Although steam is an acceptable medium for central system preheat or reheat coils, low-temperature hot water provides a simple and more uniform means of perimeter and general space heating. Individual automatic control of each terminal provides the ideal space comfort. A control system that varies the water temperature inversely with the change in outdoor temperature provides water temperatures that produce acceptable results in most applications. To produce the best results, the most satisfactory ratio can be set after the installation is completed and actual operating conditions are ascertained.
Multiple perimeter spaces on one exposure served by a central system may be heated by supplying warm air from the central system. Areas that have heat gain from lights and occupants and no heat loss require cooling in winter, as well as in summer. In some systems, very little heating of the return and outdoor air is required when the space is occupied. Local codes dictate the amount of outside air required (see ASHRAE Standard 62 for recommended optimum outside air ventilation). For example, with return air at 24°C and outside air at -18°C, the temperature of a 25% outdoor/75% return air mixture would be 13°C, which is close to the temperature of the air supplied to cool such a space in summer. In this instance, a preheat coil installed in the minimum outdoor airstream to warm the outdoor air can produce overheating, unless it is sized so that it does not heat the air above 2 to 5°C. Assuming good mixing, a preheat coil located in the mixed airstream, prevents this problem. The outdoor air damper should be kept closed until room temperatures are reached during warm-up. A return air thermostat can terminate the warm-up period.
When a central air-handling unit supplies both perimeter and interior spaces, the supply air must be cool to handle the interior zones. Additional control is needed to heat the perimeter spaces properly. Reheating the air is the simplest solution, but it is not acceptable by most energy codes. An acceptable solution is to vary the volume of air to the perimeter and combine it with a terminal heating coil or a separate perimeter heating system, either baseboard, overhead air heating, or a fan-powered terminal unit with supplemental heat. The perimeter heating should be individually controlled and integrated with the cooling control. Resetting the supply water temperature downward when less heat is required generally improves temperature control.
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