I think there is a lot of variables and don't think your flat wrong. Cl loss should be minimal at night b/c no sunlight. I run my filter during the day b/c I find it heats my pool more effectively by circulating the water while the sun is out. My cl was 7ppm on Sunday and dropped to 2 ppm today. But thats just my pool. Since the cl level didn't drop at night I would think that there is minimal to no cc or it would drop at night as well trying to fight whatever was in the pool. Have them check the cya level again. Does he have Ben's kit or get the results from a pool store? A lot of times the pool store can be inaccurate. As already mentioned, a cya level of 40 should keep the cl loss to around 1-2ppm per day.

I agree 15 hrs seems a bit much. Again depends on the pool though. My pump runs from 9:30am-7pm.