Quick answer
Your furnace or heat pump only makes the heat — the ducts decide where it goes. In a typical house, ENERGY STAR estimates 20 to 30 percent of the air moving through the duct system is lost to leaks, holes, and poor connections, which shows up as rooms that never get comfortable and bills higher than the equipment's rating promised. Sealing and correcting duct design is often the highest-return fix in the whole system.
- ENERGY STAR: typical duct systems lose 20–30% of their air to leaks, holes, and bad connections.
- Uneven rooms — hot upstairs, cold back bedroom — are usually a duct problem, not an equipment problem.
- Duct sizing follows ACCA design standards (Manual D), not rules of thumb.
- Leaky ducts in crawlspaces and attics pull dirty air into the system you breathe.
- New high-efficiency equipment on bad ducts wastes the efficiency you paid for — test before you upgrade.
How much are leaky ducts really costing you?
More than almost anyone expects, because the losses are invisible. ENERGY STAR estimates that in a typical house, 20 to 30 percent of the air that moves through the duct system is lost to leaks, holes, and poorly connected joints. You pay to heat that air, and it warms a crawlspace or attic instead of your living room.
In the PNW the leak location makes it worse: most local duct runs live in vented crawlspaces and attics — spaces that are cold and damp all winter. Supply leaks dump heated air outside the envelope; return leaks suck crawlspace air (and whatever's in it) into the system that feeds your bedrooms. Comfort, efficiency, and air quality all take the hit at once.
What does good duct design actually look like?
Duct design is engineering, not guesswork. The industry standard is ACCA Manual D, which sizes each trunk and branch from the home's room-by-room loads so every register receives the airflow the design calls for. When ducts are sized by habit instead — or when decades of remodels have grafted runs onto a system never designed for them — you get the classic complaints: a bonus room that's an oven in July, a back bedroom that never warms up, a system that runs constantly but satisfies nobody.
This is a specialty of ours: Home Energy Group fabricates custom sheet metal in our own shop, so duct design and replacement isn't limited to whatever fittings are on the truck. Odd chases, tight crawlspaces, additions that need a properly engineered extension — we build the solution rather than force a stock part.
When should ducts be tested or sealed?
Three moments matter most:
- Before replacing equipment. A high-efficiency furnace or heat pump breathing through leaky, undersized ducts cannot deliver its rated performance — this is why duct evaluation is part of our installation quotes, especially for heat pump conversions where airflow matters even more.
- When comfort is uneven. If some rooms never reach the thermostat's setting, test the delivery system before blaming the equipment.
- When dust or smells appear at registers. That often means return leaks pulling from crawlspace or attic air.
Sealing itself targets accessible leaks at joints, boots, and plenum connections with mastic and proper mechanical fastening — and honest triage matters: some duct systems justify sealing, some sections justify replacement, and we'll tell you which is which.
What results should you expect?
Sealed, properly designed ducts deliver conditioned air where the design intended: rooms even out, run times drop, filters stay cleaner because the return side stops vacuuming the crawlspace, and equipment stops working overtime to overcome its own delivery losses. If a system was losing air at the scale ENERGY STAR describes, recovering it is one of the highest-return improvements available — often cheaper than the equipment upgrade a comfort complaint might otherwise have triggered. Pair it with the seasonal habits in our maintenance checklist and both comfort and costs hold steady.
About this guide
Written and reviewed by the Home Energy Group team — a family-owned HVAC company celebrating 25 years of comfort & care across SW Washington and the Portland metro, licensed in Washington (WA LIC# HOMEEGI003J7) and Oregon (OR CCB# 184255), and an authorized Daikin & York dealer. Guidance reflects the systems we install and service every week. Last reviewed July 3, 2026.
Common questions
How do I know if my ducts are leaking?
Common signs: rooms that never match the thermostat, high bills relative to your equipment's efficiency, dusty registers, and ducts routed through crawlspaces or attics with visible disconnected or crushed sections. A proper duct evaluation confirms it — we check ducts as part of system quotes.
Is duct sealing worth it, or should ducts be replaced?
It depends on the ducts. Sound, accessible ductwork with leaky joints is a sealing job. Crushed flex runs, rusted trunks, or systems fundamentally undersized for the house justify redesign and replacement — which we fabricate in our own sheet-metal shop. An honest assessment tells you which you're in.
Should I fix ducts before buying a new furnace or heat pump?
Test them before, yes. New high-efficiency equipment on a duct system losing 20–30% of its air can't deliver what you paid for — and heat pumps are especially sensitive to airflow. Sometimes duct repair alone resolves the comfort complaint that started the replacement conversation.
