A simple dehydrator build with temperature control

Step-by-step process (with pictures) of how I built a large capacity food dehydrator, including optional thermostat control of a heating element and ventilation. Includes all parts and detailed instructions.

Recently after many weeks of successful mushroom hunting, I quickly had more mushrooms than I knew what to do with, and I would hate to throw them out. Our apple tree also had a massive amount of fruit this year. One could save time and buy a dehydrator, but after looking online at the various options and trying to decide on what to do with some old roof sheathing (plywood) I decided to build one.  This post details the build process step-by-step and includes useful tips and links (note, affiliate links support this website and future content). This is an intermediate project that anyone can do, it is well worth the effort! 

A variety of wild picked mushrooms and garden peppers about to go into the dehydrator.

This dehydrator has proven to be more useful and fun to use than I expected, we have used it to make home-made beef jerky (it can easily run over 180 deg. F!), apple chips, dried peaches, herbs, and others. After some research into similarly inspired projects I combined what I considered the best ideas into the design but kept it easy to make. It has a high capacity and can be used with different heating and ventilation units by simply plugging them into two outlets that are controlled by a built-in thermostat.  I've also included some notes on modifying it to be more suitable for jerky with an internal fan.  The total cost of this build for me was around $75. A list of materials used and links to purchase important parts and tools are provided. Hope you enjoy!


Legal Disclaimer: This project involved basic electrical wiring, even simple electrical work has inherent safety risks and can result in serious injury or death.  If you do not feel comfortable making electrical connections ask for help from a licensed electrician.  subcriticalflow.com cannot be held responsible for any injuries you may incur from building or using this dehydrator.

The box

The main task in building this dehydrator is construction of the frame which is simply a box made of plywood. Please feel free to modify the dimensions to suit your needs but the main points to consider when laying this out are:
  1. What are the dimensions (length and width) of your trays you wish to use in your dehydrator?
  2. What capacity or number of trays you do you need? 
The dehydrator is a simple plywood box, with a hole on top for the exhaust fan and a series of holes on the bottom of the walls for incoming air.

Some things to keep in mind if you design your own box, the width and length of your trays will determine the width and length of the interior of your box. You will want to make your interior box width about 1/2" wider than the length of your trays or drying screens if you build your own so that they can slide in and out easily, they will be supported by 1/2" or so thick wood strips. As for the capacity question, if you only need to dehydrate a few pounds of food at a time (most foods dehydrate in 6 to 24 hours), then you can shorten your box. Just remember that if you greatly increase the size of your box the heating and insulation should also be bumped up a bit because you will have great heat loss. 

Overall dimensions

The box interior dimensions are 17" wide by 16" deep (front to back) and about 34 1/2" tall. 

Again, the width and depth were determined by the dimensions of the stainless steel cooling racks I use as drying racks which are 16.5 x 11.5", I kept the extra 4 1/2" depth to be able to keep the food a bit away from the walls to improve the drying efficiency, I also wanted to experiment with adding a horizontal fan inside and have enough room without interfering with the drying racks, but this is optional you could reduce the depth of your box to be more inline with the size of your racks.

A note on drying racks/material:

I recommend stainless steel cooling racks because they are food safe and easy to clean, and will last forever unless you drop a sledge hammer on them (you could probably hammer them back into shape). After reading into this and considering building my own racks from wooden frames and screens I learned that aluminum or galvanized screen can discolor your food if it is acidic and may leach into it. Plastic screen (avoid fiberglass) like polypropylene can crack in high heat or tear easily, and I'm not fully comfortable with plastic material being in contact with food either.  Some claim to be food safe you should research how they do with higher temperatures up to 200 degrees F. Save yourself some trouble and just get the stainless racks!

Cut list:

I used 3/4" plywood leftover from roof sheathing but recommend 1/2". I decided to use the 3/4" material because I had it laying around but it is overkill and makes the final unit a bit heavy although it works fine. Because it was a lower grade plywood I used an orbital sander, like this one, starting with 80 grit pads to smooth off the surfaces for a nicer finish. Note, these dimensions are for 3/4" thick material, if you use 1/2" you will need to increase the width of the top and bottom pieces by 1/4" or so. 
  • 16" x 36" (2 pieces for side walls)
  • 18 1/2" x 36" (back wall)
  • 17 1/8" x 36" (door)
  • 17 1/8" x 15 1/4" (2 pieces for top and bottom) 
  • 15 " x 3/4" x 5/8" (24 for for rack supports or your preference)
Use a circular saw or a table saw to cut your pieces from the plywood. I used a circular saw and the Kreg rip cutting jig which is affordable, but use what you have. 

Assembly of the box

If you have never built a wooden box don't worry it is not too difficult and this project doesn't require precision to be successful. If there are small gaps in your box seams once complete you can fill them with silicone caulk. 

Before assembling the box, cut out the ventilation fan hole (see image below) in the top piece with a jigsaw and drill inlet holes in the walls. Find the center of the top piece and draw a 4 7/8" circle with a compass, note you may need to adjust this if you use a different fan.  The fan should be able to slide into this how so it lays flat and does not fall through. 



View of dehydrator box from above, the front door is at the bottom of the picture and covered in aluminum foil for reference.

Next, drill 1/2 or 3/4" inlet holes (about 2 inches from the bottom) in the side and back wall pieces every couple of inches angled slightly upwards from the outside in (picture below).  You could also drill these holes in the bottom of the door but I didn't. Tip: put a piece of scrap material under your wall pieces to avoid tear out. 

Side view of inlet holes near bottom of side walls and castors.


I used a pocket hole jig and screws without glue to assemble the box and it is very strong and square. It would also be fine to drill pilot holes and screw, or nail (with glue) the box together but the angled pocket holes gives a lot more strength to plywood joints because they avoid screwing directly into end grain.  

If you use pocket holes, layout your screw locations on all the pieces after you label them (back, side, top, bottom) and drill all the holes about 6-8 inches apart. Start by laying the back wall piece on a table or work surface and attach one of the side walls, a brad nail gun or corner clamp may be useful to hold the pieces aligned while you screw. The back and side walls are the same length and should be aligned lengthwise and connected at 90 degrees with the side wall should sit on top of the back (when the back piece is lying down). Next you can attach the top and bottom pieces which fit inside the back and side walls and are flush with their outer ends. Now you should have the back and one side wall connected lengthwise and along the top and bottom pieces, now the other side wall can be attached to the back and the top and bottom pieces from the inside. Look at the pictures above to see how the pieces should fit together once complete. Tip: set your drill to a low torque to avoid over driving screws and striping the plywood.


Now your box should look similar to the first picture without the wood strips installed.

Cut and install drying rack supports

I used some scrap pine for the drying rack supports, you could also use 2x building studs or whatever you have. If you have a table saw you can easily cut these, if not you can use quarter round or square molding from a hardware store. Once they are all cut clean off any rough or splintered edges with sand paper. Next determine the spacing you wish to have between the trays, I recommend 1 1/4" spacing. Cut a scrap piece of straight wood the same thickness of your spacing (1 1/4") that is about 15 inches or so long. Use this piece of wood to help place your tray supports and align them evenly on the inside of both side walls. All you need to do is lay down your box on its side and put the spacing piece up against the inside wall along the top of the box, now you can take a rack support strip so that the wider side (the 3/4" thick side) is laying down on the wall and attach it to the wall. I used a pneumatic nail gun with 1 inch brads with wood glue in between to attach the strips. Continue to use your spacer piece placed up against the previously installed support strip to install 12 strips on one side of your box then flip the box over and do the same on the other side starting from the top down. The result should be 24 perfectly aligned strips to support 12 drying racks and will look similar to what is shown in the first picture above but note that I only used 12 strips in mine (6 on both sides), looking back I recommend 24 strips (to support 12 drying racks) in case you ever want more capacity. 

Apply finish (optional)

At this stage I applied some raw linseed oil to the entire dehydrator including the door to protect the wood. I wouldn't recommend boiled linseed oil because of some of the toxic additives that may release at higher temperatures? You could use other finishes, natural plant-based oils like raw linseed are typically safe, and if fully cured most finishes are safe but do your research on their heat resistance. You would even paint the box if you are comfortable with it. 

If you decide to use raw linseed oil I would recommend mixing it with mineral spirits at a 1:3 ratio, this will help it penetrate deeper, make it easier to apply, and might cure a bit faster. 

Connect the door

This is a good time to connect the door to the dehydrator. This is simply done using two piano-style hinges. Put the box, door side facing up, on a work table and lay the door inside its front. The door should fit fairly snugly in between the two side walls and flush with the top and bottom of the box. You can lightly clamp the door in place. Next lay the hinges on top of the edge of the box so that one half the hinge extends over the side wall and the other half over the door, putting one hinge at the top of the box and the other at the bottom. Keeping the hinges centers, mark and drill small pilot holes where each screw hole is located and then attach using the screws provided.

Piano hinge used to attach door to side wall.
Piano hinges used to attach door.

Install a door latch

I built this door latch using a scrap piece of wood cut into an L shape that pulls the door in tightly when shut using a clamp latch on the side of the box. It easily moves into closing position by attaching it to the front of the door using a small hinge. I did this because it does not only pull the door closed by also pulls it horizontally towards the side of the box to minimize any gaps along the door which helps to keep heat in the dehydrator.
View of custom made door latch.
This method of making the door latch is optional, you could design another method or buy a latching system. But if you decide to make this, simply cut out or use an L shaped piece of wood attach it to the front door so that when closed the L pulls the door tightly against the side wall using the small hinge. Align, pre-drill, and screw in a mounting block that runs parallel with the L-shaped wood on the outside of the adjoining side of the box which holds the other part of the latch, make sure when the L is closed there is an appropriate gap so that when the latch is tightened it pulls the door in flush but it is not too close or too far away.  

Install wheels or bottom supports

You will need to put some sort of supports under the dehydrator, I used four 3" lockable castors which allow me to move the dehydrator easily but you could also use other supports. This is important because you will need some access under the box for the wiring to your heating element which will run outside and under the box up to the top where the thermostat is located.  

Install electrical box and fixture for the heating element

Next, drill a 2" hole in the center of the bottom of your dehydrator using a hole saw or jig saw. This will be used to run wiring to an electrical box which will be used to hold the heating fixture (bulb fixture).  The reason for the large diameter is to make it easy to connect a cable clamp to the electrical cable to the bottom of the electrical box so that it cannot be pulled loose accidentally, this is very important for safety! 

You can use an old discarded extension cord for this like I did or buy a 14 gauge cord that is about 5 feet long and keep the male 3-pronged 120v plug attached. You will directly wire in the other end of the cable into the light fixture later.  Install a 3/4" cable clamp into the bottom hole in a 4" electrical box and tighten it to the box leaving the cable clamp open. Next, attach the electrical box inside the dehydrator on the bottom using screws so that it is centered over the 2" hole.  I recommend this 4" electrical box which is safe in damp conditions and a porcelain light bulb fixture to hold your heating element. Now run your cord through the bottom hole of your dehydrator and through the electrical box through the clamp with about 4 inches of the wires out of the cable sheathing using a cable stripper. Also strip about 1/2 to 3/4" of bare wire from the ground, neutral, and hot wires with a wire stripper and attach them to the bulb fixture, this is typically done by bending a partial loop (using the small hole in the wire stripper) in the three wires and pinning them down with the screws inside the fixture, making sure to connect the ground screw in the correct location of the fixture. Once the connections are made, pull any slacking cable through the bottom of the box and attach the fixture to the top of the electrical box with the supplied screws.  Now finish up by tightening the cable clamp from below the dehydrator which should be accessible via the 2" hole that was drilled. Hold the male plug end of your cord and make sure that it can reach at least 8" above the dehydrator so it can be plugged into the box holding the thermostat.

The thermostat box and wiring (optional)

Before continuing, I think it is important to mention that installing this thermostat is optional. At this point you could plug a heating element, like this 250 watt infrared bulb into the bottom of your dehydrator and put the fan in the top and start dehydrating. From my experience this configuration will run at around 150-160 degrees F. with the fan on. You could experiment with different heating elements, a fan damper, etc to adjust your temperature and airflow manually. The neat thing about the thermostat setup is that it allows you to control the ventilation or heating element separately or in tandem, plus it tracks the temperature inside the dehydrator very accurately. For example you could plug your exhaust fan into the wall and the heating element into the thermostat so that the fan never stops running however the heating element will only kick on when the temperature falls below a certain level. When making beef jerky, I typically do the opposite using the thermostat, in other words I keep the heating element on and only turn the exhaust on if the temperature goes above say 175 degrees F. 
Building the thermostat controller was a new thing for me and I found it enjoyable and will likely use this approach again if I ever need to control heating/cooling in other projects like a greenhouse. 
The thermostat I used was the very affordable (about $16 dollars) Inkbird ITC-1000F which is commonly used by home brewers for their fermentation chambers. Hey, maybe after this project you can pick up a new hobby? I housed mine in a small electrical subpanel box simply because it was what I could find and was inexpensive. Although this works fine and is very sturdy, looking back I would recommend using a plastic "project box" because they are much easier to cut out the holes needed for inserting the thermostat and receptacle.  

Steps:

Once you have your project box, take the tightening latches off the Inkbird and draw a rectangle in the front of the box that the Inkbird can fit into but the front will not fall all the way through. Also, draw a rectangle that is about a half inch smaller than the front cover for a regular 15 amp two plug receptacle on the top of your electrical box. Next cut these holes out with a Dremel/rotary tool and a cut-off wheel.  Also knock-out or drill the appropriate size hole for a cable clamp for the power supply cord and drill a small ~3/8" hole in the back of the box for the temperature sensor to exit the box.  The wiring diagram provided on the Inkbird will not work for this approach and is more useful for direct wiring. 


Follow a good wiring diagram, here is the one I followed from the American Homebrewers Association: 

Wiring diagram for connecting the Inkbird ITC 1000F (and other Inkbird 1000 series) to a receptacle for separate heating and cooling circuits. 

Here the black wire is the hot, white is neutral, and the green is the ground. Important, one key thing you need to do that is not shown in the diagram but is critical for this circuit to function is to remove one of the tabs on the receptacle with needle nose pliers (see image below), in this diagram it is the one the left side of the receptacle, the side where the ground connection is, the tab on the receptacle will be in between the screw that holds the wire that connects to port #2 on the thermostat and the neutral wire that comes directly from the power cord. 
Side view of metal tab that needs to be removed
from one side of the receptacle with needle-nose pliers.


Once you have broken this tab, install your Inkbird into the front slot of your box, attach a cable clamp to the power supply hole you made and pull the power cable through it and into the box, separate a good amount of the individual cable wires (~ 6 inches) and strip the last 1/'2" or so. Also, from your spare cable, cut and strip 3 pieces of hot labeled wire that are about 6-8" and 4 neutrals, you can always shorten them later. Now follow the diagram to make the connections using appropriate sized wire nuts or locking connectors for the gauge wire you use in this box (typically 14 AWG). The connections to the thermostat itself are made by sliding the ends of the wires inside the terminals and pinning them down with screws that come with it.

A view of the thermostat box after making the wire connections from above.

At this point it would be useful to mark which end of the outlet is connected to the cooling circuit and which plug is for the heating circuit on the outside of your box lid. Don't forget to also connect the provided temperature sensor to the thermostat and run the probe out through the box. I attached the whole assembly to the top of the dehydrator directly with screws. It can be tricky but I tightened the receptacle to the electrical box simply by tightening the screw on the from cover of the receptacle and positioning the receptacle so that it extends under the box lid, then by tightening the cover screw it pulls the receptacle body up against the box lid. Finally, tighten the cable clamp that holds the power supply cord into the thermostat box (image below). 

View of thermostat box from the back and the cable clamp holding power supply.


Drill a small hole in the middle of the back of the dehydrator to run the temperature probe and position the probe towards about one third of the way to the middle of the box (from the wall) and about two thirds of the way up, where most of the food will be. Now plug in your thermostat to make sure things are working and the temperature reading seems correct. Plug the heating element into the heating circuit, and it should turn on, next try setting the temperature below the ambient temperature and plug the fan into the cooling circuit and make sure it turns on. 



Front view of finished dehydrator with thermostat installed.


Final touches

You could fire her up and start drying, however I highly recommend putting some sort of reflective or radiant barrier (e.g. aluminum foil) on the inside walls of the dehydrator to increase its efficiency and reduce the heating time. Even better would be a layer of insulation with radiant barrier outside of that, like the stuff they use in attics, or you could experiment with a thin layer of food sage insulation and cover it yourself with aluminum. 

View of the inside of my finished dehydrator, I used HVAC tape to hold down aluminum foil inside but a much better insulation system could be used.

Congratulations! 

I hope you enjoyed this post, please make a comment if you find it useful or have any questions, I'll do my best to answer any. If you decide to make one yourself I would like to hear about it, please share in the comments!

Usage, experiments, and anecdotes 

I've experimented with a couple different incandescent light bulbs as heating elements as well as the big 250 watt infrared which can get the dehydrator over 180 deg. F! It seems like a 50-75 watt incandescent running constantly might be perfect for making Biltong because it runs around 80 deg with the fan running constantly. 

To improve airflow without adding more exhaust/cooling I experimented with mounting a small motor with a fan blade attached inside the dehydrator about halfway up on the back of the unit. This worked very well for making beef jerky as I was able to cover the ventilation hole on top with some foil and increase the temperature greatly (maybe too much) while still drying well. 

Beef jerky made using the dehydrator with horizontal airflow fan attached inside.

The way I did this was I took a small ac motor (probably 5 volts or so) out of an old space heater someone threw away and mounted some cheap polycarbonate (heat resistant) R.C. airplane propellers to it, like these, I couldn't use the fan blades provided because they were the wrong direction. You can see the red propeller blades in the back of the picture above if you squint.  

Full material list with Amazon links:

Get the plywood and wood for the tray supports from your local hardware or lumber store referring to the cut list. You will also probably need a drill if you don't have one, this Dewalt kit is a great option for a good price (currently on sale at time of this post). You will probably need a circular saw in case you don't have a table saw or other means, here is a great Dewalt saw on sale for the holidays. 


Stainless steel drying racks

Useful tools/materials that will help but aren't required:

Pocket hole jig (don't forget screws)

Note, the links above are Amazon affiliate links that help support this blog if you make a purchase through them. All the tools mentioned I either own or are familiar with and I recommend them highly, I am not sponsored by any company these are just based from my experience.




Comments

Popular posts from this blog