I have a 1/2hp for mine and you have to have water running through it consistently for a while for the built in sensor to register a temp. Only when there's a real temp being registered will it kick off to bring it into tolerance. Be careful running it without any flow. You can freeze it up. Not sure how that works, but my understanding is that there are components that can freeze up and hose the chiller. My two cents..
150 drilled. 3x250w 14k MH, 2 superactinic VHO. Reeflo Orca 250, 1/3hp drop-in, 16w UV, LR, SPS, Fish, Softies. Austin, Tx