What those pipes are
Condensing furnaces and many tankless water heaters are direct-vent appliances. The Building America Solution Center, a Department of Energy resource, explains that the intake can be a separate pipe from the exhaust, or the outer ring of a pipe-within-a-pipe where the inner pipe carries exhaust. Because condensing exhaust runs around 100°F, these units can use PVC for exhaust venting, which is why you see white plastic pipes instead of a metal flue. If you have two pipes, both matter: a blocked intake starves the burner, and a blocked exhaust has nowhere to send combustion gases.
The same guidance says the intake and exhaust should sit above the anticipated snow line, far enough apart to prevent frost on the intake, and high enough that frozen condensation on the ground won't block the exhaust. Plenty of older installs don't meet that, especially on walls where snow drifts or where the shovel pile lands.
Shutdown is the usual result, but not a promise
In one CPSC lab test of a high-efficiency furnace, carbon monoxide in the test chamber stayed negligible with the vent up to 95 percent blocked, and the furnace shut off immediately once blockage passed 95 percent. The same furnace leaked measurable CO when its vent was disconnected inside, and it locked out when a blocked condensate drain tripped a pressure switch.
That's one furnace. CPSC's broader review of real poisoning investigations found that modern furnaces and boilers had been, and continue to be, involved in CO exposure incidents, and that the industry standards don't protect consumers under conditions that commonly occur in the field, including partially blocked and disconnected vents. So an appliance that keeps running isn't proof its venting is fine, and one that keeps shutting off is telling you something.
What to check after a snowfall
The EPA's winter guidance is simple: clear outside furnace, stove, fireplace, radon, and dryer vents of snow and ice to keep CO and other pollutants from building up indoors. For these pipes, that means:
- Find them before it snows. Note which wall they're on and mark the spot so you can find it under a drift.
- After snow or a big wind, check from solid ground. Brush loose snow away by hand or with a broom, and keep the shovel and snowblower discharge pointed elsewhere.
- Don't chip at ice on the pipe, pour hot water into it, or push anything inside. PVC cracks, and a cracked or pulled-apart pipe is a CO problem, not just a heat problem.
- Watch for ice that comes back. A lump of frost on the intake or an ice mound under the exhaust after every cold snap points to how the pipes were placed, which is something to have fixed rather than cleared all winter.
If the heat quit and the pipes were buried, clearing them may let it restart. If it shuts down again, or it cycles on and off, stop resetting it and call for service. Our guide to a furnace that keeps turning on and off covers other common causes. This is a side-wall venting issue; if you have an older appliance on a chimney instead, see how to check a flue for backdrafting.
What to ask the technician
Call an HVAC technician for a furnace, and a plumber or water-heater service tech familiar with your model for a water heater. CPSC recommends professional inspection and service of the heating system every year, including checking vents for blockages, corrosion and disconnections. Ask them:
- Are both terminations above the snow line we actually get on this wall, and far enough apart per the installation manual?
- Is the vent pipe sealed and connected all the way back to the appliance?
- Is the condensate drain clear, and where does it go?
- If ice keeps forming, can the terminations be raised or moved, and does the manufacturer approve that kit?
- What fault code did it log, and what did you fix, not just reset?
Fold this into the annual visit; our furnace tune-up guide lists what the report should cover.
If a CO alarm goes off
Don't go looking for the source or stop to clear the vent. CPSC says to move outside to fresh air, call 911, and stay out until responders say you can return. Early CO symptoms feel like the flu without a fever: headache, fatigue, shortness of breath, nausea and dizziness. Our guide on when a CO alarm means leave and call walks through it.