Heat pump defrost controls
Why we need defrost in heat pumps
Because the heat pump absorbs heat from the outside air, it reduces the temperature of that air. With that reduction, water is condensed out of the air. If the temperature of the air that has passed through the coil is below freezing, this water will condense as ice.
Therefore, we need a method of removing frost periodically from the outdoor coil.
For this we use a defrost control. This control must accomplish 3 things:
It must have a way of detecting ice on the coil.
It must include a timer to control the time in defrost.
It must have a timing function to only allow defrost to start after a specific amount of time has passed.
Functionally, it must reverse the unit to cooling so that the outdoor coil is now a condenser. This takes heat from the inside and moves it to the outdoor coil. To concentrate the heat in the coil to melt frost, the outdoor fan is shut down.
In the early years of heat pumps, there were many types of controls used. One of the first and most dependable type was the time-temperature defrost.
This control incorporated a timer that would allow defrost to begin after 30, 45, or 90 minute intervals. The control would not allow defrost to start if the a temperature probe that was mounted on the outdoor coil to sense the coil temperature was not below its set point. The set point was usually 32F or 26F. Thus, the time period had to pass, then, if the temperature was below the set point, the unit would go into defrost. Once it went into defrost, if the temperature probe reached around 50F the unit would go out of defrost. If, for some reason, the temperature probe did not bring the unit out of defrost, a timer would bring the unit out of defrost after 10 to 12 minutes.
While this control was effective, it resulted in defrost when there was no frost on the coil.
The video below explains the operation of the defrost control.
Therefore, we need a method of removing frost periodically from the outdoor coil.
For this we use a defrost control. This control must accomplish 3 things:
It must have a way of detecting ice on the coil.
It must include a timer to control the time in defrost.
It must have a timing function to only allow defrost to start after a specific amount of time has passed.
Functionally, it must reverse the unit to cooling so that the outdoor coil is now a condenser. This takes heat from the inside and moves it to the outdoor coil. To concentrate the heat in the coil to melt frost, the outdoor fan is shut down.
In the early years of heat pumps, there were many types of controls used. One of the first and most dependable type was the time-temperature defrost.
This control incorporated a timer that would allow defrost to begin after 30, 45, or 90 minute intervals. The control would not allow defrost to start if the a temperature probe that was mounted on the outdoor coil to sense the coil temperature was not below its set point. The set point was usually 32F or 26F. Thus, the time period had to pass, then, if the temperature was below the set point, the unit would go into defrost. Once it went into defrost, if the temperature probe reached around 50F the unit would go out of defrost. If, for some reason, the temperature probe did not bring the unit out of defrost, a timer would bring the unit out of defrost after 10 to 12 minutes.
While this control was effective, it resulted in defrost when there was no frost on the coil.
The video below explains the operation of the defrost control.
The video below shows how the time-temperature defrost control operates.
Demand defrost controls
Because defrost is a reduction in efficiency by taking heat from the conditioned space, it should be done as little as is possible and still keep the frost off the coil. Considering the inefficiencies of the time-temperature defrost control, a number of other solutions that will only defrost when frost is actually on the coil have been tried.
One method is air pressure differential. With this method, an air pressure sensing tube is placed between the outdoor fan and the coil. As frost builds up on the coil it gets harder for the air to pass thru the coil. This results a lower vacuum between the coil and the fan. When the vacuum reaches the proper amount, the unit goes into defrost. As with time-temp, the unit cannot go into defrost unless the coil temperature was below 32F and until a certain amount of time had passed. Usually 1 hour. It also could not be in defrost for more than 10 minutes.
This control could conceivably not go into defrost at very low temperatures if the air is dry.
The temperature differential defrost system is the most common type defrost used today.
This one mounts a temperature sensor in the air flow to the outdoor coil, and another one mounted on the refrigerant line of the coil. This type measures the temperature difference between the 2 sensors. As the coil is blocked with frost, the temperature of the coil reduces as compared to the ambient sensor. The ambient air temperature must be below 32F for the defrost to begin. Once the temperature difference between the 2 sensors reaches a preset amount, and enough time has passed since the last defrost cycle, the unit will defrost. As the coil temperature increases as the frost is melted, as with most controls, to around 50F, or the timing period expires, the unit returns to normal operation.
The video below explains this control.
One method is air pressure differential. With this method, an air pressure sensing tube is placed between the outdoor fan and the coil. As frost builds up on the coil it gets harder for the air to pass thru the coil. This results a lower vacuum between the coil and the fan. When the vacuum reaches the proper amount, the unit goes into defrost. As with time-temp, the unit cannot go into defrost unless the coil temperature was below 32F and until a certain amount of time had passed. Usually 1 hour. It also could not be in defrost for more than 10 minutes.
This control could conceivably not go into defrost at very low temperatures if the air is dry.
The temperature differential defrost system is the most common type defrost used today.
This one mounts a temperature sensor in the air flow to the outdoor coil, and another one mounted on the refrigerant line of the coil. This type measures the temperature difference between the 2 sensors. As the coil is blocked with frost, the temperature of the coil reduces as compared to the ambient sensor. The ambient air temperature must be below 32F for the defrost to begin. Once the temperature difference between the 2 sensors reaches a preset amount, and enough time has passed since the last defrost cycle, the unit will defrost. As the coil temperature increases as the frost is melted, as with most controls, to around 50F, or the timing period expires, the unit returns to normal operation.
The video below explains this control.
Below is a video giving the defrost sequence using a wiring diagram.