A sump on a 28g is not very practical but it can be done. You are going to need an overflow box to get the water to your sump.
http://www.aquariumguys.com/eshopps-...rflow-box.html One thing you have to keep in mind is when the power goes out. The sump will need enough volume to handle the drain back from the overflow box without spilling over.
An easier alternative may be to get a canister with the heater built in or place the sump above the tank and draw water off the tank and make a spillway for the return to the main tank. This is a very popular way method in Asia.