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launboy 11-11-14 12:20 AM

Simple, Efficient, Defrost
So back in my hottub heatpump experimenting days my biggest problem was by far and wide defrost. I was running my 8,000 btu window a/c with the condenser in nice 104* water, and my evaporator in anywhere between 50* and 15*F Temps. I even ran it a few times while it was snowing.

I tried the Clixon, and a couple other methods, but none were reliable with the simplistic methodolgy I was using and the wide temperature range.

Something that I consistently noticed was that airflow dropped when the coil frosted up, so it got me thinking, there's a fool proof way to determine when the coil is no longer moving air and pulling in heat.

My idea, a flap in the exiting air with a flap and switch, when coil frosted over, air flow reduced, flap lowered, defrost initiated. Obviously a flap isn't ideal, but it pushed me to my next idea, a pressure differential switch. No frost = little differential, frost = more suction between coil and blower wheel. So I Googled it, and I'm not the first guy to try this.

Heat Pump Defrost Cycle Information

He had extraordinary success with his design, and it's exactly what I was scheming. This is the direction I plan to take with my defrost control.

Seeing all the threads regarding different defrost methods and problems, I figured this may be of interest to many here.

Happy Hacking,


jeff5may 11-11-14 08:01 AM

With the evaporator fan not running, while the unit was defrosting, how would the unit know when the hx was frost free? Without another sensor, defrost wouldn't automatically terminate.

mejunkhound 11-11-14 09:07 AM

Most commercial HP defrost cycles either are timed or terminate at a preset evap temperature.

Saw this thread, so thought I'd throw in a VERY simple method I use.
Since am a retied old fart, and home most the time, we have a mirror on a tree outside that allows us to see the evap coils from a few different windows we go by during the normal day.
Also have a manual internal push button switch - look out and see frost, push the button and defrost cycle starts, simple as that. This was before we added out DIY GSHP.

If one wanted to get fancy, LED with photosensor to detect frost to activate switch.

The option most in use here is having the GSHP operates when outside air temp is below 42F, above that the air-air HP functions. For the air-air HP, 4T Evap coil with 2T compressor assures that evap coil never drops below 32F, so 'frost-free'.

jeff5may 11-11-14 04:53 PM

I read through the cited article and the author has multiple sensors in play. Besides the evaporator pressure sensor, the system described has thermometer and pressure switch sensors and a thousand dollar controller. The system was remotely logged and trended before a custom defrost program was written.

launboy 11-11-14 10:32 PM

You guys have made a few good points.

Yes, this guy has a thousand dollar control and many sensors attached to the unit, but he also has a program written to calculate instantaneous SEER and integrate it with his home automation. Point being although he does not do it cheap, it could be done very cheap. In my mind all that is really needed beyond standard heatpump parts are the pressure transducer, and a timer(to terminate defrost). If you want to get fancy, a thermistor on the output of the evap would give you more of a demand termination solution. This is how I plan to run mine.

Mejunkhound, the LED and photosensor is something another member, Xringer I believe, has done some experimenting with, though I'm not sure what his results were.


takyka 11-12-14 03:11 AM


Originally Posted by jeff5may (Post 41597)
With the evaporator fan not running, while the unit was defrosting, how would the unit know when the hx was frost free? Without another sensor, defrost wouldn't automatically terminate.

I have a temperature sensor attached between the fins of the evaporator close to the intake and one at exhaust pipe end. When HP stops, The temperature start to rise at both. If there is frost at intake, the intake sensor stop to rise at 0C (freezing point) and remains there until all ice melts down. Meanwhile the exhaust sensor's temperature continue to climb to a few degrees higher than ambient temp. As long as the heatpump is cycling (even if outdoor temperature is below freezing), with these two sensors one can clearly identify frost buildup and meltdown.


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