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What is HVAC System Performance?

Performance is a term that is being tossed around the HVAC and building industries fast and loose these days. Unfortunately what’s typically being claimed as HVAC system performance is a combination of prescriptive measures and some performance elements, with no measurement of delivered system performance.

While some of the newest industry standards are now pointing to some elements of performance, they still don’t prove a system is truly performing. It’s like saying your car is performing well as long as your engine is running, you have good tires, and your transmission works ok. You could have all three of those elements, and even measure their individual performance, but unless you test the car on the road and measure its performance as a whole (for example, its acceleration, top speed, braking, fuel consumption, etc.), your other measurements are just a hope of performance.

So what’s performance in an HVAC system? Let’s continue with the car analogy. For a vehicle to perform well, its engine must efficiently deliver the rated horsepower. This is akin to air conditioning equipment delivering the right amount of Btus at its rated EER (Energy Efficiency Ratio), which in simple terms is Btus over Watts. So the first step is to measure actual BTUs of the equipment to get a baseline of installed equipment performance.

Next we need to measure delivered airflow and Btus at the registers. When this is done properly, it takes into account the effects of the duct system on the equipment. In other words, poor duct system sizing and installation affects equipment performance.

Measuring delivered air flow also accounts for performance of ducts themselves, both in terms of leakage and radiant losses from their surface. When we know the Btus of the equipment, by measuring Btus into the conditioned space, the difference is what is lost by the air distribution system.

This is like measuring vehicle performance where the rubber meets the road — literally. If we know the engine’s performance by itself, then attach it to the transmission and therefore, the “rest of the system,” we’ll know how the vehicle performs as a whole. Why is this important? To begin with, we can identify if poor performance is caused by the engine, or coming from the transmission forward.

The final piece is comfort. This is where the car analogy ends, because in a vehicle, most of the energy delivered by the engine is powering the drive train and ultimately making the car move forward or backwards. In an HVAC system, we typically don’t have just one register in the house or building, so the final performance measure is making sure the Btus are distributed proportionately to where they’re needed.

Room-by-room load calculations help determine how many cfm of airflow you need at each register to keep every room comfortable. This is also where air balancing comes in. Air balancing techniques can be used to test if rooms are getting the right amount of cfm.

Once you measure each of these factors, you can determine if a system is truly performing. If we’re to become performance-based as an industry, we can’t ignore these realities. Other methods that only include some elements of performance, but not actual “rubber meets the road” performance, have missed the mark, and will continue to — if they don’t change.

Every month we hear from hundreds of NCI members and students reporting both old and new systems that meet local utility or government program criteria as “highly efficient,” but are testing at less than 60% delivered performance.

Is 100% possible? Of course not. However, most systems can, within a reasonable budget, be brought into the mid to high 80% delivered Btu range. That’s with very good equipment performance, and a well designed, installed, insulated, and sealed duct system.

If you could take your customers’ systems from 60% to 85% delivered efficiency, that would mean a 40% improvement in performance! If we did that in all or most of the homes and commercial buildings across North America, we could eliminate the need for building new power plants, and reduce more greenhouse gases than virtually any other measure ever taken — and we’d have the performance data to prove it, rather than computer models and wish-a-Watt hopes.

Of course what really matters is what you can do with this when you’re sitting across the kitchen table from your customers, and showing them how they can get the performance, comfort, and value that they’re paying for and deserve.

Dominick Guarino is Chairman & CEO of National Comfort Institute (NCI), (, a national training and membership organization focused on helping contractors grow and become more profitable. Email him at [email protected] or call NCI at 800/633-7058. For a demonstration of the principles he describes here, visit Performance Town at Mechanical Systems Week. Go to for more information..

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