Most garages that stay cold all winter are not missing a heater so much as they are missing the right plan for one. Garage furnace installation usually means hanging a compact gas-fired unit heater from the ceiling joists, running a gas line to it, and venting its exhaust safely outdoors. Get the sizing, venting, and clearances right, and a two-car garage can go from 25°F to a workable 55°F in well under an hour.
The short answer for most homeowners: a standard two-car garage with some insulation needs roughly 30,000 to 60,000 BTU, the heater mounts in a corner near the ceiling aimed across the space, and the gas and vent work belongs to a licensed HVAC or gas contractor. Your job is choosing the right unit, prepping the space, and knowing what a proper install looks like so you can hold the contractor to it.
- What Garage Furnace Installation Involves
- Sizing a Garage Unit Heater in BTUs
- Venting Options: Power Vented, Direct Vent, and Separated Combustion
- Gas Line, Electrical, and Thermostat Requirements
- Step-by-Step: How a Proper Install Goes
- Carbon Monoxide and Fire Safety
- Troubleshooting Common Garage Heater Problems
- Garage Heater Installation Cost Ranges
- When to Call a Licensed Professional
- Frequently Asked Questions
Installed cost typically lands between about $1,200 and $3,500 for a ceiling-mounted gas heater in an attached garage, depending on the gas run length, venting path, and permit fees in your area. Detached garages with a long trenched gas line can climb well past that.
What Garage Furnace Installation Involves
A garage furnace installation is different from putting a furnace in a house. Instead of a closet unit feeding ductwork, you are almost always dealing with a self-contained unit heater: a burner, heat exchanger, and fan in one box that blows warm air directly into the room. Many people search for a ceiling mounted gas heater for garage use, and that is exactly this category.
The job breaks into five pieces of work:
- Sizing the heater in BTUs for your garage volume, insulation, and climate.
- Mounting the unit securely to ceiling framing with the manufacturer’s hanger kit or threaded rod.
- Gas supply: extending a properly sized gas line with a shutoff valve, sediment trap, and leak test.
- Venting the combustion gases outdoors through a wall or roof.
- Electrical: a dedicated 120-volt circuit for the fan and controls, plus a thermostat.
Popular compact models from several makers are sold as “garage heaters,” and installing one of these low-profile ceiling units follows the same steps no matter whose name is on the cabinet. Read the specific installation manual before anyone lifts a wrench, because clearances and vent lengths vary between models.
Sizing a Garage Unit Heater in BTUs
Oversizing wastes money and causes short cycling. Undersizing leaves you in a jacket on January mornings. A quick rule of thumb uses garage volume and the temperature rise you want.
Start by multiplying length by width by ceiling height. A 24 by 24 foot garage with 9-foot ceilings is about 5,184 cubic feet. Then estimate the temperature rise: if the coldest design day outside is 10°F and you want 55°F inside, you need a 45°F rise.
A commonly used estimate is cubic feet times temperature rise times a factor, divided by a constant, and the easier path is a sizing table based on typical insulation levels:
| Garage size | Insulated walls and ceiling | Little or no insulation |
|---|---|---|
| One-car (about 300 sq ft) | 20,000-30,000 BTU | 30,000-45,000 BTU |
| Two-car (about 500-600 sq ft) | 30,000-45,000 BTU | 45,000-60,000 BTU |
| Three-car (about 800-900 sq ft) | 45,000-60,000 BTU | 60,000-80,000 BTU |
Northern climates, tall ceilings, and uninsulated overhead doors push you to the high end. If you plan to keep the space at 65°F for a workshop, go up one step. If you only want the chill knocked off for car maintenance, the low end is fine. A contractor can run a proper heat-loss calculation, and it is worth asking for one on detached or poorly sealed buildings.
Venting Options: Power Vented, Direct Vent, and Separated Combustion
Venting is where most DIY garage heater stories go wrong, and it is the single most important safety decision in the whole project. Garages contain gasoline vapors, solvents, and paint fumes, so the combustion design matters.
Power-vented units
A small inducer fan pushes exhaust through a horizontal or vertical vent pipe. These units still draw combustion air from inside the garage. They are common and affordable, but they need adequate combustion air and must be mounted high enough to keep the burner away from floor-level vapors.
Separated-combustion (direct vent) units
These pull combustion air from outdoors through one pipe and exhaust through another, often via a concentric vent. The burner is sealed from the garage air. For attached garages, dusty workshops, or any space where you use solvents, a separated-combustion model is the safer choice and is often required by local code.
Vent routing basics
Keep the vent run within the maker’s maximum length and elbow count, slope horizontal runs as the manual specifies, and terminate the vent away from windows, doors, soffit vents, and the overhead door opening. Use only the vent pipe type the manual lists. Never tie a unit heater into an existing water heater or furnace flue unless the manual and your inspector allow it.
Gas Line, Electrical, and Thermostat Requirements
The gas side is licensed-professional work, full stop. A contractor will size the pipe for the total load on your system and the length of the run, add a manual shutoff within reach of the heater, install a drip leg (sediment trap), and pressure or leak test every joint. Natural gas and propane use different orifices and pressure settings, so confirm the unit is configured for your fuel before it is hung.
Most ceiling units need a 120-volt supply for the fan and ignition. If you do not already have a spare circuit or a nearby junction box, a licensed electrician should add one. Anyone working on existing wiring should turn off the breaker and confirm power is off with a non-contact voltage tester before touching a conductor. Thermostats for these heaters are usually low-voltage (24-volt) controls that switch the unit’s internal relay; line-voltage thermostats are a different product and are not interchangeable.
Mount the thermostat about 5 feet off the floor on an interior wall, away from the heater’s airflow and the overhead door, so it reads the room rather than a draft.
Step-by-Step: How a Proper Install Goes
Even if a pro handles the gas and venting, knowing the sequence lets you prep the space and spot shortcuts.
- Pull a permit. Gas appliance installs almost always require one, plus a final inspection.
- Pick the location. A corner, aimed diagonally across the floor, usually gives the best air throw. Keep the unit away from the path of the overhead door and its opener rail.
- Confirm clearances. Measure from the unit to the ceiling, side walls, and floor per the manual. Many units need the bottom at least 7 to 8 feet above the floor, and parked vehicles must not sit directly under it.
- Hang the heater. Fasten the mounting bracket or threaded rod to solid framing, not drywall or a single thin strap. Two people and a lift or sturdy platform make this far safer.
- Install the vent. Cut the wall or roof penetration, run the approved pipe with the correct slope, seal and flash the termination.
- Run the gas line. The gas contractor connects with a shutoff and sediment trap, then leak tests.
- Wire power and thermostat. With the breaker off and verified dead, connect the supply and low-voltage thermostat wires per the diagram.
- Start up and check. The installer confirms ignition, gas pressure, burner flame, and that the exhaust leaves cleanly outdoors.
Plan on a half day to a full day for an attached garage with a short gas run.
Carbon Monoxide and Fire Safety
Any fuel-burning heater produces carbon monoxide, and garages already carry CO risk from running engines. Put a CO alarm in the garage and on every level of the house, especially near sleeping areas, if the garage is attached. Have the heater and its vent inspected once a year before the heating season.
Keep gasoline cans, spray paint, and solvents stored in a closed metal cabinet far from the heater, and never use aerosol sprays or open solvents while the burner is running. Maintain the listed clearance to combustibles, which means no lumber racks, cardboard boxes, or shelving jammed up against the cabinet. If you ever add accessories like a louver kit or remote sensor, use only listed parts intended for that unit.
Electric wall or ceiling heaters are an alternative when you cannot vent to outdoors, but they need a dedicated 240-volt circuit sized for the heater with headroom, and that is also electrician work.
Troubleshooting Common Garage Heater Problems
- Heater short cycles: Often oversized, or the thermostat sits in the heater’s airstream. Relocate the thermostat first.
- Burner lights, then shuts off: Commonly a dirty flame sensor or blocked vent. Turn off gas and power, and call a technician; the pressure switch is a safety device and should never be bypassed.
- Fan runs but no heat: Check that the gas shutoff is open and the thermostat is calling for heat. Beyond that, it is a service call.
- Rust or corrosion on the heat exchanger: Car wash spray, road salt, and chlorine pool supplies all attack heat exchangers. Stainless models hold up better in harsh garages.
- Uneven heat: Adjust the louvers downward toward the floor, and consider a ceiling fan on low reverse to push stratified warm air down.
If you ever smell gas, leave the garage and house, do not flip switches, and call the gas utility from outside.
Garage Heater Installation Cost Ranges
Costs vary widely by region, but these general ranges help with budgeting:
- Unit heater itself: roughly $400 to $1,500, with separated-combustion and stainless models at the top.
- Labor for hanging, venting, and startup: about $500 to $1,500.
- Gas line extension: a few hundred dollars for a short interior run; $1,000 or more for a trenched line to a detached garage.
- New electrical circuit: commonly $200 to $600.
- Permits and inspection: often $50 to $300 depending on your jurisdiction.
Insulating the garage door and walls before you buy often lets you step down a size, which pays for itself quickly in fuel savings.
When to Call a Licensed Professional
Gas appliances and gas lines are licensed-professional work. That includes the gas connection, pressure setting, fuel conversion, venting, and leak testing, and it typically requires a permit and inspection. A licensed electrician should add any new circuit, and a licensed HVAC contractor can confirm sizing and combustion air. Your part of a successful garage furnace installation is clearing the space, choosing a sensible location, and verifying the finished job passes inspection. If the project involves cutting roof framing or a new opening in a fire-rated wall between the garage and house, a pro should handle that as well.
Frequently Asked Questions
Can I install a garage unit heater myself?
You can plan the location, prep the space, and help with mounting, but the gas line, venting, and startup should be done by a licensed contractor with a permit. Mistakes in these areas cause carbon monoxide and fire hazards.
How many BTUs do I need to heat a two-car garage?
An insulated two-car garage usually needs about 30,000 to 45,000 BTU. Uninsulated garages or cold northern climates may need 45,000 to 60,000 BTU.
How high should a ceiling mounted gas heater be hung?
Follow the manual, but many units need the bottom mounted at least 7 to 8 feet above the floor, with required clearances to the ceiling and walls and nothing parked directly below.
Do garage heaters need to be vented outside?
Yes, vented gas unit heaters must exhaust outdoors through approved pipe. Separated-combustion models also draw fresh air from outside, which is safer in garages with fuel or solvent vapors.
How much does garage furnace installation cost?
Most attached-garage installs run about $1,200 to $3,500 including the heater, labor, gas line, and permit. Long gas runs to detached garages add significant cost.
