AWHP - defrost heating strips

I've noticed two rows of electrical heating strips positioned in evaporator coils of air-to-water heat-pumps destined for colder climates.

Can someone explain how these electric strips are operated in regards to the defrost cycle? How does the defrost cycle work for AWHP's?

With this kind of AWHP, there was no reversing valve evident (at least from what I observed). The electrical heating strips seem to be standard on all their heat-pumps - even S.E. Asian conditions where frost very seldom, if ever, occurs (except the very cold climes of northern Asia.)

What are simple, inexpensive ways in which to initiate & terminate a defrost cycle, for an AWHP? Links & literature would be gratefully appreciated.

Thanks for your applied wisdom. :)

Use electric strip to produce heat to defrost.
the most easy way to intialte and terminate defrost cycle is time interval, for example, every 40minutes, the defrost starts

Quote:

---

Originally Posted by desA

What are simple, inexpensive ways in which to initiate & terminate a defrost cycle, for an AWHP? Links & literature would be gratefully appreciated.

Thanks for your applied wisdom. :)

---

What is controller make and model# they are using?

^ None yet - it's still in the design stages.

Over here, we don't usually need to consider much in the way of defrost issues, but I'd like to expand the reach of the units to regions where the occasional cold air spell could arrive.

.............................

One method is by monitoring suction pressure and evaporator temperature. After unit has reached set suction pressure and been at or below that pressure for set time, compressor stops and electric heater starts, then temperature is monitored and heaters continue to heat until evaporator reach predetermined end temperature, or maximum defrost time is reached.
Than fans are started for some time to dry coil and after that set time is elapsed, compressor is started and fans are regulated. Than, next defrost cycle is prohibited for set time regardless of suction pressure.
This description fits one of the possible defrost scenarios in Carel Microchiller controllers.