Quote Originally Posted by tedkidd View Post
I was led to believe these things understood defrost based upon temperature. Given the source of the contradiction, I had to dig. I guess pressure increase does not trigger defrost. What does is really not very clear to me but probably will be to you:

Ideal Defrost / Intelligent Defrost - (I have the Carrier VNA service training manual)

What is not clear is it somehow knows how long defrost takes, and based upon that sets the next defrost run interval.

This thing is pretty amazing. The controller can manage refrigerant charging check, pump down, tells suction pressure, compressor rpm, suction temp, superheat, exv position, airflow, lbs of refrigerant to add or subtract based on lineset length...

And in a mild climate where the btu cost of propane has a snowballs chance in hell of approaching the cost of high cop heat pump, having a few btu of electric resistance combined with the heat pump for a few hours a winter is going to be WAY WAY cheaper than paying the incremental on the furnace. Paying the additional cost of burning propane particularly when you factor the heat pump CAN run with the resistance (like an additional stage) and the propane CAN NOT.

I simply have not heard of 50c kwh electric, if it exists it's very rare, and that's still cheaper than propane at most temps.

Forward energy view is electric will become more and more closely tied to natural gas. The only source that can hold a candle to the btu cost of these high COP heat pumps is Natural Gas, and that requires some pretty cold temperatures. This is very likely to remain true for a long time.
On demand defrost uses the temp difference between the air and the liquid line to determine if there is ice/frost on the coil to a point that it needs to be defrosted. After the outdoor temp drops below X degrees(varies with manufacturer) they won't go into defrost except for once every 6 hours of compressor run time to make sure the outdoor coil doesn't become oil logged.

Carrier tries to mimic the efficiencies of on demand defrost. But misses the boat a bit. Its better then just time and temp defrost, but still activated by only coil temp and compressor run time(with respect to outdoor temp being under 50). But when its 10 degrees outside, Carriers Ideal will still do needless defrost cycles. On Demand won't.