Mean fluid intake 2.5l,mean urinary output 700ml.creatinine,bun,uroanalysis,proteins all normal.no hot days,no much sweating.what could cause low out?

Staying hydrated: You probably began your experiment dehydrated. There are of course "insensible losses" or volume losses that can't be measured-- mostly this is from the humidified breath. That can amount to 0.5 L right there. You also lose water in the stool... You could account for that. Try the experiment when your urine is clear- when you are not mildly dehydrated. I think you will get better results.