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An Easy HVAC Retrofit for Old Houses

A high-velocity, mini-duct HVAC system can bring an old house up to date with minimal damage to its historic fabric. By Jefferson Kolle

    Smaller than a compact disc, high-velocity mini-duct vents are an unobtrusive choice for heating and cooling old houses.

    Smaller than a compact disc, high-velocity mini-duct vents are an unobtrusive choice for heating and cooling old houses. (Photo: Courtesy of Spacepak)

    In Maryland, the summers are brutally hot. Just ask the Sisson family: Starting around April each year, their three-story 1942 brick house became more like a brick oven, even with the help of a few window-unit air conditioners.

    They briefly considered installing a traditional central air conditioning system—“but if we’d done that,” says Welmoed Sisson, “we would have lost all our closets because that’s where the giant metal ductwork would have had to go.” Fortunately for the family’s comfort and the house’s well-being, Welmoed’s husband, Bob—in her words, “an avowed technogeek”—discovered an alternate solution: a high-velocity, mini-duct HVAC system.

    As the name implies, the system pushes conditioned air at high speeds through small ducts. For old-house owners, the main attraction is that the small (3″ diameter), flexible ducts can be snaked through walls and on top of ceilings without the need to cut big chases or drop a ceiling. And unlike the large wall-, floor-, or ceiling-mounted air registers and grilles required for a traditional HVAC system, mini-duct systems deliver conditioned air through grilles that are smaller than an audio CD.

    While high-velocity mini-duct systems are often used to cool old houses that are heated by radiators, they also can be used to deliver heat. Two companies in the U.S., Unico and SpacePak and one Canadian company, Hi-Velocity Systems, manufacture complete systems that also can be mixed and matched with other companies’ furnaces and air-conditioning components.

    HVAC Basics

    Having some basic knowledge about how traditional heating and cooling systems work will help you understand what’s involved with installing a mini-duct system. An air conditioner does make rooms cooler, but what it’s actually doing is removing the heat from the air in your house. More accurately, it moves the heat from inside the house to the outside.

    Central air-conditioning systems are comprised of both indoor and outdoor components. A closed loop of copper tubing contains a refrigerant, commonly known as Freon, that is constantly circulated from the inside of the house to the outside, then back in.
    You’ve undoubtedly seen the outdoor component of an air-conditioning system—the compressor/condenser unit, which is usually set on a concrete pad on the shady side of the house, making a fair amount of noise and giving off a large amount of heat.

    At the Flower Hill Museum in Austin, a vent blends in with the flooring.

    At the Flower Hill Museum in Austin, a vent blends in with the flooring. (Photo: Peggy Lee Oster)

    Inside the house is a matrix of ductwork connected to a plenum, which is a large air distribution box. From the plenum, one system of ducts delivers cooled air to the rooms through vents on the floors, walls, or ceilings. A second duct system, known as the return, pulls warm air from the rooms. Connected to the plenum is an air handler with a fan and an evaporator inside.

    Here’s how it all works in concert: Pressurized liquid refrigerant enters the house inside the loop of copper tubing, where it’s carried to the evaporator. The evaporator then sprays the refrigerant through a nozzle, causing its temperature to drop as it evaporates inside the tubing and turns to a gas. At the same time, warm air from the return ductwork passes over the evaporator, causing the heat in the air to be absorbed by the cooled, gaseous refrigerant. The cooled air is then pushed through the air handler, into the plenum, out through the delivery duct, and through the vents into the room.

    Back to the now-hot, gaseous refrigerant: It’s pumped outside the house into the compressor/condenser unit. The compressor further pressurizes the gaseous refrigerant, which raises its temperature. The high-pressure, high-temperature refrigerant gas enters the condensing coil, where it is cooled to a liquid state by a fan blowing outside air across the condensing coil. The fan blowing across the coil releases the heat into the atmosphere and cools the refrigerant, turning it back to a liquid before it re-enters the house, and the whole process continues.

    Forced-air heating systems use the same ducts as the A/C, but the air is heated by a gas- or oil-fired furnace and pushed through the ducts into the rooms.

    The Mini-Duct Approach

    In the kitchen of the Flower Hill Museum, ducts were snaked through a pantry closet to avoid disturbing the original floors.

    In the kitchen of the Flower Hill Museum, ducts were snaked through a pantry closet to avoid disturbing the original floors. (Photo: Peggy Lee Oster)

    A high-velocity, mini-duct system operates similarly to conventional systems in that it also moves heat out of a house by absorbing it indoors and releasing it outside. Both systems use refrigerant, an air handler, evaporator, and a compressor/condenser.

    So how is a mini-duct system different? To understand the distinction, imagine that a mini-duct system is a plastic drinking straw and a traditional system is a cardboard paper-towel tube. To get an idea of how much space a mini-duct system takes up in your house, think about how many straws you can stand upright in an average coffee mug versus how many paper-towel tubes. Now think about blowing air through a plastic drinking straw and then through the cardboard tube. In order to exhale the same amount of air, it has to travel much more quickly through the straw.

    The velocity of air that comes through a mini-duct vent has a lot to do with both where the ducts are located in a room and the number of ducts used. Going back to the straw and cardboard tube, imagine blowing that air on a stick of burning incense. When you blow through the tube, chances are the smoke will just continue on its lazy way to the ceiling. But blow through the straw, and the smoke will disperse and move through the room. That’s due to the high speed of the air and something called Bernoulli’s principle: an increase in air speed creates a vacuum behind the air flow that pulls more air back toward the source of the flow.

    The result is that the air in a room will be better mixed. With a traditional A/C system, the air can stratify: Slow-flowing, cool air pools near the floor, while temperatures in other areas of the room can be 5 to 7 degrees higher. SpacePak’s Ted Brown says setting a mini-duct thermostat to 72 degrees will feel the same as a conventional A/C thermostat set to 68 degrees. “We’ve had customers call us to say that their thermostats must be broken because the rooms feel so much cooler than where they’ve set the dial.”

    The system’s air handler and trunk lines are easily installed in extra attic space.

    The system’s air handler and trunk lines are easily installed in extra attic space. (Photo: Courtesy of Unico)

    Mini-duct systems also have the advantage of removing up to 30 percent more humidity than a traditional system—drier air feels cooler than more humid air of the same temperature. You also can operate a mini-duct A/C system on dehumidify mode, a great way to reduce energy costs if you don’t need to cool your house but just want to knock down the mugginess in the air.

    Heating your house with a mini-duct system has the same benefits as cooling: quick, unobtrusive installation and superior air circulation. Adding heating capacity to a mini-duct system simply requires adding a source of heat that can warm air to be pushed though the ductwork. Both Unico and SpacePak manufacture their own heating sources, from electric furnaces to gas-fired boilers to a reverse-cycle heat pump that absorbs heat from outside and moves it to the inside of a building. Their mini-duct systems also can be hooked up to a variety of other manufacturers’ oil, gas, or electric boilers or furnaces, or can be retrofitted to some existing systems.

    Installation & Costs

    The first step to installing a mini-duct system, says Dave Corcoran, owner of D&D HVAC in Somers, Connecticut, is to make a heat-loss calculation for the building (taking into account such factors as geographic location, solar orientation, insulation levels, and number of windows and doors) to decide its air conditioning needs. The air handler is installed first, often mounted in an attic. From there, Corcoran’s crew installs the main trunk lines—6″- or 7″-diameter insulated ducts that feed the air handler and can be laid on joists or hung from the rafters.

    For the ultimate mini-duct camouflage, Unico offers a variety of wood-finished vents to match the grain and tone of hardwood floors.

    For the ultimate mini-duct camouflage, Unico offers a variety of wood-finished vents to match the grain and tone of hardwood floors.

    Next, the 3″-diameter flexible ducts are run from the trunk to the ceilings and walls in various rooms, where they’re attached to small vents in the ceilings, walls, or floors. To get to rooms on lower floors, the 3″ ducts can be snaked through interior or non-insulated walls or through existing chases.

    Inline sound attenuators (similar to car mufflers) are installed on the ducts to reduce any wind noise from the high-velocity air. “These systems aren’t any more noisy than traditional ones,” Corcoran says.

    The size of the ducts makes them fast to install; a typical installation takes just a few days. “It’s rare that we can’t find a way to get the ducts to a room,” adds Corcoran. “For traditional systems, you run the ducts and cut in the grilles where you’re able to.
    With a mini-duct system, you run the ducts and install the vents where you want them.”

    The cutting-edge technology does make mini-duct systems pricey (on average, about 25 to 40 percent more than a conventional HVAC system), but material costs are only one factor in the equation.

    “A conventional system takes much longer to install,” points out John Bal-dasaro, national sales manager for SpacePak, “and in most cases, when the HVAC guys are finished, it’s time to bring in the subcontractors. You may need a carpenter to box out the ductwork or lower a ceiling. And then you need a drywall or plaster guy, maybe a painter or even an electrician if switches or outlets need to be moved.” That adds up to more time that your house is in disarray and more money to get the extra work done.

    It’s also important to take into account the total system performance when comparing costs. The Energy Conservatory estimates that duct leakage in traditional forced-air systems accounts for up to 25 percent of energy costs. Based on both laboratory and on-site testing, Unico estimates that its insulated ductwork has “less than 5-percent duct leakage,” and Baldasaro claims that SpacePak has a 0-percent leakage rate. Ducts that don’t leak mean that more of the air you’re paying to cool reaches its intended location.

    Plus, lower humidity levels and more constant air temperatures mean that thermostats can be set higher. “The industry rule of thumb is that every 1 degree Fahrenheit you raise your thermostat accounts for a 3-percent savings in air-conditioning costs,” says Randy Niederer, Unico’s marketing manager.

    Before the advent of mini-duct systems, old-house owners often had to choose between preserving the integrity of their home or staying cool enough to enjoy it. Now, they can have both—invisible cooling and intact period rooms.

    Published in: Old-House Journal December/January 2012


    Ron McNutt November 23, 2013 at 4:35 pm

    Can these high speed ducts be pushed through existing AC /heater ducts? I have a heat pump/AC on my (flat) roof and large ducts run across the roof to each room.

    Barbara Iaquinta May 11, 2014 at 12:34 pm

    Don’t think this can be snaked through walls. Brick and plaster. Would this still work??

    anibarro hunt July 24, 2014 at 4:41 pm

    A small, 1948 built house. Single level, no attic, no basement, only below house crawl space. Single furnace and one divided single vent, no existing duct work, no insulation. How would this work for us?

    Haroun Alhambra August 15, 2014 at 8:01 am

    Three story building, six apartments, formerly heated by steam radiators. No cooling apparatus in place; no vents in place for heating or cooling. How does this system work in a building such as this? The roof is flat and there is a large basement the full size of the building. Thank you.

    Brad Gryan September 10, 2014 at 2:02 pm

    Hi, I have a few questions about the article on “An Easy HVAC Retrofit for Old Houses”. Will this be able to snake through plaster walls with minimum damage?

    Thank you,
    Brad Gryan

    Mandy D Carl September 11, 2014 at 9:04 pm

    I purchased an 1870 circa old home that the spacepac system was suggested to us to install in order to get central heat and air in the home. Do you have a list of distributors?

    guyy cook September 16, 2014 at 11:47 am

    I have 3 story old house that runs of a wood or coal furnace I want get rid of it and those eye sore raditors. The attic is the master bedroom with accses to either side of the attic that’s not psrt of the masterbedroom. Is their any way to run this type of system in my house

    Donnie Cain November 12, 2014 at 1:24 am

    We live in Nebraska (extreme hot and cold) and have a 1 1/2 story historic rusticated concrete block house with lath and plaster walls. All is in it’s original 1906 condition. A central air/heat system was placed in the 80′s (American Standard heat and an outside central air unit) but the duct work is minimal at best especially up stairs. The wood floors are cold but the underneath can be easily accessible for mini ducts. The original perimeter large floor ducts and grates are used for return air and all the heat comes up thru 3 large grated in the center of the house. Will mini ducts substantially help us heat and cool. And are their any dealers or consultants close.

    Debbie December 7, 2014 at 3:06 pm

    Does anyone respond to these comments? I have questions as well.

    Bruce Linday March 3, 2015 at 6:58 am

    Is there a contact/installer in Southern New Jersey [west coast]?

    Richard April 17, 2015 at 1:30 pm

    All of the questions above are best directed to the manufacturers listed in the article; Unico or SpacePak. Unico has a long list of completed projects with installing contractor contact information on their website under “examples of our work”.


    Recs July 8, 2015 at 7:40 pm

    Hi Bruce we are in central jersey and used Uniqlo in Philadelphia. It has been purchased by Silas. They dropped us from their coverage area so we now get service from Princeton Air. We have a 2.5 story Victorian house built in 1906. We have two unites. One in the basement for the first floor and one under the eaves on the third floor. We have two vents in most rooms. We have plaster and lath walls and balloon construction. The AC is great. The air is very dry so it is true u can keep the house warmer; we set our Tstat at 76. We have no closets so ducted AC would be a mess and need to be framed up etc. my only advice would be to consider a mini split system for a floor if it is relatively open and do hi velocity for the rest of the house. We did ours so long ago the minisplits were iffy or we would have done it on the third floo/finished attic. We have had our units for more than ten years. They are not SEER rated and take quite a bit of an owed. The newer units are more efficient. I hope this info helps

    Amy R August 5, 2015 at 7:29 pm

    We have been searching and searching for someone in Washington State who has even HEARD of these systems. Sure wish we could find someone because this would perfectly suit our needs.

    Ellen Spencer September 21, 2015 at 3:55 pm

    We have a 1920, 2200, sq. ft. farmhouse with two floors and no attic. Can the air handler go into the basement? We can put conventional air conditioning in to service the first floor, but it is difficult to get it pumped up into the small 5 upstairs bedrooms.

    Teresa Theesfeld June 19, 2016 at 11:17 pm

    I have a house built in 1890 that has only a tiny crawl space for an attic, but a large basement. Can I put the condenser in the basement? Can you suggest some contractors in St. Paul, MN?

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