Drains smell worse in winter mostly because the house is closed up, so sewer gas that would drift out an open window stays inside. Heated, drier air and unused guest baths let trap water evaporate, and fans, dryers and fireplaces in a sealed house can pull gas through a weak trap seal.
Each drain’s trap holds water as a seal against the sewer, and a trap nobody uses slowly loses it. Winter is when fixtures sit idle: the guest bath before the holidays, a spare tub, a laundry standpipe, a garage floor drain. The guide to sewer smell in the house sorts these causes by room.
Air pressure is the less obvious cause. Bath and kitchen fans and clothes dryers pull air out of a home, and the U.S. Department of Energy notes they can lower the air pressure indoors. A fireplace or a furnace that burns indoor air adds to the draw. With the windows shut, a thin trap seal or an air admittance valve stuck open can let that air in from the drains. A partly blocked roof vent, in DFW more often leaves or a nest than ice, makes it worse.
The fix is routine. Run water in every unused sink, tub, shower and floor drain every week or two, including the laundry standpipe. Sniff any air admittance valve under a sink while a fan runs. When the dryer or fireplace is running, crack a window so the house has another source of air.
One honest limit: refilling traps only fixes a dry trap. If the smell returns within a day of running water, the gas is likely coming through a cracked drain line, a failed toilet wax ring or a vent problem, and refilling will not stop it.