VAV System Heating and Cooling at the Same Time
A 110,000 sf, five-story speculative office building uses VAV air handling units with hot-water reheat and 55 F supply air. After the first tenant occupies floors 2 and 3, the property manager logs three recurring complaints: interior zones are cold all year; the south perimeter is warm on winter afternoons while reheat is active in the same zone; and the third floor feels stuffy at 2:00 p.m. even though CO2 sensors read 650–700 ppm. The commissioning report notes that the outdoor-air damper at AHU-2 has been stuck at its minimum position since startup, VAV boxes are set to a 30 percent minimum, and reheat valves have been open during 60 percent of occupied hours. The owner will not replace air handlers and wants a scope of work for the service contractor.
1. Explain the mechanism behind simultaneous heating and cooling and identify the sequence-of-operation changes that address it without new equipment.
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The system is fighting itself on two fronts.
Interior zones: with 55 F supply air and a 30 percent minimum, an interior zone with no envelope load receives far more cold air than it needs at part load. The zone drops below setpoint and the only available response is reheat, which runs all year. The 60 percent reheat runtime is the symptom; the excessive minimum is the cause. Recompute the minimum using the ASHRAE 62.1 multiple-space equation and the actual system ventilation efficiency - it is often 10–20 percent, not 30.
South perimeter in winter: sunlight adds gain while the box sits at minimum and the reheat valve is open. A single-duct box with reheat cannot cool and heat simultaneously, so the control sequence should force reheat off above setpoint with a real deadband, and the supply air temperature should reset upward (to 60–62 F) at low load so the box is not delivering refrigeration into a heating zone. Verify the discharge-air thermistor and the valve are not leaking or misreading.
Also check static pressure reset - constant high duct pressure pushes minimum airflow higher than the setpoint.
2. CO2 reads 650–700 ppm, well under a typical 1,000 ppm control setpoint. Why is that not proof of adequate ventilation, and what would you do to establish the truth?
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A comfortable CO2 number only proves that the sensor's location is being ventilated.
- Sensor location. Return-duct or AHU-mounted sensors average the whole system and hide a starved zone. Zone sensors mounted near a diffuser read dilution, not breathing-zone concentration.
- The damper is stuck. If AHU-2's outdoor-air damper has been frozen at minimum since startup, demand-controlled ventilation cannot respond to occupancy and economizer free cooling never engages. Fixed minimum ventilation at design occupancy may still look acceptable on a CO2 chart while the building is short of air at actual peak density.
- CO2 is a proxy for people, not for anything else. It says nothing about VOCs, filtration, humidity, or air movement. A stuffy complaint at 700 ppm with a cold supply is more likely a temperature, stratification, or diffuser-throw problem than a ventilation-rate problem.
Establish the truth with a measured outdoor-air intake traverse or balometer reading, converted to per-person and per-area rates against the 62.1 calculation; a zone-by-zone CO2 injection test; and calibration verification on every sensor. Repair the damper actuator first - everything else depends on it.
3. Write the functional performance test you would require before the service contractor is paid.
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Require a written test procedure approved before work starts, executed over at least two weeks of occupied operation with trending enabled.
- Damper and economizer. Stroke AHU-2's outdoor-air damper full range, verify actuator response and position feedback, and demonstrate economizer changeover against the high-limit setpoint.
- Ventilation rate. Traverse the outdoor-air intake at design and at 50 percent flow; compare with the 62.1 calculated requirement and record the as-found versus as-left numbers.
- Minimum airflow. Measure airflow at every VAV box at minimum and maximum, verify against the new setpoints, and confirm boxes hold minimum without hunting.
- Reheat runtime. Trend reheat valve position for two occupied weeks. Accept when simultaneous heating and cooling occurs only during morning warm-up and recovery, not throughout the day.
- Temperature control. Demonstrate a 2 F deadband with no reheat active above setpoint, and verify supply air temperature reset staging.
- Zone conditions. Log space temperature and CO2 at three previously complaining locations and compare against the pre-repair baseline.
Payment should be tied to the trending report, not to the work order being closed.