oil heat furnace coil

how to install a oil heat furnace coil? 2 years ago i had a problem with my furnace coil and it wasn't producing enough hot water so the heating company advised me that i have to install a electric water heater so it will work like a reservoir so i did...and everything was working fine for last two years and now recently i have a problem with the hot water pressure in the whole house. the hot water pressure is too low. is it possible to replace the heating coil to solve this problem or there is something else wrong???? i called the oil company who have the contract for the furnace for the tune up on the furnace and the guy who did the tuneup have no clue about this problem......so may i know what is wrong and how to fix it????