Can you spot the design mistake?

Every once in a while we come across problem jobs with design issues. We like to share these so that everyone can learn from these mistakes and can hopefully avoid them in the future. 

We never share the identity of the designer, contractor, owner, or specific location. And we usually mix up sizes of the equipment to help with the anonymity.

Project location: Somewhere in Kansas City Metro

Goal: Bring in 6,250 cfm of 100% outside air with an ERV and a split system

ERV design conditions:

  • 6,250 cfm supply air
  • 6,000 cfm exhaust air
  • Summer entering air 100/73
  • Summer leaving air 82/67

20 ton split system design conditions:

  • 100 degree ambient
  • Return air conditions (None)
  • EAT 82/75 (all from ERV)
  • LAT 56/54

One afternoon we received a call that there may be an issue with the split system because the space temperature wasn’t being satisfied. The investigator thought he confirmed this when he noticed the delta-T across the split system evaporator was low (only 14 degrees).

Here were the actual field-measured operating conditions:

  • 87 degF ambient
  • 81.2/75 entering air
  • 67/65 leaving air 
  • 6,250 cfm

First things we checked was how much total cooling the split system was producing:

Total cooling = cfm * 4.5 * dH

where dH is the change in enthalpy

Total cooling = 6,250 cfm * 4.5 * (38.8-30.2)

Total cooling = 241,000 buths (approx 20 tons)

The unit was performing as it was supposed to, producing approximately 20 tons. 

So why is our discharge temperature so high? Do you notice the design mistake? 

The ERV was designed for the wrong entering WET BULB.

In Kansas City, we typically design for 78 degree wet bulb, and this was designed at 73 degrees. How big of a difference do those 5 degrees make?

A properly sized ERV would have had more latent capacity, and the temperature leaving the ERV (and entering the air handler) would have been closer to 81/67, not 81/75 as seen on site.

If you plug 81/67 degF entering air temps into a total capacity calculation, you will find that the leaving air temperature would be 55/55 with 19.6 tons of capacity. 

Another big takeaway is this: It is almost impossible to design a 100% outdoor air make-up air system with a standard ERV and a unitary split or RTU. To do this, you would need an ERV with an effectiveness close to 100%, or you would need a larger split system with a 6 or 8 row evaporator coil. 

Are you working on any design build projects that we can help you with?

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Brad Telker

Vice President of Sales, Applied Systems Group at cfm Distributors, Inc.
Brad joined the cfm team in 2006, and now as Vice President of Sales, Applied Systems Group he focuses on business development, as well as helping contractors and engineers find creative and unique solutions to any size HVAC project. When he’s not at work, Brad enjoys reading, running and spending time with his family.
Brad Telker
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