Weak Water Heater Draft? Here’s How to Fix It
(plumbing101.i234.me) — If your propane water heater “puffs” occasionally and shows weak draft pull even after a flue extension, you’re not alone. Many homeowners run into similar venting issues, especially in attached garages with poor airflow.
Question:
“My propane water heater was replaced 4 years ago and passed inspection. It sometimes puffs on the draft, but CO detectors never go off. I extended the flue pipe for a better vertical rise, but the draft still feels weak. Is this normal?”
Answer:
Yes, mild “puffing” can be normal — but a weak draft means air isn’t flowing properly through the vent.
Here’s what could be happening and how to improve it:
- Garage airflow: An unheated, sealed garage often creates negative pressure. Add small vents or slightly open a door during heater operation.
- Cold flue effect: In cold weather, metal vent pipes stay cool, delaying draft. Wrapping the first few feet with pipe insulation can help warm it faster.
- Flue design: A flue should rise at least 1/4 inch per foot with minimal elbows. Too many bends reduce pull. See Home Depot’s vent installation guide for details.
- Roof height and wind: The flue cap should be 2 feet higher than any roof surface within 10 feet to prevent downdraft.
- Combustion air: Propane needs fresh air. If your laundry area is sealed off, install a louvered vent or small intake grill.
Tip: When performing a smoke test, you should see steady upward movement near the draft hood after the burner runs for 30–60 seconds. Weak or swirling smoke indicates backdraft risk.
If the draft doesn’t improve after airflow fixes, consider a power-vent water heater. These use a small fan to ensure proper exhaust even in low-draft situations.
Weak draft is common in garages with cold air and short flue runs, but with a few adjustments, you can improve safety and efficiency. Always verify your CO detectors are working properly and positioned near living areas.
I’m Chris Mayer, writing for Plumbing 101.