A couple years back I bought a 80something year old house. It has a Trianco DBC 100 Oil combi boiler that I'm guessing to be about ~15 years old. The system also has an old hot water tank strapped to the ceiling. It's practically a space heater in a (usually) unheated basement. I wrapped the tank with some spare fiberglass insulation, and that helped cut back on the boiler cycling frequently. We've had problems a handfull of times where sediment (or corrosion) caused clogs between the tank and boiler. The original plan was to switch to natural gas sooner than later, but the recent low oil prices, and a spray foamed attic have reduced the urgency.
I'm under the impression that storage tanks on combis can be unnecessary. What are the chances this tank is a holdover from an older setup? Does having the storage tank help with hot water demand? It's a two-bath house with dishwasher and washing machine. We don't normally run all four at the same time...but it could happen if we had a house full of visitors around the holidays.