Measuring Thermal Energy in Hydronic Systems

So you’ve decided to track how much energy your HVAC system consumes in order to increase the system’s energy efficiency — great! But how will you do it? There are multiple methods that can be used to measure thermal energy in hydronic systems. We’ve outlined several methods below to make it easier for you to select the best technology for your application.  

Measuring thermal energy requires three components: a fluid flow sensor, two temperature sensors (one for inlet temperature, one for outlet temperature), and a calculator. Although thermal energy can technically be measured without a calculator, doing an energy calculation by hand is very complex and can introduce manual calculation error into the measurement. For that reason, we recommend incorporating a calculator into your system for the most accurate energy measurements. This calculator can be either an independent BTU controller, part of your building management system, or integral to a complete thermal energy system. A complete system includes a flowmeter, temperature sensors, and a calculator that are all calibrated together, eliminating the need for multiple pieces of equipment. 


Building Your Own System With Separate Components

Paddlewheel Flow Sensor, Series PFT

The most cost-effective way to obtain a thermal energy measurement is to use separate components to build your own measurement system. For example, you can combine an insertion paddlewheel flow sensor (like Dwyer’s Series PFT) or a multi-jet water meter (like Dwyer’s Series WMT2) with a BTU controller and two temperature sensors. If you want to limit the system to two components, you can instead pair the BTU controller with a thermal dispersion transmitter, such as Dwyer’s Model TDFT


Complete System: Ultrasonic

To measure flow rate, ultrasonic flowmeters transmit an ultrasonic pulse through the fluid. Two sensors are located within the inline tube of the body and each sensor transmits an ultrasonic pulse through the pipe and fluid to the other sensor. As the velocity of the liquid changes, so does the pulse rate. The transit-time difference is then used to measure the flow rate. 

To measure heat energy, two temperature sensors measure the difference in temperature between the flow supply and return of the flow system being monitored. The temperature difference, in combination with the volume of water that has flowed through the system, is then used to calculate the energy transferred to or from the water. Furthermore, ultrasonic technology has no moving parts to wear or break, leading to lower long-term maintenance costs.

Ultrasonic Energy Meter, Series TUF

If you are installing a heat meter in a new system or are able to shut down your line for installation, consider an inline ultrasonic heat meter, such as Dwyer’s Series TUF. The TUF is accurate, stable, and has serial communication outputs for easy transfer of data.

If you are interested in minimizing installation costs, or have an application where the heat meter cannot come in contact with the fluid, consider a clamp-on ultrasonic heat meter, such as Dwyer’s Series UBT. The UBT is compact, lightweight, and non-invasive. Its integral LCD displays energy rate and has selectable output options to fit your application. The UBT comes with all components needed for a successful installation, decreasing both installation time and cost. 


Complete System: Electromagnetic

Electromagnetic heat meters generate pulsating magnetic fields in the probe to induce a voltage into a conductive fluid flowing through a pipe. Electrodes then measure the induced voltage, which is converted to a flow rate while using various outputs to convey the data to connected systems (e.g. display devices or data acquisition systems). Energy calculations are based on flow and temperature measurements, compensating for density and heat content. Electromagnetic technology maintains accuracy through changes in temperature, density, and viscosity. 

Insertion Thermal Energy Meter, Series IEFB

Dwyer’s Series IEFB is a highly accurate insertion thermal energy meter that can easily be inserted into a pipe (via a hot-tap valve) without shutting down a line, saving on installation time and cost. Similar to the ultrasonic flowmeters mentioned earlier, the IEFB does not have any moving parts to wear or break, thus increasing the overall lifespan of the unit and reducing maintenance costs. All three system components (flowmeter, temperature sensors, and calculator) are calibrated together, leading to improved measurement accuracy. It has an integral LCD display that provides clear readings of the meter’s values and a variety of output options to suit your applications. 

If you have any questions when selecting technology for your application, the Dwyer Applications Engineers are available to assist by phone at (219) 879-8868 x6402, or by email at