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When it comes to making changes to save energy, building owners like to see low-cost or even no-cost measures. Many of these smaller fixes are relatively obvious to a third-party set of eyes with design and operational experience (perhaps hired to perform an energy audit), but may be missed by those "in the trenches."
So before you start replacing expensive equipment, work with your building engineer or maintenance technician, and conduct an audit to make sure your existing equipment works properly.
Some of the simple things you may find include:
Probably the most underexplored area, though, is thorough examination of the hydronic coils, primarily the cooling coils. Cooling coils, especially those with tight fin spacings, oftentimes act as filters, trapping not only particulate but also biological masses on the fins. This not only impedes the proper volume of air getting through the coils and thus to the spaces served by the HVAC system, but also reduces the heat transfer of the system, causing the compressors to work harder and thus use more energy. And it's the biologicals that can lead to that unpleasant "dirty socks" smell. So, proper cleaning of the cooling coils can not only save energy but also improve IAQ and IEQ. Please note the emphasis on "proper."