If the volume of air getting to them is insufficient, intake air for combustion gets drawn in through available openings, and the chimney then has back-drafting being open to the outside.Test it with some tissue or TP (OK TP is quite scarce and valuable during COVID-19), a single piece will do.

Please note that specific information such as the property address and names of those directly or indirectly involved in this particular real estate transaction were intentionally omitted to maintain confidentiality given delicate subject matter of/affecting health, life and safety.IntroductionHaving provided carbon monoxide (CO) testing of gas utilization equipment for well over ten years during the 1980's and 1990's in addition to having learned about the production, mitigation, prevention, testing, and harmful effects of elevated levels of carbon monoxide due to incomplete combustion of natural gas utilization equipment from a recognized leader/foremost authority on carbon monoxide testing in the industry by the name of Jim Davis—senior carbon monoxide/combustion trainer and consultant for the National Comfort Institute (NCI), who began traveling the country during the 80's and 90's conducting carbon monoxide training seminars for building and HVAC contractors, building inspectors, utility companies and virtually anyone interested in learning about carbon monoxide testing and proven methods to mitigate and prevent gas utilization equipment from producing elevated levels of carbon monoxide in flue gas known to cause CO poisoning—has without a doubt proven invaluable in heightening my awareness of conditions conducive to/indicative of incomplete combustion known to result in elevated/unsafe levels of CO in flue gas.

Larger multifamily projects are required to be built with non-combustible construction.  

I'm a combustion engineer these days, but I specialized in modern dynamics and controls. 

Things like exhaust connection, the air for combustion intake, chimney liners - those all play a role in making sure that the heating system does not fill the house with CO.

It's secure, lets light in, and would be a lot safer surrounding a dryer vent than wood (in case you had heat build up, or combustion, which can happen if the lint builds up too much in there).

One gentleman I know does it indoors daily, and I only know because I observe it, I can't smell a thing, perhaps because its steam, rather than combustion, based.

A normal byproduct of combustion is Carbon Dioxide.  

The only codes I am aware of that may be related to your issue are those that pertain to clearance to combustibles of flue piping.

In my jurisdiction a CO monitor is mandated if you have a combustion source at your property, as is at least one smoke alarm per floor irregardless of a combustion source.