I know , but i have it air tight so frost cant get in. so a vent would deem the box pointless.
I lived in the tropics for the first 40 years of my life, and even I know that having an item in a sealed (airtight) box wouldn't stop it from freezing. We know this because we use airtight plastic containers (Tupperware) to put things into our deep freeze.
Condensation is caused by warm, moisture laden air coming in contact with a cold surface. One of the reasons why the attic of your home needs a constant flow of air through the eaves is to keep it dry.
So your box needs to breath. You need to provide a dry heat source, ie a tube heater for example, and a small outlet vent on the side at the top and a small vent at the bottom. This will allow air to be drawn up through the bottom, warmed and slowly escape out the top vent, taking the damp air with it. I would buy a frost thermostat and put it in a water proof box to control the heat from the tube heater.
You also need to make sure the r/o isn't leaking water, because even a couple of drips will create a damp problem. The other problem is that as the r/o is working, the water going through it will be very cold and so the surfaces of the r/o are also going to create condensation droplets.
When we first started, I bought a trailer with 3 x 125litre plastic containers on it. It had a 3 membrane r/o with it. Each membrane was an 80 GPD unit, so I just processed water directly from my r/o at the back tap (no protection) every night - in the winter it took all night to fill those containers. If it was freezing during the day I unplugged the r/o and either put it on the kitchen sink or in the bath for the day. I did the same over the weekend. I also developed hearing difficulties as the wife continually moaned about it.
A couple of years later I rigged up a 1000 litre IBC tank in the garage and fitted my r/o unit to the wall in the garage. During the freezing cold I wrapped a towel round the r/o and hung a mechanics lead light underneath it with a 100 watt bulb. That worked for 6 years even during the freeze of Dec 2 years ago. The garage was so cold that the transfer pipe from the transfer pump to the coupling at the door completely froze - thankfully I had drained the transfer pump prior to this. The only time the r/o was unplugged and brought inside is when we went away for the Christmas new year holiday week down to London.
This year it have upgraded my R/O to a 4040. I have a box on the garage wall I have made with a front panel. The box is 1200mm high, 980mm wide and 250mm deep. I have a 40 watt tube heater in the bottom and a frost thermostat at the top. The frost stat has been set to kick in around 5 degrees and will switch off at about 6 degrees. When its very cold I put the front cover on to 'seal' the box up. (It does have a couple of breather holes at the top which also have the output and waste pipes through them and the water inlet at the bottom also is through a bigger hole than the pipe actually requires.
When the weather is very cold and also at night, I put the front panel on, but during the day it comes off. I am planning on getting a big thick towel to put across the front so the whole box can breath and still be protected if the temperature should drop quickly.
