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PROBLEM: Heat either Does Not Reach or Exceeds the Set Temperature
Heat Either Does Not Reach or Exceeds the Set Temperature
Possible Causes
• Thermostat mounted crooked.
• Heat anticipator not set correctly.
Possible Repairs
• Remove cover of thermostat and loosen screws holding unit to wall. Level the thermostat. Re-tighten screws and replace cover.
• To adjust the heat anticipator, please see Heat Anticipator Adjustment.
Thermostat Heat Anticipator Description
Thermostat Heat Anticipator
The heat anticipator is an electrical resistor device mounted in the center of many mechanical thermostats that fine tune the point at which the thermostat turns off the furnace burners. It anticipates the flywheel effect of a space heating up and turns off the burners a short period of time before the space reaches the desired temperature.
The heat anticipator essentially consists of a simple thin wire mounted to a disc which is attached to the bimetallic coil. There is an adjustment arm which touches the wire and functions to vary the electrical resistance of the wire which in turn varies how hot the wire gets. This in turn warms the bimetallic coil causing it to shut down the gas burners early as determined by the heat anticipator.
Adjusting the Thermostat Heat Anticipator
Thermostat Heat Anticipator Detail
When the heat anticipator is out of adjustment it can cause the furnace to exhibit symptoms of short cycling (turning on and off frequently) or to exceed or never reach the desired thermostat heat setting. To adjust the heat anticipator perform the following steps:
• Remove the thermostat cover
• Make sure the thermostat is level. If it is not level the mercury switch will not work properly.
• At the center of the thermostat find the small disc with calibration marks as shown in the photo above. It will probably have the word "longer" on it. The disc will have fastened to it a lever arm and indicator relating to the calibration marks. This is the heat anticipator adjustment lever arm.
• If the furnace is cycling on and off too frequently, move the heat anticipator adjustment lever closer to the "longer" setting by one calibration mark.
• If the furnace is exceeding or never reaching the desired set temperature, move the adjustment lever away from the "longer" setting by one calibration mark.
• Once the appropriate adjustment is made let the furnace run and the temperature stabilize for a period of 2-3 hours.
If necessary, repeat the above procedure.
• If the problem persists and you cannot resolve the problem with these steps, you may need to replace the thermostat.
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Solutions to Case of Duct Sweating
With the ever-increasing popularity of putting the air conditioning ducts outside the conditioned space, many contractors and homeowners in humid climates are now experiencing sweating ducts. Most of these sweating ducts are carrying cool supply air, although sometimes return air ducts can sweat, especially if the house is kept very cold and the return ducts run through cool vented crawlspaces.
Ducts sweat because the outside surfaces of the ducts are below the dew point of the air in their immediate surroundings. The primary ways to stop ducts from sweating is to raise the surface temperature of the ducts or to lower the dew point of the air surrounding the duct by some form of dehumidification. The dew point of air is the temperature at which the amount of water vapor in the air is the maximum amount the air can hold.
Make the Ducts Warmer
There are several ways to raise the outside surface temperature of the ducts. The first is to raise the temperature of the air inside the ducts. One of the easiest ways to do this is to raise the room temperature or the return air temperature. Homes with low thermostat settings are more prone to having ducts sweat. If the room temperature is 70F, and the thermostat is turned up to keep the room at 75F, the chance of the ducts sweating is reduced. If the return air temperature is raised 5F, and the temperature drop across the indoor coil is kept at 20F, the supply air temperature will go up 5F.
For supply ducts, another way to raise the outside surface temperature is to increase the airflow in the system. Half of the systems are supplying 200 cfm per ton or less. When a system is operated with low airflow, the cooling coil gets colder, so the supply air is colder than normal. This makes the outside surface of the duct colder also. Therefore, if the supply ducts are sweating, measure the system airflow to be sure it's adequate for the system.
Make the Environment Warmer
Another consideration is the temperature of the air surrounding the ducts. Raising this temperature will raise the outside surface temperature of the ducts. If the ducts are installed in a cool crawlspace, they are more likely to sweat than ducts installed in a hot attic.
In many cases, consumers are advised to add exhaust fans to their attics. Running these fans excessively and bringing in cooler outside air can cool the duct surface temperature in the attic to below the dew point and start forming condensate on the duct surfaces.
Adding exhaust fans to a crawlspace can also aggravate the problem. Because crawlspaces are typically cool, if the air outside has a dew point above the surface temperature of the ducts, more moisture is added to the crawlspace in the form of humidity. The condensation will become more rapid, increasing the amount of liquid water present in the crawlspace.
Dehumidify the Environment
Another way to decrease the amount of duct condensation is to dehumidify the space around the duct. This would normally be done by installing a dehumidifier, such as when the ducts are in a crawlspace. By closing the crawlspace and installing a dehumidifier, the air in the crawlspace is dried out, lowering the dew point temperature of the air. With a lower dew point of the air surrounding the duct, it's less likely for the ducts to sweat.
Insulate the Ducts
You can increase the outside surface temperature of the duct by raising the air temperature inside the duct, by raising the temperature of the air surrounding the duct, or by adding insulation to the duct. This can be as simple as using proper installation techniques when installing the duct wrap on sheet metal ducts. Often, when installers wrap the duct, they compress the insulation to "make it look nice." However, by pulling the duct wrap tight during installation, duct wrap with an R-6 rating can be effectively reduced to R-3 or less.
Be careful when adding insulation to an already insulated duct. If the vapor barrier on the outside of the new insulation is not well sealed, moist air can get between the layers of insulation. This can cause condensation to occur inside the insulation, destroying the insulation R-value and putting you back where you started.
Beware of Turbulence inside Ducts
Air turbulence inside ducts can affect the R- value of the duct insulation. If you install an elbow or bullhead tee near the discharge of the blower, you will create turbulence inside the duct that increases heat transfer out of the duct, making it cooler. In some cases, this can make the difference between ducts sweating and not sweating. Adding turning vanes inside the fitting can fix this problem and increase airflow as well.
Watch Room Humidity
Be careful when increasing the airflow in an air conditioning system to increase supply air temperature. Doing so may reduce duct sweating, but will also reduce the dehumidification done by your air conditioner or heat pump.
Reduce Duct Leakage
Another duct-sweating dilemma is caused by poorly sealed sheet metal ducts with duct wrap over them. When air leaks out of the duct, it's trapped by the duct wrap vapor barrier. The cold supply air flows through the insulation on its way to the nearest tear or opening in the duct wrap vapor barrier. In the process, the surface of the vapor barrier assumes the supply air temperature. Therefore, the duct sweats over the path from the duct leak to the duct wrap leak. If the metal ducts are not sealed at all, the entire surface on the duct wrap vapor will sweat.
For this reason, I recommend ductboard and flex over wrapped sheet metal ducts in high humidity areas. Properly fabricated ductboard has tight corners, keeping the supply air away from the outer surface of the duct. Therefore, the full rated R-value of the duct is achieved.
The same holds true with flex ducts. When installing flex ducts, you only need to seal it at the ends, where it connects to the starting collar and the boot. You don't need to seal three to five elbow gores at every bend, and the pipe every five feet. As long as you don't crimp the flex, or use very narrow straps to support it, the full rated R-value of the flex is achieved.
It's also possible for ducts to sweat at the exit point of the air leaking from the duct. This is common at poorly fitting or poorly sealed flex duct starting collars, and is especially true in cool crawlspaces. The trunk duct surface around the leaking starting collar will be chilled, and will sweat.
Cuss the Truss
Another thing that causes ducts to sweat is when they are run through trusses or supported by the building structure in a crawlspace. It the duct is supported by the web of a truss, air is trapped between the duct and the truss. Because no new heat is added to this air, the cool supply air in the duct gradually cools the pockets of air outside the ducts.
Running ducts next to insulation or other ducts can have the same effect. These ducts will then sweat where they touch or come close to other supply ducts, building insulation, or building materials in any closed space.
Play the Duct Detective
As you can see, there are many causes of sweating ducts. Careful installation and insulation practices will prevent most of them. |
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DUCTWORK IN ATTIC
Ductwork in the attic usually run from two sources:
- From furnace installed anywhere in a house or in attic
- From air handler installed in a house or in attic
There are several reasons for running ductwork in attic:
- House is very big and spacious and there are not enough walls for all heat runs and cold air returns
- It is an existing house
- It is in deep south
Here is how one of the mechanical inspectors writing about this kind of installation:
Ductwork in an attic is normally the largest energy problem in the home or building. The reasons why this practice should be avoided are comfort complaints, heating & cooling losses in the attic ductwork and air handler, higher energy bills, maintenance difficulties and system failures.
In the Southeastern part of Pennsylvania, the average temperature in the winter is approximately 30 degrees and the average temperature in the summer is approximately 90 degrees. The average temperature in a properly ventilated attic should be within 20 degrees of the outside temperature. (50 in the winter – 110 in the summer) The average temperature of the air in the supply ducts in the winter is 120 degrees and 60 degrees in the summer. This is a temperature differential between the attic air and the air inside the ductwork of 70 degrees in the winter and 50 degrees in the summer.
To have a correct thermal envelope would be to have the highest point of the house be the highest R value. Attic insulation of 10+ inches (R 30), would be the best. Also all penetrations into the attic should be sealed and all joints in the ductwork and attic air handler should be sealed. However, most attics are not built with any regard to proper sealing and ventilation or heat and cooling losses.
Most HVAC contractors install cooling systems in the attic based on the theory that cold air falls down. However, the real reason most of them install them in the attic, is that it is a lot easier and less expensive to install the system. There are a number of problems with this type of installation:
ATTIC AIR HANDLER and DUCTWORK PROBLEMS:
- The attic air handler and ductwork system is normally insulated to R-4 and is installed above the attic insulation causing minimal resistance to heat and cooling losses through the ductwork and air handler.
- The size and cost of the system will have to be increased due to the cooling and heating losses through the thinner attic ductwork insulation. A larger system will be required to compensate for this inefficiency.
- The systems will have to run longer to make up for the losses in the attic ductwork.
- There is also inefficiency, due to the fact that the cooling system is generating cooling in the hottest part of the house and the heating system is generating heat in the coldest part of the house in the winter.
- On a typical attic installation the temperature differential from the attic air handler unit to the supply register on the other side of the house can be as much as 8 degrees. This amount of cooling loss cannot be made up with just over sizing the system.
- Additionally, most attic access doors/hatches are not insulated which will allow more winter heat to escape up into the attic.
- The attic heat in the summer time will also migrate down into the house through the ducts and attic access, making the system have to run longer to cool the warmer air.
- This design is very wasteful in the winter. The house heat will rise up by stack effect into the supply and return ducts in the winter, making the 10 inches of attic insulation mostly ineffective. This air will flow into attic ducts and air handler to be lost through the thinner insulation and leakage points in the ductwork.
- The heating system will have to recycle on and off more often to make up for this stack loss.
- Delivery duct leakage into attics increases the cost of operation because the system has to run longer to make up for the leakage lost to the attic.
- Return duct leaks allow frigid winter air to be introduced into the system, increasing the heating load.
- Return duct leakage in the summer pulls very hot humid air into the system, increasing cooling and dehumidification loads.
MAINTENANCE DIFFICULTIES:
In many attics the air handler system is installed in the corner, sometimes behind the supply and return ductwork. The only way to get to the filter for regular cleaning or replacement (monthly) is to climb over the ductwork, sometimes damaging the ducts. Most homeowners don’t even go into the attic to change the filter, even if the air handler is easy to access. Because of this hard to access filter, most filters are not changed on a regular basis, causing the filter and cooling coil to become dirty and clogged, reducing the efficiency and air circulation of the already inefficient system. The house will have increased operational costs and the furthest rooms from the air handler will not be cooled or heated efficiently or adequately.
SOLUTIONS
- Keep the air handler unit and ductwork inside the thermal envelope.
- Installing the air handler in a first or second floor closet with proper insulation and access for service.
- Install a proper ductwork distribution system.
- Install the full supply ductwork system below the upper floor ceiling, boxed into a soffit that would run down the center of the hallway of the house.
- All ductwork joints should be sealed.
- All attic ductwork should be sealed and double insulated to reduce heat and cooling losses into the attic.
- An air handler that must be installed in the attic should be installed inside an attic room that has insulated stud walls and ceiling, as well as sheathing on the exterior of the room.
- Also, install an exterior grade door for access and to maintain a proper thermal envelope.
- All ductwork penetrations into the room should be sealed.
Most homeowners, many HVAC contractors and architects are not aware of the problems associated with attic ductwork installations. With the above information we hope to provoke some alternative designs.
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56. Air Handler Installation*# – the chapter describes an air handler installation in the attic. The chapter has 10 pictures; 3  pages. $1.86 (Instant Access).
*Register Installation Quiz.
PDF files:
Air handler Installation Manual.
Proper Subcooling Charging Techniques.
#From code:
M1305.1.3 Appliances in attics.
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Sharp Bends in Air Conditioning or Heating Duct Work Reduce Airflow
 Sharp bends in ductwork restrict airflow (and violate ASHRAE or SMACNA guidelines for duct installations). Restrictions in airflow through duct systems increase the heating or cooling system operating cost and reduce the comfort of building occupants.
This photograph shows flex-duct in an attic making a too-tight 180 degree turn, crimping and restricting airflow in the duct system.
Excessive or Unnecessary Duct Lengths Increase Cooling or Heating Cost
 Excessive length of ducts is often found where flex-duct is installed by an amateur. If you see a length of flex-duct snaking across an area with multiple unnecessary twists and turns, the combination of length and unnecessary bends reduces airflow, with the costs just cited above.
This photograph shows unnecessary lengths of small-diameter flex duct left by the installer. The small diameter of these ducts also tells us that we're looking at a high-velocity air conditioning system that uses a combination of small-diameter ducts and higher air velocity to deliver cooling air to the conditioned space. Flex duct runs should be as short and direct as possible without at the same time causing inappropriately sharp bends or kinks.
Pinched or Crimped Duct
 Other common duct routing errors include sharp bends in duct work, mismatched sizes of duct work among sections, flex duct which has become crimped or pinched to restrict air flow such as in the photo at left, and of course ducts which have become disconnected.
GOODMAN GRAY FLEXDUCT - Deterioration in Hot Locations
 
This Goodman gray flex duct or "flex duct" air conditioning duct work material is a defective product. In hot attic spaces or where exposed to UV light, the plastic of this flexible air conditioning duct material disintegrates leaving its fiberglass insulation exposed to also disintegrate, leak, or possibly blow into the building living space. Replacement of the duct work is required - a significant expense.
Where this duct is found in a building it should be replaced.
OWENS CORNING FLEX DUCT - Deterioration
 This Owens Corning gray plastic-covered, fiberglass insulated duct material is also a defective product. As with the Goodman flex-duct problem described at GOODMAN GRAY FLEXDUCT, in hot attic spaces or where exposed to UV light, the plastic of this Owens Corning flexible air conditioning duct material disintegrates leaving its fiberglass insulation exposed to also disintegrate, leak, or possibly blow into the building living space. Replacement of the duct work is required - a significant expense. Notice that not all Owens Corning flex-duct products will fail in this manner and unless you specifically find evidence of this deterioration, replacement of the flex-duct in a building may not be warranted. Where this duct is found in a building it should be replaced.
Flexible Duct Performance & Installation Standards PDF
Proper Attic Insulation & Air-sealing PDF
Building with Ducts in Conditioned Spaces PDF
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Attic Ductwork Installation
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