Every energy code in the country limits simultaneous heating and cooling in commercial buildings. ASHRAE 90.1 restricts it. The IECC restricts it. State codes that reference either standard restrict it. The concept is straightforward: do not spend energy heating air that you just spent energy cooling. And yet, Pacific Northwest National Laboratory found that compliance with this requirement is among the lowest of any building controls measure in new construction.
The PNNL study, published as report PNNL-26348, examined the implementation of energy code controls requirements in newly constructed commercial buildings. The findings are striking, not because they reveal a new problem, but because they quantify how widespread an old problem actually is.
The 10 Percent Compliance Rate
PNNL-26348 evaluated compliance across 14 control measures that energy codes require in new commercial buildings. Of all measures tested, limits on simultaneous heating and cooling had the worst compliance rate: approximately 10 percent.
To put that in context, the best-performing measure, zone isolation, achieved 76 percent compliance. Most measures fell somewhere in between. But simultaneous heating and cooling sat at the bottom of the list by a wide margin.
The researchers investigated why. The answer was not that contractors and commissioning agents did not understand the concept. Everyone involved in the study understood that heating and cooling the same air simultaneously wastes energy. The problem was more specific than that.
ASHRAE 90.1 does not simply say "do not heat and cool at the same time." It specifies that VAV terminal boxes operating in reheat mode must limit airflow to no more than 30 percent of design flow while reheating. This is the operational detail that separates code compliance from code violation. A VAV box that reheats at 100 percent of design airflow is heating far more air than necessary. One that limits reheat airflow to 30 percent uses dramatically less energy to achieve the same zone temperature.
When PNNL's researchers pressed commissioning agents on the 30 percent limit, no one indicated they checked the design VAV box schedule for this limit or addressed it in functional testing. The requirement exists in the code. It gets written into specifications. But the verification step, actually checking that each VAV box limits its airflow during reheat, is consistently skipped.
What Noncompliance Costs
PNNL calculated the financial impact of poor compliance across all 14 control measures studied. The potential recovered energy cost savings through better compliance was substantial: $168 per thousand square feet per year, or $2,122 per thousand square feet over the expected life of the control measures. The researchers estimated that approximately 12 percent of total building energy cost could be saved through better compliance with these measures.
Simultaneous heating and cooling was one of the top three measures contributing to those lost savings, alongside automatic dampers and thermostat setback.
For a 100,000-square-foot commercial building, the annualized cost of noncompliance across all 14 measures works out to approximately $16,800 per year. Over a 12 to 15 year equipment life, that is $212,000 in energy costs that code-compliant controls would have prevented.
These numbers come from modeling, not from utility bill analysis. Real-world results will vary with climate, building type, occupancy, and energy prices. But the order of magnitude is consistent with what other research has found. The DOE-funded PNNL study PNNL-25985, which modeled 34 building control measures across 14 building types in 16 climate zones, found that properly tuned building controls could reduce total commercial building energy consumption by approximately 29 percent nationally. Eliminating unnecessary simultaneous heating and cooling is one of the measures that contributes to that total.
Why This Keeps Happening
The PNNL researchers identified several reasons for the persistent noncompliance.
The compliance process ends too early. Energy code compliance is typically verified before a certificate of occupancy is issued. But controls are operational systems that need to be tested under real load conditions, which may not exist during the certificate-of-occupancy inspection. A VAV box that is programmed with the correct minimum airflow setpoint during commissioning can be overridden later by a facilities technician responding to a comfort complaint. Once the building is occupied, nobody checks the code compliance again.
Commissioning does not always verify the right things. Functional performance testing during commissioning tends to verify that equipment starts, stops, and responds to commands. Checking that a VAV box limits reheat airflow to 30 percent of its design maximum requires knowing the design maximum for each box, which means reviewing the mechanical engineer's VAV box schedule, a document that the commissioning agent may not have or may not cross-reference during testing.
Overrides accumulate over time. In the real world of occupied buildings, comfort complaints drive behavior. When an occupant reports that a space is cold, the most common response is to increase the airflow or the heating setpoint. Over years of operation, minimum airflow setpoints creep upward across the building as individual overrides accumulate. Each override is a small decision that makes sense in isolation. Collectively, they push the building out of compliance and into simultaneous heating and cooling waste.
There is no ongoing enforcement mechanism. Building energy codes apply at the time of construction or major renovation. There is no periodic re-inspection for controls compliance. A building that was code-compliant on day one can drift into noncompliance over years of operation, and nobody has the authority or responsibility to catch it.
The Connection to Building Automation
This compliance gap is directly connected to how building automation systems are programmed, commissioned, and maintained.
A properly programmed BAS can enforce simultaneous heating and cooling limits automatically. The minimum airflow setpoint for each VAV box during reheat can be programmed into the controller. Cross-condition alarms, for example an alarm that triggers when a reheat valve is open while the zone is above setpoint, can flag problems in real time. Trend data can show patterns of reheat activity that indicate drift or misconfiguration.
But those protections only work if they are set up correctly in the first place and maintained over time. The PNNL research suggests that in many new buildings, they are not.
For existing buildings, the same principles apply. Any building with VAV terminal boxes and reheat capability should have its minimum airflow setpoints reviewed against the original design schedule. If the design schedule is not available (and in older buildings it frequently is not), a controls contractor can calculate appropriate minimums based on zone size, occupancy requirements, and equipment capacity.
What Building Owners Can Do
The research points to several practical steps for building owners.
Request a VAV reheat audit. Ask your controls contractor to pull trend data on every VAV box in your building and identify which zones are running reheat at airflow rates above 30 percent of design. This is a diagnostic exercise that a competent contractor can perform remotely if your BAS supports trending and remote access.
Verify minimum airflow setpoints. For each VAV box, compare the programmed minimum airflow setpoint against the mechanical engineer's design schedule. If the minimums have been overridden upward, investigate why and determine whether the override is still justified.
Set up cross-condition alarms. A simple alarm that triggers when a reheat valve is more than 20 percent open while the zone temperature is at or above cooling setpoint will catch simultaneous heating and cooling in real time. This alarm logic requires minimal programming effort in any modern BAS platform.
Include controls compliance in your preventive maintenance program. Annual verification that VAV box setpoints match design intent is a low-cost maintenance task that prevents high-cost energy waste from accumulating over years.
The PNNL research confirms what experienced building operators already suspect: the most expensive energy waste in commercial buildings is not caused by old equipment or extreme weather. It is caused by control systems that are not doing what the code requires and the design intended. The good news is that fixing it does not require new equipment. It requires attention to the programming, commissioning, and maintenance of the controls already in place.