When you say cold, what is the temp of the tank? What size is the tank? What is the wattage of the heater? Most importantly do you have a thermometer? I use stelth heaters, but I agree the temp set on the heater doesn't always match the temp of the tank, that's why a thermometer is important, you may have to adjust the setting on the heater up or down. Installing the heater at a 45 degree angle helps as the heat wouln't rise directly into the thermostat and shut the heater off before the water heats up to the set point. Sometimes two heaters are required to get the correct temperature and good insurance as a back up incase one stops working. On my Ram tank I have two thermometers one digital and one old fashioned hang on so I can make sure the tank stays at 84.