Cooling # what causes HVAC to freeze? A frozen coil almost always means one of two things: not enough airflow, or a low refrigerant charge. Start by checking the filter, which is safe and free. Switch cooling off and run the fan alone to thaw it. If it freezes again, the likely cause is a refrigerant leak, and that requires a certified technician. ## Why does this matter? Ice on the indoor coil or the outdoor unit — airflow starvation versus low refrigerant charge. ## Why does a coil freeze? A coil freezes when its temperature drops below freezing while there is moisture in the air around it. There are only two ways to get there, and telling them apart saves money. **Not enough airflow.** The coil needs a steady flow of warm house air to stay above freezing. If the filter is blocked, registers are closed, the blower is weak, or the ducts are undersized, that air does not arrive and the coil chills until it ices over. This is the most common cause and the cheapest. **Low refrigerant charge.** Less refrigerant means lower pressure, and lower pressure means a lower evaporating temperature. The coil freezes even with correct airflow. And because refrigerant is not consumed, low charge means a leak. The practical test: switch cooling off, run the fan alone to thaw it, change the filter, and restart. If it freezes again with a clean filter, the second cause is the likely one. ## 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/). ## What should a bid show you? Three bids rarely quote the same job, so comparing headline price compares three different things. The one that was done properly shows its work, and you can check it line by line: - Model numbers for the outdoor unit, indoor coil, and air handler or furnace. - The **AHRI reference number** for that exact combination. A rating applies to a matched system, not to an outdoor unit on its own. - The SEER2 and, on a heat pump, the HSPF2 of that combination. - The Manual J result the sizing came from. - The measured static pressure and any duct work included. - The permit, and who pulls it. - Labour warranty, separate from parts, and whether registration is required. [The full red-flags checklist](https://hvacbuddy.shop/hiring/). ## What should you know about refrigerant? Refrigerant circulates in a sealed loop. It is not consumed the way fuel or oil is, so a system that is low on charge **has a leak**. Adding refrigerant without finding and repairing that leak treats the symptom, vents the new charge over the following weeks, and costs you the same money again next season. It is also regulated. Only Section 608 certified technicians may purchase refrigerants intended for use with stationary refrigeration and air-conditioning equipment. The restriction covers ozone-depleting substances and non-ozone-depleting substitutes — including HFCs — in cylinders, cans or drums. Wholesalers must record the purchaser's name, the date of sale and the quantity. There is one exception that confuses people: small cans of refrigerant for *vehicle* air conditioning are still sold without certification. That exception is for cars and does not extend to the equipment in your house. For new equipment, the 700 GWP limit in force since January 1, 2025 is why current systems ship with R-454B (GWP 465) or R-32 (GWP 675) rather than R-410A (GWP 2088). R-410A inventory built before 2025 can still be legally installed — EPA removed the installation deadline on May 21, 2026. [The full timeline](https://hvacbuddy.shop/refrigerants/). ## 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. ## Related questions - [what causes HVAC to freeze up?](https://hvacbuddy.shop/questions/frozen-coil-and-ice/what-causes-hvac-to-freeze-up/) - [how to keep HVAC from freezing in winter?](https://hvacbuddy.shop/questions/frozen-coil-and-ice/how-to-keep-hvac-from-freezing-in-winter/) - [how to tell if HVAC is frozen?](https://hvacbuddy.shop/questions/frozen-coil-and-ice/how-to-tell-if-hvac-is-frozen/) - [how to protect HVAC from freezing rain?](https://hvacbuddy.shop/questions/frozen-coil-and-ice/how-to-protect-hvac-from-freezing-rain/) - [should i cover my HVAC during an ice storm?](https://hvacbuddy.shop/questions/frozen-coil-and-ice/should-i-cover-my-hvac-during-an-ice-storm/) - [why is my HVAC freezing up?](https://hvacbuddy.shop/questions/frozen-coil-and-ice/why-is-my-hvac-freezing-up/) - [How correct equipment size is decided](https://hvacbuddy.shop/sizing/manual-j/) - [What SEER2, EER2 and HSPF2 mean](https://hvacbuddy.shop/efficiency/) - [Refrigerants and the phasedown](https://hvacbuddy.shop/refrigerants/) - [Incentives: what expired and what remains](https://hvacbuddy.shop/incentives/) Sources (2) - 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) - Air Conditioning Contractors of America — [Manual J Residential Load Calculation](https://www.acca.org/standards/technical-manuals/manual-j) (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.