Blog

Mastic can be brushed on flexible duct124. Mastic is a versatile air duct sealant that can be applied using various methods, including brushing, to seal joints and seams in flexible air ducts24. When applying mastic to flexible duct:

1. Ensure the surface is clean and dry before application7.
2. Use a brush to apply the mastic directly on the duct, not on the insulation or sleeve7.
3. For gaps larger than 1/4 inch, use fiberglass mesh tape in combination with mastic for a more secure seal7.
4. Allow the mastic to dry completely after application5.

Mastic sealant is preferred over duct tape for sealing flexible ducts because it creates a more durable, flexible, and long-lasting seal that can withstand temperature changes and vibrations4. When applied correctly, mastic sealant can last up to 20 years or longer, significantly improving HVAC system efficiency and reducing maintenance requirements4.

According to North Carolina building codes, there are specific requirements for constructing duct collars when building a large ductboard supply duct into the furnace:

1. All connections, including the duct collar, must be sealed with water-based duct mastic1. This is critical for meeting the new North Carolina Energy Conservation Code (NCECC) requirements.
2. Tape can be used during installation, but should not be relied upon as the primary method of air sealing ductwork1. Mastic should be applied over the tape to ensure airtightness.
3. The duct collar should be properly sized and well-sealed where it meets the air handler1. This is particularly important due to the large size of plenums, as poor connections at the air handler can lead to significant leaks.
4. When connecting the ductboard to metal components like the furnace, a metal collar or sleeve should be used to ensure a secure and airtight connection4.
5. The duct collar must be constructed to meet the pressure class requirements of the system4. Each size in a pressure class has minimum specifications for joints and reinforcements.
6. Proper support for the duct collar and adjacent ductwork is required. Metal straps with a minimum width of 1 inch (25 mm) and equivalent to or heavier gauge than the duct material should be used for support8.
7. The duct collar should be designed to allow for proper airflow and minimize pressure drop in the system4.

Remember to consult the specific North Carolina Mechanical Code and SMACNA guidelines for detailed requirements and best practices when constructing duct collars and connections.

A duct leakage tester works by pressurizing the duct system and measuring the airflow required to maintain that pressure, which indicates the amount of leakage. Here’s how the process typically works:

1. Preparation: All supply and return registers are sealed off to isolate the duct system12.
2. Setup: A calibrated fan (duct tester) is connected to the duct system, usually at the return side3.
3. Pressurization: The fan pressurizes the duct system to a standard pressure of 25 Pascals, which approximates typical operating conditions1.
4. Measurement: A manometer measures the duct pressure (typically on Channel A) and the fan pressure (on Channel B)3.
5. Calculation: The airflow through the fan at the test pressure is measured, quantifying the duct leakage. This is typically expressed in cubic feet per minute (CFM)1.
6. Analysis: The leakage rate is often calculated as CFM per 100 square feet of conditioned floor area. For example, a leakage of 4 CFM per 100 square feet is commonly referred to as a 4% leakage rate5.

The test can be performed as a “total duct leakage test” or a “leakage to outside test,” depending on whether the goal is to measure all leaks or only those leaking to unconditioned spaces1. This method allows HVAC professionals to accurately assess duct system performance and identify areas needing improvement.

Interpreting the results of a duct leakage test involves understanding the measurements and comparing them to established standards. Here’s how to interpret the results:

1. Measurement Units: Results are typically expressed in CFM25 (Cubic Feet per Minute at 25 Pascals of pressure)35.
2. Leakage Rate: The leakage rate is often calculated as CFM per 100 square feet of conditioned floor area. For example, 4 CFM per 100 square feet is referred to as a 4% leakage rate1.
3. Acceptable Thresholds:
   • For rough-in tests (before drywall installation), total duct leakage should not exceed 3% or 3 CFM per 100 square feet of conditioned floor area1.
   • If the air handler is included in the rough-in test, the acceptable threshold increases to 4% or 4 CFM per 100 square feet1.
   • For post-construction tests, a leakage rate of 4% or less is generally considered acceptable8.
4. Interpreting Results:
   • Lower CFM25 values indicate less leakage and better duct system performance.
   • Higher values suggest more significant leaks that may require attention.
5. Compliance with Building Codes:
   • Check local building codes for specific requirements. Many jurisdictions have adopted the 2018 or 2021 International Energy Conservation Code (IECC) standards4.
6. Energy Efficiency Programs:
   • Programs like ENERGY STAR may have their own criteria for acceptable duct leakage rates3.

If the test results exceed the acceptable thresholds, it indicates that the duct system needs sealing or repairs to improve its efficiency and performance.

Did somebody say building codes?

Yes, there are specific building codes related to duct leakage. The International Energy Conservation Code (IECC) and various state-specific codes provide requirements for duct leakage testing and acceptable leakage rates. Here are some key points:

2021 IECC Requirements

1. All ductwork must be tested for leakage, even if it remains within the building’s thermal envelope12.
2. For ducts outside the thermal envelope: Leakage must not exceed 4.0 CFM per 100 square feet of conditioned floor area at 25 Pascals2.
3. For ducts inside the thermal envelope: Leakage must not exceed 8.0 CFM per 100 square feet of conditioned floor area at 25 Pascals2.

Earlier IECC Versions (2012-2018)

1. Total leakage limit: 4 CFM per 100 square feet of conditioned floor area at 25 Pascals4.
2. Rough-in test limits: 4 CFM per 100 square feet with air handler installed, 3 CFM without air handler4.

State-Specific Codes

1. North Carolina Energy Conservation Code (2018):
   • Total duct leakage limit: 5 CFM per 100 square feet of conditioned floor area6.
   • High-efficiency option: 4 CFM per 100 square feet of conditioned floor area8.
2. ENERGY STAR Version 3 Rev 11 criteria:
   • Rough-in: ≤ 4 CFM25 per 100 square feet or ≤ 40 CFM25, whichever is greater3.
   • Final: ≤ 8 CFM25 per 100 square feet or ≤ 80 CFM25, whichever is greater3.

It’s important to note that building codes can vary by jurisdiction and are subject to updates. Always check with local authorities for the most current requirements in your area.

HVAC professionals use a variety of specialized tools and measurements to evaluate furnace, air conditioning, and duct systems. Here are some of the essential tools and measurements they employ:

Airflow Measurement Tools

1. Duct Leakage Testers: These instruments measure the airtightness of HVAC ductwork, providing accurate leakage detection and duct performance measurements12.
2. Anemometers: Tools like the Fieldpiece STA2 In-Duct Hot Wire Anemometer measure airflow velocity in cubic feet per minute (CFM)3.
3. Airflow Capture Hoods: These are used for measuring airflow at registers and grilles3.

Temperature and Pressure Tools

1. Dual Temperature Digital Thermometers: Used for conducting Delta T tests, measuring temperatures in return and supply air ducts3.
2. Psychrometer Probes: Devices like the Fieldpiece JL3RH Job Link® System Flex Psychrometer Probes measure various parameters including relative humidity, dry bulb, wet bulb, dew point, and enthalpy3.
3. Manometers: Used for measuring static pressure in ductwork and HVAC systems.

Measurements and Tests

1. Total External Static Pressure (TESP) Test: This test measures the resistance to airflow in the ductwork and HVAC components3.
2. Delta T Test: Measures the temperature difference between return and supply air to assess system performance3.
3. Live CFM Testing: Measures the actual airflow through the system, crucial for assessing efficiency and capacity3.
4. Friction Loss Rate: This measurement helps determine the static pressure of air throughout the ductwork4.

Ductwork Evaluation

1. Duct Sizing Calculations: HVAC professionals use formulas and software to calculate proper duct sizes based on factors like square footage, friction loss rate, and CFM4.
2. Multi-point Airflow Measurements: Following ASHRAE standards, technicians take multiple measurements across the duct cross-section to ensure accurate airflow readings5.

By using these tools and measurements, HVAC professionals can accurately diagnose issues, optimize system performance, and ensure efficient operation of furnace, air conditioning, and duct systems.

Yes, changing to a different MERV (Minimum Efficiency Reporting Value) rated air filter can indeed affect the pressure in an HVAC system. Higher MERV filters generally introduce more resistance to airflow, which can increase static pressure in the system25.

Impact of MERV Ratings on Pressure

• Higher MERV filters typically create more resistance, potentially increasing static pressure2.
• Low-MERV filters (<4) have an average pressure drop of 0.10 inches water column (i.w.c.)2.
• Mid-MERV filters (8) can increase pressure drop to 0.19 i.w.c.2.
• High-MERV filters (11) may further increase pressure drop to 0.32 i.w.c.2.

Considerations When Adjusting Filters

1. Filter Size: Larger filters generally allow more airflow, reducing pressure drop8.
2. Filter Thickness: Deeper pleats or increased pleat numbers can lower pressure drop without changing MERV rating5.
3. System Compatibility: Ensure your HVAC system can handle the pressure drop of higher MERV filters5.
4. Airflow Velocity: Pressure drop varies with air velocity; lower velocity typically means lower pressure drop5.

Alternatives to Manage Pressure

• Modify return ductwork to increase filter surface area5.
• Consider separate air filtration equipment for high filtration needs without impacting HVAC performance5.
• Implement a static pressure reset schedule in newer DDC systems to optimize airflow and energy efficiency9.

When adjusting HVAC pressure using filters, it’s crucial to balance filtration efficiency with system performance to avoid straining the HVAC equipment10.

There could be several reasons why your current thermostat is not using the common wire (C-wire) in the wiring harness to the upstairs split heat AC unit:

1. Your thermostat may not require a C-wire to function. Many older or simpler thermostats, especially non-smart models, can operate without a C-wire as they don’t need constant power3.
2. The C-wire may be present in the wiring bundle but not connected. Sometimes, the C-wire is left unused and tucked away in the wall4.
3. Your thermostat might be battery-powered. Some thermostats are designed to run on batteries alone, eliminating the need for a C-wire connection5.
4. The thermostat could be using alternative power methods. Some smart thermostats can operate without a C-wire by using “power stealing” techniques, drawing small amounts of power from other wires4.
5. Your HVAC system might not have a C-wire terminal. In some older systems, especially those with only heating, a C-wire may not be available2.

To determine the exact reason, you would need to:

1. Check your thermostat’s model and specifications.
2. Inspect the wiring behind the thermostat to see if there’s an unused C-wire.
3. Verify the wiring configuration at your HVAC control board.

The common wire, also known as the C-wire, in furnace and HVAC thermostat wiring serves a specific purpose:

1. It provides a constant 24-volt AC power supply to the thermostat15.
2. The C-wire completes the circuit in the cooling and heating system, allowing the thermostat to receive continuous power and function effectively2.
3. It acts as a return path for electrical current, ensuring the thermostat has a stable power source at all times25.

This constant power supply is particularly important for modern smart thermostats and those with advanced features like Wi-Fi connectivity, touch screens, and programmable settings2. The C-wire eliminates the need for batteries or power-stealing techniques, which can lead to more stable operation and prevent potential system malfunctions24.