Annual maintenance should include:
There are additional precautions and control strategies, as found in various catalogues and in the ASHRAE Fundamentals Handbook and in the Carrier System Design Guide — Piping Section, when the entering-air temperature to the coil falls below 35°F. These conditions occur when IDT coils are used for pre-heat and/or face and bypass applications.
Freeze up protection:
In spite of the precautions listed above, a coil may still freeze up. An oversize capacity coil, at partial load, with a modulating steam control valve will occasionally freeze. Freezing occurs in the 20°F to 35°F range of entering-air temperatures. A better installation would be an undersize coil, with an on/off control valve with thermostatic control in the outside air, set at 35°F air temperature, installed downstream of the first coil; or setting the minimum steam pressure at 5 psig.
Source: Carrier (www.shareddocs.com)