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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.