Proper peak-load analyses help ensure your facility will be able to weather the coldest winter and hottest summer conditions. This is when your equipment will be functioning at its highest capacity and defines the size of equipment you will need.  It is important to be prepared for these scenarios but they only account for a small fraction of actual operating conditions. Equipment specification that overcompensates for these worst-case scenarios can dictate that normal operations run poorly. This quickly translates to high operational costs and occupant discomfort.

Too frequently, ERG finds that buildings are performing poorly because the installed HVAC equipment is too big. It is not uncommon for equipment capacities to be more than double the actual loads, so ERG routinely calculates and re-verifies building loads during our energy audits, retro-commissioning projects, and to double check design teams for new build. Oversizing equipment not only has a high initial cost for the owner, but it dooms buildings to poor performance, and can go unnoticed for years.

So why is it that HVAC systems are so often oversized?

The simple answer is that if it is undersized, the building will be uncomfortable in extreme weather, and it will be quickly noticed and fixed, often at high cost. Thus, it is always safer for engineers and contractors to round up in the name of preparation for the worst-case scenario. Oversized equipment is also lower risk to have during startup, and when contractors are paid a markup on equipment, or engineers are paid a percentage of project cost, both the short-term financial incentives and risk assessments work against properly sizing equipment.

Over many years and cycles of replacement, this rounding-up can compound. In general, contractors and owners will look to replace “like for like,” specifying equipment to have the same capacity as was installed before, but sometimes they will again round up, increasing the capacity in order to be “safe”. Sometimes un-needed extra capacity can also be added when there is a building use change or a poor diagnosis of other operational problems. Adding capacity – like an additional roof-top-unit – can be a quick fix, but over time it becomes system bloat.

Just replacing what is there (plus safety factors), or adding new building capacity for new building functions – without a re-evaluation of existing capacity and ongoing need – will lead to increasing system capacity (bloat) over time. The opposite should be happening for cooling equipment because the changes in building energy efficiency – such as improved lighting and appliance efficiency – have been reducing the cooling loads on the building.

In operation, this growing mismatch between the building loads and the equipment on site means the system will run poorly and wear out faster than necessary. Oversized equipment operates intermittently, constantly cycling on and off to meet the needs of the building. This provides poor humidity control, operates inefficiently, and wears out motors, compressors, and other mechanical equipment faster than it would if they were properly sized.

When you can reduce the size of replacement equipment, you save in three ways: you save up-front capital by not buying equipment or capacity you do not need, you save day-to-day by operating the equipment in its most efficient range, and you save by increasing the longevity of your equipment and thus save premature replacement costs.

The best way to determine accurate building loads is measurement. For existing buildings, this can be done with good record keeping. If you have recently tuned your building’s HVAC controls with a retro-commissioning effort, evaluating equipment parameters on the hottest and coldest days of the year can give you a good indication of your peak loads.

The next method for load verification is modeling. Building Energy Modeling (BEM) is a versatile, multipurpose tool that is used in new building and retrofit design, code compliance, green certification, qualification for tax credits and utility incentives, and even real-time building control. Loads analysis is a portion of BEM that focuses on a space-by-space assessment of the heating and cooling necessary determined by a multitude of factors (climatic conditions, building materials, equipment, occupancy, etc.)

Contact us today to determine how much heating and cooling capacity your facility really needs and save money in the process.