Sunday, October 19, 2008

Install External Sensor on Resul CL-280

I have been wondering for a while whether low temperature help the plants a lot or not. Many peoples in forum seems to agree that low temperature improve the plants grow. So I decided to add Resun CL-280 chiller to my 3ft tank (October 11th), to see whether it is great improvement or just marginal. The average air temperature is around 30 degree C, target water temperature is around 25 degree C. Tank size is 200 liters, minus rocks, substrate, wood and plant mass, probably left about 150 liters. The setup is "Tank -> Eheim 2217 -> Chiller -> CO2 reactor -> Tank"

Resul CL-280 spec:
- Power: 280W (1/10 hp)

- Rec Pump: > 900 l/h

- Max tank size: 300L (8 degree below air temp) and 200L (15 degree below air temp)

It took about hour pluss to chill from 28 degree C to 25 degree C. After that, chiller will go to stand by mode and turn on every 5 minutes for about 2 minutes. It is stop when the temperature at 25 degree C and on when temperature is rise to about 27 degree C. This is not what I expected, water temperature should not fluctuate than much for short period of time. It was tested at night with light off, the only heat generator are filter pump and chiller it self. Furthermore the thermometer inside the tank show constand 25 degree C. To confirm this, I bought couple of digital thermometer and place in different area of the tank. To my surprise all of them show around 25.7 degree C when the chiller start turn on.

Did some research on the net and found the following:

If the water flowrate is too slow, the thermostat will read false reading and cause the chiller to start and stop regularly in short period of time. The reason is because water does not leave the heat exchager fast enough before reaching targeted temperature threshold. As a result chiller with shutting down the compressor. When the warm water from the tank gradually replaces the water inside the chamber, the chiller will start the compressor again.

There are few ways to fix my start/stop problem:

1. Change to more powerful filter or use independent pump.

I don't really like the idea, I felt that my fishes are already struggling with current 1000l/h filter. Other reason is money, new filter is not cheap. Also, I do not like extra pipe.

2. Add external thermostat

I should be able to get external thermocoupler, hook it up and put the sensor inside the tank. That should give better accuracy.

3. Use external temperature controller

Currently doing DIY temperature controller, will post once it ready.

I decided on second solution, adding external thermostat. I open up the chiller, wirring is prety staright forward, you can easily find which connector is for sensor.

The sensor is located inside small box with thin silver insulator around it, wonder if the insulator really effective. If not, the heat from the radiator might effecting the sensor reading. The tank water just enter and exit the box, no compartment to direct the water flow. It is just a simple box with coil for heat exchanger.

The sensor is normal resistive thermocoupler, inside the plastic chasing. Plastic is an insulator, I wonder is the sensor response effected because of this.

I went to Sim Lim (Electronic component shopping center) to get external thermostat and what do you know, it is not as expensive as local fish shop quote me. The fish shop quote me for S$150 but it is only S$15 at Sim Lim (Acez Instrument Pte Ltd, #B1-38). Actually, it was spare part from Fox temperature controller. By the way, Fox temperature controller is only S$65 (There goes my Temperature Controller project, total cost of DIY might not be much different than from the shop). I also manage to get the same connector use on the chiller for only S$0.80. There is slight drawback, the resistance of new sensor is slightly lower than original sensor, measuring at same temperature. It was about 600 ohm lower. Thinking of adding fix resistor to compensate but on second though, my chiller temperature control display is only 2 digit. 0.3 or 0.4 degree will not make any much different.

This is what the external sensor look a like, it made from metal or maybe encapsulate inside metal. The cable is quite long. Either way response should be faster compare to the original plastic thermostat.

After adding the external sensor, the Chiller kick in every 2 and a half hours, prety good. There is still slight problem though, by default the chiller will turn on the compressor when the temperature reading 2 degree above the set value. I wonder if this will stress the fauna? I guess DIY temperature controller were I can set the maximum and minimum threshold still good idea.

I measurement chiller power ussage using device similar to Kill-a-Watt over 24 hours. The chiller used 1.17kWh over 24 hours, with the rate of 0.3045 per kWh, 1 day equal to S$0.356, 1 month = S$10.688. Additional ~S$10 per month isn't that bad, I can leave with it.


  1. Very simple and effecive DIY! I have the same model of chiller and would like to try this DIY. Is the temperature probe connection the red and white wires with connector as is on the third photo? Also, what is the ideal resistance for the replacement probe?

  2. The replacement probe does not come with connector. You can get the connector from sim lim as well, top floor conner shop, cant' remember the name. Polarity wise not really matter but I connect mine white with white and black with red.

    I do not know what is the ideal resistance because it is variable based on temperature. However, it is not that critical. You can always check the different with standard thermometer and set the Chiller offset setting accordingly.

  3. just to clarify, all i need to do is buy the $15 sensor and connect it to the sensor board of my chiller will do??

  4. Yes but before that you need to make sure you have the correct sensor type. There are 2 types of thermistors, PTC and NTC. To make it simple, just bring the old sensor to the shop.

  5. how do i take out the thermistors?? i acutaully is intend to buy resun cl200 or 280 and install the external sensor with it

  6. You need to open the cover, disconnect the sensor and connect your new sensor. That is all.

    Alternatively you can take out the sensor from the heat exchange chamber and re-use it instead of buying new sensor. However, you need to find something to replace it, otherwise the water will leak out.

  7. i intend to buy this chiller too for my planted tank. The temp right now with lights on is 32.3 degrees. So will this chiller work out for my three and a half feet tank. Want to bring down the temperature to 26 or 25 degrees for my plants etc. Even i am planning to connect it to my filter instead of buying a seperate pump for it. Might even go for the external sensor as you pointed out.


  8. Hi Robert

    I have intend to mod my chiller with a external probe. The problem is when I opened up the chiller. I saw the probe which is white in colour, shall I just remove the white colour wire which is attached to the compartment? Because I saw you remove both the white and the red wire. I thought the white wire is the temperature probe and not the red one so why did you removed the red one and re-attached it again?


  9. do not remove the white plastic thing, that is part of the sensor. The metal sensor that I have is different, that was bought separately.

    Different chiller might use different wire color. Mine happen to be red and white.

  10. Hi,

    thanks for sharing, may I know what is the spec of the temperature sensor and value.


  11. Sorry, not a clue about the spec, I don't event know the manufacturer. Just make sure you have the correct type, PTC or NTC.

    I don't really care about the temperature range and rate of resistance change because I'm only interested around 25-30 C.

  12. hii Robert..
    if i want to change the sensor on my resun chiller.. i need to find PTC or NTC type?
    want to order online. so that i need to know the type first.

  13. you need multimeter to do that. There is no marking on my sensor. I'm not sure if the chiller manual mention about it.

    You can actually re-use the sensor. Just take it out and extent the wire and don't forget to plug the hole where the sensor was

  14. so is cl280 ptc or ntc? since i am using the same module as you

  15. Forgot, but you can bring the sensor along to the shop and ask the shop the similar model. That what I did.

  16. Hi Robert

    I was trying to open up my chiller CL-280 but i notice there are wire connections via the switch on the side. How do you open up the whole casing?

    Adding on, the original sensor length is less than a meter or much longer? I also trying to figure out how to have the thermostat to run to the tank as there doesn't seem to have a hole on the casing for it.

    Thanks alot for the advices as I am a total noob when comes to wirings.

  17. I remember it is not difficult to do, the whole cover can be open but you do not need to do that, just unlatch the sensor connector and connect it to your external sensor.

    The original sensor cable is very short maybe only 50cm or less. You can tun your external sensor cable via the grill at the front or you can also make a hole in the chasing.

  18. Thank you for another fantastic blog. Where else could I get this kind of information written in such an incite full way? I have a project that I am just now working on, and i am sure this will help me a lot..and I have been looking for such information since from few days....Thanks!!
    Home Alarm Installation

  19. Hi sorry maybe out of the topic here I just gt a chiller I tink is the same model as urs but I gt no ideal wad ur are talking about it can any1 help me pls as I nid to get an external termostar in order to control the temputer I needed for my haven started planted

    1. There are simple solution actually, get temperature controller from ebay (link below). Quite cheap, about the same price as buying external probe. However, do check for the spec make sure it suitable for your application.