and will protect the HVAC equipment from being clogged or damaged by air pollutants. For this reason, it was decided that this type of filter would be used to calibrate the air filter alarm, though theoretically the alarm should work no matter what type of air filter it is used on.

3.2 Designing the Alarm Set Up

The alarm was designed so that the LDR would be on one side of the air filter and a light emitting diode (LED) would be on the other, pointed directly at the LDR. The way in which the alarm would work is that as the air filter becomes more and more clogged, less light from the LED is able to fall upon the LDR therefore increasing the resistance. Once the air filter becomes clogged to a certain point, the resistance would become great enough to trigger the alarm to go off. This alarm was designed to have minimal expenses and the way in which it was built is found in the following section.

4. Experimental Work

4.1 Building the Air Filter Alarm

The air filter alarm was built on a breadboard using simple components such as resistors, variable resistors, one integrated circuit (IC), LEDs, and of course the light dependent resistor. A PSpice simulation of the basic circuitry for the alarm can be found at the end of this paragraph. A dirty air filter, one which had been used for exactly 30 days in a typical home, was then used to calibrate the alarm light to go off when it was placed between the LDR and the LED. Although there was no time to test the alarm in a real household over the period of a month, the alarm should go off when the air filter is as dirty as the one with which it was calibrated. It should go off no sooner or later. Sketches of the design of the final alarm to be installed within a household can also be found in the attachments at the end of this report.

4.2 Building a Testing Apparatus

A testing apparatus was set up in order to simulate the amount of airflow over an air filter for the period of a month. This testing apparatus was built using a small ventilation fan, 3 feet of ducting, a register box, an air filter, and the alarm itself. The fan was put at one end of the ducting with the air filter at the other end and the alarm was placed with the LED on one side of the air filter and the LDR on the other. According to the energy expert at the Arizona Public Service (APS) Company a very general estimate of the number of hours an air conditioning system runs is 990 hours per year, or 82.5 hours a month.[7] Therefore, it was determined that the fan should run for 82.5 hours before the filter would become clogged enough for the alarm to go off.

4.3 Testing the Alarm

On July 3, 2001 the air filter was attached to the end of the testing apparatus, the fan was turned on, and the testing of the air filter alarm began at 9:30 a.m. The fan ran for the full 82.5 hours and the alarm did not go off. There was found to be only a minimal change in the voltages of the alarm circuitry and the air filter itself did not appear dirty at all. This showed that the fault was not with the air filter alarm, but with either the testing apparatus or the calculated hours that it was supposed to run. The following data shows the voltages of the 8 pins on the IC when the filter is clean, when it is dirty, and when it had been run for 82.5 hours on the testing apparatus.

IC PINS Clean Filter Dirty Filter Tested Filter
1 0.70 V 0.70 V 0.70 V
2 2.16 V 2.09 V 2.14 V
3 1.74 V 2.30 V 2.02 V
4 0.00 V 0.00 V 0.00 V
5 2.09 V 2.10 V 2.10 V
6 1.79 V 1.81 V 1.80 V
7 0.51 V 0.50 V 0.51 V
8 0.57 V 0.55 V 0.57 V

5. Results

5.1 Why the Alarm Did Not Go Off

The air filter alarm did not go off during the testing, but this should not be attributed to problems with the alarm. The number of hours a typical HVAC system runs seemed to be inaccurate when it was applied to the testing apparatus built for this project. The APS energy expert referred to this information from the APS standard cost book which shows that it takes 5545kW to cool a typical 1800 square foot home and then assumed that the home would have a 4 ton AC using 5.6 kW.[8] However, the testing apparatus for this project was only using a small ventilation fan and 3 feet of ducting to pull air across the air filter. Also, the test was ran within a lab on the fifth floor of a building where there would not be as much dirt as would be found in a typical household. Therefore when the apparatus was built and the fan was run for the 82.5 hours required in a months period of time, the filter did not appear dirty and the alarm did not go off. It had nothing to do with the effectiveness of the air filter alarm, as a matter of fact, it showed that the alarm was working in that it did not go off because the air filter was not dirty.

5.2 Alternative Use For the Testing Apparatus

Even though the alarm was not able to be tested effectively in a real time simulation, the testing apparatus still proved to be useful. By simply hooking up an ammeter to the fan and measuring the current drawn across it when both clean and dirty air filters were on the other end of the ducting, the difference in the energy consumption of the fan when clean and dirty air filters were used was able to be found. The following is the work for those calculations.

5.3 Finding the Difference in Power Consumption

Power is measured by:

P = I V cos()

where P is in the power in watts, I in the current in amps-rms, V is the voltage in volts-rms, and cos() is the power factor of the fan motor. The energy consumed is then measured by

E = P T

where E is the energy in joules, P is the power in watts, and T is the time in hours.

Air Filter Current Reading (mA) Power (Watts) Energy (Joules)
Clean 415 0.415 V cos() 0.415 V cos() T
Dirty 430 0.430 V cos() 0.430 V cos() T

Because the voltage, power factor, and time would be constants, the difference in the energy consumption between the clean and dirty filters can be found as a percentage. The dirty air filter consumes 3.61% more energy than the clean air filter. This may not seem like a substantial difference, but one must consider this difference over an extended period of time. If one was to gather up their electric bills for a ten year period of time, estimate the portion which was spent on running their HVAC system (which should prove to be a considerable amount), and then take 3.61% of that amount, they would most likely find that having a clean air filter would save them a great deal of money.

6. Summary and Recommendations

In this project an air filter alarm was built in order to alert a homeowner that their air filter needs to be changed. Clean air filters prevent damage from being done to the HVAC equipment, allow for less energy to be consumed by the HVAC unit, and some even provide a healthier environment in which to live. It is very cost effective to stay up to task on the maintenance of air filters, therefore it would be cost effective to buy an alarm to prompt you to do so. The alarm is a very simple circuit using LEDs, an LDR, resistors, variable resistors, and one comparator, or integrated circuit. This would allow the alarm to be in the $20.00 range: a very affordable price for the amount of money saved when the air filter is clean. It has been shown that it provides many benefits to the homeowner to have a clean air filter and since most people do not remember to change or clean their air filters on time, this alarm would be of great use to the general public.

There is still much work to be done on this project before it can be created for commercial use and there is a good deal of future recommendations to be considered as a follow up to this project. The air filter alarm needs to be tested within a real HVAC system for the full period of time that it takes for most air filters to become clogged. The alarm needs to be tested with all types of air filters in order to see how the LDR sensor reacts to different materials and whether or not it will go off around the time that the manufacturers of the different filters claim that they need to be cleaned or changed. A final design of the household alarm needs to be built in such a way that it will be easy for any person to install and so that different lighting sources such as sunlight from windows and lamps within the home do not alter the input of the LDR sensor. A dust protective housing needs to be designed to go around the LED and the LDR of the alarm. This is the most prominent work in the project left to be done.

7. Acknowledgements

The work done on this project would not have been possible without the help of Dr. Keith Holbert and Dr. Khaled Nigim. Their experience in electrical engineering made it possible for them to bring many new ideas to the participants of the REU program and to help in the construction of the circuitries for the various projects built.

The REU program was a great learning experience which helped the undergraduates understand what the work of a real engineer in the field is like. The basic task life cycle of an engineering project was taught and followed by the participants in order to show the most effective way to solve a problem or reach a goal. It was taught that preparation, planning, execution, and conclusion are very integral parts of any project. Targets must be identified, goals and objectives must be established, research activities and strategies must be developed, progress must be evaluated, and findings must be presented in an effective and organized manner in order to conduct an efficient research project.

This program brought a realization of what it means to do research in the engineering field. Problems occurred, there were delays in progress for various reasons, some experiments turned out well, some did not, and this was all part of the learning experience. Every engineering student should benefit from some sort of experience such as this so that they can know what the field they are moving into expects from them and what they can do to be better engineers.

8. References

  1. Consumer Energy Information: EREC Fact Sheet (2001). Energy-Efficient Air Conditioning, p. 3.
  2. The Home Depot Product Index (2001). Heating and Cooling: Learn About Air Filters.
  3. James Vincent, A+ Certified Technician, Authorized Velleman Distributor.
  4. Input Devices and Sensors: Analog Devices (2001). Pressure Measurements, p. 8.
  5. Sensor Design (2001). Light Sensors: Photocell, pp. 11-12.
  6. Custom Vac (2001). Air Filtration, p. 1.
  7. Arizona Public Service Energy Expert.
  8. Arizona Public Service Energy Expert.


Sketches of a basic model for a household air filter alarm are given in the attached pdf file.