Hi, i would place it on the outlet off your second cylinder, as your first cylinder is basically acting as a storage tank only this is preheated first before it reaches the second cylinder. Its a strange set up and why the didnt used unvented pressurised cylinders in the first place. It depends what set up you have to control the water temperture but as above it shouldnt be above 65, i usaully used a blending valve set at 55c as you shouldnt need the water any hotter and will protect the pumps. I also believed the manufacturer request you to fit one tho i might be wrong