Refrigerator Not Cooling but Freezer Works: Diagnostic Tree
Freezer cold, fridge warm: a step-by-step diagnostic tree in the order a tech checks it.
Read the guideMobile Home Furnaces: Downflow, Ratings, and Why House Furnaces Do Not Fit
Mobile home furnaces are downflow units — the blower pushes heated air down into a duct system that runs under the floor. They are also built for the higher static pressure of small, tightly run mobile home ductwork, and they must be manufactured-home rated. The names you will see on that data plate are Coleman, Miller, Revolv, and Intertherm. A house upflow furnace does not swap in. Wrong airflow direction, wrong rating, and an inspection and insurance problem before it ever lights.
Downflow is not a preference
In a site-built house, the furnace usually sits in a basement or a closet and pushes air up into ducts overhead, or horizontally through a plenum. In a mobile home the ducts are in the belly, under the floor, and the furnace sits above them. The blower has to push down. Put an upflow unit in that closet and the supply air has nowhere to go but back into the return, or out of the closet door.
You can sometimes find a downflow conversion kit for a house furnace. That still does not make it a manufactured-home furnace, and it does not fix the rating.
The rating and the label
Just like the water heater, the furnace is part of the HUD-code package. The data plate has to show it is listed for manufactured housing. That listing covers the clearances to combustible material, the venting arrangement, and the airflow characteristics the home was designed around.
An inspector reads the plate. So does an insurance adjuster after a fire. A house furnace in a mobile home closet is one of those findings that gets a policy cancelled or a claim questioned, and the homeowner usually finds out at the worst possible moment.
Why the duct system matters to the furnace choice
Mobile home ductwork is small, long, and often restrictive. The furnace is designed to move its rated airflow against that higher static pressure. A house furnace rated for a low-static, generously sized duct system will run at the wrong airflow in a mobile home: the limit switch trips, the blower runs hot, the temperature rise climbs out of spec, and the heat exchanger takes the beating.
That is why the furnace and the ductwork have to be looked at as one system. It is also why so many “furnace problems” in these homes turn out to be a crushed crossover duct.
Roof jack venting
Mobile home gas furnaces vent through a roof jack — a combined vent and combustion air assembly that goes straight up through the roof. The jack, the flashing, and the vent connector have to be intact, correctly pitched, and matched to the furnace. A rusted-out jack, a sagging vent connector, or a roofer who stepped on the assembly is a venting problem, and venting problems are carbon monoxide problems.
2026 price ranges
| Item | Typical 2026 range |
|---|---|
| Manufactured-home rated downflow furnace, unit only | $1,200–$2,500 |
| Hired installation | $800–$1,500 |
| Roof jack replacement, with install | $300–$700 |
| Blower motor, part | $120–$350 |
| Hot surface igniter, part | $25–$70 |
| Flame sensor, part | $15–$45 |
| Control board, part | $150–$450 |
| Service call and diagnostic | $100–$250 |
Repairs that mirror a house furnace
Most of the guts are the same as any gas furnace. The igniter cracks and the burner never lights. The flame sensor coats over and the burner lights for a few seconds and drops out. The blower motor gets dry bearings and starts squealing, or the run capacitor dies and the blower will not come up to speed. The control board fails and the sequence stops halfway through. None of that is mobile-home-specific, and the troubleshooting order is the same: call for heat, watch the sequence, find the step where it quits.
Symptoms that point at the ducts instead
- Weak airflow at the far registers while the near ones are strong.
- One half of a double-wide never gets warm or never gets cool, while the other half is fine.
- The furnace runs, the burner stays lit, but the house never reaches setpoint.
- The limit switch trips repeatedly and the blower runs long after the burner shuts off.
Those are duct symptoms. Before you replace anything in the furnace, check the crossover duct and the belly. That is a separate page in this guide, and it is usually the cheaper answer.
STOP: If you smell gas, get out and call the gas company. Do not troubleshoot it, do not test for leaks with a flame, do not flip the furnace switch on the way out.
STOP: If you find a cracked heat exchanger, a rusted-through flue, or a rusted roof jack, stop using the furnace and call a pro. That is a carbon monoxide path into the home, and it is not a homeowner repair.
STOP: If the furnace has a scorched board, burnt wiring, or a breaker that trips when the blower starts, leave the power off. Line-voltage and control wiring faults are pro territory.
Calculators and guides give ranges and rules of thumb from field experience — they are not a diagnosis of your system. When a guide says stop, stop: gas, refrigerant, and mains electrical work belong to licensed pros. Get two or three written quotes before signing anything.
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