Honest answers

A blocked intake is a very common reason a 90%+ two-pipe furnace shuts itself off in snow or ice. One of the two white plastic pipes running outside is the intake — that’s how the furnace breathes. When snow, ice, or debris covers that opening, the furnace can’t get air and turns itself off. EfficiencyMax is a damper that fits in a PVC tee on the intake and opens a backup air path automatically if the intake is blocked, so the heat can stay on. It does not fix a blocked exhaust, a frozen condensate line, or a failed pressure switch.

Look at the two white plastic pipes on the outside of the house. If snow, ice, leaves, a nest, or other debris is covering one of the openings, that pipe may be blocked. On a two-pipe high-efficiency furnace or boiler, a blocked intake can shut the heat off. If the pipes are already clear and the furnace still won’t run, that is a different problem — call a technician.

The damper goes on the intake only, not the exhaust. Both pipes can be white PVC, so color alone does not tell them apart.

1. Use the manufacturer’s labels and the manual. Look for INTAKE or COMBUSTION AIR versus EXHAUST or VENT, and confirm against the pipe diagram in your exact model’s installation manual. Do this without removing panels.
2. Treat other signs as clues only. Temperature, visible vapor, or a condensate drain position may hint which pipe is which, but they are not reliable identification.
3. If you cannot confirm it, stop. Do not cut or modify either pipe until the intake is positively identified. Ask a licensed HVAC professional. The damper goes on the intake only.

If you can do so safely, check and clear the two white pipes outside. Then follow your furnace’s reset instructions (often a power cycle at the switch or breaker). A blocked intake is a common “no-heat” service call; a typical emergency furnace repair runs $300–$1,200 (Angi, 2026). EfficiencyMax is a $44.95 backup for 90%+ two-pipe gas furnaces and boilers, so a blocked intake is less likely to take the heat down in the first place. It does not replace a repairman, and it is not a carbon monoxide detector. If you’re not comfortable clearing the pipes, or they are already clear and the furnace still won’t stay on, call a professional.

Call a professional if the outdoor pipes are already clear and the furnace still won’t stay on; if you are not sure which pipe is the intake; if you smell gas; or if a carbon monoxide detector sounds. Also call if you suspect a blocked exhaust, a frozen condensate line, or a failed pressure switch — those are different problems, and this part does not fix them.

A blocked intake can cause a 90%+ two-pipe furnace to lock out, which some people see as a pressure-switch trip, because the furnace can’t get combustion air. Clearing snow, ice, or debris from the two white pipes outside is a sensible first check. A failed pressure switch is a different problem this part does not fix. After you clear the pipes, follow your furnace’s reset instructions. If the lockout continues, call a technician.

EfficiencyMax is for 90%+ two-pipe PVC gas furnaces and boilers — it fits about 90%+ of those. It is not for 80% furnaces, metal flues, electric furnaces, heat pumps, or a small closed mechanical room that needs a fresh-air makeup opening. If your furnace has two white plastic pipes running outside, you are in the typical fit group. Confirm on the fit-check list.

The damper opens automatically, so your furnace still gets air.

Yes, it adds a second air path, on purpose. Normally the flapper stays shut and the system runs sealed. It opens only when the outdoor pipe is starved, so the trade becomes some room air for a while versus no heat at all. Room air near laundry chemicals, chlorine, or solvents isn’t good for a furnace over the long run. That’s why it’s a backup and not the main supply, and why you clear the intake when you can.

We can’t promise it won’t. Furnace makers usually tie their warranty to installation per their instructions, and none of them has endorsed this part. It doesn’t touch the furnace itself; it glues onto the intake pipe. If the warranty matters to you, check your furnace manual or ask your installer before you add it.

Not currently.

It sits on the flapper so you can see the flapper’s position at a glance.

One damper fits both. Glue it to a 2" tee, or to a 3"×3"×2" reducing tee on 3" pipe.

Either. Arrows on the damper must point straight UP. A vertical tee is typical; a horizontal run is also OK. In both layouts, the damper face stays vertical, with the adjustment screw at the bottom.

Your local store sells it for less than it would cost us to ship it. Scan the QR code, or open the parts page, for the exact part.

Because blockages tend to happen at night, in a storm, or while you’re away. The damper covers you during the hours you aren’t watching.

No. The damper is not for primary combustion air. Do not install it in a confined space with less than 50 cubic feet of space for every 1,000 BTU/h of all the gas appliances in the space (ANSI Z223.1 / NFPA 54, 9.3.2.2). Your furnace should still breathe outdoor air almost all the time. See the safety limits.

U.S.: $44.95, free shipping. Canada: $44.95 USD + flat $20 shipping; Canadian taxes and import fees may be due on delivery.

The original name, used for the same patented part.