A heat pump quote and a duct quote are two different conversations, and plenty of homeowners find that out after the equipment is already on the pad. So let's settle it up front: most Puget Sound homes keep their existing ductwork when they install a heat pump. New ducts are the exception, not the rule.
But the exception is real, and it costs real money. Here's how we decide — and how you can get a rough read on your own house before anyone shows up.
Why the question exists at all
A gas furnace pushes air out of the supply registers at roughly 120–140°F. That's hot enough that a modest volume of air carries plenty of heat into a room. A heat pump delivers air around 90–100°F — warm, comfortable, but nowhere near as hot. To move the same amount of heat with cooler air, the system has to move more of it.
The rule of thumb installers use is about 400 CFM of airflow per ton of capacity. A 3-ton heat pump wants roughly 1,200 CFM moving through the house. A furnace-era duct system that was quietly getting by at 900 CFM will now be the bottleneck.
That's the whole issue in one sentence: heat pumps don't break ductwork, they expose it.
What a duct bottleneck actually feels like
- Registers farthest from the air handler barely blow
- A noticeable whoosh or whistle at supply vents and return grilles
- The blower runs loud even on low speed
- Upstairs stays warm in summer, cool in winter, no matter the thermostat setting
- The system short-cycles or the outdoor unit ices more than it should
None of those symptoms prove the ducts are the problem on their own. They just put ductwork on the suspect list.
The five things we check during the estimate
1. Static pressure
This is the number that matters most and the one skipped most often. We take a pressure reading across the air handler with the blower running. Most residential equipment is designed for about 0.5 inches of water column of total external static pressure. Read 0.9 or 1.1 and the duct system is choking the equipment — the blower is working against restriction, which costs efficiency, capacity, and blower life.
High static almost always traces back to returns, not supplies. Older Washington homes frequently have one undersized central return and nothing else. Adding a second return or upsizing the existing one is often a modest modification, not a full replacement.
2. Physical condition in the crawl space and attic
Western Washington is hard on ducts. Damp crawl spaces corrode sheet metal, saturate fiberglass insulation, and turn old flex duct brittle. When we crawl under a house we're looking for:
- Flex duct sagging between joists, kinked at bends, or crushed where someone stored something on it
- Disconnected branch runs blowing conditioned air into the crawl space (more common than you'd think)
- Torn outer jackets and compressed insulation
- Rusted-through trunk sections and failed mastic or tape at every joint
Ducts over 20–25 years old that show several of these at once are usually replacement candidates. One sagging run is a repair.
3. Leakage
Leaky ducts don't just waste energy — in a crawl space they pull damp, musty air into the house and push conditioned air out of it. Sealing joints with mastic, replacing crushed sections, and re-supporting sagging flex often recovers more comfort than any equipment upgrade.
4. Sizing versus the load calculation
Every honest heat pump proposal starts with a Manual J load calculation, not square footage. Once we know the required tonnage, we can compare the existing trunk and branch sizes to what that airflow needs. Sometimes the fix is surgical: a larger trunk section, two new branch runs to starved rooms, a bigger return drop. Sometimes the layout was wrong from day one and patching it is throwing money at a bad design.
5. What's wrapped around them
In pre-1980 homes, old duct insulation and joint wrap can contain asbestos. If it looks suspect, it gets tested and abated by the right licensed trade — not torn out by an HVAC crew. It's an honest cost we flag early rather than discover mid-install.
Repair, modify, or replace
| Situation | Typical call |
|---|---|
| SituationDucts intact, static pressure near target | Typical callReuse as-is, connect new plenums |
| SituationGood ducts, one undersized return, high static | Typical callModify — add or upsize a return |
| SituationSealed metal trunk, tired flex branches | Typical callReplace the flex, keep the trunk |
| SituationSagging, disconnected, rusted, 25+ years old | Typical callFull replacement |
| SituationNo ductwork at all | Typical callPrice ductless instead |
Ductwork replacement runs $3,000 to $8,000 in standard Washington markets and $4,000 to $10,000 for larger or more complex layouts. Targeted repairs, sealing, and return modifications land well below that. A full replacement typically takes 2–4 days; simple modifications 1–2 days, and both are best scheduled alongside the equipment install so the crew only opens up your crawl space once.
When ducts aren't worth saving at all
Some homes shouldn't get ducts. If your house has no duct system — hydronic baseboard, wall heaters, an old oil furnace pulled out decades ago — retrofitting ducts into finished walls, soffits, and ceilings is invasive, expensive, and often architecturally ugly. That's the classic case for ductless mini-splits at $8,000 to $14,000 installed in standard markets, sized zone by zone. We break the trade-offs down in mini-split vs. central heat pump.
Manufactured homes are another category. Under-floor duct systems in older manufactured housing are frequently beyond economical repair, and a mini-split or a hybrid approach usually beats rebuilding them.
What good duct work looks like when it's done right
Custom-fabricated commercial-grade sheet metal, properly sized transitions, mastic-sealed joints, supported flex with no kinks or long sags, insulation rated for the space it lives in, and balanced airflow verified at the registers before the crew leaves. Then a permit and inspection — mechanical permits are required across the cities we serve, and we pull them on every job.
Every Varsity installation carries a 10-year parts and labor warranty, and most equipment changeouts finish in one day when the ducts don't need major surgery. When they do, we tell you before you sign, not after.
Get a real answer for your house
Duct decisions can't be made from a phone call or a photo. They come from a load calculation, a pressure reading, and someone with a work light under your floor. That's part of every estimate we do — request one here or call (253) 785-4622, Monday through Friday, 8 to 6.
If your ducts are fine, we'll say so and reuse them. That answer is free.