Heating # Furnaces and combustion Gas and electric furnaces, AFUE, venting, and the combustion safety line. ## What does this topic cover? Gas and electric furnaces, AFUE, venting, and the combustion safety line. This section gathers 8 published questions on the topic. Each carries a safety verdict where one applies, and every regulatory claim traces to DOE, EPA, the IRS or ACCA with the date it was verified. ## 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/). ## 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/). ## 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 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/). ## 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. ## Questions - [how do you troubleshoot a spark ignition HVAC?](https://hvacbuddy.shop/questions/furnaces-and-combustion/how-do-you-troubleshoot-a-spark-ignition-hvac/) - [is it roseburg HVAC companies furnace repair recommendations?](https://hvacbuddy.shop/questions/furnaces-and-combustion/is-it-roseburg-hvac-companies-furnace-repair-recommendations/) - [is HVAC and furnace the same?](https://hvacbuddy.shop/questions/furnaces-and-combustion/is-hvac-and-furnace-the-same/) - [AC furnace unit?](https://hvacbuddy.shop/questions/furnaces-and-combustion/ac-furnace-unit/) - [how much to replace HVAC and furnace?](https://hvacbuddy.shop/questions/furnaces-and-combustion/how-much-to-replace-hvac-and-furnace/) - [is it littleton HVAC furnace repair recommendations?](https://hvacbuddy.shop/questions/furnaces-and-combustion/is-it-littleton-hvac-furnace-repair-recommendations/) - [where can you buy a furnace?](https://hvacbuddy.shop/questions/furnaces-and-combustion/where-can-you-buy-a-furnace/) - [what is HVAC furnace?](https://hvacbuddy.shop/questions/furnaces-and-combustion/what-is-hvac-furnace/) 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.