"... there are often code restrictions for using hot water heaters for space heating alone"
Wow, would it matter that HTP includes instructions in the Phoneix install manual for heating application?
Not usually.
Most of those codes seem to require an ASME certification on the space heating boiler, unless you're running it as a combi space-heating/hot-water system. It may or may not be allowed in your area. (I'm heating my place with a hot water heater too, but it's also heating my domestic hot water, with the potable & heating sides isolated by a heat exchanger.) I'm not saying it makes any sense to require an ASME stamp on a hot water heater as bullet-proof as an HTP, but that seems to be what's required, even if it's all low-temp low-pressure space heating.
If you're heating hot water with an existing standalone gas-burner it probably has enough burner to serve both in a New Orleans location. You probably have enough radiation to run it all at domestic hot water temps too- have you run a room-by-room heat load calculation, and compared the room radiation to the actual room loads?
A lot of older heating systems were oversized for the loads back before the walls and attic got insulated, and the windows got swapped out for double-panes, etc. which means with a tightened up and better insulated house it doesn't need 160-180F water anymore, and may not even need 140F water. To get to the "right" solution needs a bit of analysis on just how much heat is needed, and how much radiation is available for delivering that heat. The 99% outside design temp for New Orleans is +34F. Even though it gets colder than that from time to time, a typical 2x4 R11 wall house with clear-glass double-panes and R19 in the attic would typically come in at 7-9 BTU/hr per square foot of conditioned space, unless it leaks air like wind-tunnel and is on a pier foundation with no floor insulation (both of which are worth fixing anyway.)
Take a 13 x 15' =195' corner bedroom with two 12 square foot windows, and assume it's over a conditioned crawlspace (or slab-on grade), with 9' ceilings and R19 in the attic. The U-factor of a 2x4 R11 wall is about 0.1 BTU/hr per square foot per degree, the ceiling is about U0.06, and if double paned clear-glass it's about U0.5. Assuming a 72F interior design temp, that's 38F degrees above the 34F outside design temp. The heat load is then:
Windows: 24' x U0.5 x 38F= 456 BTU/hr
Walls: You have 9' x (13' + 15') - 24' of window= 276' of wall area
276' x U0.1 x 38F= 1050 BTU/hr
Ceiling: 195' x U0.06 x 38F= 445 BTU/hr.
Add it up and you're at about 1951 BTU/hr.
One sleeping human puts out about 250BTU/hr , so make that 1700 BTU/hr
That's a load/floor ratio of 1700/195' = 8.7 BTU/hr per square foot. There are some air leakage to factor in, but there's also plug loads reducing the load- it's the right order of magnitude. With a 1700 BTU/hr load and 10' of fin-tube baseboard that's 170 BTU/hr per running foot, which can be done with 120F water. With 140F water you'd be able to deliver 2x as much BTU per foot.
It's worth taking the time to sketch out the heat loads this way, based on your actual construction, even if it isn't necessarily worth paying somebody else to do it for you. Then you'll at least have a base line from which to work that's grounded at least somewhat on the physical reality of the place, and not some rule of thumb.
