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Smell gas? Leave now, then call 911 or your gas utility from outside. CO alarm sounding? Get everyone out and call 911.Gas smell or CO alarm? Get out, then call 911.

Symptoms

Six folders, by what you notice

Sounds4
Smells3
Leaks and ice3
Won't run right5
Lights, alarms and codes4
Comfort and bills3

Photo Credits

Every photograph on Hum Drip Hiss comes from Wikimedia Commons, used under the license its author chose; this page names each one.

Updated

Photographs

The 24 photographs below are shared on Wikimedia Commons under public-domain dedications or Creative Commons licenses. Each was cropped to fit the page layout, and where a maker's name or logo showed on equipment, a meter or an alarm, it was blurred or cropped out so no brand is shown as part of a symptom. No photo shows a real fault in a reader's home; each illustrates the kind of equipment a page describes.

Photographs used on this site, with author and license (checked October 2026)
PhotographAuthorLicenseOriginalEdits
Icicles under an outdoor unitShixart1985CC BY 2.0File pagecropped
Analog hygrometer low humidityJengodCC BY-SA 4.0File pagecropped
Blue gas flame in darkSappPublic domainFile pagecropped
Breaker panel door closedBrokenSphereCC BY-SA 3.0File pageblurred maker names
Ceiling return grilleTeepetersenCC BY-SA 4.0File pagecropped
CO alarm close upThe EnergySmart Academy from Santa Fe, NM, United States of AmericaCC0File pageblurred maker names
Condensation on windowShixart1985CC BY 2.0File pagecropped
Condenser beside brick wallFanFan61618CC BY-SA 2.0File pagecropped
Condenser top fan grilleFamartinCC BY-SA 4.0File pageblurred maker names
Dirty indoor coil finsGerardolagunesCC BY-SA 4.0File pagecropped
Electric meter on sidingFamartinCC BY-SA 4.0File pageblurred maker names
Furnace and coil cabinet in basementFamartinCC BY-SA 4.0File pagecropped
Furnace control board wiringIdahoprogrammerCC BY-SA 4.0File pagecropped
Furnace plenum and ductworkFamartinCC BY-SA 4.0File pagecropped
Gas burner flames in heaterJohanCC BY-SA 3.0File pagecropped
Gas meter and regulatorHomeSpot HQCC BY 2.0File pageblurred maker names
Gas range burner flameLisafernCC0File pagecropped
Indoor humidity monitorShixart1985CC BY 2.0File pagecropped
Outdoor unit in snowfallShixart1985CC BY 2.0File pagecropped
Setting digital thermostatShixart1985CC BY 2.0File pagecropped
Testing a CO alarm on wallU.S. Consumer Product Safety Commission (CPSC)Public domainFile pagecropped
Thermostat base circuit boardDvortygirlCC BY-SA 3.0File pageblurred maker names
Two-story colonial houseFamartinCC BY-SA 4.0File pagecropped
Two-story house in snowFamartinCC BY-SA 4.0File pagecropped

Creative Commons licenses can be read in full at creativecommons.org. CC BY-SA photos remain under the same license after cropping.

Illustrations and charts

The drawings marked "Illustration" and the chart-style infographics were made for this site with an image-generation tool and then checked by eye. Every label and number on an infographic repeats a figure from the page it sits on, with that page's sources. Drawings show how equipment is laid out in general; they are not plans or repair instructions.

Report a problem

If you hold the rights to a photograph here and the credit is wrong, the file page linked in the table shows how the original was published. Credits are re-checked when photos are added or replaced. For how the rest of the site is put together, see about this site, or go back to the symptom list on the front page.