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Furnace Runs but the House Never Warms Up: Sealed Return Leak in Las Vegas

If your furnace runs but the house never warms up, the system is making heat that never reaches the rooms. A clogged filter, leaky or disconnected ducts, or a gap that lets heated air slip straight back into the return are common causes. That’s what we found on November 24, 2025, in a Las Vegas home in the 89117 ZIP. The return and supply sides of the rooftop unit were open to each other, so heated air looped right back into the unit and overheated it. After we sealed the gap, the thermostat climbed 6 degrees before we left.

What We Found

The tenant told us the thermostat never seemed to reach its setting, in heat or in cool. The system is a rooftop package unit on a tile roof, about 2 to 3 years old. We confirmed the problem at the thermostat, then went up to the unit. Return air temperatures were abnormally hot. Where the unit meets the ductwork, the return and supply sides had a gap between them, so air leaving the unit was pulled right back in. That made the system overheat quickly and kept it from ever heating the house properly.

How We Fixed It

Sealed the gap between the return and supply sides with mastic tape and mastic sealant for an airtight seal.
Ran the system and checked temperatures.
Adjusted the blower fan speed and the gas pressure.
Turned off the thermostat’s schedule at the homeowner’s and tenant’s request.

How it turned out

The system reached proper operating temperatures, and the thermostat reading rose 6 degrees by the end of the visit.

Why does my furnace run but the house stays cold?

The furnace is heating air, but too little of it is getting to the rooms. Check the filter first. After that, the usual causes are leaky or disconnected ducts in the attic, a leak on the return side, low gas pressure, or a furnace that overheats and shuts its burners off early. Our ductwork troubleshooting page explains how duct problems steal heat.

Can a return leak make a furnace overheat?

Yes. When heated air leaks back into the return, the furnace heats the same air over and over. The temperature inside the unit climbs fast, and the high-limit switch can shut the burners off before the house is warm. The furnace runs longer, the house stays cool, and parts work harder than they should.

Will the same leak hurt cooling in summer?

Yes. The tenant noticed the thermostat falling short in cool mode too. Cooled air that loops back into the return never reaches the rooms, which is one reason an AC isn’t cooling well even when everything seems to run.

Expert insight

“If the furnace runs and the house never reaches the setting, check where the air is going. On this rooftop unit a gap let air cross between the return and supply sides, so warm air never reached the rooms. We sealed it with mastic and the thermostat reading came up during the visit.”

J.C. PauliAir Duct Cleaning Manager

Safety note: HVAC repairs involve high voltage, refrigerant and, on gas furnaces, combustion gases. Leave them to a licensed technician.

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