Effective 10 November 2026.
Heat and Air Handbook gives general information only. It is not professional advice, and it is not a substitute for a qualified technician who has seen your system.
If you smell gas: leave the house now. Don’t switch lights or appliances on or off, don’t use a phone inside, and don’t try to find the leak. From outside or a neighbour’s home, call 911 or your gas utility’s emergency line (PHMSA). If a carbon monoxide alarm sounds: get everyone outside into fresh air and call 911 or the fire department from outside (CPSC).
General information only
Everything on Heat and Air Handbook (heatandairhandbook.com) is general information, published to help readers understand the central air conditioners and gas furnaces found in US homes. It is not professional advice, and it does not take your equipment, your home, your venting, your gas piping, your wiring or your local rules into account. Your system’s maker, your manual and a qualified technician decide what’s safe for your equipment, and local codes decide what needs a permit or a licensed installer.
Not a contractor, maker or parts seller
Heat and Air Handbook is not an HVAC contractor, a technician, an equipment maker, a parts seller, a retailer or a utility. It does not install, service or inspect equipment, sell parts or equipment, or refer anyone to a contractor, technician, gas fitter or electrician. We never diagnose a reader’s system, read a code for a reader or review a quote, by email or any other way.
No view on your system
Nothing on this site says that your furnace or air conditioner is safe, what is wrong with it, that it is fine to keep running, that a repair is within your skills, that an installation meets your local code, or whether you should repair or replace it. Those are decisions for you, with a qualified technician who has seen the equipment, and some are decided by your building department.
Steps and your own risk
Gas, flame, carbon monoxide, line voltage, 240-volt outdoor units and capacitors that can hold a charge after the power is off can all cause serious injury, death, fire or damage. Where a page describes a step, it names the maker’s document the step comes from, and where the maker says a job is for a qualified technician or service agency, the page says so. Goodman, Lennox and Trane tell owners not to service the unit themselves, Carrier says work other than filter care is for trained service personnel, and Trane’s owner guide says do-it-yourself repairs on an in-warranty unit may void the warranty. If you are not trained, not comfortable, or the problem comes back, stop and call a qualified technician. Never bypass a safety switch or sensor. Never relight or run a furnace that has been under water. You are responsible for any work you choose to do.
Makers and models differ
Where a page describes what a maker says, it names the document it read, by number and edition. Your unit may be a different model, use a different control board or come with a different edition of the manual, and what one maker says is not a rule for another. Furnace codes in particular depend on the board: the label inside your furnace door, and the manual for your model, decide. Where we have not read a maker’s document, the page says “not confirmed”.
Local codes, permits and licensing
Building, mechanical, fuel gas and electrical codes are adopted by states and local governments, often with their own changes. Where a page cites the International Residential Code or another model code, it is citing the model code in a named edition, not the law where you live. Whether work needs a permit, an inspection, a licensed gas fitter, a licensed electrician or a licensed HVAC contractor is set by your state and local government, and some states require a licence for gas work. Your building department decides. Federal law requires anyone who opens a refrigerant circuit to be certified under Section 608 of the Clean Air Act, and there is no homeowner exemption.
Rules and figures change
Federal efficiency standards and refrigerant rules are amended, challenged in court and delayed. The 95% furnace minimum for furnaces made from 18 December 2028, for example, was under legal review when we last checked. Makers revise their manuals and control boards. Energy prices change every month. A rule is right only for the equipment and period it applies to. Each rule and figure on this site shows its source and the day we read it, and it may have changed since. Check the current rule with the original source before you rely on it.
Tools show estimates, not decisions
Calculators and lookup tools do arithmetic on what you enter and show the rules, maker documents and assumptions they name. Their results are estimates and information. A code lookup gives a maker’s meaning for a code as printed in the document named; it is not a diagnosis of your furnace. A cooling cost estimate depends on your usage, your price and the ratings you enter; real bills vary with weather, installation and how the system is used. A repair or replace check sets out points to weigh, using your own quotes; it does not say what to do. Defaults that are our assumption are labelled, and you can change them.
Warranties
Work on your equipment, including by you, can affect a maker’s warranty, and some makers tie it to yearly service: Lennox’s installation instructions for its ML180 furnace call for a yearly check by a licensed professional technician to comply with its limited warranty (508547-01, 12/2025). Your warranty document and the maker decide what it covers.
Accuracy
Pages are checked against the sources they cite, but no check catches everything. Check important details with the original source before you rely on them. See also our terms of use.
How our content is made
Every page is fact-checked against its sources before it is published. Pages appear under the Heat and Air Handbook name. Our editorial policy and how we check page explain the process.
Advertising and independence
Heat and Air Handbook is free to read and funded by display advertising. Advertisers have no say over what we publish, and an ad on a page, including an ad for an equipment maker, an HVAC contractor or a parts seller, is not an endorsement or a referral. We use no affiliate links, accept no paid placements, sell no leads, make no contractor referrals and publish no product rankings. Names of makers, agencies, code publishers and other organizations are used only to identify them, and we are not affiliated with them.
External links
We link to regulations, agencies, makers’ documents and other websites so you can check what we say. We do not control those sites and are not responsible for their content.
Questions and corrections
If you think something is wrong, email contact@heatandairhandbook.com with the page address and a source, and we will look at it. We can’t diagnose a system, read codes for a reader or recommend contractors. See also our corrections page.