Heating and cooling # Indoor air quality Your heating and cooling system affects indoor air quality through filtration, humidity control and ventilation, but it cannot fix every air problem. The most important thing to understand is that a filter too restrictive for your system reduces airflow, which hurts capacity and can damage equipment. ## Why can't you just fit the highest MERV filter available? Because a filter does two things at once: it captures particles, and it resists airflow. A higher MERV captures more and resists more. Every duct system has a design static pressure it can work against, and a filter that pushes past it starves the system of airflow. Starved airflow reduces capacity, can freeze an evaporator coil in cooling, and makes the blower work harder for less result. Filter marketing almost never mentions this, because the tradeoff is inconvenient to the sale. The right answer is the highest MERV your system's measured static pressure can actually support — which is a question about your ducts, not about the filter aisle. ## What can the system genuinely do about air quality? - **Filtration** removes particles from the air that passes through it — and only from the air that passes through it, when the blower is running. - **Dehumidification** happens as a by-product of cooling, which is why an oversized system leaves a house cold and clammy: it does not run long enough to wring moisture out. - **Ventilation** brings in outdoor air deliberately. Most older houses do this accidentally, through leaks, which is neither controlled nor efficient. ## Explore ### [Indoor air quality](https://hvacbuddy.shop/questions/indoor-air-quality/) What your system can and cannot do about dust, allergens, and stale air. ### [Filters and MERV](https://hvacbuddy.shop/questions/filters-and-merv/) MERV ratings, change intervals, and the airflow tradeoff nobody explains. ### [Humidity control](https://hvacbuddy.shop/questions/humidity-control/) Why an oversized system leaves the house clammy, and what to do about it. ### [Ventilation, ERV and HRV](https://hvacbuddy.shop/questions/ventilation-and-erv/) Bringing in outside air on purpose, and recovering the energy while you do it. ## Which efficiency numbers can you actually compare? If you are comparing efficiency figures, check which generation they belong to first. On January 1, 2023 DOE changed the test procedure and renamed three metrics: SEER became **SEER2**, EER became **EER2**, and HSPF became **HSPF2**. The current procedure measures at a higher external static pressure, much closer to what equipment meets once installed in a real house on real ductwork. DOE is explicit about the consequence: "Although SEER2, HSPF2, and EER2 ratings are similar to ratings using the original metrics, they are not directly comparable. Compliance with the new standards for each model must be determined based on its measured SEER2, HSPF2, or EER2." In practice: a SEER figure from a 2019 article cannot tell you whether a quote reading SEER2 is good, bad or identical. That is why we publish no conversion factor. We do not publish a SEER-to-SEER2 conversion factor. DOE states that ratings on the new metrics are "not directly comparable" to ratings on the original metrics, and that compliance must be determined from the measured SEER2, EER2 or HSPF2 value. Approximate multipliers circulate widely, but they vary by equipment type and configuration, and using one to compare an old article's number against a number on today's quote is how homeowners end up believing they were offered less efficiency than they were. Compare like with like: ask for the SEER2 figure, and check it against the AHRI certificate. The nationwide minimum for a split-system air conditioner is 13.4 SEER2, and for a split-system heat pump 14.3 SEER2 with 7.5 HSPF2. [What each metric measures](https://hvacbuddy.shop/efficiency/) · [minimums by region and state](https://hvacbuddy.shop/efficiency/standards/). ## What actually drives the cost? We publish no national average price, and the reason matters: an average across every house, region and system type is not a number you can act on. A replacement that includes duct modification, an electrical upgrade and a permit is a different job from a straight swap, and both read as "system replacement" on a one-page proposal. What actually drives your number: - **Equipment capacity and tier**, which should come from the Manual J rather than the old nameplate. - **The ducts** — whether they can move the air the new equipment needs, and what it costs if they cannot. - **Electrical work**, if a circuit or panel change is required. - **Permit and inspection**, usually required. - **Refrigerant** — current equipment uses R-454B or R-32; remaining pre-2025 R-410A stock may price differently. [What a bid should itemise](https://hvacbuddy.shop/costs/) · [operating cost calculator](https://hvacbuddy.shop/calculators/). ## Why do the ducts matter as much as the equipment? Equipment capacity is rated at a specific airflow. If the ducts cannot move that air, the equipment cannot deliver what its label promises, and the homeowner concludes they were oversold. The number that reveals it is **static pressure**: the resistance the blower works against, measured in inches of water column. It is to a duct system roughly what blood pressure is to a person. A contractor who measures it before quoting a replacement is doing the job; one who does not is assuming your ducts are fine. Duct design has its own ACCA standard, **Manual D**, and it is the step most often skipped on a replacement because the ducts already exist and reusing them costs nothing up front. [What Manual D determines](https://hvacbuddy.shop/ductwork/manual-d/) · [ducts and airflow](https://hvacbuddy.shop/ductwork/). ## How is the right equipment size decided? Nearly every persistent comfort problem traces back to equipment size, and size is decided before the first box arrives. The standard is the ACCA **Manual J** load calculation (ANSI/ACCA 2 Manual J - 2016), which totals the heat gain and loss of your specific house: orientation, window area and type, insulation levels, air infiltration, internal gains, and your local design temperature. What a Manual J is *not*: square feet per ton, and not the old unit's tonnage. The first ignores everything that makes one house different from another. The second assumes whoever sized the old equipment got it right, and propagates their error for another fifteen years. Oversized equipment reaches the setpoint and shuts off before it has dehumidified, so the house ends up cold and clammy at the same time. Every start is also the hardest moment in a compressor's life, so wear accelerates. And you pay for the surplus capacity twice — once to buy it and again to run it. Ask for the completed Manual J in writing, not the claim that one was done. [What it contains and how to read it](https://hvacbuddy.shop/sizing/manual-j/). ## How can you verify any of this yourself? Everything regulatory on this site traces to a primary source and carries the date it was read: DOE for efficiency standards, EPA for refrigerants and Section 608, the IRS for tax credits, and ACCA for load calculation. The sources for this page are listed at the end with the date each was consulted. If you find a different figure elsewhere, check two things before deciding which to believe: whether the page says which generation its efficiency metric belongs to, and whether it carries a visible review date. Most of the outdated content in this category fails both tests. Sources (2) - Air Conditioning Contractors of America — [Technical Manuals — Manual J, Manual S, Manual D](https://www.acca.org/standards/technical-manuals) (accessed 2026-08-17) - U.S. Department of Energy — [Central Air Conditioners — Conservation Standards guidance (Final, October 5, 2022)](https://www1.eere.energy.gov/buildings/appliance_standards/pdfs/2023_CAC_Standards_FAQ_10-5-2022_Final.pdf) (accessed 2026-08-17) By [HVACBuddy Editorial Team](https://hvacbuddy.shop/authors/editorial-team/). Last reviewed: 2026-08-18. HVACBuddy publishes independent information. We do not install or service equipment, we do not dispatch contractors, and we do not sell leads. Nothing here is engineering, tax, or legal advice.