Furnace Service Maine

What Is a High-Speed Ducting System?

A small-duct, high-velocity (SDHV) system — also called a high-speed air system — uses a specialized air handler that moves air at much higher velocity than conventional systems. Instead of large rectangular ducts and 6" branches, SDHV systems use 2" or 3" flexible tubing and small circular outlets (about the size of a CD) to deliver conditioned air at 1,000–2,500 feet per minute. This high-velocity jet creates an "aspiration" effect, pulling room air into the stream and rapidly mixing the air in the space — virtually eliminating hot and cold spots.

SDHV systems are manufactured by several companies, with Unico, SpacePak, and Hi-Velocity being the most well-known brands in the Northeast market. While the system brand determines the air handler and duct kit specifications, the principles are universal.

Why Choose High-Speed Ducting?

Retrofit-Friendly

This is the primary reason SDHV systems are chosen in Maine. Older homes — Victorian, Cape Cod, antique farmhouses — often have no existing ductwork and no easy way to run large conventional ducts. A 2" flex tube can be snaked through a 2.5" hole drilled through studs and plates, run inside walls, under floors, and through closets and knee walls without opening up large sections of the structure. In many old-house retrofits, SDHV is the only practical way to add central heating and air conditioning.

Superior Dehumidification

SDHV systems with central A/C remove significantly more humidity from the air than conventional systems. The high velocity and specialized coil design achieve a longer refrigerant contact time with the air, pulling out more moisture per ton of cooling. This matters in Maine's humid summers.

No Bulky Ducts

Conventional ductwork dominates basement ceilings and can require soffits or dropped ceilings in living areas. A 2" SDHV tube disappears into a wall or floor, and the 2.5" round outlets can be placed inconspicuously in walls, floors, or ceilings.

How the System Works

A high-static-pressure air handler (purpose-built for the system brand) connects to an insulated, flexible supply plenum — a larger 7" or 10" trunk tube — that runs through the home. Small 2" or 3" branch tubes tap off this supply plenum and run to each outlet. Return air flows through a conventional return duct or a series of return grilles back to the air handler. The air handler contains the heating coil (for hot water or electric) or connects to a furnace, plus an A/C coil.

Pairing with a Hot Air Furnace

Some SDHV manufacturers offer systems that integrate directly with a gas or oil furnace as the heat source. In other configurations, the system uses its own electric or hydronic heat coil. When paired with an oil or propane furnace:

• The furnace's heat exchanger provides the heat, but the SDHV air handler's blower does the air moving — at much higher pressure than a standard furnace blower.
• The air handler is sized to move sufficient CFM for the system tonnage and the home's heat load.
• Fuel system, venting, and controls for the furnace are standard per Home Installation guidelines.

Outlet Placement

SDHV outlets are small — typically 2" diameter — and must be placed to maximize the aspiration/mixing effect:

• In walls: typically at baseboard level, angled slightly upward and toward the center of the room.
• In floors: pointing upward, near exterior walls.
• In ceilings: angled toward walls or placed to sweep air across the ceiling.
• Each room typically requires 2–4 outlets depending on size and heat load.
• Outlets should not point directly at occupants — the high-velocity jet, while not harmful, is uncomfortable at close range.

Return Air

Return air for SDHV systems is typically handled through one or two centrally located return grilles — often at high-wall or ceiling level. The return duct is usually a conventional 6"–8" round duct or a single large rectangular duct. Because SDHV systems operate at higher static pressures, undersizing the return is particularly penalizing — always size returns generously.

Installation Steps

Noise Considerations

High-velocity systems generate more airflow noise than conventional systems. Most of this comes from the outlets, not the ductwork. Using the manufacturer's approved velocity-reducer fittings at outlets and ensuring the system is not over-pressurized will minimize noise. A well-designed and balanced SDHV system is no louder than a conventional forced-air system at the registers.