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WARNING there are lots of pictures to follow. I used the general idea found from the github page on immersion heaters and ran with it. The steps below are for a drawer style set up. I believe this could easily be modified using rigid airline tubing bent into a coil and placed in the mesh bags for the open bug mealworm farm kit.
Started with a sterilite drawer system from walmart
Reflectix insulation from lowes which is basically like reflective bubble wrap.
Cut to fit the drawers and used hot glue to adhere the sidewalls to bottom. The ideal of the sidewalls is to direct the heat upward. Also the insulation is not glued directly to the drawers so this can be modified later if necessary.
Using standard airline tubing used for aquariums I made a coil pattern in the bottom of the drawer. I used clear scotch tape to keep the tubing in place and directed the tubing out the back of the drawer through two holes I made using a 1/4 drill bit. Be sure to leave about a foot extra tubing going in and out of the drawers to allow for the drawers to be opened completely without snagging issues.
Insulated a 5gal bucket and used and old bathroom trashcan as the actual reservoir for the water. I used duct tape to make a seal between the trashcan and bucket for when the water evaporates and drips back down from the lid.
I used a 200watt submersible heater and a 120gph submersible pump.
Now to connect it all. The pump comes with a 1/4" connector so a small section of 1/4" inner diameter tubing is required. Also needed is a reducing coupler. I used a 1/4" to 1/8" barbed coupler and secured the airline tubing side with a zip tie because of the pressure coming behind it.
To split off to each drawer I made a ring of sorts with 1/4" t-connectors. The idea of the ring was so that one drawer was not necessarily getting more flow than the others and so the heat would be fairly consistent throughout. I did the same configuration for the return tubing back into the bucket.
I used a small section of tubing and and on/off valve to go to each drawer from the T and a coupler and another small section of tubing for the return lines. The on/off valve gives me the ability to adjust flow to specific drawers and even turn a drawer completely off if not using it.
Ok finally everything is connected and water is flowing through all the lines.
Now to prevent heat loss to and from the drawers I used left over reflectix and some foam to wrap the exposed tubing.
Now for some temperature measurements. Placed directly on the coils I was getting temps around 82-83. About 1 inch off the coils temps were around 80 and place about two inches off the coils the temps were around 77. This was consistent in all 3 drawers and this is with the heater set on its highest temperature setting.
The entire project took me about 4 hours to complete and everything used was purchased from Amazon, Walmart and Lowes.
My cost were as follows: Airline tubing 50ft - Walmart ~$6.00 Reflectix insulation - Lowes $16.25 1/4" ID tubing 20ft - Lowes $5.47 1/4" Coupler 15ct - Lowes $4.36 1/4" T connector 10ct - Lowes $4.65 1/4" On/off valve 3 ct - Lowes $3.86 Submersible Pump 120gph - Amazon $13.99 200w Submersible Heater - Amazon $13.24 1/4" - 1/8" Barbed Coupler 25 ct - Amazon $3.75
Everything else mentioned I already had around the house so that brings my total cost to $71.57 This price could be reduced if I had a re-do. I just went with was convenient at the time.