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jeff5may · 12-25-14, 01:55 PM

So without modifying the plumbing in a prefab unit, there are a few things you can do to defrost the units:
1. Bimetal switch defrost thermometer (klixon thermostat)
2. Digital sensor and control board (thermistor or digital thermometer sensing)

With dehumidifiers, the defrost sensor is usually in a good place. All you must do is separate the airflow between heat exchangers to isolate the heat transfer you want to discard. With the newer portable spot coolers, indoor air is sucked in by the unit and the flows of hot air and cold air are exhausted seperately. To heat with a portable aircon unit, the cold air flow is exhausted out a window or vent to outdoors. For sub-freezing outdoor conditions, the dehumidifiers are best run in this configuration also.

This configuration has been criticized by many due to the fact that the living space is operated in a "negative pressure" condition. Since we are exhausting a portion of the airflow, that portion has to come from outside the living space. In reality, unless the unit is heating your whole living area, and this area is very airtight, the increase in heat output that the unit produces (due to sucking in warm room air, the unit operates at much higher suction pressure than if it were sucking in cold outdoor air) more than offsets the relatively small portion of infiltrating cold air. Plus, the unit hardly ever needs to be defrosted.

Some of the newest portable units have split the airflows completely in the design process. These units are the easiest to modify to provide heating during the winter. However, the performance of these units suffers when it gets really cold outside. At some point, there is a temperature below which the unit will begin to frost up (needing defrost) in which case it would be more efficient to draw in at least some indoor air to raise the evaporator operating temperature.

There is a third method to defrost the evaporator: active heating. This mode works like a freezer, where the unit basically shuts down every so often to defrost the evaporator with a resistance heating element. This is the least efficient method of defrosting, but it does work. The sensing and control method must be planned well to avoid any unnecessary defrosts.

12-25-14, 01:55 PM	#13
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	So without modifying the plumbing in a prefab unit, there are a few things you can do to defrost the units: 1. Bimetal switch defrost thermometer (klixon thermostat) 2. Digital sensor and control board (thermistor or digital thermometer sensing) With dehumidifiers, the defrost sensor is usually in a good place. All you must do is separate the airflow between heat exchangers to isolate the heat transfer you want to discard. With the newer portable spot coolers, indoor air is sucked in by the unit and the flows of hot air and cold air are exhausted seperately. To heat with a portable aircon unit, the cold air flow is exhausted out a window or vent to outdoors. For sub-freezing outdoor conditions, the dehumidifiers are best run in this configuration also. This configuration has been criticized by many due to the fact that the living space is operated in a "negative pressure" condition. Since we are exhausting a portion of the airflow, that portion has to come from outside the living space. In reality, unless the unit is heating your whole living area, and this area is very airtight, the increase in heat output that the unit produces (due to sucking in warm room air, the unit operates at much higher suction pressure than if it were sucking in cold outdoor air) more than offsets the relatively small portion of infiltrating cold air. Plus, the unit hardly ever needs to be defrosted. Some of the newest portable units have split the airflows completely in the design process. These units are the easiest to modify to provide heating during the winter. However, the performance of these units suffers when it gets really cold outside. At some point, there is a temperature below which the unit will begin to frost up (needing defrost) in which case it would be more efficient to draw in at least some indoor air to raise the evaporator operating temperature. There is a third method to defrost the evaporator: active heating. This mode works like a freezer, where the unit basically shuts down every so often to defrost the evaporator with a resistance heating element. This is the least efficient method of defrosting, but it does work. The sensing and control method must be planned well to avoid any unnecessary defrosts.