Duct Booster Fan: How It Works and Whether It Fits Your Home

What a duct booster fan is, and the three kinds

A duct is the tube that carries heated or cooled air from the indoor unit to a room, and a register is the vent cover where that air enters the room. The trade article says in-duct booster fans are typically wanted in the areas furthest from the air handler, or where the duct size is too small. The air handler is the indoor unit that holds the blower, the part that pushes air through the ducts. The article's author shares a surname with the company that made the DB-2 manual cited below. Our reading: the article may be maker-written, though it does not say so, and it is 15 years old, so check any one product against the maker's manual for the exact model.

The first kind sits inside the duct itself, and the article says the most popular residential sizes are 6-inch and 8-inch round. The second kind fits into the boot and replaces the existing register, and the article says it is popular in finished construction, where there is no practical access to ducts. The boot is the metal fitting at the end of the duct where the register attaches. The article says economy register fans use small propeller fans in a plastic housing, while deluxe ones use a cross-flow blower with a metal housing. The third kind is a transfer fan, which the article says moves air from an adjacent space and is switched by a separate thermostat or wall switch.

The article adds that fans can also go on return-air ducts, for tight rooms when doors are closed or where the return ducting restricts flow back to the air handler. Return air vent too small covers an undersized return in its own right. Two fan makers' manuals describe their in-duct fans the same way, as products that boost the heated or cooled air already moving in a forced-air duct. A forced-air system is one whose blower pushes heated or cooled air through ducts.

What a booster fan changes, and what it does not

The trade article says weak airflow is usually not a shortage of air handler output, and it names the prime culprit as pressure drop from the duct's size, length and layout. Pressure drop is the push the air loses as it travels through the duct, its bends and its fittings, and cfm, or cubic feet per minute, is the amount of air moved. The Department of Energy's Building America guideline says the ducts, fittings and outlets all add pressure loss between the equipment and the room. It adds that each component's pressure loss must not exceed the static pressure the equipment can work against. The guideline defines static pressure as a resistance to airflow from coils, filters, elbows, grilles and duct friction. Ductwork in Florida gives the airflow and static pressure numbers to ask for, so this page does not repeat them.

The same guideline says the proper amount of air follows each room's heating and cooling loads. It says too little air undercools a room, and too much air overcools it. Our reading: a fan that sends more air to one room changes the share every other room gets, and it does not make the air colder or add cooling capacity. Our reading: that is why a fan can fix one room and still leave a second problem behind, such as a room that now runs too cold.

Causes a booster fan does not fix

A fan fits only when the duct run itself is what limits the air. The documents below describe causes that sit outside the fan, so a fan leaves them in place.

• A weak return path: the Building America Solution Center says inadequate return air pathways can cause pressure imbalances from room to room, with drafts and temperature differences between rooms or floors. For closed bedrooms it lists door undercuts, transfer grilles and jump ducts as ways to let the air return. For new homes it says ENERGY STAR sets a closed-door limit of 3 pascals either way between a bedroom and the main house. It allows 5 pascals for rooms with a design airflow of 150 cfm or more. A pascal is a small unit of air pressure. Our reading: a fan that pushes more air into a closed room needs a return path that can carry it, so ask for a closed-door pressure reading afterward.
• A supply system that is out of balance: the Solution Center's balancing guide lists this as its own cause. It says dampers and register controls are meant to fine-tune airflow, not to correct significant duct sizing errors. Our reading: a duct that is too small for its room is where the trade article places a fan. Two-story house uneven cooling covers zoning and damper balancing.
• Leaks and crushed flexible duct: ductwork in Florida covers both, including what a Department of Energy-hosted guide says about compressions. Our reading: a fan on a leaking or crushed duct pushes against the same defect, and it may push more air out of the leak. Duct sealing vs duct repair separates the two fixes.
• A system that cannot keep up with the heat: if every room runs warm, the shortage is in the system, not in one duct. Our reading: a fan in one duct then only moves the shortage around. AC tonnage and sizing explains how a house's size is worked out, and why your AC is not blowing cold lists simpler checks.

What two makers' manuals set as limits

The two manuals below describe different products, one from 2000 and one undated, and both copies are hosted on sellers' sites. They show the kinds of limit to look for, not the limits of any fan you are offered.

Suncourt's manual says its in-line fans are designed to boost dry conditioned indoor airflow in the ducts of forced-air systems, at air temperatures between 40°F and 140°F. The manual also says a fan must never be exposed to temperatures over 140°F. Tjernlund's 2000 manual for its DB-2 limits that fan to branch ducts serving individual rooms, not the main supply. It bars installing that fan where the air in the duct exceeds 200°F. Our reading: the two ratings differ by 60°F, so the number to ask for is the one printed for the exact model.

Suncourt's manual says a group of its models is suitable only for a duct that supplies a single room. It says never to connect its fan to a variable-speed (ECM) furnace blower, and to consult the system maker or a qualified HVAC contractor before connecting to a blower. ECM stands for electronically commutated motor, a kind of motor used in many variable-speed blowers. Variable-speed air handlers explains that kind of blower.

Suncourt's manual says to keep the fan accessible and to clean it at regular intervals, no less than twice a year. Tjernlund's manual says that where filters are properly installed and maintained, no cleaning of the blower wheel should be required. Our reading: the two makers disagree on cleaning, so ask which routine applies to your model and who will do it. Tjernlund's warranty excludes products that are installed improperly or wired incorrectly.

Florida homes: hot attics, humid air and permits

The Florida Solar Energy Center reported attic air temperatures of up to 136°F at a test home in Cocoa, even with a radiant barrier in place. The summer was 1997, going by the retrofit date the report gives, and the home had a flexible duct system in the attic that carried the cooled air. Our reading: that is one home in one county in one summer, not Tampa Bay data, but it shows Florida attic air can come within a few degrees of a 140°F limit. Suncourt's manual also says the fan's power cord should be secured where it is not subject to temperatures above 140°F. Our reading: the manual's wording, "temperatures over 140°F", does not name the air around the fan, so ask the maker whether attic air counts.

The same manual describes its fans as designed for dry conditioned air. Ductwork in Florida covers how cold ducts sweat in humid attic air. Our reading: ask whether the model you are offered is rated for the damp, cool air in a Florida duct. For duct insulation, see ductwork insulation. Whether adding a fan to a duct needs a permit is a question for your building department. HVAC permits in Florida explains how permits and contractors work.

What to ask before you agree to a booster fan

Put these six questions to any contractor who suggests a fan, in writing.

1. What is short of air in this room, and how was it measured? Ask for the room's airflow at the register and for a look at the return path, not a guess.
2. Why a fan rather than a fix to the duct, the return path or the balance? Ask what each of those would involve and why it was ruled out.
3. Which exact model is it, and what air temperature, moisture and surrounding temperature is it rated for? Ask for the rating printed for that model, because the two manuals above differ.
4. Is it made for a branch duct that serves this one room, and is it approved for my blower type? Ask for the maker's statement in writing.
5. How will it switch on and off with the blower? The trade article says this is typically done with a pressure switch that senses positive pressure when the air handler runs. It adds that temperature sensors are another option, but the drift of cheaper snap-action thermostats can cause erratic operation.
6. Who installs and wires it, who pulls any permit, and how will the room's airflow and closed-door pressure be checked afterward? Ask what happens if it does not help: can it be removed, and is that in the quote?

An answer that is not written down cannot be checked after the work. Cutting into a duct and connecting a fan's power are work for a licensed contractor.