Dwyer Engineering Blog

Office Building VAV Rooftop Unit Replacement

Posted by Dave Driver, PE

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Feb 9, 2016 5:41:08 PM


VAV Replacement Pitfalls

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When the time comes to replace that old VAV rooftop unit, a building owner may solicit proposals from contractors for more efficient, but same capacity, direct replacement equipment.  

Before signing that purchase order, here are four considerations that could justify a more in-depth look. 

  • Is the capacity right?
  • Is the new unit heavier?
  • Does the new unit fit?
  • Is gas available?

Is the replacement equipment capacity right?

That old rooftop unit was probably sized for inefficient lighting and outdated office equipment throughout the building.  Modern light fixtures and Energy Star office equipment have significantly reduced the interior heat loads associated with these systems.  Perhaps also the roof was replaced a few years ago, and insulation increased.  Properly sized equipment costs less and is more energy efficient than oversized equipment.  On the flip side, does the proposed replacement unit have adequate capacity for today’s required outside air ventilation rates?  Nobody wants undersized equipment either.

On one office building VAV rooftop unit replacement study with two different capacity units, we discovered that the units were inadvertently swapped, so that the larger unit was installed where the smaller unit was scheduled to be on the original drawings, and vice versa.  A direct replacement scenario would have propagated that mistake for another 20+ years.  

Is it heavier?

If the replacement unit is heavier than the old one, structural analysis will be required to ensure that the roof structure is capable of supporting the added weight.   Beware of curb adapters - these are usually required when the supply and return duct configurations of the new unit differ from the original unit, which is a common scenario.  A curb adapter can add north of 1,000 lbs for even a small VAV rooftop unit.

Will it fit?

A replacement unit might be physically larger than the old unit, and will most likely have different clearance requirements for servicing.  While this might not be an issue on a wide open roof, it certainly could be a problem when trying to shoehorn a replacement unit into a tight roof screen area, or near a parapet or similar roof structure.  Encroaching on the equipment manufacturer’s minimum clearance requirements could lead to headaches servicing the equipment down the road.  Is the height of the equipment a concern from a street visibility standpoint?  Again beware of curb adapters, which can add a couple of feet to the height of the new equipment.

Is Gas Heat available?

It may be feasible to bring natural gas to the building and replace the old electric unit with a gas heat unit, which will save on energy bills.  Even if gas heat is not on the property, the gas company will often extend a gas service to the building at no cost to the owner, in anticipation of future revenue.  

Conclusion 

As VAV rooftop units age, efficiency declines, reliability and comfort suffer, and energy bills rise.  Replacement with the new, improved model of what you already have seems logical, but that may not be a good choice. Replacement of central building HVAC equipment is a big investment - with a little engineering analysis ahead of time, the pitfalls of a simple direct replacement can be avoided.  

Office Building HVAC Consult

 

Topics: Washington-DC, Rooftop Units, Office Building HVAC


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