Direct-vent gas heaters have the advantage of combusting the fuel right in the room where heat is needed, instead of remotely like a central heating furnace. Some models have fans/blowers. Most models may be side wall vented and some have the option of chimney venting. Orifices can be jetted to work with propane (LP) or natural gas.
The two variables of of heat loss are a) geographic location, and b) building performance. Together they will determine the quantity of heat needed to maintain comfort.
Geographic location drives the building's “design day." Your design day is the difference in temperature between the inside and outside of your house on the theoretical coldest day of the year; so in the northeast we calculate a day that is -10ºf outside and comfortable 68ºf inside. This is referred to as the “delta-T” value. A typical delta-T for the northeast is 78º.
Building performance is the big one because there’s nothing we can do about geographic location. Today we can build a really tight thermal envelope, but most of our heating pollution comes from the millions of homes built over the last 100-150 years. We are getting better at buttoning-up those old buildings with insulation and weather sealing but weatherization is still never going to be enough to overcome the heat loss from a 78º delta-T. That means we need heating systems that can quickly put a whole lot of heat back into those old houses on the days that are really cold.