Stainless steel and aluminum connectors can be used if listed for the natural draft water heater venting and if permitted in a particular jurisdiction. The reason behind recommending only galvanized connector for the water heater venting purposes is that the IRC (International Residential Code) lists galvanized steel as a first approved material.
It is also the least expensive vent connector and the most popular & commonly accepted installation type. Personally, I didn’t even see one full length stainless steel water heater vent connector while performing home inspections for almost 13 years, except for some short connector pieces mixed with galvanized.
Besides, after over a decade of seeing what combination of materials people attempt to use for water heater venting I chose to make it… less confusing. At least that was the idea. But you are right, stainless steel connectors last forever and maybe I should list all possibilities .
Before attempting any installation check with your local building department if the following paragraphs from 2012 IRC (http://publicecodes.citation.com/icod/irc/2012/icod_irc_2012_24_sec001.htm) have been adopted in your area.
G24188.8.131.52 (503.10.2.1) General.
A vent connector shall be made of noncombustible corrosion-resistant material capable of withstanding the vent gas temperature produced by the appliance and of sufficient thickness to withstand physical damage.
G2427.7.1 (503.7.1) Construction.
Single-wall metal pipe shall be constructed of galvanized sheet steel not less than 0.0304 inch (0.7 mm) thick, or other approved, noncombustible, corrosion-resistant material.
G24184.108.40.206 (503.10.2.3) Residential-type appliance connectors.
Where vent connectors for residential-type appliances are not installed in attics or other unconditioned spaces, connectors for listed appliances having draft hoods, appliances having draft hoods and equipped with listed conversion burners and Category I appliances shall be one of the following:
1. Type B or L vent material;
2. Galvanized sheet steel not less than 0.018 inch (0.46 mm) thick;
3. Aluminum (1100 or 3003 alloy or equivalent) sheet not less than 0.027 inch (0.69 mm) thick;
4. Stainless steel sheet not less than 0.012 inch (0.31 mm) thick;
5. Smooth interior wall metal pipe having resistance to heat and corrosion equal to or greater than that of Item 2, 3 or 4 above; or
6. A listed vent connector.
Vent connectors shall not be covered with insulation.
Exception: Listed insulated vent connectors shall be installed in accordance with the manufacturer’s installation instructions.