Atmospheric venting: the chimney question
An atmospheric tank vents by natural draft: combustion air comes from the room and exhaust rises through a draft hood into a chimney or B-vent sized for the appliance. Bradford White's Defender atmospheric line takes a 3 in or 4 in connection, and its high-input models take 4 in only. The unit needs no electricity, which is why these tanks keep heating through outages, but the chimney is a condition of the installation: it must be sound, correctly sized and lined for a gas appliance, which many older masonry flues are not.
The same homes often lack combustion air after energy retrofits. Sealing the basement without providing air to the tank is how atmospheric units start backdrafting, and is the most common reason a replacement moves to one of the sealed-venting options.
Power vent: a blower instead of a chimney
A power vent tank pushes exhaust under positive pressure through PVC, ABS, CPVC or polypropylene pipe to a wall or roof termination, so no chimney is needed. Bradford White's TTW power vent family allows up to 200 equivalent feet in 4 in pipe, with a 7 ft minimum run, and its blower and electronic ignition run on a 120 VAC circuit, which is the trade: hot water stops when the power is out, and there are more parts to maintain.
Combustion air on a power vent still comes from the room. Where the room cannot supply it, the power direct vent version pulls air from outside through a second pipe, which is the version for tightly sealed mechanical rooms.
Direct vent: sealed and powerless
A direct vent tank is the no-chimney answer without the blower: a co-axial pipe, pipe inside pipe, draws combustion air down the outer passage and exhausts up the inner one by natural draft, terminating through a side wall. Bradford White's RG2DV family runs on a millivolt control, so it needs no electricity at all. The cost is flexibility: the vent is short and must use the manufacturer's kits, which fixes the tank's location near an outside wall.
Condensing tankless and condensing tanks
Condensing equipment recovers heat from the flue gas, so the exhaust is cool enough for 2 in or 3 in PVC-family pipe under positive pressure from a fan. A tankless unit such as the Rinnai RU199iN allows up to 65 equivalent feet in 2 in pipe or 150 ft in 3 in, direct-vented, room-air or common-vented per its manual. The condensation is acidic and needs a drain, and on tankless units the condensate trap must be kept filled and clean or the unit reports blockage codes.
Which one fits your house
The four systems answer four site conditions: a sound chimney favours an atmospheric tank; no chimney with air to spare favours a power vent; a sealed room favours a power direct vent; and a short, flexible plastic-vent route with a condensate drain favours a condensing tankless unit. The assessment checks the flue, the vent route, combustion air and condensate drainage before any equipment is proposed, because the venting decision usually decides the equipment list.