I'm assuming you're looking at a water to water system. The district side of the HX will be (better be...), as should the customer side, be tight, and leak free. The big killer of closed-loop hot water heating systems is make-up water, due to leaks. Leaks can include losses through relief valves due to improperly selected and installed expansion tanks.
The main problem I've encountered with district heating system HXs, is where there are chronic leaks on the closed loop water side of a steam-to-water arrangement. The continuous trickle of oxygen-filled make-up water has corroded the system to the point where the HX (in the basement) routinely plugs with black goo. I've told them that this muck is actually the inside of their heating system, and that they've absolutely got to fix the leaks and flush the system, but the shop has been downsized to the point that there's not even enough bodies left to fight fires, let alone get ahead of problems like this.
If there's glycol in one or both loops, then it must be chemically checked annually. Glycol can break-down into some interesting acids, and really go after your system.
If the HX is for domestic hot water, deposits will typically be from hard water. This will happen no matter what kind of system you use. Install pressure gauges and inlet/outlet thermometers across the DHW side (this should be done on any HX to troubleshoot and monitor it), and get base-line readings on a new, clean HX. Someone must check the installation periodically (don't laugh - it happens sometimes) and when the delta-P drifts up, it's time to pull the HX out of service, and clean it.
If it's a district steam system, really the only way you can get deposits on the steam side is if there's serious carry-over back at the boilers.
