Home Experts: Local News
Costly and dangerous home heating and cooling mistakes
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Story Updated: Aug 2, 2011
Home heating and cooling systems differ from other appliances
A home's heating and air conditioning equipment comes with an efficiency rating. That number represents the most it can do. Unfortunately, it can do much less. Home air systems aren't like refrigerators, washing machines, and other appliances, which only have to be plugged in and perhaps hooked up to a water supply. In their normal situations, the appliances will operate at or very near their peak capacity and therefore at peak efficiency.
Unless a homeowner is very lucky, the home's heating and cooling system is not operating at its full capacity. Many parts of the home can adversely affect the system, and thus reduce the efficiency of the equipment, raise the energy bills, and increase the carbon footprint. If that unlucky homeowner buys a three-ton, 36,000 Btu system, she or he will get less than 36,000 Btu's of conditioned air into the home from those little grills in the ceiling, floor, or walls. However, the cost to operate that system remains at the 36,000 Btu level. To make bad matters worse, if the house really needs all of those Btu's, the system will not be able to heat or cool the home on really cold or hot days.
One example of how a poor duct system can reduce the system's energy efficiency
A homeowner requested a whole house energy and comfort audit because on cold days his furnace couldn't maintain a comfortable temperature inside the house. His regular contractor, unable to figure out what was wrong, had shrugged the problem off. After all, the furnace had heated the house most of the time and had done so for years.
On the initial walk-through, the inspector did a mental uh-oh when he looked at the return grill. It was small. He knew its size, but he put the tape measure to it for the homeowner's sake. Twelve by twelve inches. One square foot. For a reference point, consider that heating and cooling engineers require that the usual three-ton system have a 20-inch diameter duct to carry the air from the return grill to the furnace. (This is no big secret. The furnace comes with a manual that instructs the technician on how to install the system, and the manual should specify the proper size of the duct. If it doesn't, the authoritative manuals of ACCA, Air Conditioning Contractors of America, will provide the information and should be made available by all heating and cooling contractors to their personnel.) A 20-inch diameter duct has a cross-sectional area of 314 square inches. A 12 by 12 inch grill has an area of 144 square inches. The grill was less than half the size of the duct required for optimal operation.
The inspector abandoned the walk-through and stood near the furnace. The thermostat was nearby. He listened to the furnace start and stop. It would come on and run a few minutes, go off for few minutes, then repeat the cycle. The house air temperature never reached the heat limit set on the thermostat.
An example of how a poor duct system can create grave danger
The inspector didn't need to look inside the furnace to diagnose the problem. He knew what the problem was, and he knew the furnace was operating dangerously, not because the machine was defective, but because of the tiny return.
Furnaces have a safety switch that turns them off if the heat exchanger overheats. The heat exchanger is a slab of sheet metal with small ducts that carry the heat and the waste gases generated by the gas fire and vent it out through that metal "chimney" jutting through the roof. The furnace's fan blows the air from the return grill across the heat exchanger to heat the air that is then delivered through the supply ducts into the house.
The heat exchanger can't withstand the heat of the gas fire without help. That help is the air coming from the filter grill. By absorbing heat from the heat exchanger to heat the house, that air also serves as a coolant that allows the heat exchanger to withstand the heat. Getting the right amount of air from the filter grill allows the furnace to operate safely at its maximum capacity. This customer's return filter grill was so small that all thought of optimal performance went out the window, and attention became riveted on safety, although performance was dramatically and adversely effected.
The explicit danger
Defective heat exchangers can kill people. Furnace burners can produce carbon monoxide, and defective heat exchangers can deliver that carbon monoxide into the house. Although the odds are low, every winter brings news stories about families killed by carbon monoxide, so it must be taken seriously.
The remedy
The inspector recommended that the homeowner replace the furnace and build a return system with a 30 by 30-inch filter grill, and a properly sized duct. The furnace was still working, but its heat exchanger had been stressed by being overheated thousands of times. The safety switch had done its duty for years and should have been framed. The homeowner wisely accepted the recommendation.
A home's heating and air conditioning equipment comes with an efficiency rating. That number represents the most it can do. Unfortunately, it can do much less. Home air systems aren't like refrigerators, washing machines, and other appliances, which only have to be plugged in and perhaps hooked up to a water supply. In their normal situations, the appliances will operate at or very near their peak capacity and therefore at peak efficiency.
Unless a homeowner is very lucky, the home's heating and cooling system is not operating at its full capacity. Many parts of the home can adversely affect the system, and thus reduce the efficiency of the equipment, raise the energy bills, and increase the carbon footprint. If that unlucky homeowner buys a three-ton, 36,000 Btu system, she or he will get less than 36,000 Btu's of conditioned air into the home from those little grills in the ceiling, floor, or walls. However, the cost to operate that system remains at the 36,000 Btu level. To make bad matters worse, if the house really needs all of those Btu's, the system will not be able to heat or cool the home on really cold or hot days.
One example of how a poor duct system can reduce the system's energy efficiency
A homeowner requested a whole house energy and comfort audit because on cold days his furnace couldn't maintain a comfortable temperature inside the house. His regular contractor, unable to figure out what was wrong, had shrugged the problem off. After all, the furnace had heated the house most of the time and had done so for years.
On the initial walk-through, the inspector did a mental uh-oh when he looked at the return grill. It was small. He knew its size, but he put the tape measure to it for the homeowner's sake. Twelve by twelve inches. One square foot. For a reference point, consider that heating and cooling engineers require that the usual three-ton system have a 20-inch diameter duct to carry the air from the return grill to the furnace. (This is no big secret. The furnace comes with a manual that instructs the technician on how to install the system, and the manual should specify the proper size of the duct. If it doesn't, the authoritative manuals of ACCA, Air Conditioning Contractors of America, will provide the information and should be made available by all heating and cooling contractors to their personnel.) A 20-inch diameter duct has a cross-sectional area of 314 square inches. A 12 by 12 inch grill has an area of 144 square inches. The grill was less than half the size of the duct required for optimal operation.
The inspector abandoned the walk-through and stood near the furnace. The thermostat was nearby. He listened to the furnace start and stop. It would come on and run a few minutes, go off for few minutes, then repeat the cycle. The house air temperature never reached the heat limit set on the thermostat.
An example of how a poor duct system can create grave danger
The inspector didn't need to look inside the furnace to diagnose the problem. He knew what the problem was, and he knew the furnace was operating dangerously, not because the machine was defective, but because of the tiny return.
Furnaces have a safety switch that turns them off if the heat exchanger overheats. The heat exchanger is a slab of sheet metal with small ducts that carry the heat and the waste gases generated by the gas fire and vent it out through that metal "chimney" jutting through the roof. The furnace's fan blows the air from the return grill across the heat exchanger to heat the air that is then delivered through the supply ducts into the house.
The heat exchanger can't withstand the heat of the gas fire without help. That help is the air coming from the filter grill. By absorbing heat from the heat exchanger to heat the house, that air also serves as a coolant that allows the heat exchanger to withstand the heat. Getting the right amount of air from the filter grill allows the furnace to operate safely at its maximum capacity. This customer's return filter grill was so small that all thought of optimal performance went out the window, and attention became riveted on safety, although performance was dramatically and adversely effected.
The explicit danger
Defective heat exchangers can kill people. Furnace burners can produce carbon monoxide, and defective heat exchangers can deliver that carbon monoxide into the house. Although the odds are low, every winter brings news stories about families killed by carbon monoxide, so it must be taken seriously.
The remedy
The inspector recommended that the homeowner replace the furnace and build a return system with a 30 by 30-inch filter grill, and a properly sized duct. The furnace was still working, but its heat exchanger had been stressed by being overheated thousands of times. The safety switch had done its duty for years and should have been framed. The homeowner wisely accepted the recommendation.