Heating and Cooling a Garage Workshop

A garage that is too cold to work in is a garage you stop using. This guide shows how to estimate the heat a workshop needs, compares the common heater types, covers insulation and ventilation, and sets out the safety rules around fuel vapors and carbon monoxide.

How much heat do you need?

Heat loss depends on floor area, ceiling height, insulation, air leaks and the temperature difference between inside and outside. A quick rule of thumb sizes a heater at roughly 20 to 30 BTU per hour per square foot for an insulated garage in a moderate climate and 40 or more for an uninsulated one in a cold climate. A 24 by 24 foot two-car garage is 576 square feet, which suggests about 11,500 to 17,300 BTU per hour if insulated and 23,000 or more if not.

A more careful estimate uses the temperature rise. The Workshop Heating (BTU) Calculator takes the size of the space, the ceiling height, the insulation level and your outside design temperature and returns an estimate you can compare with heater ratings. Treat results as a starting point and round up a little, especially if the door is opened often.

Heater types compared

Type	Strengths	Watch for
Electric (240 V) forced air	No combustion, simple to install, no fumes	Running cost; needs a dedicated circuit
Natural gas or propane unit heater, vented	Strong output, economical to run	Needs venting, gas line and correct clearances
Infrared radiant (tube or panel)	Warms people and objects, good for high ceilings	Positioning; keep flammables away
Wood stove	Cheap fuel for some owners	Sparks and fumes near finishes; clearances
Ductless mini-split	Heats and cools efficiently	Higher upfront cost
Portable kerosene or unvented propane	Cheap to buy	Combustion gases inside; not suitable in most garages

Insulation comes first

A bigger heater only pays the bill for heat that leaks away. Insulate the walls and ceiling, seal the gaps around the door and add weatherstripping. The garage door is usually the weakest point, and an insulated door or a kit can make a real difference. Radiant barriers and spray foam in the rafters also help in a metal building. An attached garage's shared wall must meet fire-separation code, so do not remove or compromise the drywall.

Electric heaters and the circuit

One kilowatt of electric heat gives about 3,412 BTU per hour. A 5 kW heater produces about 17,000 BTU per hour and draws about 21 amps at 240 volts, which needs a 30 amp circuit at least. Check the nameplate and use the Wire Gauge Calculator for the run. See Wiring a Garage for the wiring basics.

Fuel vapors and ignition sources

Garages hold gasoline, solvents, paint thinners and sometimes propane. Their vapors are heavier than air and settle near the floor, where they can ignite from a pilot light, a spark or a hot element. Choose heaters certified for garages, which means those with sealed combustion or hung high enough off the floor, usually 18 inches or more, according to the manufacturer's instructions. Never use an open-flame heater in an area where you spray finishes or work with solvents.

Carbon monoxide

Combustion heaters produce carbon monoxide, a colorless, odorless gas that is deadly. Vented heaters send it outside; unvented ones release it into the room. Running a car in a closed garage does the same, and carbon monoxide can seep into the house through an attached wall. Install a carbon monoxide alarm in the garage and in the adjacent living spaces, check batteries regularly and never run a vehicle or fuel-burning equipment with the door closed.

Cooling and ventilation

Summer heat matters too, especially for painting, curing finishes and working on a hot engine. Ventilation with a high-volume fan and an intake opening on the opposite side can exchange the air in a garage several times an hour. A rule of thumb for fume removal in a paint area is to keep air moving continuously across the work and out through a filter. For real cooling, a mini-split or a portable air conditioner works if the space is insulated. A mini-split provides both heating and cooling, and is a good fit for a serious shop.

Humidity and rust

A cold garage that warms up quickly can sweat, putting moisture on tools and projects. Keep the temperature steady, vent damp air and use a dehumidifier in humid regions. Store bare metal parts with a light oil or a rust inhibitor.

Operating cost

Estimate the cost by multiplying the energy you use by the price. A 5 kW electric heater running continuously for 6 hours uses 30 kWh, which at an illustrative 15 cents per kWh costs $4.50. Gas and propane are often cheaper per BTU, but equipment and installation cost more. Insulation reduces whichever cost you have.

Common mistakes

Buying a heater by size only and ignoring insulation.
Using an unvented combustion heater in a closed space.
Placing a heater where fumes from solvents can reach it.
Forgetting the clearances to combustibles in the manual.
Skipping a carbon monoxide alarm.

Frequently asked questions

How many BTU do I need to heat a two-car garage?

A rough guide is 20 to 30 BTU per hour per square foot for an insulated garage in a moderate climate and 40 or more for an uninsulated one in a cold climate. A 576 square foot garage would need about 11,500 to 17,000 BTU insulated. Use the calculator for a closer figure.

Is it safe to use a propane heater in a garage?

Only if it is designed for the purpose and installed according to the maker's instructions. Unvented heaters release combustion gases, including carbon monoxide, into the room, and open flames near fuel vapors are a fire hazard. Use vented, garage-rated heaters and a carbon monoxide alarm.

How much does it cost to run an electric garage heater?

Multiply the heater's kilowatts by the hours of use to get kilowatt-hours, then multiply by your electricity price. A 5 kW heater running 6 hours uses 30 kWh.

Tools used in this guide

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Figures in this guide are general guidance for planning. Always follow your vehicle, tool and product manufacturers' instructions and local codes, and read our disclaimer.