HVAC Savings Upgrades – Cutting Fan Watts In Half or More

There is a simple rule with regards to watts consumed by electric motors. Reduce torque demand and watt draw will reduce.

HVAC Motor SavingsElectric motor torque demand = watts cubed. If you can reduce the work demand of the motor, you can reduce the watts by a factor – to the power of 3.

That formula means that we can get dramatic results by making some improvements in the HVAC system. If the duct design is inadequate and the motor is working hard to get the air through the system – then fan watt draw will be higher than it needs to be. The solution is to reduce the amount of work the motor has to do to move the air.

One might say: “My system was installed correctly, therefore this is not really an opportunity to save energy.” That would be a wrong assumption probably 95% of the time.

Nearly all HVAC systems on residential and commercial installs fail the pressure test parameters – that the manufacturers require for the system to operate as intended. You guessed it – if you think you can just call up any AC contractor and have them replace the equipment and get 16 SEER out of it – that is incorrect. Unless it is installed according to manufacturers spec at least – you will never get 16 seer out of it. Just ask the AC contractor if they test the “static pressures” of the system and correct the ducts until it passes. (Warning: Dont just believe what they tell you – get an outside third party to verify the install – building departments do not check this stuff.) I commonly find 5 ton AC systems only getting 65% of the indoor air they are supposed to through the system. That means the fan is also working very hard to move that air, and fan watts are going to be high. As well the rest of the AC system is now suffering. This is because low airflow will reduce the systems capacity and Energy Efficiency Ratio by a factor of at least 65% if the airflow is 65% lower than it needs to be; at least a 65% reduction in efficiency because the fan watts will be high too!

If you can get more airflow than the required, and get it with low duct pressures – the Energy Efficiency Ratio of the whole system can be increased over what the manufacturer says is possible. Yes – that is right – we can exceed the manufacturers tested SEER and Energy Efficiency Ratio ratings – if installed very well — I have 3rd party studies on this from reputable measurement and verification consultants – well on the way. I will post how to get these kinds of results eventually when the study is officially released.

Here is a hint – a big part of the solution is listed below.


The solution: Fix the duct problems. Then install a DC replacement fan motor (if the system doesn’t already have a DC fan motor).

Here are simple duct design tables based on successful residential installs. They show the area of filter needed and the sizes of return ducts needed. 90% of the duct design problems are in the return ducts to the system. These tables below are now published as part of the updated California Energy Code, and are now published in the book available on Amazon.com - Measured Home Performance: Guide to Best Practices for Home Energy Retrofits in California.

Click on the tables below to expand:

Return Filter Area Design

Return Duct Area

(We will cover installing a DC, or Variable Frequency Drive, motors in the very next blog post.)

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Posted on February 25, 2012, in Right Size and tagged . Bookmark the permalink. 8 Comments.

  1. Great article, Chris! I do have one technical point to make, though. Blowers with PSC motors (the majority of the residential market) consume *less* energy as static pressure increases.

    Blowers with DC (ECM) motors use more energy as static increases, but those with constant CFM drive logic (most variable speed blowers) will maintain the selected airflow. They do this by automatically applying more power.

    So a high static duct system either causes loss of airflow (PSC), or increased energy consumption (ECM), but not both.

    There is an exception to this… an ECM blower without constant CFM drive logic. In that case, you can have both a decrease in airflow and an increase in power as static pressure increases. However, non-variable ECM offers *by far* the best cost-performance for a properly designed (low static) duct system.

    • Christopher Cadwell

      Good points. I agree with all points on the motors.

      This is why we need to reduce duct pressures to 1) Increase airflow if required for climate needs and 2) lower static pressures. Then change the motor to a DC motor that wont use up a lot of watts, because it wont be working to overcome the static pressure issues.

      More to come in the next post.

  2. Myself and Bruce Manclark are giving a talk at ACI National in Baltimore March 26-30th on this very subject. I have some infield data to back up these assumptions. Great article!

  3. There is perceptibly a lot to realize about this. I believe you made various good points in features also.

  4. Hello there, I discovered your web site by way of Google at the same time as searching for a related subject, your site got here up, it seems to be great. I have bookmarked it in my google bookmarks.

  5. How would I go about finding someone who can test, measure, and augment a typical system???

    • Christopher Cadwell

      Depends on where you are located. Search for someone who is Building Performance Institute Certified for AC and Heat Pump. They will know how to test and repair the system for optimal performance.

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