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Modern gas boilers here (furnaces aren't very popular, I assume from a pollution point there's no difference) have external intake and exhaust. Would those stil

• lazide 4y ago
  Any furnace, boiler, etc. which allows any exhaust gas to mix with indoor air is a major risk of Carbon Monoxide poisoning. Which does sometimes happen. If it's functioning correctly, exhaust gas and indoor air do not directly contact each other at all, they both go on opposite sides of a heat exchanger.
• nrclark 4y ago
  In the US, furnaces are required to be in a closet and have a carbon monoxide shutoff. A furnace will disable itself if local CO levels get above some threshold. So if your furnace's exhaust is working correctly, it shouldn't have much effect on airborne pollution in your house. And if your exhaust -isn't- working, then your furnace will shut itself off. I don't know the rules around gas boilers, but I'd expect that they're probably the same. Gas stoves are very different from an air pollution perspective. Kitchens are high-traffic areas in a lot of houses, and stoves are usually out in the open from an airflow perspective. Lots of houses don't have an exhaust hood that vents to the outside. And lots of cooks aren't in the habit of turning on their exhaust fans unless something has burned.