It is probably for yearly servicing. The furnace should be serviced once a year for it to work properly and not backfire. It could also be that it is a leased item not owned. Who knows, since you are a tenant, he can charge what he wants on his lease agreement as long as it's not illegal. When you go look at the boiler, there should be a record right on it (a sticker) that lists any repairs or general services with the dates on it. When a company comes out, it is recorded on the item. Look around the boiler and see...then at least you have something to go back to your landlord with. Good luck!!