HVAC Changeover Temperature Calculator

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HVAC Changeover Temperature Calculator

HVAC Systems and Components
Heat Transfer Engineering

Changeover (Balance) HVAC Temperature Calculator

For all systems using a primary-air system for outdoor air, there is an outdoor temperature (balance temperature) at which secondary cooling is no longer required. The system can cool by using outdoor air at lower temperatures. For all-air systems operating with up to 100% outdoor air, mechanical cooling is seldom required at outdoor temperatures below 55"F, unless the dew point of the outdoor air equals or exceeds the desired indoor dew point. An important characteristic of in-room terminal unit systems, however, is that secondary water cooling may still be needed, even when the outdoor temperature is considerably less than 50°F. This cooling may be provided by the mechanical refrigeration unit or by a thermal economizer cycle. Full-flow circulation of primary air through the cooling coil below 50°F often provides all the necessary cooling whle preventing coil freeze-up and reducing the preheat requirement.

Preview Changeover HVAC Temperature Calculator

The outdoor temperature at which the heat gain to every space can be satisfied by the combination of cold primary air and the transmission loss is called the changeover temperature. Below this temperature, cooling is not required.

The following empirical equation approximates the changeover temperature at sea level. It should be fine-tuned after system installation (Carrier 1965):

Eq. 1
t_co = t_r - [ q_is + q_es - 1.1 · Q_p · ( t_r - t_p ) ] / Δq_td

where:

t_co = temperature of changeover point, °F
t_r = room temperature at time of changeover, normally 72 °F
t_p = primary-air temperature at unit after system is changed over. normally 56 °F
Q_p = primary-air quantity, cfm
q_is = internal sensible heat gain, Btu/h
q_es = external sensible heat gain, Btu/h
Δq_td = heat transmission per degree of temperature difference between room and outdoor air

changed from winter to summer operation at the same time, so the room with the lowest changeover point should be identified. In northern latitudes, ths room usually has a south, southeast, or southwest exposure because the solar heat gains on these exposures reach their maximum during winter.

If the calculated changeover temperature is below approximately 48"F, an economizer cycle should operate to allow the refrigeration plant to shut down.

Although factors controlling the changeover temperature of inroom terminal systems are understood by the design engineer, the basic principles may not be readily apparent to system operators. Therefore, it is important that the concept and calculated changeover temperature are clearly explained in operating instructions given before operating the system. Some increase from the calculated changeover temperature is normal in actual operation. Also, a range or band of changeover temperatures, rather than a single value, is necessary to preclude frequent change in seasonal cycles and to grant some flexibility in operation. The difficulties associated with operator understanding and the need to perform changeover several times a day in many areas have severely limited the acceptability of the two-pipe changeover system.

Source:

ASHRAE Handbook HVAC Systems and Equipment