Framing in a home's crawlspace
Framing isn't the issue, airflow is. If you plan on enclosing both the hot water heater and furnace, use open slotted panels on a bifold door to allow air flow. If you have a fresh air return (from outside) for the furnace, this must be directed into the room of the furnace with sufficient space for air to flow into the the furnace, usually the face or a special opening on the side. For space you can use the wall cavity much like cold air returns are used in finished rooms.
There is no threat to framing being in contact with a stone/brick chimney, obviously keeping the ash shoot opening clear. With metal exhuast or AA and AAA steel chimney framing cannot come within 2&quot; of the steel. Some building codes may exceed that.