Cem 350 hvac ductwork ii 11 2015

HVAC Ductwork Issues IICEM 350

Facility Systems DesignNeil OpferInstructor

UNLV CEM Program

Monthly Heating Costs vs. HVAC Duct Leakage Percent

Monthly Cooling/ Heating Costs

10% 15% 20%

$125 $12.50 $18.75 $25.00

$175 $17.50 $26.25 $35.00

$225 $22.50 $33.75 $45.00

$275 $27.50 $41.25 $55.00

$325 $32.50 $48.75 $65.00

HVAC Ductwork Problems

The owner converted the original duct work system to make it run over the attic to distribute air. It’s nothing wrong to run air duct over attic, but difficult to do it right. When air ducts penetrate interior (warm - room temperature) and attic (cold - exterior temperature), condensation is very easy cause problem in ductwork and ceiling penetrations, especially in winter cold climate days. To eliminate the condensation problem, it has to rely on the insulation around the air duct (air tight seal to ductwork and the vertical penetration between interior and attic). However, an air tight seal is very difficult to achieve or overlooked during construction.

HVAC Maintenance Issues - 1

MAINTENANCE DIFFICULTIES: - 1 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.

HVAC Maintenance Issues - 2

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.

HVAC Maintenance Issues - 3

SOLUTIONS - 11.Keep the air handler unit and ductwork

inside the thermal envelope. 2.Installing the air handler in a first or second

floor closet with proper insulation and access for service.

3.Install a proper ductwork distribution system.

4.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.

HVAC Maintenance Issues - 4

SOLUTIONS - 21. All ductwork joints should be sealed. 2. All attic ductwork should be sealed and

double insulated to reduce heat and cooling losses into the attic.

3. 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.

4. Also, install an exterior grade door for access and to maintain a proper thermal envelope

HVAC Attic Ductwork - 1

Ductwork in the attic runs from two sources: 1. From the furnace installed anywhere in the house or in the attic. 2. From the air handler installed in the house or in the attic.

HVAC Attic Ductwork - 2

There are several reasons for running ductwork in the attic: 1. House is very big and spacious and there not enough walls for all heat runs and cold air returns. 2. It is an existing house. 3. High level of incompetence.

HVAC Attic Ductwork - 3

Here is how one mechanical inspector described this kind of problematic 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.”

HVAC Attic Ductwork - 4

 SYSTEM DESIGN – 1 – Winter Climate In a winter climate, average winter

temperature is approx. 30 degrees and the average summer temperature is approximately 90 degrees. Average temperature in a properly ventilated attic should be within 20 degrees of the outside temperature. (50 in the winter – 110 in the summer)

HVAC Attic Ductwork - 5

 SYSTEM DESIGN – 2 – Winter Climate 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.

HVAC Attic Ductwork - 6

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

HVAC Attic Ductwork - 7

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.

HVAC Attic Ductwork - 8

1. 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.

2. 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.

3. The systems will have to run longer to make up for the losses in the attic ductwork.

HVAC Attic Ductwork - 9

1. 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.

2. 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.

HVAC Attic Ductwork - 101. Additionally, most attic access

doors/hatches are not insulated which will allow more winter heat to escape up into the attic.

2. 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.

1. 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.

2. The heating system will have to recycle on and off more often to make up for this stack loss.

HVAC Attic Ductwork - 11

HVAC Attic Ductwork - 12

1. 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.

2. Return duct leaks allow frigid winter air to be introduced into the system, increasing the heating load.

3. Return duct leakage in the summer pulls very hot humid air into the system, increasing cooling and dehumidification loads.