过去，住宅热泵使用两种除霜控制方法之一:需求或时间/温度。按需除霜控制系统通常测量室外空气温度和室外盘管温度之间的差值，或通过室外盘管的空气压降，并在测量值超过预设阈值时启动除霜。因此，按需除霜控制系统直接响应室外盘管上的结冰。历史时间/温度除霜控制系统使除霜以固定的间隔进行。 由于除霜间隔是一个固定值，因此这里将这些系统称为固定间隔时间/温度（FITT）除霜控制系统。最近开发了几种除霜控制系统，利用时间/温度方法，但根据室外空气干球温度改变除霜间隔。这种系统在这里通常被称为可变间隔时间/温度（VITT）除霜控制系统。VITT系统监控室外空气干球温度，并使用测量数据改变除霜间隔。 然而，对于任何固定的室外温度，VITT系统都会以恒定的时间间隔解冻。引用:俄亥俄州辛辛那提市ASHRAE学报第87卷第2部分

Residential heat pumps historically have utilized one of two methods of defrost control: demand or time/temperature.Demand defrost control systems typically measure either the difference between outdoor air temperature and outdoor coil temperature, or the air pressure drop across the,outdoor coil, and initiate a defrost when the measured value exceeds a preset threshold. Demand defrost control systems are thereby directly responsive to the formation of ice on the outdoor coil.The historical time/temperature defrost control systems cause defrosts to occur at fixed intervals. Because the intervals between defrosts are a fixed value, the systems are referred to here as fixed interval time/temperature (FITT) defrost control systems.Several defrost control systems have been developed recently that utilize the time/temperature approach, but vary the interval between defrosts depending on the outdoor air dry-bulb temperature. Such systems are generically referred to here as variable-interval time/temperature (VITT) defrost control systems. VITT systems monitor outdoor air dry-bulb temperature and use the measured data to alter the interval between defrosts. However, for any fixed outdoor temperature, the VITT systems defrost at constant intervals of time.