VAV box reheat is active, damper is open to maximum ... - BetterBricks

VAV box reheat is active, damper is open to 

maximum position 

Introduction 

This condition can exist in almost any variable-air-volume (VAV) system regardless of VAV box type or 

source of reheat. Please refer to your operation-and-maintenance (O&M) manuals to determine what type 

of VAV boxes you have installed. 

An air handler delivers air to all of the zones it serves at the same temperature as determined by the 

control-program logic. Some zones may require cooling while others, usually on the building perimeter, 

may require heating. The air temperature is set to satisfy the zones needing cooling, and the air delivered 

to the zones needing heating will be reheated by an electric or hydronic coil in the VAV box for that zone. 

Before the heating coils are energized, the damper in the VAV box should be set to the minimum position. 

As more heat is needed, the airflow should be as low as possible to minimize the volume of cooled air 

that has to be heated. 

There are conditions (noted below) where having a damper open at 100% does not necessarily mean that 

airflow is 100%. Also, there are special situations where reheat is used for humidity control and different 

control logic may apply. 

Reheat systems using heating water have no minimum requirement for coil air velocity, whereas electric 

reheat coils require a minimum air velocity to prevent the coil from overheating. This is typically around 

70 cfm/kW. Electric reheat coils can also be provided with stages of 1 kW through 5 kW. They can also be 

modulated using special controllers. These can greatly reduce electrical demand compared to staged 

reheat, which is often in 5 kW increments. 

How This Wastes Energy 

The VAV-box primary air damper (from the AHU) should be at its minimum position before the reheat 

energizes. It is possible for the damper to be open more than minimum. This allows simultaneous heating 

and cooling which is prohibited by most, if not all, energy codes. 

Possible Causes for This Symptom 

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an 

energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your 

current system that would probably require a capital project to change. Follow the steps described after 

the table to determine the possible cause of this symptom. If you find a problem, perform the suggested 

trend logging to confirm that the problem exists and, later, that you have solved the problem. 

Checked Type Description 

Explanation AHU is in heating mode, zone needs additional heat. 

Problem DDC controls are overridden or reset, keeping damper open. 

Problem VAV box controller has failed. 

Problem Inlet static pressure is not adequate to maintain minimum air flow rate. 

Problem Excessive static pressure prevents the damper from closing. 

Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance 

BOpTlsMstrPrb22.doc • 1

How to Find the Problem(s) by Inspection 

Inspection Step 1 

Inspect the direct-digital-control (DDC) system to see if the AHU is in cooling or heating mode. If it is in 

heating mode, then the zone reheat is providing the additional heat that the AHU can’t provide. This can 

occur during morning warm-up. 


If the damper control has been overridden in the DDC programming, determine why before correcting the 

problem. Some reasons for override may be: 

• The space has a high minimum make-up-air requirement such as for a fume or exhaust hood. 

• The damper was overridden during the cooling season to meet a temporary need for space 

temperature and was not subsequently reset. 

• Tenant complaints about poor air circulation, or “dead-air” conditions. 

The first condition has no simple solution and may actually be part of the original design. The second and 

third conditions require a review of the space-cooling loads to ensure that proper primary air is available 

under all conditions. The diffuser distribution should also be checked to make sure the supply air is not 

short-circuiting to the return-air plenum. 


Inspect the damper operator to make sure it is operating properly. Cycle the damper using the DDC 

system while observing its response locally. If there is no change in position, the damper operator may 

have failed. A DDC technician can determine whether this is the case. Make sure there are no obvious air 

leaks at the flex connection to the VAV box. 


The AHU may have a pressure-control problem that is starving the VAV box of primary air, forcing the 

damper to be wide open just to meet minimum airflow requirements. Inspect the AHU static pressure (SP) 

setpoint (in inches of water, typically denoted as "wg) and variable-speed-drive (VSD) speed to see if the 

static pressure design conditions are being met. This condition can also be caused by excessive duct 

leakage or a closed fire damper upstream of the VAV box. 


In conjunction with Inspection Step 4, the AHU static pressure may be too high and the VAV box damper 

actuator does not have enough torque to overcome it. This typically occurs in VAV boxes close to the air 

handler where the duct static pressure is the highest. 

How to Confirm the Problem(s) by Trend Logging 

Trend log the following: 

• AHU supply-air temperature (SAT) 

• AHU static pressure (SP) 

• VAV box SAT, if available 

• Damper position 

• Heating coil percent capacity (feedback and not output from the DDC system) 

Graph the heating coil percent with respect to the other four points. Look for correlations in the trends. 



If the damper position is constant when you would expect variation, then the motor operator or DDC-box 

controller (including the pressure transducer and its tubing) may be the problem. 

If the damper position is constant, the duct static pressure is not satisfied, and the AHU VSD is at 100%, 

then the system capacity is inadequate, or there may be a large duct leak somewhere in the system. 

(Contractors have been known to remove terminal units and leave the medium-pressure ducting wide 

open for added ventilation during renovation projects.) 

If the AHU SP is being maintained and the VSD is cycling as needed to maintain the setpoint, there may 

be a closed fire damper upstream of the VAV box. Inspect any other VAV boxes being fed by the same 

branch duct (downstream of the last fire damper) to see if they exhibit similar problems. 

Example of Normal Operation 

If your graph looks similar to the one below then your system is operating as it should and the problem is 

resolved. The damper is at minimum position while reheat is active. When the thermostat calls for cooling, 

the reheat goes to zero and the damper opens as needed to satisfy the space requirements. Note that the 

fan static pressure is maintained throughout the day. 

Normal VAV-box operation 

Examples of Abnormal Operation 

VAV boxes are notorious for having many different modes of failure. Below are two examples of what a 

trend log of abnormal operation might look like, but your own trend-log graph will probably have its own 

unique features. 

If your graph looks similar to one below then your system is not operating as it should and the problem is 

unresolved. The reheat is energized at 50% once the damper rises above minimum setpoint. Airflow from 

the AHU to the pressure-sensor location is not a problem since it can maintain static pressure. Inspect the 

static pressure at the VAV box locally to make sure it has adequate pressure. There may be a partially 



closed damper upstream, so check other VAV boxes fed by the same branch ducting to see if they have 

the same problems. 

Abnormal Operation: reheat not de-energized 

If your graph looks similar to the one below then your system is not operating as it should and the 

problem is unresolved. The reheat is energized at 50% once the damper rises above minimum setpoint. 

Airflow from the AHU to the pressure-sensor location is a problem since it cannot maintain static 

pressure. The damper has opened to 100% in an attempt to maintain the space setpoint. This chart 

depicts a reheat system that uses heating water. If the system has electric reheat, this graph shows at 

least 70 cfm/kW of heating capacity, otherwise the reheat would be at zero. Look for fan-speed 

restrictions, open duct-inspection ports, large leaks, or removed terminal units in a tenant renovation 

area. Checking above the ceiling for air noise is a good way to locate a duct leak. 



Abnormal operation: AHU SP setpoint not met 

Labor Skills Required to Find and Resolve the Problem 

• DDC system operator/programmer 

• Service mechanic

VAV box reheat is active, damper is open to maximum ... - BetterBricks

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