kelvin_cool_ohm wrote:
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

Yes, it's fairly common to use a single heater for domestic
hot water and a Hydronic heating loop.

No holding tank is required. City water pressure (thru a
pressure reducing valve) can maintain system pressure at
all times. The heater should have a small expansion tank
on it.

A circ pump is used on the cold *return* side to the bottom
of the heater. The pump can be controlled by a room
thermostat.

GOOGLE around for combination water heater / space heating
or similar.

If Codes are an issue where you live, get input from the
inspector first.

Jim