# Heat Pump Installation for Old Houses in Seattle

> Heat pumps for older Seattle houses: gravity furnace ducts, asbestos surveys, plaster, air sealing and panel capacity, planned before work starts. Get a plan.

URL: https://seattleheatpumpco.com/heat-pump-older-homes/

Updated: 2026-10-21

An old house can run very well on a ducted heat pump, but the project has more moving parts than the same job in a 1990s house. [The Seattle Heat Pump Company](https://seattleheatpumpco.com/) plans heat pump installations for older houses in Seattle and King County, from craftsman bungalows to postwar ramblers, and the work is carried out by licensed and insured contractors.

What makes these projects different is rarely the heat pump itself. It is what the heat pump has to connect to: ducts built for a different kind of furnace, wiring and panels from another era, finishes that are expensive to open, and materials that need testing before anyone disturbs them. Planning for those at the start is what keeps a heat pump for an old house on budget.

## How much of Seattle is old housing

A lot. Census data (American Community Survey, 2020–2024) shows 22.8% of Seattle's housing units were built in 1939 or earlier, against 10.9% for King County as a whole. In some districts the share is higher still: about 36% in both Capitol Hill and the North Central area around Wallingford, Fremont and Green Lake, and about 28% in the Central Area. See the [Capitol Hill and Central District](https://seattleheatpumpco.com/service-areas/capitol-hill-central-district/) and [Wallingford, Fremont and Green Lake](https://seattleheatpumpco.com/service-areas/wallingford-green-lake/) pages for what that means locally.

Age alone does not decide the project. A 1925 house may have had its heating system replaced three times, with ducts that are now decades newer than the walls. Another may still have its original gravity furnace runs. The assessment starts by finding out which house you actually have.

## Gravity furnace ducts and what can be reused

Many early 20th-century houses were heated by gravity furnaces, sometimes called octopus furnaces for the large round ducts that spread out from the top of the cabinet. They had no blower. Hot air rose through oversized ducts to registers on the floors above, and cooler air sank back through large floor returns to the furnace.

Those ducts were designed for slow, buoyant air, not a fan pushing a heat pump's airflow. Typical issues are runs with no practical blower connection, returns that rely on open floor grilles, joints that were never sealed, and insulation wrap on the ducts or furnace that may contain asbestos. Some trunk sections may be large enough to reuse once tested and sealed; branches and returns usually need rework. There is no reliable public data on how many Seattle houses still have gravity-era ducts, so the only way to know is to look.

Even later forced-air systems in older houses tend to share one weakness: a small central return. The [existing ductwork page](https://seattleheatpumpco.com/heat-pump-existing-ductwork/) explains how static pressure and return capacity are measured, and why that measurement comes before equipment selection.

## The envelope changes the size of the system

Older houses are often assumed to need large heating systems. Sometimes they do. Often they need less than the old furnace suggests, because the furnace was sized decades ago for a house that has since gained attic insulation, storm or replacement windows, and weatherstripping.

The load calculation uses the house as it is now. Seattle's energy code requires heating and cooling loads calculated with ACCA Manual J and equipment selected no larger than the load requires, at a 24°F heating design temperature. Air leakage is one of the biggest inputs in an older house, which is why air sealing ahead of or alongside the heat pump can change the size of the system you buy. Sealing attic bypasses, the rim joist and plumbing penetrations reduces drafts, and a smaller load means a smaller heat pump, smaller duct demands and less reliance on backup heat. The [Manual J sizing guide](https://seattleheatpumpco.com/guide/heat-pump-sizing-manual-j/) explains the inputs.

The worry that a heat pump will double the bills in a leaky house mixes two things. The bill follows the heat loss and the electricity rate. On City Light's current residential rate of 13.38 cents per kWh, a heat pump averaging a COP of about 2.2 matches a 95% efficient gas furnace on energy cost, and modern units run well above that through most of a Seattle winter. A leaky house costs more to heat with any fuel; the [operating cost guide](https://seattleheatpumpco.com/guide/heat-pump-operating-cost-seattle/) walks through the numbers.

## Asbestos: survey before the old system comes out

Old furnaces, the tape and paper wrap on ducts, boiler and pipe insulation, and some older flooring and ceiling materials can contain asbestos. Puget Sound Clean Air Agency requires an asbestos survey by an accredited inspector before renovation or demolition work, and that includes houses. Owner-occupants of a single-family home may survey their own house, though a professional survey is often the practical choice when the results need to be documented for contractors.

If the survey finds asbestos in material that will be disturbed, removal is a separate scope with its own rules and, above certain quantities of friable material, advance notification to the agency. Planning the survey early keeps it from stopping work halfway through a furnace removal.

## Plaster, finishes and getting ducts where they need to go

Running new ducts through an old house is a route-finding exercise. Lath and plaster is harder to open and patch cleanly than drywall, and original millwork and floors are worth protecting. The usual routes are:

- the unfinished basement or crawlspace for trunks and first-floor branches
- the attic for second-floor or top-floor rooms, with ducts sealed and insulated
- stacked closets or chases built into a corner to reach upper floors
- existing chases from an abandoned chimney flue or old ductwork, where they are sound

Each opening has a finish cost on the other side of it. A good plan accepts a slightly longer duct run to stay in the basement, rather than cutting through a plaster ceiling, wherever the airflow allows.

## Electrical capacity in an older house

Older houses were wired for far fewer electrical loads than they carry today, and many still have smaller electrical services than modern houses. There is no public data on how many Seattle houses still have 60 or 100 amp service, so the panel gets checked on every project.

The heat pump itself usually needs one dedicated circuit and an outdoor disconnect. The bigger question is backup heat: electric resistance strips in an air handler can draw much more current than the compressor. Where the service is tight, the options include a smaller strip heater with careful controls, keeping a gas furnace as backup in a [dual fuel design](https://seattleheatpumpco.com/dual-fuel-heat-pump/), or upgrading the service. A 100 to 200 amp upgrade typically runs about $1,333 to $4,500 in the Seattle area. The [panel requirements guide](https://seattleheatpumpco.com/guide/heat-pump-electrical-panel-requirements/) covers the decision.

## Oil furnaces and old tanks

If an older house still burns oil, the project grows. Census estimates put fuel oil at 2.9% of Seattle households, about 10,600 homes, and the city's own estimate is roughly 7,000 to 10,000 still burning oil. When an oil furnace comes out, the tank has to be dealt with too: Seattle Fire Code requires a tank out of service for a year or more to be decommissioned by a certified decommissioner under a fire department permit.

That is usually one coordinated project. See [oil to heat pump conversion](https://seattleheatpumpco.com/oil-to-heat-pump-conversion/) for the heating side and [oil tank decommissioning](https://seattleheatpumpco.com/oil-tank-decommissioning/) for the tank. Seattle Clean Heat offers a $2,000 rebate for oil-heated Seattle homes converting to a qualifying heat pump through a participating contractor.

## When ductless is the better fit

Some older houses are poor candidates for ducted heat. A house heated by radiators or baseboard, with no basement, a shallow attic and finished plaster throughout, may have no reasonable route for supply and return ducts. Forcing a ducted system into that house can cost more in finish work than the equipment itself.

In those houses, ductless mini-split systems are often the more practical answer. The Seattle Heat Pump Company plans ducted systems and does not install ductless equipment, so the assessment will say so plainly when ducts are the wrong path. The [ducted versus ductless guide](https://seattleheatpumpco.com/guide/ducted-vs-ductless-heat-pump/) explains how to weigh the two.

## Budgeting for an older-house project

For a ducted heat pump on ducts that test as usable, Seattle-area projects typically fall between about $6,700 and $20,000; these are budgeting ranges, not quotes. Older houses tend to land in the upper part of that range, or above it once add-ons are counted, because they are more likely to need duct work, electrical upgrades, asbestos handling or tank decommissioning. National guides put whole-house duct replacement at about $1,400 to $5,600 (HomeGuide), with complex jobs running far higher. The [cost estimator](https://seattleheatpumpco.com/tools/heat-pump-cost-estimator/) gives a starting range, and the [oil conversion cost guide](https://seattleheatpumpco.com/guide/oil-to-heat-pump-conversion-cost/) covers tank costs for oil-heated houses.

## What's included

- Load calculation that credits insulation, windows and air sealing already done
- Assessment of existing ducts, including old gravity furnace runs
- Asbestos survey coordination before the old furnace or ducts are disturbed
- Duct route planning through basements, attics, closets and chases
- Electrical capacity review for the heat pump and any backup heat
- Oil tank decommissioning coordinated when an oil furnace comes out
- Air handler or furnace coil matched to the outdoor unit
- Outdoor unit sited for Seattle's noise limits on narrow lots
- Permits, inspections and startup commissioning

## Three starting points in an older Seattle house

|  | Forced-air ducts from a later furnace | Original gravity furnace ducts | No ducts (radiators, baseboard or stoves) |
| --- | --- | --- | --- |
| Usual path | Static test, targeted fixes, heat pump on the existing system | Duct assessment; partial reuse is possible but a redesign is common | New ducted system only where routes exist; otherwise ductless |
| What the assessment checks | Return size, leakage, static pressure | Duct sizes, condition, asbestos wrap, blower path | Basement, attic and closet routes; finish impact |
| Scope risk | Low to moderate | Moderate to high | High for ducted work |

## Questions

### Can a heat pump heat a 100-year-old house?

Yes, if the system is sized to how the house loses heat today. An old house that has had attic insulation, storm windows or air sealing often needs less capacity than its age suggests. The load calculation answers that, and the duct and electrical assessment shows what has to change for the heat pump to deliver it.

### Can I reuse the ducts from my old octopus furnace?

Sometimes, in part. Gravity furnace ducts are large and were built for warm air rising on its own, with no blower. A heat pump needs ducts sized for a fan, and a real return path. Some trunk runs may be reusable after testing; many systems need new returns and resized branches, and some need full replacement.

### Will a heat pump double my bills in a drafty old house?

Not because the house is old. The heat pump's bill depends on how much heat the house loses and on electricity rates. At Seattle City Light's current rates, a heat pump beats a 95% gas furnace above an average COP of about 2.2. Air sealing and insulation shrink the load, so they lower the bill with any system.

### Do I need an asbestos survey before the old furnace comes out?

Generally, yes. Puget Sound Clean Air Agency requires an asbestos survey before renovation work, including in houses. Owner-occupants of a single-family home may survey their own house. Old furnaces, duct wrap and boiler insulation are common places to find asbestos, so the survey happens before anyone cuts or removes them.

### How long does a heat pump project in an older house take?

The equipment installation itself usually runs a few days. Older houses add steps before it: an asbestos survey, any abatement, duct work through finished spaces, a possible service upgrade, and an oil tank decommissioning if the house burned oil. The schedule is set once those pieces are known, so the timeline is confirmed with the scope.