I'm making a rack of pork ribs and I want to try and cook them as low and slow as possible, with time not really being an issue. Just wondering if the warm setting is enough to do the trick. I'm cooking the ribs from frozen, with two cups of water in a covered pan and some liquid smoke.
The collagen in the ribs needs to get to a temperature of about 160 F to start breaking down into water and gelatin. If your oven can be set as low as 175 or 180 F on a normal bake or convection bake setting that is probably a better alternative unless you have access to an oven thermometer and can measure what temperature the warm setting on your oven attains.
You really need an accurate oven thermometer - the thermostats on domestic oven are notoriously bad, especially at controlling low temperatures.
There is a food safety issue involved here that you may not be aware of. The USDA recommends that food not be exposed to the "danger zone" of 40-140 F for more than four hours. It may take your ribs longer than that to get to temperature.
Traditional barbecuing mitigates this problem by exposing the meat to smoke, which has a preservative effect.
Another method I've sometimes used is to start the ribs on the grill, and finish them in the oven. This allows the meat to get closer to temperature quickly, while not having to maintain the temperature for an extended period of time.
If you're set on the approach you've described, I would recommend minimally getting that water nice and hot before going in the oven. The steam will act as conductive heat, getting your ribs to temperature much more quickly.
As long as the ribs reach a 160 temp it will work depending on whether your oven is gas propane or electric the warm settings all differ but most are slightly under the 200 mark with some being as low as 125 and others 150 or 175 or even higher... look in your ovens owners manual for more information.