My 1987 model HAF-315D Cromolox forced air Electric furnace will not stop, what are the things to look for..

Some thermostats allow you to change the differential value - that is how far below the heating (or above the cooling) set point before calling for heat (or cooling in the case of air conditioning). The built in 1 degree differential setting provides the most even temperature in the space at the expense of more frequent furnace firing and stopping. Many people prefer this mode as opposed to allowing wider swings in temperature for less frequent furnace firing and stopping. If you wish to change this setting, you'll need to replace the thermostat as it is not user adjustable in this model.

I presume you're talking about a gas furnace, even tho the title says Rheem gas water heater. Either way, it works the same for both.

Any black coloring on the heat exchanger is *not* from overheating. Black is from a sooty flame caused by a mis-adjusted burner. (In fact, a sooty flame is less hot). There is an adjustable "air intake plate" for air to enter the burner where the gas enters the burner. A proper flame is made by adjusting this air opening so the proper gas/air mixture feeds into the burner. If the air vent is closed too much it produces a sooty flame.
The age of the furnace is the most important aspect, and that helpful to determine a likely condition of the heat exchanger and burners. Other than a badly adjusted flame, the main thing to watch for is a badly rusted out heat exchanger. But black does not mean rust.

Air in line. Prime and shut off power to unit at least 1 minute

More likely the thermostat

Turn on the boiler and jump out the pressure by putting a wire between the 2 electric connections only after the unit fires up than put the jumper on, if it runs ok it is the pressure switch.

   By sealed unit you mean that the burner air comes from outside of the house?      You have checked the heat exchanger for cracks by removing the burners and looking with a mirror and flashlight, correct, or had a service tech so?
The furnace burners have enough air and are burning clean (blue) with just a tip of orange at the end? You bought the house new and the fireplace has never been used?  If it has been used to clean smoke for the house for whatwever reason I woudl start by havign the ducts cleaned.   Another possibility I have seen is insolation inside of ductwork breaking down.   

this is more than a fan control relay . it is a black relay that will appear to have several wires connected to it but those wires connect to the base ,it unplugs , if it is set up for ac it might have 2 relays one toward the front one toward the rear replace the front one first ,,, if on good replace one @ rear should fix if not let me know

is installed in a vertical up flow or horizontal position , if in horz position you could have a bad or miss positioned roll out switch this is a thermal switch positioned toward the burner edge that if the flame rolls out of burner area trips and then resets as it cools allowing the furnace to re-start until it reaches temperature , something to check .

MAYBE IT DOES NEED IF THE FILTER IN THE COLD AIR RETURN GRILL MAKE SURE THEIR IS NO FILTER IN THIS FIRST , AS 2 FILTERS ARE NOT BETTER THAN ONE WHEN COME TO ELECTRIC FURN AND H/P ALSO IS THIS A UP-FLOW ,DOWN FLOW OR HORZ MEANING AIR #1 RTN IN BOTTOM DISCHARGE COMING OUT OF TOP OR#2 COLD AIR COMING IN TOP AND DISCHARGE OUT OF BOTTOM OR #3FURNACE LAYING SIDE-WAYS AIR COMING IN ONE END AND OUT OTHER THESE WOULD DETERMINE FILTER LOCATIONS