HVAC Systems - What's the Difference?

This is a technical subject, but an important one...

The "Base Building Specification" within the Request For Proposals required a dedicated library-appropriate Heating, Ventilation and Air Conditioning (HVAC) system. We believe that this specification was properly made because indoor environmental comfort and air quality should be critical considerations for a significant civic building such as this.

Some have suggested that if the Central Library was embedded within a larger office building, it could save significant cost by sharing a central system with the office tenants... However, we believe that a shared heat pump system would be a "penny-wise, but pound-foolish choice" at best - and potentially a nightmare.

Here's some of the important differences between the dedicated Variable Air Volume (VAV) system proposed by Fiore-Irgens, and the shared heat pump system proposed by T.Wall.

COST TO DEVELOPER: The heat pump system will typically be less expensive as a base system for the developer because the boilers and make up air handling unit are both smaller. Additionally, a cooling tower is less costly than a chiller.

COST TO LIBRARY INTERIOR BUILD-OUT: The Library interior build-out will likely experience a higher cost to install the heat pump system as heat pumps are more expensive than VAV boxes and require more power wiring.

OVERALL COST: The overall cost of the VAV and heat pump systems are typically found to be comparable. However, the heat pump system shifts more of the cost burden to the interior buildout portion of the project as described above.

NOISE & COMFORT: Each heat pump has a compressor and fan. Heat pumps will be located above the ceiling throughout the new library. Both the compressor and fan create noise which is sometimes found to be objectionable. Also, heat pumps heat via overhead with luke warm air. This can sometimes result in drafts which people find uncomfortable. Finally, because an individual heat pump is larger and more expensive than a typical VAV box they are less likely to be used to provide zoning for small individual rooms. The sharing of thermostats can sometimes lead to comfort and temperature control problems.

INDOOR AIR QUALITY: Heat pumps only have simple filters (30% efficient). Generally, the make up air units only has 30% efficient filters as well, although higher efficiency filters can be installed. A heat pump system typically only brings in minimal outdoor air. The VAV system by comparison can bring in up to 100% outdoor air when operating in economizer mode. The VAV systems’ air handling units will have high efficiency filters installed as standard. Finally, the VAV system easily allows for a very efficient humidification system to be added.

MAINTENANCE: Each heat pump has a compressor, a fan, a filter, a reversing valve, a condensate pan, and controls. All of these items require more maintenance than a VAV box. The maintenance of these heat pump components must be done in the ceiling at the location of each heat pump through the new library. By contrast, the VAV system will have a large chiller. The VAV system’s chiller will be located on the roof making it easier and less intrusive to maintain. Another item to consider is that each heat pump will have a condensate drain and drain line. If the drain becomes plugged, water can overflow potentially causing damage to books, other media, equipment, and the library interiors below.

OWNERSHIP: A shared central heat pump system will have boilers, pumps, water treatment and a cooling tower or fluid cooler. Allocation of operating costs in a shared heat pump system will be quite difficult with different types of users and different operating hours throughout a mixed-use building. As ownership of the other components of the mixed-use facility change throughout the life of the building, each new owner brings a different philosophy of maintenance, cost sharing, etc. This can lead to a lifetime of disputes related to operating cost allocation. A stand-alone HVAC system (such as the VAV system proposed by Fiore-Irgens) eliminates these issues in their entirety.

ENERGY: Both a heat pump system and a VAV system are very efficient heating and cooling systems. Efficiency will largely depend on building dynamics and the design of the specific HVAC system. A stand alone VAV system can more easily be designed with the City’s efficiency expectations in mind. This is due to the fact that it is stand alone and decisions will not affect the costs attributable to other users in a mixed-use facility. Additionally, the Fiore-Irgens proposal includes LEED Silver certification causing the stand alone HVAC system to be designed to a high standard in order to achieve such certification.