Many boilers are indirectly heated by the central heating unit. A three way valve directs the hot water from the normal circuit to the boiler spiral. When this valve leaks during normal heating, water is fed through the boiler spiral and cools the boiler. This will increase the number of boiler heating cycles. The effect is a high gas consumption. Whilst heating the boiler, the temperature of the feeding pipe to the boiler should be higher in temperature as the return pipe. During normal operation the temperature of both pipes will drop simultaneous. In case of a leaking valve, the feed pipe will be significantly lower in temperature than the return pipe. When the feed and return pipes of the boiler would have been equipped with NTC temperature probes, a malfunction of the three way valve could have been detected. Possible implementation: - Use the 24 v ac supply available in the central heating unit as power supply and create a 5 volt DC power supply. - Place two NTC’s of 10 k in series between the power supply. At equal T the voltage in the midpoint should be 2.5 v. - Connect this voltage to a comparator (+) and the midpoint of a variable resistor to the (-). - Switch a buzzer using a small transistor if the NCT of the feeding pipe is significantly colder than the return pipe. This will constantly monitor the behavior of the three way valve and will help to keep the bills low and reduces CO2. The zip file contains two pdf files with graphs showing the temperature versus time for normal and abnormal operation.