[jeff5may]

Originally Posted by Mikesolar

The question for all makers of ASHPs is how do you best defrost for the shortest amount of time without taking heat from the conditioned space.

Adam said he is using a control board from a dehumidifier.

At least with water heating there is a large source of heat and with a reversing valve, the heat comes from tank which was produced at a good COP.

This is why I had the wild idea. With a novel, cheap, reliable magdrive/jebo/eco pump, he can use his warm hot tub as a heat source for defrost.

What you do depends on your goal.....if you will be defrosting every 1/2 hour, 5min of straight electricity is a lot of power, 2 steps forward and 1 back.

With my ASHP window shaker beast machines, I went both ways, both cheap as dirt and utterly reliable.
With the first unit, I simply chopped the lines at the top of the cxr. I got lucky with that one, because the thermostat had two control output terminals. I simply moved the cxr from the "cool" to the "heat" output connector. I plan on adding a defrost klixon to that one so it stops running when the suction line gets to around 20 degrees f. Then, when and if the line warms to above 40, the unit will start up again.

The second unit is a different story. It had been modified by a previous "pro" when the original compressor died. It's still being debugged as we speak, since Murphy's Law is still being beaten out of the unit. The main issue with the unit is leaks from adding a reversing valve to the refrigerant circuit. On a positive note, the defrost control I purchased on ebay for under 5 dollars works like magic. It's an ICM 315.

What is a cxr? I haven't seen that term.

CXR is short for chest xray seriously, cxr is short for compressor.

To help others reading this thread, I will share my learnings about defrost control boards. There are only a few OEM suppliers of these boards, ICM, ICP, and Ranco being the three largest. From these few manufacturers come hundreds if not thousands of rebranded control boards.

The simplest type of board is the ICM 300 type, aka 621 dash whatever, aka Ranco DT-1. They use a separate defrost relay. They get power from a defrost klixon thermostat, which clicks on at a set temperature corresponding to a frozen evaporator. The board delays defrost a set amount of time before initiating defrost via the relay. When the evap coil warms up enough to click off the defrost thermostat, the control board loses power, the relay is deenergized and the unit goes back to normal operation. If the defrost klixon isn't satisfied, the control board ends defrost after ten minutes. It starts timing again, and defrosts after time runs out again.

The type I bought has its defrost relay on the board. It also can be wired to run off a klixon thermostat, but it doesn't have to. It has a thermistor which does the same job without any moving parts. This one, an ICM 315, is the same as a Ranco E-15 or an Avion DFT-100. It isn't quite as universal as the above board, since it has only one set of relay contacts and a standard defrost relay has three. Otherwise, it works basically the same: preset delay time, check thermistor sensor, defrost if too cold. When thermistor senses warmth, end defrost and start timing again.

There are maybe a dozen basic variations on these two designs, tailored to fit more elaborate units. HVAC techs stock a few each of their preferred brand of boards in their truck and are covered for 99.9 percent of every unit they will encounter in the field. For the other 0.1 percent, one of the stock boards will drop in with a little sorcery or the customer can pay 500 dollars for the (rebranded) 15 digit number (same) board.

My opinion is this: if you want to make your own temperature controller defrost microcomputer, go right ahead. It can be expanded later to stream real-time chest x-rays of your unit to your iphone if you want it to. But the pros go with what works and what is common. When mother nature wreaks havoc on the world, that 5 dollar timer will probably survive. If not, tech comes and swaps board in 10 minutes for 250 dollars to get you going today. Or you can do it yourself for 5 dollars. Or you can spend countless hours trying to figure out why your microcontroller went insane.