True GDM-6-LD Build


Hi all! First off, I’m really excited to get started with the BrewPi!

I just recently purchased a BrewPi as well as a couple of temp sensors and 10A SSRs + heatsinks to get me started. (It should be arriving soon!) I want to include a lot of details regarding this build here as I go along with the build. I find there is a serious lack of complete build information on the forum. I will be using a BrewPi Spark v2 (particle photon). This is really all I will be focusing on for the next few months so I am very dedicated to the project meaning I will have the time to be thorough and do a lot of testing. I will probably include one test fermentation once I get the finished chamber running to my satisfaction. Full disclosure, I am not a complete noob when it comes to industrial electronics and uC projects but I still have A LOT to learn. I’m not an EE. Just a curious nerd. I will try to explain everything I figure out or make happen here in this thread and in plain English. I do have access to EE’s I trust that have helped me with projects in the past at my local hackerspace.

I purchased this beverage merchandiser second hand from a used restaurant supply store to use as my fermentation chamber. I have a 6.5 gal glass carboy in the refrigerator for reference. There is plenty of floor space for the carboy and a second smaller vessel for potential blowoff as well as vertical clearance for an airlock.

This is a True GDM-6-LD (link to spec sheet)

I plan to have the BrewPi directly control the compressor as well as heating element and the evaporator fan for circulation. The resistive heat cable would be woven into the top rack you see there in the picture not unlike the method in the fridge hacking guide.

A. I want to either use one of these at the top of the chamber which would sit next to the evaporator fan…

Zoo Med Reptile Heat Cable 100 Watts, 39-Feet

B. Or affix the two shelves to the sides of the chamber and use two of these, one on either side.

Zoo Med Reptile Heat Cable 50 Watts, 23-feet

I feel that 100w of reptile heat cable should be sufficient (please correct me if I’m wrong). My question is in regards to whether there would be any obvious practical benefit to either setup with respect to heat distribution. I can’t know for sure obviously without testing but it seems that the evaporator fan would be sufficient in circulating heat if I placed the heating element at the top of the chamber. Any suggestions/advice? I will probably get started with the build on Monday.

EDIT: I chose option A.


The process was extremely easy and doesn’t really warrant step by step instructions. There were maybe ten sheet metal screws in total required to remove the front/back panels and thermostat. I would just advise people to be cautious when separating the thermostat from the interior panel. Also, set the dial to zero on the thermostat before removing it so that when you put it back it corresponds to the correct setting in case you ever want to convert it back to stock.


  • 14 AWG
  • 22 AWG Two Wire Cord
  • 6" Right angle Micro USB to USB A
  • 10’ Micro USB to USB A
  • 5v 2A USB Power Supply
  • 14 & 22 AWG Ring Terminals (Red and Blue)
  • 3 Port Push-in Wire Connector (1)
  • 2 Port Solderless Splice Connector (1)
  • 1/4" Grommet
  • Permagum (Duct Sealing Compound)
  • 25A SSR (2)
  • 3 Pole Circular Connector M & F (2)
  • 1/2" Split Loom Tubing
  • Cable Management Kit
  • Black Spray Paint
  • RJ-11 Connector
  • Heat Shrink Tubing (various sizes)
  • Electrical Tape


The build was fairly easy and straight forward to my surprise. I didn’t run into any problems during the build and really didn’t do anything vastly different from the fridge hacking guide for compressor control. I did get lucky in that the neutral terminal block on the fridge had one opening which I used to connect the heat cable neutral to. Also, I didn’t use heat sinks on my SSR’s because mine came with thick aluminum plate to absorb heat. I mounted them to the back panel of the fridge using thermal tape to use the whole grill as a big heat sink. So far the panel has been cool to the touch. The other thing I did was install a rubber grommet into a hole I drilled in the sheet metal interior panel for the heat cable to pass through. I did this to prevent the sharp metal from cutting the silicone covered heat cable. The permagum was used to reseal the cable port in the fridge used to pass wires through to the back panel after I was done routing the new cables. The finishing touches included routing the cables through a cable management kit I cut to size and spray painted matte black. I then just cut several short pieces of split loom tubing to organize the cables further.

Performance Testing


I think either option will be fine. Weaving it throug the top rack is probably easy to do and makes it easy to slide it out for cleaning. The only downside is that heat goes up, but if the fan is circulating the air, I see no problem.

Looking forward to your build thread and great choice of fridge!

Thanks Elco! I will go with option A (top rack). I’ll order the heat cable now and hopefully it’ll be here by Monday so I can get some work done.

So speaking from experience here (I have a BevAir merchandiser hooked up to a BrewPi) really pay attention to the amperage the unit pulls. You will super heat the SSR and have some major issues so size it correct. We found out the hard way with an undersized SSR and ended up burning components out in the cooler.


What kind of SSR did you use? What is the amperage of your fridge?

Of course in the US, your current would be double what it would be in Europe.

@Blankly, either option would work. I found that an old hair dryer my wife quit using, set to medium power worked well for my 18 cuft fridge

My opinion is that most hair dryers are a bit too much power to heat up a small fridge. At least the one I have at home is 2000W. 100W is the right amount of power and much safer.

Hey thanks for the heads up @sunadmn! The spec sheet says it only pulls 2.3 amps. I thought that the 10A SSR’s sold in the shop would be sufficient in controlling the condenser. Am I wrong in assuming that? I am also curious as to which SSR’s you ended up with and which Bev Air fridge you have.

Also, how did you choose to control the fridge? In the True GDM series technical service manual it looks like it will be fairly easy to just unplug the thermostat and add a SSR in its place and leave everything else the same including the evaporator fan. It seems like the easiest and least involved hack with no downsides that I can see. I guess the easiest way to see if the 10A SSR’s I ordered will be up to the task is to check out the thermostat specs and see how many amps it is rated for.

Edit: So after a basic teardown it looks like the thermostat (GE #3ART55VAA4) is rated for 12A. I’m thinking it might be wise to use something rated for atleast 12A. I think I’m going to bridge the thermostat and then just add the SSR somewhere down near the terminal block where everything is connected. I’ll post pictures in a bit.

This thing is an OLD OLD (1980’s) BevAir and I had a 15amp SSR on it and it was pulling just a little over 15amp. The heat sink couldn’t get the heat off fast enough and would start to brown out the unit. This caused chaos with the internal fans and we ended up having to replace them.

Moved to 20amp SSR and everything was fine after that.


My BrewPi came in the mail three days ago. I’ve been working on setting up the BrewPi with my Raspberry Pi for the last couple of days just testing and playing with the settings to familiarize myself with the system. I just posted teardown pictures in the main post like I promised. The fridge is made to be repaired easily so it and other industrial/commercial refrigerators are ideal for this project as they are very straightforward in function and easy to route cables through.

I will post a schematic from EagleCAD or something showing you guys how I decide to wire up the relays. (I haven’t yet done it.) I am probably going to go with a pair of 25A Crydom relays and low profile heatsinks when all is said and done. The wiring will be done very similarly if not identical to the fridge hacking guide. During the teardown I discovered that the evaporator fan is always on so I am just going to leave it that way. I will be ordering the rest of the supplies soon. I may have this project and guide finished by the end of next week which will be perfect because my hops are almost ready to be harvested!

I almost forgot; I also ordered some pot/cup magnets for mounting the BrewPi like the ones @Elco showed in his post. I had a hell of a time finding someone locally that sells them but eventually stumbled upon Amazing Magnets, a company pretty local to me (SoCal) that has what I think is a match to the ones he listed. I ordered some and will report back when I get them.

Item Code: CUPT375

EDIT: The magnets I listed above work wonderfully.

Here’s some hop porn for your viewing pleasure. This was taken a few weeks ago.


Nice hops!

For those in Europe, is a great supplier for magnets:

I don’t have them in the shop, because shipping magnets by air is restricted.

Alright, so I have it all put together and working but I am still adding the finishing touches (making things look pretty). I have been testing the system with a 5 gal carboy full of water for the last 24 hours or so though and I have been getting what I think are ideal results. I am however going to wait until it is completely done before I post the grand reveal and a proper set of instructions and pictures.

Preliminarily it seems as though I am able to reach and very accurately maintain the full range of temperatures anyone would ever want for fermentation (33-90° F).

The crests and troughs in the photo below consistently reach 60.12° F and 60.01° F respectively. This 0.11° F variance seems pretty reasonable to me. I haven’t done any fine tuning. This is just the algorithm doing its thing. Does everything look normal in this picture?

EDIT: The build is now done and working beautifully. I had a lot of fun building this project! Thanks Elco!