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Old 10-28-14, 08:54 PM   #1
jeff5may
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Default Frost buildup cures

When I started experimenting with air conditioners and dehumidifiers to try to turn them into heaters, one of the first problems I ran into was frost buildup. It seemed to me that the better the units performed for heating duty, the more frost they produced when trying to gobble up heat. As outdoor temperatures plummeted, it became more and more difficult to keep the units working. There had to be a way to combat this problem.

This thread will discuss the methods and hardware that have been found to work for a variety of different types of units.

Make sure to check this thread from the beginning periodically, as I will be editing info in as it becomes useful.


Last edited by jeff5may; 10-29-14 at 05:45 PM..
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Old 10-28-14, 10:15 PM   #2
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the first unit I experimented with was an ancient dehumidifier. It probably looked like this during its younger years:


Now it looks nothing like this. It has been experimented with in multiple ways, but still retains its original charge and plumbing. The covers have all been either tossed out or harvested for the sheet metal. It's ugly, but it still works.

I learned pdq that these units were designed to distill water out of the air by chilling the evaporator coil to just above freezing at room temperature (about 65 degF or so). Even at that temperature, if the unit runs a long time the coil will frost up and freeze. Below about 60 degF, the evap coil will form frost in 15 minutes, then freeze as fast as it can pull water out of the air.

The units have a built-in frost sensor to detect when the coil has loaded with ice enough to choke the airflow. While warm air is flowing through the evaporator, the refrigerant picks up heat from the air and comes out above freezing temperature. Once the air path is clogged, the refrigerant cannot pick up much heat and leaves the evaporator below freezing temperature.


defrost thermostat is part #25

The sensor, named a defrost thermostat, is wired so that it opens the circuit to the compressor when the refrigerant line leaving the evaporator falls below a certain temperature. During this condition, the fan runs to assist thawing the frozen heat exchanger. Once the evaporator has thawed, the thermostat closes the circuit and the compressor starts again. This cycle is repeated until the humidistat is satisfied or the unit is powered down. This is known as "passive" or "warm air" defrost.

The strange thing about these "old style" dehumidifiers is that the defrost thermostat was not included in the less expensive models. These "economy" models simply ran and ran, forming a solid block of ice inside them. Obviously, this method isn't very efficient. That's why a lot of them didn't have finned tubes, but just a bare coil. That way, air could still flow around and through the iced up evaporator and draw more water out of the room air.

The defrost thermostats used in these units can be (and are) installed in window air conditioners and heat pumps to provide the same functionality. They are dirt cheap, and come in a wide range of "make/break" temperature combinations. They were widely used in refrigerators, freezers, and heat pumps until digital control boards superseded them.

Here's an old schematic:


Old school air conditioners were wired the same way, except they had a thermostat control where the dehumidistat is in this diagram. The defrost controls can be wired in the same exact way. The low-temperature range thermostats are the best ones to use for this purpose, with around 25F break, 45F make contacts. The 25F break temperature ensures that the coil is frosty, but not frozen solid. The 45F make temperature ensures that the unit will still run when temperatures drop. Of course, below about 50F outdoors, the unit may take forever to come out of defrost.

a DT45 thermostat (described above) sells for $3 on ebay:

http://www.ebay.com/itm/DT45-Refrige...item2342626452

Usually, these buggers are described by their upper limit temp (DT45) or by both upper and lower limits (L70-58F or L70/58). Sometimes, they are numbered by their upper limit and difference (L72-25). If the number has a WR in front, the numbers mean nothing as to temperature ratings. Regardless of how they are numbered, you can easily look up specs on these devices via google or bing search.



If you want to run a passively defrosted unit near freezing temperatures, the airflow through the evaporator will have to increase significantly. In most cases, it is a losing battle with air-source units.

Last edited by jeff5may; 10-28-14 at 11:33 PM..
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Old 10-28-14, 10:47 PM   #3
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I am going to modify my dehumidifier after I learn from this thread.
I have started to turn it into a heat pump a year or 2 ago but never did finish or learn the best methods. it's sitting in the basement case off waiting for me to restart the project.

it uses 200w while a dehumidifier on the kill a watt meter
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Old 10-29-14, 12:57 AM   #4
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Great information.
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Old 10-29-14, 04:54 PM   #5
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I assume my dehumidifier frosted over and did not thaw on its own is because I had it out of its warm case with the evaporator pulled away from the unit, among other things.

is the defrost thermostat & dehumidistat under the cover next to the compressor ? I am looking my unit over and it seems especially barren with a LG compressor.
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Old 10-29-14, 06:23 PM   #6
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The newer generation of units look like this one in the smaller size:



The unit pictured has a front panel from another common model laying on it. The space behind the other panel is where the bucket assembly sits. The units usually have casters and some sort of garden hose fitting you can attach to avoid having to empty the bucket constantly. They don't all look exactly the same close up, but from across a room it is difficult to tell one braand from another.



Under the surface, they are almost all the same. The smaller units have a 3000 btu (plus or minus a bit) compressor and single row, finned tube heat exchangers. The tubes are high-density finned, uber thin wall, and rifled on the inside for high heat transfer efficiency and reduced cost.

Due to their ultralight nature, these radiators will spring a leak if you look at them the wrong way! Do not try to separate the fins from the tubes, or the end caps, or bend or twist them, or... you get the point. If you are going to separate the coils and reposition them, do it with the interconnecting tubing, it is much heavier and stronger.



Above the capillary tube lies the defrost thermometer. In the newer units, it is electronic.
The capillary tubes in these units is very long. They limit the BTU throughput and force a very low evaporator temperature. In comparison, here is a replacement tube for a commercial cabinet freezer:



The controls in these newer units are no longer mechanical. They all have a control board of some kind in them and a few sensors to detect temperature and relative humidity. Defrost sensing is done by the control board, and all of them I have seen rely on a thermistor to sense temperature. They also have a humidity sensor which tells the unit how to act.



To me, this little sensor looks eerily like one of these (DHT series humidity/temp sensor):



If the unit still works as manufactured, it should be fairly easy to "jinx" the machine by replacing the humidity sensor data with the thermometer data. That way, the built in defrost control would still operate. The DHT sensors are a well trodden road in Arduino territory. Warping the data stream would not be a complicated endeavor. It could be possible to set the unit as normal, only the readout would display the approximate temperature instead of humidity. The unit could then be set as a thermostat.

If the unit doesn't operate correctly, but the relays on the control board and the defrost sensor still do their respective jobs, a new control scheme could be built to repurpose the unit for not much money. This is a perfect job for an arduino or other embedded microcontroller.



I brought this unit back to life by replacing the "pico-fuse" with a short solder jumper. If the unit draws excessive current, the solder melts just like a fuse. In the above picture, the chip at top center is a buffer between the microprocessor (the diamond) and the relays that control the compressor and fan.

If you look close, you can see four little silver circles on the circuit traces that run between the micro and buffer chip. These are "factory test points" that you can tie directly to an added controller to hijack the compressor and fan. This unit has only three relays: compressor, high fan, low fan. The fourth output has no relay on the board. If desired, one could add another "sugar cube" relay to the vacant spot on the board, and it would switch mains voltage to control another device (such as a heater, reversing valve, or water pump). The buffer chip also has "extra" unused buffer switches that could also be used if desired.

Most of the units have buffer IC's that wire up like this example circuit:


Rather than tying the input pins to a parallel port, we just tie them to digital ouput pins on the Arduino board (or other controller -SEE HERE)

Raspberry pi owners use these IC's to interface their controllers with relay shields. It seems the r-pi and its 3.3V output pins don't always trigger the relays like they should. Since the shields were originally designed for 5V (TTL) logic, they sometimes ignore the 3.3V logic "high" signals. The ULN chip was found to be the cheapest fix ever.


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Old 10-29-14, 06:50 PM   #7
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mine is like the last one you described its from around 2001 or so. If I can learn how the control board works maybe i could "control' it , if only manually for now.

or do i add a DT45 thermostat as well ?

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Old 10-30-14, 03:15 AM   #8
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Hi,

Frosting is a big issue with ASHP. Unfortunately dehumidifiers are not a good candidates for conversion if one keeps the original evaporators.
In oposition to an ASHP, in a dehumidifier is equipped with an undersized evaporator by purpose. The leading design idea is to have as low evaporation pressure (temperature) as possible. This ensures lot's of water condensing out from ambient air with the smallest possible compressor. The temperature gap between the air and HX is arround 20Kelvin.
If you ran such unit as ASHP your evaporator reach wery early the freezing temperatures and start to develop ice.
On an efficient ASHP you have to have as small temperature difference between evaporating temperature and ambient air as possible. To achieve this you need huge size of evaporator (comparing to dehumidifier), good airflow and low superheat.
Nowdays minisplit systems starting to develop ice at arround 45F as the designed dT is much smaller comparing to dehumidifier. It is done by increased surface and airflow.

If we want to make a good, efficient ASHP, we need to strech the evaporator surface area even furter than a standard minisplit. My heatpump is running on a compressor, half the power the evaporator is designed for and with good airflow. This way there is no frosting down to 2C/35F ambient and maybe 10% of the area is frosted after two hours running at -2C/28F without any defrosting.

Another important, but not obvious thing is to keep superheat low. to achieve this you have to set refigerant charge level spot on (in case of capillary metering) or better, switch to TXV or EEV. Every degree of extra superheat rise the corner temperature (where the frosting starts) roughly by the same ammount.
I run my hp with an EEV and superheat set point is at 0.5K.

To make a working HP is easy, not requires much effort. To make a good efficiet HP is much more work!

T.
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Old 10-30-14, 04:17 PM   #9
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A larger evaporator salvaged from something much larger than a dehumidifier would be the fix. Maybe a salvage yard could turn up something, say from a commercial unit.
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Old 10-31-14, 03:49 AM   #10
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Attached a picture from today morning after a foggy night. Temperature was at 0C and from 3:00am slightly below freezing. The machine was heating my house all night alone. No forced defrost at all...
The picture shows bottom 1/3rd of the evaporator, the rest is clean. You can see ice on the bottom and one patch in the midle.

T.

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