A better defrost for heatpumps

jeff5may · 12-03-17, 10:31 PM

Most all of the defrost algorithms I have seen that actually work reliably are based on Delta T. The generic boards I've employed and pretty much all of the defrost control boards in split systems' outdoor units use 10k NTC thermistors for thermometers. Some run on time and temperature, and others have demand defrost algorithms. They are more predictable and reliable than the stuff I tried to come up with.

The time and temperature defrost control has only one sensor. It reads coil temperature. When coil temperature falls below the setpoint, a timer begins. When and if the timer runs out, defrost begins. These are great for water source systems, but not so great for air source units.

The demand defrost controls have two thermistors to sense temperature. The first sensor in the same spot as the time temperature control: on the heat exchanger. The other is attached to nothing and measures ambient temperature. The controller reads the difference between the two sensors. When the coil gets a little frosty, the delta T isn't affected very much. As the frost grows, the delta T rises only when airflow is restricted. At some point the delta T becomes excessive and starts the defrost timer. If the timer runs out, the controller initiates defrost.

12-03-17, 10:31 PM	#9
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	Most all of the defrost algorithms I have seen that actually work reliably are based on Delta T. The generic boards I've employed and pretty much all of the defrost control boards in split systems' outdoor units use 10k NTC thermistors for thermometers. Some run on time and temperature, and others have demand defrost algorithms. They are more predictable and reliable than the stuff I tried to come up with. The time and temperature defrost control has only one sensor. It reads coil temperature. When coil temperature falls below the setpoint, a timer begins. When and if the timer runs out, defrost begins. These are great for water source systems, but not so great for air source units. The demand defrost controls have two thermistors to sense temperature. The first sensor in the same spot as the time temperature control: on the heat exchanger. The other is attached to nothing and measures ambient temperature. The controller reads the difference between the two sensors. When the coil gets a little frosty, the delta T isn't affected very much. As the frost grows, the delta T rises only when airflow is restricted. At some point the delta T becomes excessive and starts the defrost timer. If the timer runs out, the controller initiates defrost.