When purchasing a gas or wood furnace, boiler, water heater or fireplace, the main factors to consider are safety, purchase cost, operating cost, maintenance requirements, comfort level, dealer support, and indoor air quality.
Houses are now much tighter.
Heating equipment designed to operate in loose houses may not operate properly in tight houses. A decade ago most appliances obtained air for combustion and venting from the interior of the house. Leaks in the house replenished air lost up the chimney. Not only do tight houses not have enough air leaks, there are often more exhaust fans competing for air. The house becomes depressurized.