Sensors help improve performance of HVAC designs

Article By : Josh Wyatt

Temperature and humidity sensors can help improve performance of HVAC designs.

Heating, ventilation, and air-conditioning (HVAC) systems use sensors to regulate the operation of electromechanical equipment. Running this equipment usually consumes energy at a rate that represents a large portion of a monthly electricity bill. As the temperature outside drops below room temperature, the heating load increases. Conversely, when the outside temperature increases above room temperature, the cooling load increases.

Systems that use only dry bulb temperature measurements to determine control decisions are outdated and should be replaced with control based on a combination of sensor measurements, both for optimal occupant comfort and to lower the cost of heating or cooling a structure. Using the latest technology offerings in both temperature and humidity sensing components in these designs can help improve performance for years to come. The purpose of this article is to provide a high-level view of where these sensors are located in HVAC systems, how they are used, and their impact on accuracy and repeatability of system performance.

In an HVAC system, there are several sensors (Figure 1). They are located in the supply air ducts and the outside and return air ducts, as well as in the thermostat control unit. These sensors, either analog or digital, provide the raw data from which the controller calculates and manages the overall performance of the system.

Figure 1 This is the layout of a basic HVAC system. Source: Texas Instruments

The overall efficiency of an HVAC system depends on the inherent accuracy and repeatability of the sensors. Significant performance increases are possible by making sure that the mechanical operations of the HVAC system—operating fans, dampeners, and humidifiers—function at precise times and for the least amount of time, reducing overall system power consumption and therefore lowering energy costs.

Primitive technologies, shown in Figure 1, include passive devices such as negative temperature coefficient (NTC) thermistors and resistance temperature detectors (RTDs). Both devices are prone to drift as they age. While RTDs are very linear, the NTC thermistor is not and requires both slope and offset correction to achieve any real accuracy. That makes NTC thermistors difficult to manufacture.

RTDs can become quite unreliable over time when compared to silicon-based positive temperature coefficient (PTC) thermistor sensors, which can be driven from the same or similar single excitation current source as an RTD. These PTC sensors can also be voltage-biased like an NTC thermistor. The PTC devices share the same linearity as seen with RTDs, but without the same recurring and cumulative drift, if uncorrected over time. Here, improved sensor accuracy and reliability can add value to the dry bulb measurement use case.

Single- and dual-economizer HVAC

There is a drawback when using a system that only has temperature sensors in the outside, return, and mixed air, as shown in Figure 1. For example, on a cool and rainy day, the humidity from the outside air will be brought in. This will require extra cooling capacity in order to dehumidify the air. But because temperature sensors cannot detect this condition, the controller is unaware, adding relative humidity sensing into some of the same locations as the temperature sensors solve this issue.

So, HVACs can be arranged as single or dual enthalpy economizer systems, as shown in Figure 2 and Figure 3, respectively. The additional input from the %RH sensor to the main controller allows it to do a better job managing energy consumption while accomplishing environmental control.

Figure 2 The single-economizer HVAC system uses a combined enthalpy sensor module with access to outdoor air. Source: Texas Instruments

Figure 3 The dual-economizer HVAC system adds a second enthalpy economizer sensor located in the return air path. Source: Texas Instruments

Enthalpy is a thermodynamic property and cannot be measured directly. It’s calculated from measurements of both temperature and humidity. So, it’s extremely important to use an accurate and repeatable temperature and relative-humidity sensor; the calculation error is a combination of the individual sensor accuracy and tolerance and should be as low as possible.

A single-economizer HVAC system uses a combined enthalpy sensor module with access to outdoor air. The purpose of an economizer is to use outdoor air for cooling, whenever possible, to reduce compressor operation. It reports both dry bulb temperature and humidity, enabling the use of outdoor air at higher temperatures for free cooling of the air when the humidity is low.

When the user adjusts the thermostat set point, the HVAC controller switches the mixed-air control loop from outdoor to return air at a preset outdoor air-dry bulb temperature. In a single enthalpy economizer system, the HVAC controller module compares the enthalpy value calculated from the temperature and humidity data against preselected set point curves to accomplish the task more efficiently than a temperature-sensor-only solution.

Using enthalpy instead of dry bulb temperature with a single-economizer HVAC system lowers cooling costs in most climates. And while these systems are effective and provide improvement over temperature-only systems, using a second combined sensor module in the system adds another measurement location for data, and thus an opportunity to increase system efficiency.

A dual-economizer HVAC system adds a second enthalpy economizer sensor located in the return air path. When the user adjusts the thermostat set point or when the mixed-air temperature goes above a preset range or set point, the air with the lower enthalpy—from the outdoor or return air—is brought into the conditioning section of the air handler.

It’s a very efficient method of controlling outdoor air usage since the return and outside air comparison is continuous and automatic year-round. Moreover, it eliminates operator error by removing the need for the user to remember or know how to make the set point changes needed. It may appear wasteful to cool outdoor air at a higher temperature than return air, but the savings are verifiable, as the amount of mechanical cooling required to dehumidify air often exceeds the amount required to lower the dry bulb temperature.

In buildings where there is a substantial amount of moisture-generating activity, like a kitchen or shower, this type of control sequence can result in substantial savings compared to methods that include using a dry bulb high limit alone. Using enthalpy modules is significant, as about 50% of the cooling capacity of an air-conditioning system is used to dehumidify conditioned air by removing latent heat before the sensible heat temperature begins to be reduced.

Enthalpy calculations

In HVAC application usage, the enthalpy is derived from:

• The ambient temperature measured directly with an analog or digital sensor
• The mixing ratio, X, in kilojoules per kilogram or British thermal units (BTUs) per pound
• Atmospheric constants
• A relative humidity measurement

Enthalpy (h) can be calculated directly on a microcontroller (MCU) by using data from a single digital temperature and humidity sensor.

T is the temperature in Celsius and X is calculated with Equation 2:

B is equal to 621.9907 g/kg (valid for air), as expressed in Equation 3:

M(H₂O) is the molecular weight of water and M(gas) is the molecular weight of air.

The mixing ratio, X, is in kilojoules per kilogram or BTUs per pound, as expressed by Equation 4:

The ambient vapor pressure (P_W), in hPa, is calculated from the water vapor saturation pressure (P_WS), also in hPa.

P_WS is also calculated from temperature and some constants, and can be calculated first, using the measurement from the temperature sensor.

For a temperature range that the HVAC applications typically operate in, Table 1 lists the recommended constants A (pressure), m (mass), and Tn (temperature) to use in Equation 6.

Table 1 Constants for calculating P_ws

A single enthalpy calculation example starts with using the temperature and humidity data collected from the sensors. In the example set of calculations listed below, the measured temperature returned from the sensor combination is 25°C and the relative humidity of 52%RH.

1. Solve for P_WS = 31.67450264 hPa
2. Solve for P_W = 16.47074137 hPa
3. Solve for X = 10.28032832 g/kg
4. Solve for enthalpy, h1 = 51.43657 kj/kg, converted to BTU/lb, h1 = 22.1327738 BTU/lb

This math occurs inside the local MCU. The MCU determines whether the outdoor air is above or below the selected set point, and either sends a 4-mA signal (not OK to economize) or a 20-mA signal (OK to economize) to the logic module on the 4-20 mA current loop, back to the main controller.

When commanded to cool from the controller or commercial thermostat, the economizer logic module compares the value calculated above (h1, outdoor enthalpy) to a preselected set point control curve, as shown in Table 2. The installer selects the control curve based on geographic climate; the type of cooling equipment installed; occupant comfort; and to control the humidity, which will prevent indoor air-quality issues caused by high humidity.

Table 2 Control curve set points

The single-economizer HVAC system does require that the building occupant or maintenance technician know about the seasonal need to make the settings change or to remember to actually make the change to the control curve setting based on seasonal conditions.

Dual enthalpy calculations

A dual enthalpy calculation builds up the single-economizer example by adding a second set of sensors, located in the return air, as shown in Figure 3. When the system is set to cooling or when the mixed-air temperature goes above the high mixed-air temperature sensor range or set point, the air with the lower enthalpy—either outdoor or return—is brought into the conditioning section of the air handler.

As mentioned previously, it’s an enhanced method of controlling outdoor air usage, since the return and outside air comparison are the path toward the highest performance. From a kilowatt-per-hour consumption and usage-cost standpoint, using two enthalpy sensor subsystems can eliminate or avoid operator error after correct installation. As the seasons change, the system can adjust as needed, which will result in even more dramatic cost savings.

If h1 = 22.1327738 BTU/lb, the enthalpy is calculated for a user set point for temperature and humidity from the measured sensors exposed to outside air. If you apply those calculations in the formula for total heat, along with cubic feet per minute (CFM) of the HVAC system, convert that to kilowatt hours and apply the national average electricity rate to that value. Also, determine the time to the temperature set point being resolved and system going into idle mode, which will yield the cost of operating the system per event, day, or month.

In this example, if the user sets the thermostat to 21°C and leaves %RH alone, the enthalpy will be 17.914 BTU/lb. Equation 7 expresses the formula for total heat:

Assuming that CFM = 400, each time the user reduces the temperature, in an ideal scenario it would take approximately 2.23 kWh and roughly 7.6 minutes for the HVAC system to handle the request. The request would cost approximately $0.04, based on a U.S. national average of $0.139/kWh. This may not sound like a large cost at a glance, but it does add up. If an HVAC system was continuously cycling between the outside air temperature and a lower setpoint scenario, that could happen more than 60 times a day.

At $2.50 a day, that adds up to $75.14 over the course of a month. Inserting a different humidity set point—in this case, a higher %RH—can reduce costs more. That’s what the full value of designing a complete enthalpy sensor brings to HVAC designs and systems because there will also be a humidity sensor onboard.

Now imagine that the temperature sensors were off by just 1°C, in either direction. Such inaccuracy would result in an almost 7% “leak” in energy consumption for the system, which adds an extra $5 a month to the bill. The same cost leak is present on the %RH sensor side as well, if it’s off by >5%RH. That’s why it’s critical to design-in and provide sensors to the HVAC system that are accurate at the levels needed and repeatable so that the system’s performance over its lifetime is dependable.

Thermistor with RTD linearity

Devices like TI’s TMP61 linear thermistor can be driven from the same or similar single excitation current source as an RTD, or voltage-biased like an NTC thermistor. These devices also share the same linearity as RTDs, but without the same recurring and cumulative drift if uncorrected over time, as these linear thermistors are a silicon-based device that do not shift at all by comparison.

The accuracy, reliability, low cost, and general flexibility of TMP61 thermistors add value to the dry bulb measurement use case. When designing or specifying components for an enthalpy economizer, consider digital temperature and relative humidity sensors (HDC1010, HDC1080, HDC2010, HDC2080, HDC2021, HDC2022), which can meet or exceed most system design requirements for enthalpy from just one combined sensor component.

Using sensors that offer the required accuracy with precision and repeatability is important, because they add real value to consumers through direct savings on electric bills, supporting the installed HVAC system by making dependable and consistent calculations and decisions.

This article was originally published on EDN.

Josh Wyatt manages the temperature and humidity sensing applications team at Texas Instruments.

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