I would go with Keystoker's advice,, a $100 savings is nothing in the big picture,, You can use the coil or have it in reserve for the future, if your boilermate tank dies. There are many ideas about using, storing and heating hot water.. there are several threads on this topic on the forum.. It all depends on how much hot water at any one time you may ask for from the system..
I agree with Keystoker's comment about eventually thinking about getting rid of the oil boiler,, providing you don't have any issues getting and storing coal,, it is a great system and much less expensive... I've decided to buy another 2-year's supply of coal this month,, I don't think anything is going to get cheaper... and having three years of heating fuel in storage is a wonderful feeling !!