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I'm working on a community wifi project for an impoverished settlement. We have a router and a cable modem installed in a container house, sometimes without heating for days. Both are designed for indoor use. (Wifi coverage is provided by outdoor APs.) What should I do to protect the indoor equipment from frost? Outdoor temperatures sometimes reach -15 °C (5 °F), the container is made of metal. We would need the cheapest workable and safe solution. The cable modem is provided by the ISP and they offer no outdoor equipment (but we may be able to convince them to let us use our own one).

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    Aug 9, 2017 at 2:49

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Whenever I am deploying something outside in an enclosure, this is the procedure I use. First, make sure you start with the proper enclosure (for example NEMA 3 or 4). If you penetrate the enclosure, make sure the penetrations are properly sealed to maintain the rating. (I assume this would be the case as you need electricity and because you mention that the enclosure is made of metal and no one wants their antennas inside a metal box, right?)

Since your enclosure is going to be restricting air from flowing into/out of the enclosure, you now only need to contend with any moisture that is trapped within the enclosure with your devices.

Your next step is to reduce the moisture in the trapped air. Low moisture means less chance of condensation and with no condensation, you get no frost. For this, I prefer to use silica packs. You can buy these cheaply, but even better they come in the packaging with many items (shoes, clothes, electronics, and so on). Most people throw them away, but I recommend saving them as they have many uses.

Before you close you enclosure, add some silica packs. However, before you do that, make sure the packs are "fresh" by heating them. Heating them (120-150C/250-300F) will cause them to "release" the moisture they have gathered. For this, I use either an oven (longer, but you can easily set the temperature) or microwave on 1-2 minute cycles (faster, but you need to figure out a way to determine if you have heated them enough and some outside packages won't do as well).

Now, silica packs will not remove all the moisture from the air, but you don't really need to remove all the moisture. The waste heat from your electronic devices will help to prevent the moisture from condensing on them and most of the condensation (if you have any) would be on the inside of the enclosure itself (which will tend to be colder). Make sure you account for this by not setting any electronics or power cords directly on the bottom of the enclosure.

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  • Thank you! So basically you're saying I don't need to meet the operating temperature requirements (above 0C/32F)as long as I get rid of the humidity? The mentioned metallic container house is not an enclosure - it's an actual building of about 20m2 (~200 sq feet) area. (Antennas are placed outside, attached to proper outdoor APs, connected with ethernet cables.) As there is no rain inside, do you think we are OK with a simple enclosure without NEMA3?
    – Gabor Herman
    Sep 26, 2014 at 16:46
  • You need an enclosure (or something) that will prevent the free flow of air. If you have free flow of air, that air will bring more moisture into the environment and you won't be able to prevent condensation/frost from forming. As for the temperature requirements, most devices will operate outside of those ranges but it can shorten the life of the product or cause problems. The enclosure will help to contain some of the waste heat produced, maintaining a higher ambient temperature inside the enclosure than exists in the surrounding environment. Dealing with the heat is a separate issue.
    – YLearn
    Sep 26, 2014 at 18:04
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Buy an inexpensive cooler -- a big plastic or styrofoam one like you'd use for a picnic -- remove the lid, place it open side down over the equipment. As long as there's no drafts in the container/house-unit, the heat from the equipment will stay up inside the cooler. If it's really cold, move the wall-wart power supply/ies inside the cooler too.

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  • If you do this, you'll need to be careful that when the ambient temperature changes, the equipment doesn't overheat.
    – alx9r
    Sep 24, 2014 at 20:21
  • @alx9r, that is true, but it also works both ways. It can actually protect from excess heat in the enclosure, especially if in direct sunlight. The bottom of the enclosure itself may be enough surface area to radiate excess head depending on how much is produced while the cooler protects the electronics from the air being heated by the other surfaces of the enclosure.
    – YLearn
    Sep 25, 2014 at 2:11
  • @ylearn I don't understand your argument. The property relied upon to increase the temperature of the device is the insulation of the cooler. The waste heat of the router passing through the cooler creates a temperature differential relative to ambient temperature. For example, the combination of router and cooler might result in a measured temperature differential of 20C. No matter the ambient temperature, the inside of cooler will always be 20C higher. So if it's 40C ambient, it'll be 60C inside the cooler which is now too hot.
    – alx9r
    Sep 25, 2014 at 22:22
  • @alx9r, you are not accounting for two things. First, there is no "cooler" or insulation between the equipment and the bottom of the enclosure, which will make it free to radiate out waste heat. Second, I said this is especially true if in direct sunlight. The bottom (protected from direct sunlight) will be significantly cooler than the surfaces in direct sunlight. Those surfaces in direct sunlight may radiate more heat into the enclosure than the router produces, meaning that the cooler will insulate the router from the surrounding hotter air.
    – YLearn
    Sep 25, 2014 at 22:36
  • Thanks! It's fully OK for us if we have to remove the cooler at the beginning of spring and put it back again before the winter freeze kicks in.
    – Gabor Herman
    Sep 26, 2014 at 16:53

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