A new duct system is designed first and built second. It is the job when a Daly City house gets central heating or a heat pump for the first time, gains an addition or an ADU, or has ducts that are past saving. The design decides most of how comfortable the house will be: a correctly sized furnace on poorly designed ducts still leaves cold rooms and noisy registers.
When new ductwork is the right call
- A new system in a house without ducts. Many mid-century homes were heated by a wall furnace, a floor furnace or radiant panels. Moving to a central furnace or a ducted heat pump means designing ducts from scratch.
- Converting from radiant heat. When an old radiant system fails and the owner chooses forced air, often to add cooling at the same time, new ducts take its place. If you would rather keep radiant, see radiant floor heating.
- Additions and ADUs. A new room, a converted garage or a backyard unit.
- Replacing deteriorated ductwork. Brittle flex, insulation falling off, rusted metal or rodent damage throughout.
- Correcting a poor design. Long runs, starved rooms and one small return can cost more to fix piece by piece than to replace. Smaller layout fixes are ductwork modification.
How a duct system is designed
- Room-by-room heat-load calculation (Manual J). Each room’s heat loss and gain, from its walls, windows, insulation and exposure. A bedroom facing the ocean wind and a living room with big sunny windows get different numbers, and that is the point.
- Duct sizing (Manual D). Each branch is sized for the airflow its room needs at the pressure the blower can deliver. Return paths get the same care as the supplies.
- Layout. Trunk and branches routed to keep runs short and straight, with a return path from every bedroom and from every level of a split level.
- Register and grille placement. Supplies placed so the air mixes in the room, usually toward exterior walls and windows, and not blowing onto a bed or a sofa.
- Materials. Sheet metal for trunks and for anything in the garage; insulated flex only for short branches in an attic or crawlspace, where it fits.
- Sealing and insulation. Every joint sealed with mastic, ducts in unheated spaces insulated, and the finished system tested for leakage.
Where the ducts go in local homes
Over-garage homes. The garage ceiling is the floor of the living space, so the trunk usually runs along the garage ceiling or between the joists, with short branches rising into the rooms above. Ducts in a garage must be sheet steel and must not open into the garage, which keeps exhaust and fumes out of the house. The route is planned around the garage door track, the water heater and your storage.
Crawlspaces. Under hillside split levels the crawlspace is usually damp. Ducts are hung clear of the soil, wrapped in insulation with an unbroken vapor jacket, and sealed so they do not draw in damp air.
Attics. Attic runs are kept short, supported every few feet and insulated. On the fog side of the hills an attic stays cool for most of the year, so insulation there is about keeping heat in.
ADUs. A garage conversion under the main house shares space with the main furnace and its ducts, so the design has to decide where that equipment goes and how the two homes are kept on separate controls. A detached backyard unit usually gets its own system. For many ADUs a ductless mini-split is simpler than ducts, and we lay out both options. The ADU heating and cooling guide goes through that choice.
How installation goes
The design and a written estimate come first, and we go through both with you. On the job, finished rooms are protected, new openings are cut for registers and returns, and old ducts that are being replaced come out. Most installations take 2 to 5 days. When new ducts go in with a new furnace or heat pump, the two are planned together, because the equipment’s blower and the ducts have to match.
At startup, airflow is measured at every register, balancing dampers are set and the leakage test is run. You get the readings, so you know what the system is delivering and not only that it turns on.
What affects the price
New ductwork typically runs $2,000 to $8,000 per system. Near the low end: a single-level house or an ADU with an open garage ceiling or a roomy crawlspace. Near the top: several levels, a finished garage ceiling, a low crawlspace, many rooms, or old ducts that have to be removed first. Ducts that serve a house on a slope often need longer routes, and that shows up in the estimate too. There are no hidden fees.
Local notes
Homes in Daly City, Broadmoor and the older parts of South San Francisco were built with heating in mind, not cooling, and their original ducts, where they have any, were sized for a furnace alone. A new system is a chance to size everything for the house as it is lived in now: a home office on the sunny side, a bedroom over the garage, or a lower-level room set into the hillside.