Thanks all. I also went ahead and looked at the pump as suggested and the
arrow does show that the circulator is on the return side of the boiler. I
just wanted to be sure.

RogerT wrote:
I own a small building with 3 apartments and 3 separate gas boiler
hot water radiator heat systems. Each boiler has a circulator pump
that is attached to a pipe that goes into the side of the boiler
about half way down. Another pipe comes out of the top of the boiler.

I am assuming that where the circulator pump is located is the return
water line that goes back into the boiler and the pipe coming out of
the top of the boiler is the supply/hot water line that sends hot
water up to the radiators to supply them with heat.

Is that correct?