Q:- My husband is considering installing a vent-free gas fireplace in our living room. As a retired school teacher, I'm familiar with the effects of the CO2 gases exhaled by 40 students in a 1200 sq.ft. classroom. If we didn't open several windows top and bottom to provide fresh air inflow, symptoms of the beginning stages of CO2 poisoning (drowsiness, lethargy, etc.) would overcome my students and myself quite rapidly.

Our living room is only 400 square feet, so I'm concerned about the CO2 emissions from the unvented fireplace. I asked a local fireplace salesman to compare the CO2 output of a 30,000 btu vent- free fireplace to the amount of CO2 exhaled by an average person, and he said he had no idea. Do you?

A:- An average person at rest exhales around 5 liters of air each minute, and the exhaled gases contain CO2 at a partial pressure of approximately 40 mm Hg.. This calculates to slightly over half a cubic foot of CO2 being released every hour by the average human at rest.

Using this average, your 40 students were exhaling approximately 20 cubic feet of CO2 into your 1200 sq.ft. classroom every hour, or about 1/500th of the total breathing space volume.

A 30,000 btu/hr vent-free fireplace releases nearly 27 cubic feet of CO2 into the breathing space every hour, the equivalent of the amount of CO2 produced by 54 people at rest.

Since your living room is only 1/3 the size of your classroom, the percentage of CO2 saturation would reach about 1/120th of the total volume of the breathing space during the first hour of operation, or over four times the saturation you experienced in your stuffy classroom ( imagine cramming 54 students into your living room! ).

If you do decide to buy an unvented gas fireplace, here's some guidelines from a recent Consumer Reports Magazine article about how to operate it as safely as possible:

Observe GRI guidelines. Insist that the contractor not exceed them when sizing your fireplace.

Limit its use. Though occasional extended use of an unvented fireplace should pose little long-term health risk, we suggest limiting operation to no more than two hours at a stretch, as a rule.

Provide extra ventilation. Leave at least one window open in the space where the fireplace operates.

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