This corner of Somerset County holds more listed history than most. Millstone's historic district, a National Register listing of 1976, counts 58 structures along Amwell and River Roads, including a house from about 1755 that sheltered Washington after the Battle of Princeton. Rocky Hill's district covers about 176 acres and 145 buildings in Carpenter Gothic, Queen Anne and older vernacular styles. East Millstone and Griggstown in Franklin and Neshanic Station in Branchburg have districts of their own, and Somerville's 2019 master plan notes how much of that borough's housing is historic, with buildings in every style from Georgian and Federal to Queen Anne and Neo-Classical. None of these houses was built with ductwork in mind, and owners who want central air, or who have inherited a tired mid-century retrofit, face the same question: how do ducts get in, or get fixed, without damaging what makes the house worth keeping?
Start with how the house is heated now
The answer depends on what the house already has. A house still heated by a boiler and radiators needs ducts only for cooling, which in most cases puts an air handler in the attic with short runs down to the rooms below, a job for air duct installation. A house that got a warm-air furnace in the middle of the last century already has ductwork, often squeezed under the first-floor joists with a panned joist bay for a return, and the work there is mostly air duct repair and air duct cleaning. A house heated by stoves or a heat pump with wall units may need no ductwork at all. Knowing which of the three you have tells you whether the conversation is about new runs or old ones.
Routes that leave the plaster alone
Old houses offer more hidden space than they seem to. The attic is the usual home for a cooling air handler, and from there supply runs can drop into second-floor rooms through the ceiling, where a register is easier to place than in a plaster wall. Closets stacked one above another make natural chases from floor to floor. The cellar or crawl space reaches the first floor through floor registers set between joists, which is how the first furnaces in many of these houses delivered their heat. A back stair, a knee wall or a built-in cabinet can hide a run that would otherwise mean opening a wall.
The return is the hard part. Supplies can be small and tucked away; a return has to move the same air back and needs a larger grille somewhere central, often a hall ceiling upstairs and a floor grille near the stair downstairs. Where bedroom doors close, jump ducts or transfer grilles above the doors let air find its way back without a return in every room. Planning the return first, before any supply is placed, is what keeps a retrofit from whistling and short-cycling later.
An attic air handler brings one more risk to an old house: water. Its coil makes condensate all summer, and a clogged drain line or a rusted pan above a plaster ceiling shows up as a stain in the room below, sometimes long after the leak began. A secondary pan under the unit and a float switch that shuts the system down when that pan fills are worth asking about, and the drain line deserves a check whenever the system is cleaned.
When the old ductwork is already there
Mid-century retrofits in old houses were built for speed. Trunks were hung wherever the joists allowed, returns were made by closing a joist bay with sheet metal, and branches were run to whatever room was easiest. Those systems can often be kept, cleaned and sealed. Where a panned return pulls air across old framing and basement dust, air duct repair replaces it with real duct, and where a branch was never large enough, a new run through air duct installation fixes the room it serves.
One caution belongs in any old house. The EPA lists pipe, furnace and boiler insulation among the materials in older homes that may contain asbestos. The agency's advice runs in three parts: undamaged material is best left in place, samples should be taken by a trained and accredited professional instead of the homeowner, and removal or major repair belongs to people qualified to handle asbestos. If old wrap near the furnace or on the heating pipes has to be disturbed for duct work, that testing and handling comes first, by a specialist, before any duct work begins.
Chimneys and vent hoods that show from the street
Most of the exterior changes a duct or vent project makes are small, and in a historic district small still matters. A new dryer hood on the wall, a cap on the chimney, a rebuilt crown or new flashing all show from the street. Millstone has its own Historic District Commission, and other towns may have their own review, so the practical step is to ask before the work is ordered. Dryer vent installation can usually put a hood on a side or rear wall in a finish that disappears against the siding. Chimney repair can match the profile of an old crown and choose a cap that suits the stack. A new stainless liner inside the flue barely shows.
Protecting the interior while the work happens
Inside, the concerns are plaster dust, floors and old grilles. The work area is covered from the door to the equipment, and cleaning runs with the system held under negative pressure, pulling dust toward the machine and away from the rooms. Old cast iron or stamped grilles are lifted by hand, kept in order and set back where they came from. Where a register has to be cut into a ceiling, the opening is laid out before the saw comes out, and the dust is contained at the cut.
Where to begin
Know how the house is heated today. Ask the local commission about anything that will change the outside. Walk the attic, the closets and the cellar with whoever is planning the runs, and settle the return before the supplies. If there is old ductwork, inspect it before deciding to replace it, and if there is old insulation near the furnace, have it tested before anyone touches it. In the historic houses of Millstone, Rocky Hill and Somerville, the best result is ductwork a visitor never sees: registers in ceilings and floors, a return in the hall, a hood around the side. It starts with air duct installation planned around the house rather than through it.