I agree, JB. The days of the vent-free are limited. When old houses had high air leak rates from rattling windows and poor joining, the fresh air introduced would dilute moisture and combustion product air. Vent-free heaters provide savings in inadequately-built homes. The new houses are built so tight that fresh air exchangers are often required to give breathing comfort.
There is a limit to money-saving. There are many people in the country where saving an extra $20 may be worth risking health and safety; but for most there is no benefit. Saving money by coupon has driven many to extremes in thinking so that taking high risks for a few dollars seems worthwhile.
The National Fire Protection Association goes to precise lengths to protect the comsumer, yet people will try to skirt the safety codes to save. Think of the household appliances that do the most good for you; the stove and the heater provide the life-enhancing service. Invest in the best equipment to provide comfort with safety. The vent-free do not qualify.