i recently add an external thermostat to my resun chiller, kick in for an hr to lower 1.5 degree off for over 2 hrs with 2 no of 150 w mh, 1262, 1060, seio and rio pump on. and its not affected by chiller pump flowrate. my tank is 4 x 1.5 x 2. with lights off can tahan over 3 and the half hour.
my friend with the same setup but bigger tank at 4 x 2 x2 claim his off for over 5 hours . before that is on 5 and off for 15 min big headache.