Overview

You can make a cheap, stable medication refrigerator by plugging a chest freezer into a thermostat-controller, which will power the chest freezer on and off to maintain an appropriate temperature range.

It will hold a much narrower temperature range than most home refrigerators. This is most important if you have medications with a narrow acceptable temp range, like biologics.

You can do this for about $50 if you can find a free, working chest freezer. I spent about $300 on my setup.

Why? Medications and refrigeration

Some medications need to be kept within certain refrigerated temperature ranges. The range is usually 36-46F.

Unfortunately, home refrigerators:

  • Swing widely in temperature
  • Have warm and cold zones. Ever opened your fridge to frozen spinach on a shelf? Had your milk go bad fairly quickly because you kept it in the door?
  • Should be kept colder (for food safety/longevity) than is wise for your :medication temps
  • Are subject to many openings/closings a day, which can raise the temperature considerably

Okay, so what about medical-grade, pharmacy style fridges? Well, anything that promises tight ranges and real stability is like $10,000 USD.

:x Medication temperatures

For food safety, the FDA recommends 40°F (4.4°C) or colder.

Most refrigerated medications, like insulin and Humira, should be kept in temperatures from 36°F to 46°F (2°C to 8°C). It is critical that these medications never freeze; you shouldn’t use them if they have frozen, and at least for biologics like Humira (or Xolair, Cimzia, etc), you can develop antibodies to the medication and lose the ability to take the drug.

So what’s the big deal? Set your fridge to 40°F and call it good.

Well, yes, but also no.

  1. The set temperature is basically an average. It will absolutely swing above and below…sometimes by a lot. So if you want your food to stay 40°F or lower, you’ll need to set the temp to closer to 37°F.
  2. But then it’s still gonna swing lower! And now we’re going outside the medication storage range and risking freezing your meds, which absolutely cannot happen.

If the medication gets too warm for too long, same thing. (Yes, Humira can be kept at room-temp for up to two weeks, but going into too-warm range starts a clock - it’s not just two cumulative weeks of warm-time. Insulin has a longer clock, but the clock still starts. Other medications may be different.)

I had a home refrigerator that maintained an average of 39°F, and depending on location and time in the cooling cycle, it could still get from below freezing to above 46°F. It was a constant battle to keep the medication in just the right spot.

You should monitor fridge temperature

First consider this step, even if you’re storing your meds in your normal fridge. It’s pretty cheap to pick up a battery-powered, remote-read thermometer with configurable hot/cold alarms and a 24h hottest/coldest temperature memory.

This lets you at least find a spot in the fridge where you can be certain your meds won’t risk freezing or repeatedly go above range.

You could also upgrade to a sensor that will record data and and send it to your phone via BlueTooth. Upgrade that with a wifi gateway, and you can see the temperature of your medication at any time. With graphs! See Materials section for details.

One warning: your newfound data can be very stressful if your refrigerator has really big temperature swings. Sometimes I’ve almost wished for the simpler days of ignorance, but hey, the medications were still getting just as far out of range, I just didn’t know about it.

Or, maybe you’re lucky and your home fridge holds great temperatures! You can then skip the rest of this page. Maybe get a power station and/or keep some extra gel packs frozen and refrigerated so you can hold ideal temperatures in case of a power outage. For everyone else, read on.

Help from an unexpected source: the “keezer”

You can make a relatively inexpensive, highly temperature-stable refrigerator with a tiny bit of tinkering. To learn how to do this, I turned to a group of people who have very similar needs for a stable, consistent temperature in a very small window: homebrewers.

Enter the thing called a :keezer, like a kegerator but a “keg-freezer”—a name I find so aesthetically displeasing I generally refuse to use it (but you need to know it as a search term, so here you go). I refer to it as the “meds fridge” instead. The best part is that our meds fridge can be way simpler: no CO2, no ‘collar’ to allow for taps and lines, no kegs.

Materials

Here’s what you’ll need. I will add (non-affiliate) links to what I use, but plenty of others will work. I’ll discuss what each piece needs to achieve for you.

Optional upgrades (but recommended)

The wifi version of the Inkbird is not much more expensive than the non-wifi version and comes with an app that gives you all the benefits of a separate sensor + wifi gateway system, so I recommend doing the wifi-thermostat route if you can. (On the other hand, I suppose there’s a security factor: wifi-connected devices are often not all that secure, and a wifi-enabled thermostat affects actual fridge temp, while a wifi-enabled sensor only affects monitoring.)

Even more optional upgrade

  • A big power station battery, if you live in an area with power outages
  • Solar panels for it, if you expect long outages

Notes on specs

Chest freezer: you can often find working ones for free or cheap on Craigslist or FB Marketplace. In the US, BestBuy sells small ones pretty cheap, especially around sale time. Newer may be better in terms of efficiency and how much power it uses.

Thermostat: you need a thermometer that also controls an outlet. It should allow you to set a target temperature and a “turn on” cooling differential. For example, mine is set to a target temp of 44°F, it starts cooling with a 1-degree differential (i.e. the freezer kicks on at 45°F), and it stops cooling when it returns to the target temperature. Because of how cooling works, the temperature will keep coasting downward for awhile after cooling kicks off. :Some thermostats have more features.

Remote temp sensor + wifi gateway: a continuous record of temperatures is an enormous aid for high-stakes, expensive medications like biologics. Especially if you’re not home.

Power station: a nice backup if you have power outages. Even if you keep ice packs around or it’s frozen outside, it’s hard to keep things at juuuust the right temperature using only ice. :The power station rabbit hole goes deep but they’re pretty cool devices.

:x Other likely thermostat features

Some thermostat controllers have a ‘heat’ outlet and a ‘cool’ outlet; you’ll only be using the ‘cool’ outlet, so you can ignore the settings for a heat differential and the little ‘heating’ light that might kick on sometimes.

Most thermostats will also have an alarm setting; mine is set to alarm at 46F and 41F.

How to build

  1. Plug thermostat into the wall. Plug chest freezer into the thermostat.
  2. Set up your thermostat. Here’s the page I used for my Inkbird, but you can follow the instructions on whatever thermostat you end up with. I typically do a set temperature of 40°F with a cooling differential of 1°F. That means it’ll start cooling at 41°F and stop actively cooling when it comes back down to 40°F, with a couple degrees of coasting on either side.
  3. :Situate your probe and let the cycles even out. Hard-won lesson: any time you change something, the fridge may do some major temperature swings as it readjusts. Plan to be around to babysit if you’ve changed a setting or added/removed a decent amount of thermal mass.
  4. Optional: set up additional remote :temperature monitoring, if you didnt get a wifi-enabled thermostat. Make sure you put the sensor under similar conditions to your meds and the thermostat probe.
  5. :Adjust thermal mass for stability. Put some cardboard next to the cooling coils, and put some jugs of water in there to take up extra air space. A few of the gel packs your medication probably ships in are also a good choice.
  6. Optional: plug the chest freezer into a :portable power station battery (aka “solar generator”). Get a sense of how long it would last on a full charge.
  7. Put your meds in your new meds fridge!
  8. Profit Enjoy your newfound medication temperature stability.

:x Thermal mass and stability

There’s a couple things you want to be careful about: where the cooling coils are most concentrated, and rapid cycling of the compressor that makes the freezer start cooling.

  1. Cooling coils. Areas right next to the cooling coils are going to get much colder, much faster, especially if your temperature probe is farther away from the coils. You may want to line these parts of the wall/floor with cardboard to buffer a little.
  2. Thermal mass. Air changes temperature a lot faster than water. Insulation affects things too. If your meds fridge is mostly empty, it will cool down and warm up very rapidly, meaning you’ll cycle the motor on and off quite frequently (as often as every 10 minutes). If you take up a bunch of the space in your fridge with water - like a few jugs from the grocery store - it will slow down the temperature swings, leading to smoother, more even cycles and less frequency of needing to kick on the motor.
  3. Air circulation. Like your home fridge, the medication fridge can get cold spots. Heat rises. A little battery-powered fan, or a wired PC cooling fan (the wires are thin enough to run out of the fridge lid/door and not disrupt the seal too much), can help keep the temperature more even throughout the whole fridge.

:x Probe location

Because the probe is what controls the temperature, you want the probe to be in roughly the same situation as your meds.

Bad: Probe is sitting in empty air in the fridge, or touching the wall with the cooling coils. Meds are in a box in a box in a cooler surrounded by gel packs.

Good: Probe is in an empty meds box just like the one your meds are in. Both meds boxes are in a cardboard box on some crumpled brown paper, situated toward the middle of the fridge.

Best: Every thing from Good, plus the coldest walls of the fridge are covered by a piece of cardboard, there’s a few gel packs and jugs of water distributed around the cardboard box that holds the meds boxes, and there’s a small battery fan for air circulation. (See :thermal mass and stability for more detail.)

:x Power station

Portable power stations are a whole thing; it’s basically a giant backup battery that you can carry around with a handle, and it can run most appliances, charge your phone, etc. They’re sometimes called “solar generators” because they fill a pretty similar niche to generators during camping or power outages.

A power station like this can keep your meds at an appropriate temperature for quite awhile in a power outage—keep in mind the freezer isn’t running continuously. If you can get it set up with solar panels, and have clear skies, you could go even longer.

There’s a huge range of price, power output (given in Watts, usually normal and ‘surge’), storage-of-power (in Watt-hours), solar charging capacity, etc etc. You can go down a research and optimization rabbit hole for a while on these things. The main thing you want is to make sure its peak/surge output can match the :startup draw of your freezer.

Jackery and Ecoflow are two popular brands. I really like Anker devices for charging, and I know they make power stations too. I had one of these and it worked great with my fairly old chest freezer (newer ones tend to have lower power demands so that’s even easier).

One really interesting feature to me is pass-through mode, where you plug the power station into the wall, plug your freezer into the power station, and it “passes through” the power from the grid when all is working as normal. If grid power cuts out, it automatically switches over running from the power station’s battery after a brief delay.

In other words, automatic battery backup if the power goes out. Test this before you need it, though! I set it up wrong once and was accidentally running on battery power instead of pass-through.

:x Appropriate temperature

A chest freezer that is at freezing temperatures, if undisturbed, can keep food safely cold during a power outage for a couple days. But our chest freezer isn’t at freezing temperatures, it’s at a fairly warm refrigeration temperature. So without regular, active cooling—my fridge runs for about 5 minutes every couple hours—it’ll quickly climb toward room temperature.

:x Startup draw

The main thing you need to pay attention to is the startup power demand of the freezer’s compressor, which is what runs to make the chest freezer get cold. It takes quite a bit more power to start the motor than to keep it running, so you need to make sure that your power station can handle not just the steady-state energy consumption but the startup demand.

This page has a good explanation, but essentially you need to know both the running watts and starting watts of your chest freezer. (This is also why generators and power stations typically advertise their surge power capacity in addition to the normal power output.)

You can usually find these numbers in a user manual or on a label on the back of the freezer.

If your freezer doesn’t specify starting watts, it’s typically safe to assume running watts multiplied by 4.

Your power station needs to provide at least that many watts.