When using a tank type coolant heater you need to do several things to make it work well.
1) Install it as close to vertical as you can. 2) Make sure you have the fitting with the check valve on the bottom. 3) Route your water hoses so that they do not have an air trap in them.
4) Bleed the air out of the coolant heater water circuit before plugging the heater in. Reason being is that you can have an air pocket in the tank and that will harm the heater.
Also you need to make sure the water can thermal cycle in your system. I had a heater issue on a friend's tractor. We found the bottom hose was caked with sediment and was only allowing a small amount of water flow. It would over heat and burn the paint off the heater.
Did you get the same size as what you did have??? They make the Kat's heaters in several wattages.