Small-Duct High-Velocity HVAC, Explained

If you've been told your house "can't take ductwork," small-duct high-velocity is the system that was invented to prove that wrong. SDHV dates to the 1970s, built specifically for finished and older homes where conventional duct would mean tearing the place apart. Fifty years on, it's the establishment answer for heating and cooling heritage houses — and with modern cold-climate heat pumps, it's a full year-round replacement for oil, not just air conditioning.

How it works

A conventional forced-air system moves a large volume of air slowly through big ducts. SDHV inverts that: a smaller volume of air, moved fast, through insulated flexible tubes roughly 2 inches across.

That size difference is the whole story. A 2-inch tube fits where only wiring and plumbing could go before — up inside wall stud cavities, through closet corners, along joist bays, into the chases old radiator piping already uses. No soffits, no dropped ceilings, no opening plaster walls for rectangular trunk duct.

In each room, the tube ends at a small outlet about the size of a pot light. Air enters fast enough to stir the whole room's air into motion — the aspiration effect — which eliminates the warm-ceiling, cold-floor layering that older homes know well, and wrings out noticeably more humidity than a conventional system. Even temperatures, drier summer air, and almost nothing visible.

The hardware, briefly

• The air handler is compact and sits high in the house — an attic, a knee-wall space, a top-floor closet — because pushing conditioned air down is easier than pushing it up. From there the supply tubes fan out to every room.
• The outside unit is a cold-climate inverter heat pump, rated for reliable heating well below minus 25 Celsius. One system heats, cools, and dehumidifies.
• The return is typically a single generous return path rather than a web of return ducts, which is part of why the system disturbs so little of the house.
• The manufacturers: Unico and SpacePak are the two American pioneers; Hi-Velocity is Canadian-designed and built in Edmonton, and worth having on any Nova Scotia shortlist.

The design details that decide everything

SDHV's reputation runs on a split: owners of well-designed systems love them, and owners of badly designed ones complain about noise. The difference is three decisions made before anything is installed:

1. A real heat-loss calculation. The system is sized to the house's actual load, not its square footage. In an older home this step matters more, not less.
2. A generous outlet count. Each 2-inch outlet should carry only a modest share of the airflow. Skimp on outlets and the system forces too much air through each one — that's where the whistling complaints come from. Designed generously, each outlet runs quiet.
3. Sound-attenuating tubing at every outlet. The supply tubes are wrapped in sound-absorbing insulation, and proper practice puts a minimum three-foot attenuating section before every outlet — longer for large rooms — so the moving air is silenced before it enters the space.

None of this is exotic; it's simply design discipline. It's also why the brand debate matters less than the installer's care.

What it costs, honestly

SDHV costs more than conventional ducting where conventional ducting fits easily — the equipment is specialized and the routing is labour-intensive. In the homes SDHV was built for, that comparison is misleading: the alternative isn't cheap conventional duct, it's conventional duct plus opened walls, soffits, and plaster repair, or living with wall units you didn't want.

Published installed ranges for full systems vary too widely by home and market to be worth much, and we won't quote a number before a walkthrough. The real drivers are the heat-loss calculation, the outlet count, the routing paths your framing offers, and the heat pump pairing. What we will say: for the right house, it's a significant project that delivers something no other system can — complete, concealed, year-round comfort in a home that keeps its character. Whether yours is that house is exactly what an assessment answers, and sometimes the honest answer is that a simpler system serves you better; our guide on modern heat for century Halifax homes compares all three concealed paths off oil.

Where we fit in

An SDHV system succeeds or fails on its air distribution — the routing through the house, the outlet placement, the design discipline above. That's sheet-metal and ductwork craft, and it's our trade. We assess the home, design the layout to its actual heat load, route the tubing to spare the interiors, and deliver the complete system with the right equipment for a Nova Scotia winter. If a conventional path would serve your house better for less, we'll tell you that too — the details of what a standard conversion involves are in our oil-furnace conversion guide. Either way, the starting point is the same: show us the house, and we'll show you the path.