My brother has a gas hot water heater which he would like to
convert over to use as part of a hot-water radiator system.
Any suggestions, caveats, ideas?

Such as: does the return water go to a holding tank and the
system pump then pumps the water out of it, through the hot
water heater, and then through the house?
Or does the pump go AFTER the water heater and then pump
to the house?
Does the make-up water go into the holding tank to maintain
level?

Rick