The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

proofing box

caviar's picture

proofing box

I've read several places on TFL about hooking in an inline voltage thermostat to get the right temperature for proofing. Has anyone actually done this and if so how was it done? What i'm finding is that a heat (resistance) source has to be at least 2.0 amps. Is it any different than hooking in a light switch and is there a certain kind of digital thermostat.




BobS's picture

I bought one of these and wired it to a socket with a 15W bulb. Then I place the bulb and thermostat on opposite sides of a cooler. I use different coolers, depending what size I need. Works great

flournwater's picture

Herb,  I suspect you may have your electrical theory confused.  If P=IE, then 2 amps of current on a 120 volt circuit would mean a 240 Watt lamp.  That's far more wattage than you'd need for a proofing box. You could probably bake the bread with that much heat.  The idea offered by BobS (I really like the thermostat he selected) works great.  If you don't want to use a picnic cooler type of container you could build a plywood box (double walled and insulated if you want to go that far) but it's not at all difficult to achieve the goal of buiilding a proof box yourself.

So you don't have to cut and paste the link, here it is in a form you can just click:

caviar's picture

Thanks for the reply. I picked up a thermostat, installed it in a styrofoam box as mentioned in the site here from some time ago with a light socket and a low wattage bulb. The thermostat didn't function so I looked more carefully at the instructions where it said there had to a minimum of 2.0 amps. So I tried a 240 watt bulb and the instrument registered that it was functioning but the light did not go on. Instructions stated it was for baseboard heating. Looking at the site you sent me the type of thermostat is quite different. Not being all that knowledgeable about this stuff I guess I just picked up the wrong kinds.

Thanksfor the link. I guess I just blew a chunk of money.


Pablo's picture

Any cool, easy, cheap ideas for a proofing box that would be a little cooler than the environment?  Say maintain ~60F?


flournwater's picture

Use BobS's idea but instead of a light bulb, cut a hole in the side of the box near the top and install a muffin fan in the hole,  Control it with the thermostat and cut small holes on the opposite side (also near the top) of the box.  Connect a funnel the exterior of the box, where the muffin fan is mounted, and link that by ducting to a bucket of ice cubes so the fan pulls in cooler air when the temperature goes above 60 degrees and stops pulling in air when it reaches that temperature.

SteveB's picture
xaipete's picture

I was thinking of trying out my cooler with an ice pack in it. Slightly different subject, but I was also thinking of using a wide-mouth thermos to keep my starter at 70 degrees.


summerbaker's picture

My Cuisinart wine fridge turned out to be kind of a dud for wine since it really struggles to keep temps in the 50's F, but is great for SD starter control.  It happily keeps 68F which is perfect for my stiff starter.  If I were you I'd look into inexpensive wine fridges but stay away from Cuisinart (which was NOT inexpensive).


Scottyj's picture

Huum! and here I was going to hook up a heating element to a reostat and try that.

good thing I saw this post just save me alot of time and money. Well not somuch time as I made the proofing box already. Yes plywood and doubled insulated. The wood had been sitting for about a year and the insulation was left over from the attic. the money part is the thermostate.

drhowarddrfine's picture

p=ie is for dc voltages and current. Here we're talking about AC, which alternates direction and isn't constant, so the formula doesn't work out to 240W as you might suspect. The actual voltage needs to be multiplied by .707 to account for the sinusoidal varitions, so 84V times 2amps gives you 168Watts.



flournwater's picture

You're absolutely right, doc; I forgot about the RMS .707 element for AC.  But I'm pretty sure you'd agree that 168 watts is still far more wattage than you'd need to maintain the temperature for a good proof box.

ClimbHi's picture

done that. In the winter, my kitchen is a bit on the cool side, so I made my own proofing box.

I got a large old Coleman cooler, fitted it with a line voltage thermostat that's made for greenhouses (look here: ) and use a heating pad for the heat source. I place the heating pad on the bottom of the cooler and put a shallow pan of warm water over it. I built a shelf to sit a few inches above the pan, and I proof the dough in a container on the shelf.

It works really well for a Rube Goldberg appliance.

Logiwonk's picture

If you build the proofing box so that it can get up to 110 deg F then you can also use it as an incubator for making yogurt, or a variety of other tasty home fermented goods.  For an excellent book on fermentation techinques check out:

annimax's picture

I took a very different approach- I ended up with a labratory oven from the local univesity surplus store (think never ending garage sale for anything & everything a university might ever use). For $30, it will hold a temp of ambient room temp +5F up to 500F, with less then 1-2degrees variability. Its large enough to hold half sheet pans, has forced air convection, all stainless steel inside with 2" insulation. I originally bought it to make it into a meat a smoker, but I feel in love & let it follow me in the house-lol. Thankfully I picked up a couple of others that are a little smaller, in the $5-$10 range. One of which is trying to find a place inside, to use a more dedicated proof box. It only has gravity convection, not forced air, and it would also make a nice dehydrator space when I overflow the larger one or if I want to bake in the larger one.


TomH's picture

Can anyone clarify the apparent discrepancy between the thermostat that BobS used and the one that Caviar bought?  I ordered the one from the link that was posted and it appears I am in Caviar's situation.  The thermostat won't plug into a "standard" receptacle.  The ground prong is round and the outlet for the ground prong is flat on one side, so it doesn't fit.  I would appreciate and additional information.



BobS's picture

Hi TomH,

I'm not sure I understand your question, but let me explain a little about thermostat types and perhaps that will help. I should note that I am in the US, so this discussions uses US voltages for example. YYMV outside the US.

There is a lot of confusion because there are two types of thermostats that appear to be similar (and are sometimes called the same things in listings and at the store) but are quite different.

One type, often called a Line Voltage Thermostat, is used to control electric heat and is usually (but not always) wired in place. In a house, sometimes these appear on the electric baseboard, and sometimes on the wall. These thermostats have two important characteristics:

  1. They operate on 220 volts.

  2. They will only operate if the load they are controlling draws a significant amount of current - much more than a light bulb or a fan.This is sometimes referred to as the 'minimum load'.

These Line Voltage Thermostats do not work for controlling light bulbs, fans and small heaters. It is often hard to figure out exactly what the thermostat is from the description. Here's a line voltage thermostat:

Note that the detailed electrical parameters of the thermostat are not listed - you have to read the back of the package.

A good clue to finding a thermostat that will control a light bulb is to look for one that has an electrical plug that fits in your wall outlet and a jack into which you can plug the light bulb. There are a couple of types in this area:

  1. One type of thermostats, with varying names,  used to control smaller loads, like aquarium heaters, fans and the like.  These typically are sort of like an extension cord with a thermostat in the middle - you plug one end into an electric outlet, plug your light bulb into the other end, and set the thermostat in the middle to the temperature you want. People seem to use these for proofing boxes. I don't have any experience with them

  2. The other type, which I bought, is designed to run on 120V and control smaller electric heaters and devices. It is more hefty than an aquarium thermostat and less hefty than a line voltage thermostat. I'll place the link below. The listing does not indicate a minimum load (they never do), but I and others have confirmed that it will work controlling a light bulb for a proofing box. I like it because it is simple and very accurate. I have also used it to control a portable electric cooler to retard dough.

Here's the link to what I bought again for reference:

I hope that helps. I don't quite understand your comments about plug, etc. If you elaborate I may be able to help.



TomH's picture


Thanks for your very complete answer, and sorry for my very vague question.

I had ordered the thermostat that you reference in your post.  When I went to plug the unit into the outlet I was using to power my proofing box it didn't appear to fit.  I assumed that it was because the third "prong" on the back of the unit, the ground "prong", was round, while the ground socket on the outlet was more U shaped, that is, flat on one side.  It turned out that it was just a very, very tight fit and I had to push very hard to get it in, but in it did go.  Color my face red.

Thanks again for your suggestion on the thermostat, I'm looking forward to using the proofing box this week-end.


BobS's picture

Cool. Sometimes, in life, you just have to push a little harder.

It's normal for the ground socket has that flat side and the plug to be round. Maybe it's to give it a little wiggle room for removal.

I did have one of those thermostats fail within the warranty period. I looked up the manufacturer on the web, exchanged a couple of emails, and had another one by second-day Fedex.

My sourdough life greatly improved after I built that box. I live in the Northeast, and the house is never warm enough for my starter a lot of the year. The box, combined with overnight retardation,has allowed me to make consistently good (maybe even excellent) bread on my schedule.



TomH's picture

I can relate to the too cool conditions, living here in Minnesota.  I'm excited about how this will change my life.


dantortorici's picture

Can't tell from the Amazon listing. Is the temperature sensor built into the unit or is it wired so it can be separated?

Looking for something to control a refrigerator to use as a retarder.




BobS's picture

It can be separated, but only by maybe an inch or two.

coffeetester's picture

So I am currently using a plastic filing box. I have a 40 wat light bulb hanging down. It is very clumsy to use. I am constantly knocking the bulb trying to put my bulk proofing bins. These bins go in and out 3 times during a normal bake. Here is the concept for the 2.0


I want to use White sheathed Melamine. I want to build the box to take advange of the height under the cabinets (One reason why its clumsy is because I cannot raise the lid all the way open). This way the cabinet will look like it belongs in the kitchen.


So using the the new wood box I would like to find a low profile heating source. I would prefer a flat radiating heating source. Has any one used a Terarium Heat source like the ones on this list

Does any one else have a recomendation for a good blanket stle of heat source.


So to finish my plan I am going to use cooling racks as shelves when I want to proof dough and then remove 1 of them when I am profing my starter. I will also be making tight fitting doors with weather seals to keep draft out and warmth in. 


If any one has any good suggestions I would appreciate it.

koloatree's picture

Proof boxes that provide humidity control, is covering the bread during the proofing stage necessary?


I assume the answer is no since the humid environment, with the addition of the air flow, will not dry the bread's surface area. Is this correct?



TedW's picture

Thanks for all the contributions. That WIN100 controller looks great, but as mentioned, I'd like to be able to main temps up to 110 for yogurt as well. The Win100 controller is only rated to 90F. Are there any similarly simple / inexpensive controllers that handle the higher temps?

 Might this work?


flournwater's picture

Reviews on that item are not very good ....

MNBäcker's picture

Here's what I had ordered for my original proofing box design. It's a bit pricey, but will control both heat and humidity sources with a combined output of up to 1000W. It will also display heat and humidity. After I ended up buying a used professional proofing cabinet, I use this unit to control the accuracy of that cabinet while using it (it only has a 1-10 dil to regulate temp and humidity).

It looks like it's sold out right now at the source I linked to, but a search for "Zoo Med HygroTherm" might deliver additional sources.


TedW's picture

Thank you vey much Stephan. Question: how critical is it to actualy control the humidity level as compared to simply having a "humid" environment? I had imagined simply inserting a bowl of water or a wet towel in the box to add some humidity to keep exposed dough from drying.



MNBäcker's picture


it totally depends on your doughs and how serious you are:)

A bowl of boiling water will give you plenty of humidity - I actually use the "microwave and bowl of hot water" method when I make just one loaf.

If you have a larger proof box or need to proof your breads longer, it might be nice to be able to monitor the humidity.

Like I said, it's not necessary, but definitely nice to have.


TedW's picture

Thank you so much!

Janetcook's picture

Too much humidity will ruin your dough....I found that out this week in a proofing box I tried out.  Put 2 mugs of hot water into a large plastic box along with my dough....Created a lot of moisture and the moisture went right into my loaves.  I ended up with 2 'frisbee' loaves.

 If you do the hot water method to increase temp. while proofing just be sure to cover your loaf so it doesn't absorb the moisture.

Good Luck!

sam's picture

-> "Too much humidity will ruin your dough"

I learned that too.  Last week I  ruined a soaker + levain with far too much humidity in my attempt to make a home-based proofer-thing, but it was fun to try.  :-)



TedW's picture

I have read soooo many places in such a short time that high humidity can be disastrous. Also, the common theme is that a bit of water in a cloth in a corder works great. Lastly, in my mind the simplest tact is to have a light bulb, dimmer switch, and a thermometer.

I'll have to tend the system to get it to the proper temp, but after a time the temp will stabilize and I can leave it. Use with a big Styrofoam cooler and this system is cheap, function, measurable  and adjustable.

I'll try this and if it's cumbersome or I wind up using it a lot more, I'll go for the thermostat control. I'd go for the thermostat today but I haven't found a particularly dependable unit that doesn't cost an arm and a leg. I've added to the difficulty because I'd like to have a top temp range of 105-110 for yogurt.

Mozart's picture

I am going to be building a proofing box based on the comments in this thread. I have purchased the Win100 thermostat and plan to use a light bulb as a heat source, etc.

I am assuming that electrical cords etc. will be running into the proofing box, both to the thermostat and from the thermostat to the light bulb, etc. Doesn't this become a problem when the heat in the box begins to evaporate some of the moisture in the dough (ie. mixing electricity and water?).

Maybe I am mis-understanding how these things are to be designed. Could someone post a picture of the inside of their proofing box so I can see what the "set-up" looks like?


Janetcook's picture


I am going to post 2 links found here on  proofing boxes that might be helpful for you.

The first is the one that  helped inspire me.

The second is the one that I built and posted here awhile ago.

First box uses a light bulb.  

Mine uses a 60 watt ceramic heating bulb which are used in reptile tanks.  (They last longer as they are made to deal better with the cycling on and off when used in conjunction with a thermostat to maintain a specific temp. range. They can be purchased on line from Amazon for about 20.00.)

The thermostat that I purchased came from a reptile source too and is really simple to use and only cost about 45.00.

No problem with any of the moisture as both of these items are designed for use in humid environments.  The thermostat is the only one with a wire in the box and it isn't a bare wire. It is actually the probe used to monitor the temp.  The ceramic bulb is attached to a light socket and the wires are all outside of the box.

I maintain about a 50% - 65% HL in the box when in use and evaporation is going on.   I have to fill the shallow pan on the floor of the box every few days.  I cover my doughs with plastic crap or shower cap kind of covers and this is very effective against the dough absorbing the moisture out of the air surrounding them.

I have experience no short circuiting of anything electrical since this box was put into use and it is used for 12 hours daily.

Have fun building yours.  Mine is all on automatic pilot now and I love it :-)  It has 3 cookie sheets full of rolls in it right now plus my current leaven build jar and there is still room to spare.

Take Care,


BoyntonStu's picture

This Goodwill bought $5 invention keeps my oven set at any temp between 90* and 150*F.



barryvabeach's picture

Mozart,  mine is just a wine cooler, with a heating pad in it, so I don't think pictures would help much.  Is general, if you want to increase the humidity, put in a cup of boiling water, put a sponge in the water if you want the humidity even higher.  Some of the posters above try to avoid excess humidity, so they might use a smaller glass of water.  I am usually proofing my starter to get it going, so it isn't there long enough to make much difference in humidity.