An HVAC system is monitored or tested using a tube with a measurement instrument in the tube. The tube is connected to an HVAC system and an output of the HVAC system is measured using the measurement instrument. The tube is connected between the HVAC system and a volume to be conditioned. A reading of the measurement instrument is noted while the tube is connected between the HVAC system and volume. The volume is disconnected from the tube and a valve is connected to the tube. The valve is adjusted until the reading of the measurement instrument is approximately the same with the valve as when the volume was connected. A setting of the valve is noted and then used to test the HVAC system in the future or to test other HVAC systems simulating the same load.