Optimum Length for a HERMS coil

My HERMS project is slowly coming along and I am getting to the point to where I need to build the heat exchange coil. I am going to go with 1/2 inch copper tubing (everything else is 1/2 inch) but I am not sure as to how long it needs to be.

Is there an optimum length for the HERMS coil?

25' 1/2" seems to be the most popular.

Depends on flow rate and temperature differential. I built mine with a 50 ft 1/2 inch coil, and it's MASSIVE overkill. 25' should work alright.

Mine is about 19' becaues that is all that woulf fit but my plan was to go with 25'. klyph is right that the flow rate has a lot to do with it.

I've got 1/2" and it's about 22'-23' with the fittings. I was worried about it being too short, but it works really well. I went ahead and put a a 5500W electric element in the HLT and I can step and heat water pretty fast.

I do 10 gallon batches and put my strike water in my MT and my Sparge water in my HLT and the turn on the HERMS and start recirculating. I can heat all the water that I need for the brew day up in about 35-40 mins (summer time).

I will have to be going with 20 feet of 5/8...I am already having some problems getting the coil done without any kinks...wasted about 20 feet just trying to wrap around a corny keg.

I also had to redo the compression fittings, one of which is defective (my fault for getting an open package). So far the HERMS coil has been the biggest challenge to this project.