Enthalpy Economizers Don’t Work!

In engineering, we have what we call rules of thumb. Those are general approximations known to be “good enough”. The additional accuracy obtained by complex mathematical formula is either not worth the trouble, eclipsed by the instruments’ lack of precision, or simply a bigger source of human error. We, poor flesh beings, are easily confused.

Following this, if you want to control your rooftop packaged unit by calculating the enthalpy differential (or simply the enthalpy) instead of simply checking the outside temperature, you better have some good reason! Every layman can understand how the RTU is controlled by the outside temperature. The enthalpy differential is, on the other hand, orders of magnitude more abstract, which may lead to more errors and more difficulty in maintenance.

Turns out enthalpy economizers aren’t worth it. By that I don’t mean the gain is somehow negligible, oh no! I mean that this overly complex solution is worse than using the plain outside temperature (dry bulb). Here is a really nice article written by Steven T. Taylor, P.E., Fellow ASHRAE; and C. Hwakong Cheng, Member ASHRAE: Economizer High Limit Controls and Why Enthalpy Economizers Don’t Work.

I highly recommend you read it, but here are the most important figures:

https://frozenlock.files.wordpress.com/2011/11/wpid-fig20.png https://frozenlock.files.wordpress.com/2011/11/wpid-fig21.png https://frozenlock.files.wordpress.com/2011/11/wpid-fig22.png https://frozenlock.files.wordpress.com/2011/11/wpid-fig23.png https://frozenlock.files.wordpress.com/2011/11/wpid-fig24.png

Notice any trend? The outside temperature isn’t simply the cheapest and simplest way, it’s also the best one!


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