It looks like ventless radiant nat gas heaters are approved for many situations and achieve close to 100% efficiency. They have a safety device to prevent carbon monoxide production. If the humidity produced isn't a problem, are they still a bad idea?

other than a half hour or an hour of cooking level usage, you couldn't pay me to do open combustion in my home for extended usage. CO or not, combustion byproducts aren't that great for breathing.

especially not for something as continuous duty as primary heat.

that's not a scientific evaluation, of course. but I think we stopped using gaslights for pretty good reasons....

I tend to agree, but find it interesting that the regulations think it is fine.

I find it appalling that regulations allow it. In a reasonably tight home the moisture content of the NG combustion alone is enough to warrant venting, not to mention the particulate output that can be problematic for athsmatics,potential for oxygen depletion, etc. I suppose if you lived in a leaky breezy wreck of a house huddling next to/under an unvented radiant heater beats freezing to death. IIRC there are several US states that disallow unvented combustion heating appliances in residential apps.

In MA they are allowed provided the appliance is rated for 40KBTU/hr (input) or less, is sized appropriately for the heat load, the fuel lines are hard-plumbed, the appliance has an automatic oxygen-depletion cutoff, and the space has at least one hard-wired carbon monoxide detector. But that's not to say they're a good idea.

Do not and will not install for reasons stated above....Eric