Exploring The Science Behind Dirty Air Filters

Obviously, if the air filters in your home HVAC system are dirty, you should change them. But exactly how dirty is dirty? [Tim Rightnour] had heard it said that if you didn’t change your filter every month or so, it could have a detrimental effect on the system’s energy consumption. Thinking that sounded suspiciously like a rumor Big Filter™ would spread to bump up their sales, he decided to collect his own data and see if there was any truth to it.

There’s a number of ways you could tackle a project like this, but [Tim] wanted to keep it relatively simple. A pressure sensor on either side of the filter should tell him how much it’s restricting the airflow, and recording the wattage of the ventilation fan would give him an idea on roughly how hard the system was working.

Now [Tim] could have got this all set up and ran it for a couple months to see the values gradually change…but who’s got time for all that? Instead, he recorded data while he switched between a clean filter, a mildly dirty one, and one that should have been taken out back and shot. Each one got 10 minutes in the system to make its impression on the sensors, including a run with no filter at all to serve as a baseline.

The findings were somewhat surprising. While there was a sizable drop in airflow when the dirty filter was installed, [Tim] found the difference between the clean filter and mildly soiled filter was almost negligible. This would seem to indicate that there’s little value in preemptively changing your filter. Counter-intuitively, he also found that the energy consumption of the ventilation fan actually dropped by nearly 50 watts when the dirty filter was installed. So much for a clean filter keeping your energy bill lower.

With today’s cheap sensors and virtually infinite storage space to hold the data from them, we’re seeing hackers find all kinds of interesting trends in everyday life. While we don’t think your air filters are spying on you, we can’t say the same for those fancy new water meters.

34 thoughts on “Exploring The Science Behind Dirty Air Filters

  1. Dirty filters can cause reduced efficiency in cooling, meaning the system will run longer to cool or heat because of the reduced volume of airflow. Reducing the air flow enough will cause the coil to freeze over. A longer term experiment, or minimum airflow specification of the coil would help determine when to replace the filter. All in all this data could be used as a wear indicator for your filter increasing overall HVAC efficiency and reducing the cost of buying filters.

    1. I can personally testify to this, as I was there when the tech showed up to diagnose a frozen AC system, and, when he checking the furnace room, there by the door was the clean filter that he had told the guy six months before needed to be put in to prevent the AC freezing up. It was frozen up right from the outside unit to the furnace itself, and took FOREVER to thaw out. Meanwhile, the guy who thought that he knew more than the tech and didn’t have to worry about replacing the filter right away had to continue to swelter while everything thawed out.

  2. Well the fan may use less energy, but it is doing less work – CFM’s drop as it pulls more vacuum, so the heating and/or cooling job the overall system is supposed to be doing will use more energy or fail to maintain climate settings.

    1. Agree. Anyone who had put their hand over a fan intake would know that the fan picks up speed and uses less energy. If he would have bothered to put a timer on the HVAC system he would have noticed that his heat / cool cycle would have lasted longer with the dirty filter compared to the clean one due to the loss of CFM’S.

      1. I’d be curious how it all pans out in the end – runtime might not change much – lowering CFM you’re going to increase temperature rise (if heating) or drop (running AC). Overall you’re going to loose some efficiency I’m sure, but would be curious how much actually in the end. As long as you don’t loose enough CFM to ice up an AC coil, I’m guessing it’s a relatively minor difference.

        1. For AC – the greater the temperature delta between the evaporator and condenser, the less efficient the system. Lower flow will definitely cause a greater delta.

          For heat – this eventually reaches the point where the heat exchanger overheats and the furnace safety systems kick in and shut the thing down. I speak from experience – one place I attempted to rent had neglected to install an air filter when they remodeled. As a result, all of the remodeling crap was caked on the blower itself, absolutely killing airflow. The furnace would short-cycle every 15 minutes.

          Efficiency was severely compromised, between insulation issues and the furnace problem, the heating bill for 5 weeks where the thermostat was set down at 58 the majority of the time was $200-250. (Note: I was able to use photographic evidence of the neglected filter along with a threat of calling code enforcement to have them agree to release me from the lease about 2 weeks after moving in, given that I called within 30 minutes of receiving the key that there was a problem.)

  3. “So much for a clean filter keeping your energy bill lower.”

    The power used for the blower will be less, per unit time, but the system will run will be increased a lot. The blower is not the highest energy usage part of the system, so on that alone, the dirty filter causes increased cost since the system will need to run longer, but the blower itself will cost more to run due to the greater run time. A clean filter is a key part of energy management.

    For the types of blower most often used in these systems (squirrel cage with forward curved blades for high volume), the power used is dominated by the power o move the air. Free, open units move a lot of air, and units with blocked inlet or outlets work only against the friction and turbulence of a captive air volume, using much less power (often less than half) than doing the job right.

    There is a LOT of literature out there on this. Time is the simple, conservative, way to go. Better is to monitor pressure drop across the filter (EVERY piece of heavy equipment I have run manufactured sine the late 1950’s has a dimple mechanical indicator for this in the engine air filter, and, if equipped with a separate intake for an air compressor, there as well), as this pressure difference is an excellent indicator of intake air rate, and is an excellent indicator of a filter that is too restrictive. (think absolute air pressure sensor on the air intake system of a gasoline, or these days, diesel, engine (Generally, MAP sensor, but there are other options). This tells the engine control exactly how much air gets into the cylinder better than most airflow sensors, such as vane or hot wire, though those are often in the system as well)

  4. The point of changing the filter is to allow a relatively low pressure gradient/high volumetric flow circulation system to operate efficiently throughout its environmental volume. The higher rate of airflow, the more uniform the temperature/humidity and the more comfortable you are. Likely the change intervals are averages of some sort – mine become clogged solid in shorter periods than the recommended interval as a function of pets and a lot of indoor-outdoor traffic.

    A more useful (if somewhat ambitious) experiment would be to cut cardboard “filters” with varying degrees of occluded area to find the rolloff point of the HVAC system (that is, where it becomes noticeably unable to keep up with the heating/cooling load), and match that to the pressure drop of the filters in various stages of filth, preferably without damaging anything. Finding the time that it takes to reach this point would be that particular systems changeout interval.

  5. I agree with the procedure of 3 filters, but “recording the wattage of the ventilation fan would give him an idea on roughly how hard the system was working.”, I’m not sure about. I’ve seen filters clog, cause flow restrictions and permit air-conditioning systems to build ice which further blocks airflow and almost completely shuts down the system. I’d be very interested in a second round with longer sampling periods, flow rate, temperature differential, maybe evaporator temperature and compressor duty cycle. Now that could be conclusive. Good effort, but it would be really cool if you could dig deeper.

    1. So actually, my initial experiment idea was to use just the pressure sensors to collect the data. I actually use a pair of BME680’s, so I also have temperature and humidity. The power usage was kind of thrown in last minute, when I realized I already had a wattage sensor directly connected to that power line, and figured it would also be interesting, and maybe a better indicator.

      But to directly answer your question, yes. The plan is to keep these sensors up there pretty much forever, and see what kind of things come out of it. I ran the quick 10 minute tests to find out if there was anything in this experiment at all, which, it turns out there is. I can “see” the difference in data. (my initial fear was the BME680’s would not be precise enough to see a difference at all, which is why the abomination filter was used)

      The long term goal, is to now find the optimal point. This will likely take 2-3 filter life cycles to figure out, and will require stuff like my runtime data of the AC itself, outdoor temp, etc etc. What I would like, is to eventually hit a point where I have some “goal value” and the home automation system says “now, replace the filer”.

      It’s not a perfect experiment, but I’m now reasonably certain I can do the goal value thing.

  6. Many gas furnaces require a certain b=number of CFM through them to keep the heat exchanger cool enough. I live in an area with a lot of students who rent old houses. A very common thing in the wintertime will be a furnace not running. The not running will be from the hot surface igniter being burnt out, or the heat exchanger overtemp sensor being bad. If you take the time to watch the furnace after replacing the defective part (an alarming number of HVAC tech’s don’t) you often see that it is short cycling and turning off on a heat exchanger over temp error and not the thermostat. This is almost always caused by the students covering HVAC ducts in less used parts of the house to save on energy, but in the process they choke off the furnace. You can only hit the heat exchanger overtemp limit so many times before it starts to take it’s toll on the heat exchanger, which will warp or crack. Cooling systems usually just ice up if they don’t get enough air flow. It does not make them happy, but you don’t see the permanent damage you see with furnaces.

    1. Agreed.
      10 years of call after call stating “There’s something wrong with my thermostat, I think it needs to be replaced” only to find an issue that leads back to reduced airflow. You have one job. Change your filter regularly. (They’ll figure out about keeping the condensate drain clear eventually). Burnt out igniters, faulty hi limit sensors, welded relays, shorted controller boards due to condensate dripping from iced evaporator coils, even overloaded and burnt mains junctions, all due to restricted airflow. Any forced air system that has been properly laid out has a peak efficiency based on an acceptable bracket of flow rate. If you choke it too far it does not matter that the fan is using less power, it’s doing less of it’s intended workload and the system cannot efficiently exchange heat for the assigned internal mass of the structure, of which the air is only part. It has to deliver or remove heat to or from every floor, wall, ceiling, piece of furniture, and appliance in the house. All of this is included as your thermal investment.
      If a hoarder fills his house with junk, displacing enough of the internal air volume with static mass, the system will have to work harder to change the temperature when he finally gets around to replacing the filter, which in all likelihood has been clogged for the last 6 months. But if he keeps it clean (unlikely) and keeps the same temperature all the time, he gains some efficiency due to the slower thermal loss of the static mass. (Assuming he hasn’t blocked any registers, again, unlikely). You cannot gain efficiency by reducing airflow, or by reducing the volume of the thermal load without redesigning the system.
      I went to an old lady’s house once, and she had blocked all the vents in all the rooms except the living room which she spent all her time in. She was complaining about her energy bills. It was a bit difficult getting her to understand that although she was heating and cooling a smaller space, and the system only ran for short amounts of time, her small space was not insulated from the rest of the house, which quickly leached away at her thermal investment, causing the system to start running again in short order. The system was not designed to run efficiently for such short and frequent amounts of time. The constant motion of the air also meant that dust rarely had time to settle, especially since the system was forced to move air through fewer vents, causing it to jet out much harder at the available vents, in turn causing the filter to clog up faster.
      Iv’e spent half the time diagnosing and the other half educating. Or attempting to, that is.
      The pressure sensors are a definite plus for a trouble free system, though. Especially when coupled to an filter alert on the thermostat when the flow drops below critical threshold. There are plenty of homeowners who are very good about changing their filter on a regular basis, writing install dates on them, marking calendars, or even putting sticky notes on the fridge. But they often fail to take into account things like that renovation project that filled the air with sawdust and drywall dust(which is white, so they look at the filter and think it’s clean), or that long hot or cold spell that caused the system to run for extended periods.
      I admire the curiosity and proactive experimenting done with this project. If he were to properly calculate his volume and mass, and was able to get proper measurements on his duct work, take a good anemometer reading, and before and after temperatures, he could apply an enthalpy equation to bear against his equipment and see if it was setup properly.
      It’s all been laid out in the last century of HVAC. Nothing new. But in the end, if he comes up with a cheap and easy pressure differential sensor that sends alerts to his phone, now that will be useful!

        1. Some indoor equipment use variable-speed motors, but not all. These are typically the high-end units. Lower-end units use a high efficiency variation of the variable speed motor, which is a fixed speed ECM motor. These fixed-speed motors are replacing the PSC (permanent split capacitor) motors to help meet DOE efficiency requirements, at higher cost to the customer, no less.

        2. Premium units, but they’re pretty rare.

          Some will have fans that have two speeds (but only two) and not continuous variation.

          Probably 90% of home units out there are single-speed.

  7. New systems with R410 refrigerant can be destroyed by lack of filter replacement. In the less worse case the failure to fully vaporize the refrigerant can result in corrosion to the aluminum heat exchanger coils reducing system life. In the worst of cases you can realize hydraulic compressor lock with compressor failure. This R410 is not forgiving like the old stuff. Many air conditioners and heat pumps are designed for a maximum of 0.5 inch of water draw down in the system.

    The fan coil units often come with 4 inch deep pleated filters that should normally last at leas t6 months. But these filters often cost $40.

  8. I think the monthly ‘rule’ is there, since it’s a convenient measure of time, in relation to the typical need. There really is no one set period between the need to change the filter. I don’t run the heat much in Florida, so I can go 2-3 months, without needing to change the filter. Really depends on a lot of things, not just the passing of time. Monthly is easy to remember, lot of other things we need to do on a monthly basis. If it were every other month, we’d have to keep track, or setup some reminder, lose track of when to change. I usually pull mine out and look at it. How much light that shines through, tells me how clogged up it is. Monthly is about right, for normal usage months.

  9. I appreciate the guys ingenuity, but he has missed the point of his filter. It’s supposed to move air past his air handler, which then conditions it (or just moves it for air flow) to the desired temperature. If the filter is dirty, it can’t move the air, this makes his system run longer.

    How could he miss that?

    I also see that it looks like a 2″ wide filter could fit in there, that would make it a multi-plete, which have more surface area than standard 1″, multi-plete or not. greater surface area equals less resistance and greater are flow.

    I suggest he redo his experiment with a variety of filters and test their air flow. and while he’s at it, measure the run time of his heating and cooling equipment and measure the overall electrical usage of the system.

    If I was grading this, I’d give him an “incomplete”

  10. When considering efficiency, you don’t just look at energy consumption. You need to also consider the cooling load delivered. Energy out/energy in. With a dirty filter, you get less air flow and possibly less cooling delivered (it is a function of temperature as well). It is actually a very complicated relationship, overall efficiency and the dirtiness of the filter. Most systems are designed to be the most efficient with somewhat dirty filter because the majority of the time, the system is in that state, but other factors include the outside temperature, inside temperature, and the inside set point. So it will never be as simple as “this one way is the best way to change your filter”.

  11. I haven’t seen this clarified here explicitly:

    “Counter-intuitively, he also found that the energy consumption of the ventilation fan actually dropped by nearly 50 watts when the dirty filter was installed. So much for a clean filter keeping your energy bill lower.”

    Why is this counter-intuitive? It’s exactly the way fans work. Fans are just windmills in reverse: would you be surprised if the power generated by a windmill went down when the airflow across it dropped?

    The dirty filter restricted airflow, so the fan was doing less work, and consumed less power. However it also meant that less heat was delivered to/removed from the house, which means overall the system works more and in the end, uses more energy.

        1. Because one would expect the fan has to turn harder ro maintain airflow. Or that the fan is trying to compensate for the increases resistance.

          While this may sound odd to experrs, for less experienced people it soundsncounter intuitive. Kinda like how a vacuum makes more noise when you cover the hole. It makes more noise, sounding like its sucking harder and therefore consuming more power.

          I still dont get the reverse windmill anology so dat feel free to enligjten us why this wouldnt work this way?

          1. Most of the energy of a fan is intended to be used for doing actual work – that is, into pushing against air in order to do two things: speed it up and force it through restrictions. Only a little bit is meant to go into forcing it through restrictions, and the rest is wasted in friction and such.

            Going back to fans: If you push against almost no air, you don’t do much work. If you are blocked off, no new air comes in and the trapped air just spins around and around with the blades. The motor doesn’t have to work as hard so it speeds up. There’s more friction but less actual work to do, so it’s less power generally.

            Or, a windmill that doesn’t get hit by any air doesn’t make any power.

            The point where you do a fair amount of pushing against a fair amount of air is the one where you do the most work, and that’s closer to normal operation than to either being completely choked off or being completely unrestricted. Though, normal operation is very close to being unrestricted in the world of ventilation fans. There’s not much force to go around, and you want the energy to go into making the air move faster instead of making it go through more restrictions than are required.

  12. Side note: Something to consider, if you happen to require a filter that handles fine particulates (such as for allergy reasons):

    See if you can determine how the finances work if, in addition to the main furnace filter, you’ve got a 20 inch box fan with a cheap coarse filter for the purposes of getting the coarse junk out of the air, reducing the amount of coarse stuff hitting the furnace filter.

    It used to be possible to get active electrostatic air cleaners that could be washed instead of replaced, but I haven’t seen anything like this in a while.

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