I recently got my pool and spa remodeled and added a Pentair gas heater. I also have a roof-top solar heater. Presently the piping is setup to always go through the gas heater, though I can isolate the solar heater. My thought was to set it up so that I can bypass the heating system altogether for times when I am not going to be heating so as to minimize pumping losses. However, when I asked my pool guy about this, he said that (1) heaters typically aren't bypassed and (2) it could be problematic to leave water in the heater core.
So, my question is: is there any good reason to not bypass the heater completely if I am not concerned about the cost to do so? As a corollary, does anyone know how significant pumping losses are through the gas heater? If they are extremely small, then it's probably not worth it.