As energy prices continue to creep up, property owners are becoming more and more concerned with improving the energy efficiency of their HVAC (heating, ventilating, and air conditioning) systems. A typical HVAC system accounts for 50% of a home’s total energy use, making it the logical place to make energy saving upgrades. Achieving these energy upgrades can come through the purchase of more efficient equipment or basic repairs and modifications. While newer more efficient, safer, and quieter HVAC equipment is something that every home owner should consider, this article will address some fairly simply duct and air loss repairs that will significantly increase energy savings. We will address ducts, insulation, and length.
"In a lot of cases duct leakage is the single biggest energy loss in a residential or commercial heating and A/C system. Energy loss can be as high as 40% through damaged air ducts..."
The majority of ducts we come across in the field are damaged, under-insulated or leaking air. In fact the average duct leakage in existing residential HVAC systems is 35%. The majority of air leakage occurs at the connection points in the duct system. For example this occurs at the connection of the duct to the register boot, the plenum, and wye connections. Unless the duct has been damaged it’s rare to find leaks in the middle of a duct.
Catastrophic damages, such as disconnected ducts, rodent damage, or environmental damage, if left unchecked, usually result in complete duct replacement. So it’s important to have your ducts checked every year. Since air leakage is the number one source of energy loss it’s important that homeowners have a pressure test completed to determine actual air loss in the system. This even includes newly installed ducts. Though it’s nearly impossible to design a completely sealed duct system, one should strive for less than 10% air leakage.
Mastic and butyl tape are two of the best materials to use to properly seal and prevent air leaks. Mastic should be applied at any connection point, especially register boots and plenum connections. A common mistake we see in the field is the use of duct tape to seal air leaks. Though duct tape has the word “duct” in it, it was never intended for sealing ducts. A key indicator of a duct leak is discolored fiberglass. A lot of older ducts are insulated with yellow or pink fiberglass insulation. When the fiberglass starts to turn dark brown or black it usually means an air leak, the discoloration is accumulated dirt and debris.
Proper insulation is the second part of an efficient forced air system. Usually ducts are run through non-insulated spaces such as attics or crawl spaces. Since ducts cover approximately 40% of a home’s total square footage they become an easy source of heat loss though conduction. A 2,000 square foot house would have roughly 800 square feet of duct. Think of this as having a second living room in your, non-insulated, attic or crawl space.
If we assume that that attic space reaches 140 degrees Fahrenheit and the fan is delivering air at 60 degree Fahrenheit, we are left with a difference in temperature, also known as “Delta T”, of 80 degree. By using the heat loss formula Q = (A * T) / R we can estimate that a 2,000 square foot house ‘A’, with ducts insulated to R-value 4.2, ‘R’, would loss approximately 1.27 tons energy every hour. To put this in prospective, the typical 2,000 square foot house’s HVAC system would be powered by a 3.5 ton air handler, and therefore be losing 36% of its energy through conductive duct loss. If we re-run the same example, but insulate the ducts to an r-value of 8, we reduce the total energy loss to .66 tons. It’s clear that by improving duct insulation we drastically improve the efficiency of any HVAC system.
The third part of a properly designed HVAC system is duct length. In most cases duct length should be kept to a minimum and utilize a “spider” pattern; individual ducts should come off of the plenum and run to individual registers through the house. By doing so total duct area is reduced. Since total duct area is represented in the numerator of our heat loss formula, Q = (A * T) / R, reducing the area will reduce our total heat loss given that none of our other values change.
In summary all homes are not created equally and every HVAC system is going to be designed differently given the home’s layout, climate, and homeowner’s budget. However, every homeowner should consider the three parts mentioned above and have the ducts inspected. Often times a simple duct test, which typically runs $250 - $400, can lead to significant energy savings. Given the high cost of heating and cooling it will be money well spent. If you’d like to have your heating, cooling, and ducts system inspected, call us a Bellows Plumbing, Heating, & Air 877-477-7151. Our offices serve all cities within Santa Cruz, Santa Clara, and Marin Counties.