The Lift-Off Deep Cleaner with HeatWave Technology® does not have a heater inside the machine. You’ll want to be sure you are putting hot tap water (not boiling water) into the tank so the water stays hot.
HeatWave Technology® provides a constant source of heat to help maintain water temperature throughout the cleaning process.
If you’re still not getting heat, at this point it might be best to take your carpet cleaner to a technician. You can find your nearest BISSELL® Authorized Service Center by clicking http://www.bissell.com/support/product-supportservice-centers/ or by calling 1-800-237-7691, available 24 hours a day.