Supply Air Temperature Trouble
Updated: Mar 29, 2022
Image: Typical SAT Sensor
The supply air temperature sensor is required for all ducted heating applications in California and recommended by ASHRAE. By the nature of zoning, temperature and airflow are constantly adjusted throughout the day. Room temperature is controlled by modulating the air volume damper and the reheat valve in sequence. When in heating mode operation, the supply air temperature is maintained at set point by sequencing the heating hot water valve controlling the water flow through the reheat coil. The controller measures the supply air temperature via the installed sensor and modulates the heating coil valve open or closed to maintain its heating set point. That is the intent, however commissioning too often uncovers a breakdown between intent and installation.
In an ideal world, the owner provides direction, the engineer provides a design to meet intent, the contractor installs the design, and everything works as the owner intended. However, in commissioning, we deal with the real world, and not the ideal world. Building construction is complex defined by one-off, fast paced, budget driven projects. Creating a complex aggregation of information into equally complex construction processes can’t be done without mistakes. Mistakes are made, caused by this complexity of the design and the human limitations of differing perspectives. There are many ways to interpret designs, details, and specs. An incorrect interpretation of a detail can leave the owner with a system that does not work and no clear path to resolution.
Image: Typical Single Duct VAV with Controlling Components
VAV/CAV/Air Valves, Reheats, and Temperature Sensors
A Variable Air Volume (VAV), Constant Air Volume (CAV) or Air Valve terminal units’ reheat coil supply air temperature sensors (SAT), or discharge air temperature sensors (DAT), are located after the reheat coil. Their intent is to measure the temperature of air distributed to the zone in the supply air duct. While the thermostat in the space controls the VAV damper actuator and the reheat valve actuator, the SAT is a controlling sensor used to limit the maximum supply air temperature, preventing excessive temperatures which would lead to stratification.
Image: Maximum SAT per California’s Building Energy Efficiency Standards (Title 24, Part 6)
A single duct VAV terminal consists of an air inlet assembly, housing with an insulation liner, and a discharge outlet. The terminal controls equipment will include a flow sensor, an actuator, and a controller. The pressure-dependent control modulates the damper in response to zone temperature.
Single Duct Terminal with Reheat
The basic single duct terminal unit with reheat is similar to the single duct but has a reheat option built into the unit. The reheat option is either a water coil, or an electric heater. Accessories for the single duct with a water coil include access doors in the coil section upstream and downstream of the water coil.
Single duct with reheat is often used for zones which require a source of supplemental heat. Usually, the single duct with reheat operates at some minimum airflow rate to minimize the amount of heat required to
offset the conditioned air being supplied to the zone.
A typical detail for a VAV with a reheat coil is shown below:
Image: Typical VAV with Reheat Controls Detail
The detail includes Note 3 for the supply air temperature sensor. It is shown to be installed downstream of the reheat coil, but there is no specified distance required from the face of the coil or any other locating dimension.
Catching Up with Codes
In the past, these SAT sensors were a nice-to-have monitoring point. However, in California, since 2016, State Building Code Title 24 regulations have required measurement and control of the reheat coil valve based on the SAT.
Title 24 Part 6 Section 140.4 – PRESCRIPTIVE REQUIREMENTS FOR SPACE CONDITIONING SYSTEMS
140.4(d) Zones served by variable air-volume systems that are designed and controlled to reduce, to a minimum, the volume of reheated, recooled, or mixed air are allowed only if the controls meet all of the following requirements…
iii. The first stage of heating consists of modulating the zone supply air temperature setpoint up to a maximum setpoint no higher than 95ºF while the airflow is maintained at the dead band flow rate.
This 95ºF SAT is the upper temperature limit for air supplied to a space. When performing a heating load calculation for the conditioned zone the Engineer of Record (EOR) assumes that the air temperature flowing out of the diffuser into the space is 95ºF when the thermostat calls for full heating and the heating hot water valve is full open. Similarly, the Heating Hot Water (HHW) reheat coils are sized so the mixed, or average, leaving air temperature (LAT) off the coil is 95ºF at full heating hot water flow.
During installation, the controls contractor installs the SAT sensor downstream of the reheat coil. The Test, Adjust, and Balance (TAB) contractor balances the amount of heating hot water flow through the coil and the controls contractor programs the HHW control valve. The program, in compliance with Title 24, limits the supply air temperature to what is measured at 95ºF. This programming limit stops the reheat coil valve from opening further if the installed SAT sensor reaches 95ºF. In theory, this is compliant, but in practice the only works if the sensor is (1) correctly calibrated, (2) installed correctly, and (3) is accurately reading the uniform air temperature (not stratified or uneven) being delivered to the diffusers.
In our regular walkthroughs on this project, we noted that a number of sensors were installed in incorrect locations, however, we were unable to observe all of the installations.
The missed installations came back to bite us when, during functional testing, we discovered that a few SAT sensor readings were different when measuring with calibrated temperature sensors at the diffusers. We would expect a loss of one or two degrees, as the heated air travels down the duct in unconditioned space, but on average we were seeing 11ºF to 12ºF of temperature difference with some readings larger than 15ºF.
After creating an issues log and bringing this up to the installing contractor, the response was that the sensors were “installed per plan” and operating correctly. No amount of data could convince the contractor that the sensors were not operating as intended.
We believed the cause of the discrepancy was due to the sensors being installed too close to the coil and not necessarily a calibration issue because we recorded significant temperature differences at different locations in the same plenum (see readings from Terminal 1 with an 11ºF difference in the same duct at different elevations).
Image: Example of a SAT sensor installed 6” from the coil
There are several reasons why the sensor location can result in erroneous readings. The primary reasons are based on two modes of thermal heat transfer: convection, and radiation.
Convection occurs when fluids (water and air) move and transfer thermal energy in the process. Heating hot water enters a reheat coil and cooler air moving through the coil is heated. However, the temperature of air immediately after the coil is stratified and not uniform. That is, the air temperature is higher at the bottom of the coil than at the top due to the difference between the heating hot water entering the coil at the bottom (at 180oF) compared to exiting the coil at the top (140oF).
Image: Typical HHW Coil Water Temperature Difference
Sensing air temperature at the bottom of the duct just after the coil will result in measurements up to 40oF higher than the top. This is the reason for the 11oF difference between readings of the same coil at the same distance from the coil, but at different elevations.
The temperature difference drops and becomes more uniform as you move further down the duct and the air temperature is mixed and constant.
Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than zero. A coil heated to an average of 160oF will produce radiation effects. This heat radiates away from the source equally in every direction and is more spread out the farther you are from the source. Put your hand inside a VAV with reheat on the upstream side of the coil while it is heating, and you will feel this effect.
This thermal radiation will effect the temperature reading of the downstream SAT sensor. The closer the sensor is to the coil, the higher the affect. This is the same reason why outdoor air temperature sensors should be mounted in the shade.
Issues are not always resolved
Going back to our project, the contractors correctly pointed out that the VAV with reheat detail only shows a temperature sensor installed downstream of the reheat. There was no dimension provided clarifying the distance between the coil and the sensor. The sensor was technically installed per plan.
We brought this issue up to the owner after not getting any traction from the contractor. Our main concern was since the Building Management System (BMS) readings were ~12oF less than the temperature of air being supplied to the space through the diffusers, the coils would underperform and spaces served may not reach heating setpoints (Recall that when designing the system, the EOR create a load calculation with the assumption of 95oF heating air temperature). Ultimately the owner decided the impact of fixing the SAT locations at the end of the project was not worth the benefit of a few degrees of heating in the mild climate of Southern California.
Speaking with other controls contractors the reason for this hesitation to fix the issue became more apparent. Most SAT sensors can be ordered from the factory with a 5’ lead wire. It takes a special order (and more money) to have a 15’ wire delivered to the site. Therefore, some controls contractors may opt for the standard 5’ wire and hope the controller is installed close enough to the coil to allow for proper temperature mixing.
It is now in our standard procedures to ensure the drawings, details and specification for control sensors are clear in intent and instruction. Our design review checklist includes recommendations on SAT sensor distance from coils for terminal unit reheat details. Our installation checklists include verification that the SAT sensor for VAVs with reheats is installed at least two duct diameters (if round duct) or at least two times the largest dimension (if rectangular duct) downstream from the face of the coil. We make this observation well before ceilings are installed.
In an ideal world, sensors operate exactly as intended, measuring their environment exactly as it is in the real world. But in reality, all sensors have anomalies, both inherent and applied that affect their measurements. For SAT sensors downstream of coils, stratification, convection momentum, and flow can hold the uneven profile of temperatures after the coil. In addition, radiation will play tricks on sensors in its path.
Commissioning is the systematic quality assurance process of proactively verifying facility equipment and systems meet expectations. Expectations include accuracy. Accurate air temperature measurements are notoriously difficult to obtain in air-conditioning equipment because of space constraints. However, SAT is important for control of reheats and in diagnostic algorithms and temperature sensor locations should be aptly scrutinized.
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