The expansion tank can be plumbed into anywhere in the hot water system and located wherever convenient, including slongside the calorifier. That will be no problem. Make sure where you fit it, there is easy access to the car tyre style valve on the tank that is used to repressurise it. The air in the internal bladder will lose pressure over time, like a car tyre and need topping up. It is a good idea to make this easy to get at. The valve may be hidden under a plastic cover. I find I need to top it up once, perhaps twice a year, but this will vary, depending on it's age etc. By the way, Tracy D'arth has passed away.