An considerable amount (about 39 quadrillion British thermal units) of the energy produced was consumed by the residential and commercial sectors, which is mostly building related for heating, cooling, and lighting. More than one third of this energy is consumed by so-called heating, ventilating, and air conditioning (HVAC) systems. To reduce this energy consumption the installation of a sufficiently large number of sensors is required to obtain a detailed picture of the actual state of the HVAC system by sensing parameters like temperature, humidity, air flow velocity, flow, but also pressure, density, and CO2-concentration. However, placing various sensors to measure all these parameters in each room is neither energy efficient nor necessary. Simply measuring the volume flow rate at strategic points will reveal inefficiency and cost-reduction potential. The aim of this work is the design and a proof-of-concept of a thermal flow sensor that is only based on PCB technology. ^The calorimetric principle is envisioned for the readout method where two operation modes are investigated, constant current and constant temperature mode. At the constant current mode, a specific current heats up a copper lead of the PCB sensor and surrounding sensing elements evaluate the excess temperature which is flow dependent. At the constant temperature mode, a controller keeps a specific excess temperature between the sensing elements at a constant level. With increasing flow velocity, the controller has to adjust the heating to maintain this specific excess temperature, therefore, the heating voltage becomes a function of the flow velocity. An analytical model is set up followed by FEM simulations to verify that the targeted principle is feasible. Several sensor designs are developed, fabricated, and then characterized with measurements. ^These measurements focus on resistance values as well as the transient behavior of the copper leads, experiments with a lock-in amplifier reveal the flow dependency of the sensor, and experiments in a real HVAC system prove the concept feasible.