I need to purchase sheet metal screws for two residential HVAC DIY projects. I'm assembling basic rigid round peices, reducers, elbows, register boxes etc.
I'm wondering what the HVAC guys reach for when buying a box/bucket of screws.
Do you go TEK or ZIP, #8 or #10, and what length is a standard "go to" fastener?
I understand the TEK is self-tapping where as the ZIP is a piercing screw. Are they personal preference or is there one preferred in the HVAC trade over the other?
In my region the self-piercing (sharp tip) screw in #7x1/2 size is the standard. They pop through 28 or 30 gauge sheet almost instantly and they work decently well in heavier 24 or 26 gauge sheet too. They'll easily pierce through the first ply of metal and continue on into another ply (or several - I sometimes put them through 5 plies when securing an S cleat!). Sometimes the inner ply of sheet metal is a little too far away or deflects a little too easily. We keep #7x1 size on hand for those times because they'll grab the inner sheet from a greater distance.
The self-drilling type don't grab multiple plies of metal as well so they're less-commonly used in ducting. But equipment cabinets (furnace, AC coil case) are made of heavier metal than the ducts are; I use the self-drilling screw when attaching to heavier gauge metal where a self-piercing screw won't pierce. Again, usually 1/2 length is plenty. I'm not actually sure whether we typically get these in #7 or #8, but definitely not #10.
This is difficult to answer without just throwing an opinion out, but I go with the piercing screws for one reason and I think it's objective enough to count as a non-opinion. Self-drilling screws have the potential to throw shavings which is undesirable when working in an assembled duct.
Neither thread type is good at pulling faces together, unless your first face has a hole larger than the thread, so if neither face is pre-drilled/punched, then you [may see note below] have to DIY drill at least one of them..
I don't know your terminology [TEK vs ZIP]. To me they're self-drillers or self-tappers. Self-drillers need no pre-drilled hole, of course, they make their own. Self-tappers must always be pre-drilled.
If the elements you are fixing together have one hole already, then self-drillers every time… on a drill-driver. You cannot do them by hand.
If you have no drill-driver, your choice is down to self-tappers only.
For self-tappers, if neither face is pre-drilled, that means you also need two drill-bits to make two hole sizes - one that doesn't tap followed by one that does. This is just a time-waster.
This puts it firmly back in self-driller territory, where you only have to drill the first face.
The swarf argument is not in the game. Both methods leave swarf.
Note: There is a trick with self-drillers [& I suppose self-tappers, though I've never tried it with them], that if you drill through the first face without hitting the second, then let the bolt spin a few seconds after it's hit the end of the thread, that will strip the thread in the first face & allow you to pull the second face in tight, so long as you don't over-tension as the second face pulls in. This takes a bit of practise to get the drill tension just right, but is easy enough once you have a rhythm.
If you cannot leave sufficient gap between faces once assembled, you can do this pre-fit then back them out again. You've damaged the first face sufficiently that it will not impede the tightening action as you pull up the second face.
I used to get through self-drillers by the kilo. I love them ;)
