What HVAC 'experts' pretend not to know about ductwork.
What HVAC 'experts' pretend not to know about ductwork. 
Prevent getting sicker
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When ducts are installed in an attic, the energy waste can exceed 30 percent of the capacity of the cooling unit. Ducts for a 2-1/2 ton central air conditioner have the approximate wall and ceiling area of a 10-by-10 foot room. DO THE MATH. Air conditioning ductwork is usually insulated to R6 (the 'newest' flex duct code minimum requirement,) about half the insulation of an ordinary wall and 1/5th the insulation value of today's ceilings. They are in effect, insulated to 1960's insulation standards. |
Problems:
Ducts in hot attics in the summer: The ducts in an attic are not subject to the same temperature difference of inside to outside as the rooms below; but usually are in 120F or higher attic temperatures, which is like putting a poorly-insulated room in an area that is twice as hot as the rest of the house. Any breaks in the insulation cover on the outside is a clear path for hot, humid air to reach the colder duct metal or plastic inside and condense water inside the insulation, making it even more useless.
Ducts in cold attics in the winter: In the winter in northern states, the same poorly-insulated ducts are exposed to temperatures close to that of the outdoors. This is like having the poorly-insulated ten-by-ten room left with the door of the unheated room open to the house for the entire winter. Moisture from the house goes into the ducts to collect there, just as the walls used to sweat in the poorly insulated houses before 1950.
Interpreting that into cures, some moisture lay in the evaporator pan to start mold and algae, so the hope of a contractor is somebody comes back every year to 'winterize' at a service charge.
As the usual cause is insufficient insulation to prevent moisture from collecting in the duct during the cold weather, requiring that the entrances and exits of the ducts weren't sealed during the winter, that could be another 'winterizing' that people go through.
The REAL cure is sufficient insulation around the ducts in cold spaces so that house moisture doesn't collect there.
A 'traditional' problem. When air conditioning was first installed in homes back in the 60's the ductwork was made for heating. When ductwork was made for heating only, it was uninsulated, so would sweat when cold. Also, ductwork was too small for air conditioning as heating was done at a 70 degree rise from 70F air; but cooling was accomplished at a 20-degree drop from 75 to 55F air. Today, as building heat LOSS is closer to building heat cooling GAIN so heating and cooling units are closer to the same btuh, the same air that heats has to flow three times as much air to cool, requiring larger ducts.
When air flow is low for the air conditioning cooling machine, the air coming
through the cooling coil dwells at the coil longer to become cooler. This lowers
the air temperature in the duct, making the surface colder, which causes the
moisture in the air to condense on the surface. The cure is to insulate the
duct so the surface of the insulation on the duct is warmer - above the condensing
temperature, while sealing it away from letting moisture inside the insulation.
OR
Increase the air flow through the coil, so the air is warmer - above the dew
point of condensation, but still cools the rooms. (Yes, you can still cool
a room with warmer air that is still cooler than the room when there is enough
of it.) Manufacturers of the more expensive variable-speed air conditioners
make use of this knowledge to vary the fan speeds of their units to produce
more or less condensation INSIDE the AC coil (while recommending more insulation
on the outside of the duct.)
So your solution is to insulate the ducts and/or add more duct to increase air flow or get a smaller outdoor unit to cool the air less inside the duct. All parts of an AC must match. Getting a bigger unit than ductwork can support "because it won't work too hard then" or "because I get a bargain at the same price as a smaller one" has its downsides.
THE SOLUTION: Make an improved industry of INSULATING ductwork, especially when in an attic, to R 15 with thicker insulation, perhaps pre-manufactured ducts with closed-cell foam between metal. Another solution is to KEEP the ducts INSIDE the heated and cooled building envelope within walls and soffits. Placing insulated square, small duct branches or high-velocity small ducts UNDER the ceiling batts can help reduce cost. That way, it would reduce the extra work some homeowners have to do every year to remove their grill covers, put saran wrap or cardboard behind them, replace the covers to prevent the house air from reaching the cold ducts, then reversing the procedure when the AC is needed. One energy and cost saver: Although high-velocity duct systems require more powerful blowers to push air through the small duct, this additional energy cost can often be more than made up for in that the duct exterior surface is less by far than 'usual' low-pressure duct. A 2" duct, insulated to the same low R-6 rating has 1/3rd the surface area, thus less than 1/3rd the energy loss of a 6-inch duct PER FOOT. Additionally, the smaller duct can be placed UNDER a ceiling insulation batt to make it one of the most-insulated duct one can find in houses AND by doing so, eliminate moisture collection with the extra insulation on top of the small ducts. |
Why isn't it done properly? The sticking point against having so many jobs done RIGHT is the cost of insulating properly. When installers don't have the experience to know that the R6 flex duct standard is the MINIMUM insulation an installer can 'get away with,' and that the homeowner hasn't a clue about what insulation means, the usual reason for selection is PRICE. If all ducts were mastic-sealed so there were no leaks, and insulated properly, then there would be no complaints; but lower-price often is a final decision.
There are your solutions:
1. Block off the air from getting to the ducts in the winter. If you don't
mind doing that, then you can defend the practice of having insulation thin
enough to allow the house moisture to settle in the ducts in cold regions.
This will allow the installers to give a 'good price' or 'beat the bid' at
the inconvenience to the customer of plugging and unplugging holes depending
upon which season.
2. Install ductwork with enough insulation on it to not have to plug and unplug holes, or not having to change the ducts if you forgot because the mold has grown and or dried out inside the ducts.
A duct like a wall, adequately insulated, doesn't ever have the problem. As the contest is to do it the cheapest, the problem due to inadequate insulation by installing the minimum required by code, is always a fresh yearly topic as people discover it.
Why don't manufacturers stop the problem? As evidence of such problems are about smells, the response is about ductwork itself being a medium for mold, bacteria, and algae. This is not favorable for ducted system manufacturers, so it is not favorable to manufacturers to respond to posts about that subject.
Like citizens stopping wars or not, it isn't easy decide which is better, to let such problems go on or to take a position to improve the situation - and not usually the assignment of corporate officers to limit unhealthy practices through added expense detrimental to sales. That is 'Nader-style whistle-blowing government interference,' and more like 'shoveling sand against a tide in HVAC.'
An 8x24-inch trunk duct about 40 feet long
Add in the branch ducts from the trunk to the registers:
A 40-feet-long trunk duct with its branch ducts equals a poorly insulated 10 x 10 ft x 8 ft high room.
Pictures of a blower/coil dripping water from the attic to the room below.
Seeing an uninsulated trunk duct and R6 flex duct attached to the 92.8% efficient furnace is not too surprising:
