When to use a blow-off tube??

Sometimes. I made a stout using 2X Coopers kits and 1kg of dextrose to make a 23L brew in a 30L fermenter. After 2 days I had foam coming out of the airlock, and lots of it so in that case an blow off tube would have been a good idea.

I always use a blow-off tube for the first 3-4 days. Sometimes I'm glad I did, other times, it wouldn't have mattered. But cleaning out an airlock is no fun - so I recommend err on the side of expecting one.

Temp can have a large impact, as well; if you ferment hot, you'll get a more vigorous initial result. Keep your temps on the low side of the recommended range for your yeast, and you'll see fewer blow-offs.

I've used one blow off tube, I believe. I think it was about three years ago.

I use a 7.5 gallon bucket for primary (almost always), so I have plenty of headspace for a 5.25 gallon batch. I also always ferment on the cool side of the yeast strain's optimum temperature, so I rarely get super vigorous fermentations. Even using a pacman starter last week, my IIPA (OG 1.072) is fermenting at 60 degrees and there isn't any krausen peeking out.

I know that a warmer fermentation can have explosive results, as can using a big percentage of wheat. I also think that some people don't use a large enough fermenter to give them enough headspace. If any of those things are happening, then a blow-off tube is probably a good idea!

I would say always. Unless you are a real pro and can predict exactly what will happen it seems like it's always possible and cleaning up after it blows isn't fun.

Thanks guys.

Lately I've used one on every batch. Since I don't move the carboy around any its not a big deal to leave it connected until I rack to secondary. That's when I switch to an airlock. Better safe than sorry. Then again I make a 2L starter on a stirplate whenever using liquid yeast which can get out of control if the temp isn't monitored.

I have started using a blow off exclusively for the first few days for both my carboy and fermenting bucket. I've only had one "explosion", with an air lock, but that was enough for me to see the light. I always go to an airlock for secondary.

+1 on always. Yeast, being living beings, are unpredictable. I've had brews blow that I thought wouldn't, and quiet fermentations on predicably active ones. Another option is a few drops of Fermcap. I haven't tried it, but hear it works to keep the krausen under control.

I have switched to a blowoff tube as well, mostly for ease of cleanup and sanitizing. It is much easier to clean and sanitize a big honkin hose than a bunch of plastic bits and the bung from an airlock. Also fewer parts to sanitize means less chance of an infection as well

i use one when necesary... like this

Is the siphon tube pushed through the hole in the lid wide enough to handle the blow off? If not, how do you put the large tube in the primary without allowing other stuff get in?