Does the electric consumption of HVAC fan increase as the filter gets dirty?

It occurred to me that it is possible that the power consumption of the HVAC fan (in my case for the furnace + AC) may increase as the filter gets dirty. This would be very useful in terms of having an objective indicator of filter dirtiness, as I’ve been getting wildly different advice from different techs (of the same company!) about how often to replace our particle filter (which has high surface area).

Per filtration theory, the pressure drop through a filter does increase as more particles an dust deposit on it. So it is plausible that the fan would work harder (and use more power, detectable by the HS110 that I have the furnace on).

But it is also possible that the fan will use the same power all the time, and I’ll just get less air circulation as the filter gets dirty.

Comments? Anyone has experience with this? Perhaps @halkat971 ? (Since he posted a beautiful analysis of what a furnace does at Gas Furnace analysis)

(The technician will change the filter in a month, so I can do the experiment then, but I was wondering if anyone knew the answer already. Our furnace is hard to access in the attic)

Consuption will decrease as it gets dirty.

Since there is nothing to tell the motor the filter is clogged the motor maintains the constant speed selected. As the filter gets dirty the amount of air flow reduces and thus the work done by the motor and amperage/wattage reduces.

Taking the thought one step further, air flow reduction across the evaporator coil will cause the compressor to see reduced load and thus a reduction in amperage/wattage. BUT the reduced airflow will cause the unit to run longer to maintain temperature. The reduced air flow across the evaporator coil will also cause a reduction in coil temperature. This will put it into an improved dehumidification mode, for a period of time. Ultimately the coil will freeze when the filter gets dirty enough. If your system is equipped with a low pressure switch the compressor will cycle on it to avoid compressor failure.

Lastly, typical filter frames are not 100% tight. Dust will bypass the filter and collect on the evaporator coil. Enough dirt on the coil and the efficiency of your system will be degraded until the coil is pulled and cleaned. Costing you $$$. Regular filter maintenance can save you electric and repair costs. It’s a shame your installer didn’t provide more accessible filtration.

Sorry for the long dissertation.


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That’s great information for many basic systems.

However, I have a “smarter” system, and its controller sets an expected “air flow” (this determination is highly variable: for cooling: it will do low air flow for a longer period to decrease humidity; higher airflow if there is a significant temperature “demand”; lower airflow if only some sections of the house need temp changes [because other sections are automatically shut off because unoccupied or temp OK]).
In short: lots of factors determine what the “demand” for airflow is.

However, say the system decides X airflow is needed, the blower will ramp up to use as much power as necessary to generate that airflow. If you have dirty filters, or any other imbalance, this will cause the blower to need to work harder (and consume more energy) to get the same airflow.

In the above example, this is quite an advanced system. However, I’ve also seen relatively “basic” systems (10+ years old) also equipped with similar “airflow-based” blower settings. In that case, the desired airflow may be set by installers via switches/jumpers on the air handler circuitry.

So, it depends at least partially on your specific system. Could test it by closing all your registers or significantly obstructing the air return grille. Obviously not something to do long-term, just for a few minutes test. :wink::nerd_face:

In my experience, this kind of “airflow” control is only going to be on fully variable-speed air handler units.
If your blower ramps up/down over a span of 20+ seconds (with a gradual “hill”/ramp energy usage), then it’s probably variable speed.
If you see a big spike followed by relatively constant energy usage, then it is probably constant speed (and will have minimal ability to respond to dirty filter, etc).

Thanks. To be clear this smarter system is just part of your HVAC system you bought, and not something you installed, correct?

The experiment of restricting the airflow with the fan only running is a good one, and a lot easier to do than changing the filter. I may do that this evening. Although my fan waveform does seem to match your description of the constant flow one.

Thanks a lot for the info. This is good to know, and when the technician visits next month, I’ll ask him if he can look at the coils and see if they are dirty.

I am told our filters get dirty slowly because our house is very clean (my wife is an artist who values cleanliness a lot, and we don’t have pets), but I may start changing the filters twice a year instead of once a year as it was recommended by several of the technicians.

Also, if indeed the power use of the fan-only goes down with dirtiness, any indication of how much the power will go down at the point that I should replace the filter? E.g. 3%? 10%? 25%? With this info I could go into “scientific filter changing” instead of guessing.

Correct. This is a relatively “top of line” new install approximately a year ago (the energy usage is extremely variable: though I can determine it pretty effectively, Sense struggles to capture more than the minimal amount of usage from this very complex/“random” usage pattern). I live in hot climate: dehumidifying and cooling are both extremely important for this unit (much less need for heating).

I also have a considerably older (10+ years) heat pump unit (with single stage outdoor unit, but variable speed blower)… this exhibits the variable “airflow” behavior as I mentioned in previous post.

I’ve also monitored another house with an extremely old heat pump (roughly 18 years old). That was obviously a constant speed.
That unit was just replaced with a brand new, but more entry-level install (the “silver” line instead of “gold” or “platinum”). That has constant speed air handling and constant speed outdoor unit.

I’d definitely be interested to see what your results are on this. Unfortunately, none of the 4 units I’ve tracked in detail have had an HS110 for the air handler. So I couldn’t differentiate the much smaller “blower” usage from the comparatively massive energy usage of the outdoor unit.

I wouldn’t personally expect to see anything as dramatic as 25% increase for a dirty filter. And if you have a constant speed blower, probably 0 difference anyway (for the air handler). As other posters mentioned, you may get very slightly lower energy usage for the overall system. Which makes sense, but I haven’t observed that to any significant degree myself myself (note that this decreased energy per point in time is almost certainly outweighed by the whole system needing to struggle for a longer period to move enough air to change the house temperature).

Good luck! Let us know what you find out! :+1::nerd_face:

Many variables can change a motor wattage. That’s why I don’t think you’ll be able to create an alarm for a dirty filter. A dry vs wet evaporator will trigger your alarm. If you have zoning amp will change when a zone damper close. What you need is a differencial pressure switch or transmitter across the filters.


A differential pressure transducer had crossed my mind. Anything you can recommend that would be house-friendly?

I do have one of these installed for our radon fan: Not sure if that would be the right range for the filter? Also a Wifi or Z-Wave version that I could see more easily than having to go into the attic would be more useful.

And would there be then a clear criterion about which increase of differential pressure (absolute or relative) means that the filter should be changed?

A dirty filter will impact system performance. Dirt will also accumulate on your coils and fan blades and possibly in the fan itself. As the efficiency of you system decreases, the power consumption increases. A dirty filter will have the same impact on your heating system as well.

A clean system (and filter) is a happy system!

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so you are saying that rather than inefficiency causing the system to use more energy, it would actually use less?

Inefficiency may improve comfort by reducing humidity. But probably consume more energy doing it. But it would be a fairly narrow band. Then the cost would increase as the system becomes more inefficient.

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The current reduction would be hard to use to determine a predicted alarm point. Considering the minimal cost of filters (use pleated, high efficiency, rather than fiber glass) Observing the filter media at changes and determining the amount of dust collected should dictate frequency. If you keep your home reasonably clean semi annual filter changes, as a general rule, should be adequate.


Meh, I just use a dehumidifier in conjunction with my air conditioner. Then I can turn the thermostat up to higher temps without worry