HERMS coil length - 15' x 1/2" too little?

A while back I bought 50' x 1/2" copper coil that I cut into 15' and 35' segments for a pre-chiller (I live in TX) and an IC, respectively. Shortly thereafter, I discovered that it's more effective to pump ice water through the 35' coil with a cheap aquarium pump, rendering my 15' coil relatively useless.

I've recently decided to go with an eHERMS setup using my 10-gallon water cooler HLT, and I'm wondering if anyone has used as little as 15' for their HERMS coil. I'd like to keep using my 35' as an IC and put the 15' to use.

I've done some searching and see a lot of people using 25' or so, but it seems to me that 15' might be sufficient given the insulation of the water cooler and the fact that I won't be using HERMS for ramping temps (if I need to step mash, I'll do it w/ multiple infusions), so temp changes after doughing in will not be huge.

Any thoughts?

There's really no reason it wouldn't work. If you want you could try it with the 15' first, measure the temp coming out of the herms and see if it is the same temp as your hlt. If it is, then it's working fine.

Good point. I think I'll solder some straight portions of copper tube to come up through the cooler lid, so there's at least 15' of submerged tubing. The more I think about it, the more it seems that 15' should be enough - especially with the insulation of the cooler.