My house currently is heated by a hot-water boiler, with radiators in the main rooms. The bedrooms have no radiators (it's an old house and I guess when it was built they figured the central radiators were enough, which they are if you leave the bedroom doors open a bit).
The boiler is nearing the end of its life and I'm wondering if there is any reason why I couldn't replace it with wall-mounted ventless gas heaters (like this, as an example only: https://www.homedepot.com/p/Dyna-Glo-30-000-BTU-Natural-Gas-Infrared-Natural-Gas-Wall-Heater-IR30NMDG-1/206556628). I'd make sure the total BTUs met the requirements of the space. (I'd also make sure my carbon monoxide detectors work, just in case, given that these are ventless.)
Would this create any code violations or other problems? It's common in my area for folks to install one or two of these in their houses as auxiliary heat, but I've never heard of anyone using them as their only heat source, so I'm curious.