One 5KW diesel heater will keep an insulated two-car garage at about 55°F (13°C) when it is 14°F (-10°C) outside. Take the insulation away and the same garage needs more than three times that heat, which no single diesel air heater delivers.
That gap is behind most of the “it barely takes the chill off” threads from people who bought a diesel heater for a garage. The chart below uses the same heat-loss method vehicle heater installers have used for decades. The rest comes from building diesel heaters since 2003.
Key takeaways:
- Insulated 1-car or 2-car garage: one 5KW heater is enough down to about 14°F (-10°C).
- Shed or small workshop: a 2.5KW heater covers an insulated space up to about 10 × 12 ft.
- Uninsulated garage: 10–13 kW of heat for a 1- or 2-car space. Insulate the door and ceiling first.
- “8KW” labels: many share the housing and fan of 5KW units. Size on real output, not the sticker.
- Running cost: about $0.14–0.47 an hour for a 5KW heater, with diesel at $3.75 a gallon.
- Portable or fixed: a portable plugs into a 110V outlet and moves with you. A fixed heater inside the garage heats more efficiently.
In this guide
How Much Garage One Diesel Heater Can Heat
A 5KW diesel air heater covers about 300–400 sq ft (28–37 m²) of insulated garage. With nothing insulated, the same heater handles barely 100 sq ft (9 m²).
The table shows how much heat each space loses when you hold 55°F (13°C) inside with 14°F (-10°C) outside. We used the standard formula: surface area × insulation factor (K) × temperature difference, plus one air change per hour. The K values for garage construction are our estimates, so read every number as roughly ±30%.
| Space (L × W × ceiling) | Well insulated | Walls only insulated | Uninsulated wood frame | Bare steel building |
|---|---|---|---|---|
| Shed / small workshop 10 × 12 × 8 ft | 1.5 kW 5,000 BTU | 2.8 kW 9,500 BTU | 5.3 kW 18,000 BTU | 7.8 kW 26,500 BTU |
| 1-car garage 12 × 22 × 8 ft | 2.8 kW 9,500 BTU | 5.1 kW 17,500 BTU | 9.7 kW 33,000 BTU | 14.2 kW 48,500 BTU |
| 2-car garage 20 × 20 × 8 ft | 3.8 kW 13,000 BTU | 6.9 kW 23,500 BTU | 13.1 kW 44,500 BTU | 19.2 kW 65,500 BTU |
| Large 2-car garage 24 × 24 × 9 ft | 5.5 kW 19,000 BTU | 9.8 kW 33,500 BTU | 18.4 kW 63,000 BTU | 27.1 kW 92,500 BTU |
| 3-car garage 32 × 24 × 9 ft | 7.1 kW 24,000 BTU | 12.5 kW 42,500 BTU | 23.4 kW 79,500 BTU | 34.2 kW 117,000 BTU |
| Shop building 30 × 40 × 12 ft | 12.1 kW 41,000 BTU | 20.8 kW 71,000 BTU | 38.2 kW 130,500 BTU | 55.7 kW 190,000 BTU |
Heat needed to hold 55°F (13°C) inside at 14°F (-10°C) outside. “Well insulated” means insulated walls, ceiling and garage door.
Colder where you live? At 0°F (-18°C) outside, add about 35% to every number. If you only want 40°F (4°C) to protect the car and stored paint, take about a third off.
Which heater matches your number
- Under 2 kW (7,000 BTU): a 2.5KW heater, either the Apollo-P1 portable or the fixed Apollo-V1.
- 2–4 kW (7,000–13,500 BTU): one 5KW heater, the Apollo-P2 portable or the fixed Apollo-V2. The Apollo-V2 measures 4.7 kW on Boost. We size to 4 kW so the heater isn’t pinned at maximum all day.
- 4–8 kW: two 5KW heaters, or insulate the door and ceiling until your garage drops into the single-heater column.
- Above 8 kW: a diesel air heater is the wrong class of heater. See the last section before you buy anything.
Insulation Matters More Than Square Footage
Insulating a 2-car garage cuts the heat it needs from 13.1 kW to 3.8 kW, a drop of more than 70%. No heater upgrade comes close to that.
The garage door is usually the weakest surface. A bare steel door loses heat roughly ten times faster than an insulated stud wall, and the gaps around it leak air all day. Foam door panels and a new bottom seal cost far less than a second heater.
The ceiling comes next. Warm air collects at the top of the room, and open joists under a cold roof let it straight out.
Why an “8KW” label doesn’t close the gap
Many heaters sold as 8KW use the same housing, burner and fan as 5KW models. Forum owners who have checked theirs report closer to 4–4.5 kW of real heat. If an “8KW” heater is the same physical size as a 5KW heater, plan on 5KW output.
We size our own heaters the same way. The Apollo-V2 carries a 5KW rating and measures 4.7 kW on its top setting, and 4.7 kW is the number behind the table above.
Portable or Fixed Diesel Heater for a Garage or Workshop
If you heat the same garage every winter, a fixed 5KW heater mounted inside it is the more efficient choice, and it costs less. A portable diesel heater for a garage makes sense if you rent, won’t drill the wall, or also need heat at a cabin or campsite.
| Portable (Apollo-P1 / P2) | Fixed (Apollo-V1 / V2) | |
|---|---|---|
| Where the heater sits | Outside the garage. One duct carries warm air in. | Inside the garage on an L-shaped wall bracket. Exhaust and combustion air pipes run out through the wall. |
| Air it heats | Outside air, drawn in around the unit | Garage air, so it reheats air that is already warm |
| Power | 12V DC, or 110V AC from a wall outlet | 12V DC only. Needs a 110V-to-12V power supply or a battery |
| Fuel | 5-litre (1.3 gal) tank built in | Separate tank, bought on its own |
| Drilling | None | Two pipe holes through the wall |
| Moves to cabin or camp | Yes | No |
| Price | Higher | Lower, plus a fuel tank |
The portable’s single duct is the trade-off to understand. Sitting outside, it pushes in outside air that it first has to warm up from 14°F. The same volume of warm garage air leaks back out through the gaps, so part of every kilowatt goes to heating the outdoors.
A fixed heater inside the garage skips that loss. It keeps pulling in 50°F garage air and sending it back warmer. For a fixed install you’ll also need a separate diesel fuel tank, because the Apollo-V2 box doesn’t include one.
Power is where the portable wins. The Apollo-P1 and P2 switch between 12V and 110V with a lever inside the heater, so you plug straight into a garage outlet. A fixed 12V heater needs a power supply that handles the 8A glow-plug draw at start-up, and most owners use a 20–30A unit.
The 5KW Apollo-P2 measures 16.5 × 13.8 × 9.4 in (42 × 35 × 24 cm) with a 5-litre tank on top. In a garage it sits outside and ducts warm air in.
Compare both options on our portable diesel heater and Apollo-V2 5KW pages.
Placement, Exhaust and Fresh Air in a Garage
A diesel air heater burns fuel in a sealed chamber and sends the exhaust out through its own pipe. That protects your garage only if the pipe is routed correctly and the combustion air comes from outside.
- Slope the exhaust down all the way to the outlet. Condensate trapped in a low loop can freeze or push exhaust back. Fit the muffler outside the building.
- Use a proper fitting where the pipe goes through the wall. Near the heater, the exhaust pipe gets hot enough to scorch timber and fiberglass, so never run it bare through studs or insulation. Our stainless exhaust skin fitting takes the 24mm pipe and has an insulating mat between the pipe and the wall.
- Keep the outlet away from anything that breathes. Leave at least 8 in (20 cm) between the exhaust outlet and the combustion intake. Keep both away from doors, windows and the garage door opening.
- Portable heaters stay outside. Set the unit on level ground, exhaust pointing away from the building and downwind. Run the warm-air duct through a wall port or window panel, never through a half-open door. Don’t move it inside and pipe only the exhaust out. Its exhaust runs through a flexible pipe and muffler inside the case, joints you can’t see or check.
- Fixed heaters mount inside, level. The heater can’t be mounted on its side, so bolt an L-shaped mounting bracket to the garage wall and sit the heater on it. The single exhaust joint is right under the heater where you can see it. Fit heavy-duty exhaust clamps there and check it every fall.
A steady downward run lets condensate drain out. A sag in the pipe collects water that can freeze solid overnight.
Our diesel heater installation guide covers the drilling, fuel line and wiring for a fixed install.
Attached garage? Fit a carbon monoxide alarm in the garage and another in the room next to it. For the full exhaust and CO checklist, read our five diesel heater installation precautions.
Running Costs and Power Supply
A 5KW diesel heater costs about $0.14–0.47 an hour to run, based on 0.144–0.475 L/hr of diesel at $3.75 a gallon. A full Saturday in an insulated 2-car garage, mostly on High, comes to about $3.20.
| Setting | Heat output | Diesel use | Cost per hour | Hours per gallon |
|---|---|---|---|---|
| Boost | 4.7 kW / 16,000 BTU | 0.475 L/hr | $0.47 | 8 |
| High | 4.0 kW / 13,600 BTU | 0.403 L/hr | $0.40 | 9.4 |
| Medium | 3.1 kW / 10,700 BTU | 0.315 L/hr | $0.31 | 12 |
| Low | 2.0 kW / 6,800 BTU | 0.201 L/hr | $0.20 | 18.8 |
| Ultra Low | 1.4 kW / 4,900 BTU | 0.144 L/hr | $0.14 | 26 |
Measured figures for the 5KW Apollo-V2 at 12V. Diesel priced at $3.75 per gallon.
Electricity barely registers. Once it’s running, the heater draws 0.77–2.2A at 12V, or 9–26W. The glow plug pulls about 8A for the first minutes of start-up and again during shutdown.
Three habits that keep a garage heater running
- Never cut the power while it’s running. Switch off at the controller and let the 4–5 minute cool-down finish. Pulling the plug on a hot heater can cook its electronics, so don’t wire it through a plug-in outlet timer.
- Don’t leave it on Ultra Low for days on end. Long low-flame runs build carbon in the burner, which leads to diesel heater smoke and failed starts. Give it regular time on High, and follow our diesel heater maintenance guide.
- Keep the voltage up on battery power. Below about 12V under load, starts get unreliable. Our guide to choosing a 12V battery for a diesel heater has the sizing math.
When a Diesel Heater Is the Wrong Tool for Your Shop
If your number in the chart is above about 8 kW (27,000 BTU), a parking-style diesel air heater is the wrong class of equipment. That covers most uninsulated 2-car garages, bare steel buildings and anything near 30 × 40 ft.
Those spaces need 40,000–190,000 BTU. Look at vented indirect-fired shop heaters, ceiling-hung radiant tube heaters, or a forced-air torpedo heater for short bursts with the door cracked for air. We’d rather say that here than sell you a heater that runs flat out while you work in a jacket.
A roll-up door that stays open half the day is the other case. Radiant heat warms you and the bench instead of the air, and it’s back to full effect the moment the door comes down.
Frequently Asked Questions
Are diesel heaters safe to use in a garage?
Yes, when the exhaust leaves through a sealed, downward-sloping pipe and combustion air comes from outside. The fuel burns in a sealed chamber, so only warm air enters the garage. The risk comes from poor installs: leaking exhaust joints, an outlet near a door, or a portable parked inside. Fit a CO alarm, especially in an attached garage.
Can a 5KW diesel heater heat a 2-car garage?
Yes, if the garage is insulated. A fully insulated 20 × 20 ft garage needs about 3.8 kW (13,000 BTU) to hold 55°F (13°C) at 14°F (-10°C) outside. That sits inside a 5KW heater’s 4.7 kW real output. Uninsulated, the same garage needs about 13 kW, and one heater will only take the edge off.
How many square feet will an 8KW diesel heater heat?
Size it like a 5KW heater: about 300–400 sq ft of insulated garage at 14°F (-10°C) outside. Many 8KW-labeled units share their housing, burner and fan with 5KW models and deliver 4–5 kW. A heater that truly put out 8 kW (27,000 BTU) would cover an insulated 3-car garage.
How long will a diesel heater run on a gallon of diesel?
About 8 hours on full power and 26 hours on the lowest setting, for a 5KW heater. Medium gives you about 12 hours. A 2.5KW heater such as the Apollo-V1 burns 0.136–0.252 L/hr, so a gallon lasts 15–28 hours.
Will a diesel heater stop my tools from rusting?
It helps, but it won’t stop rust on its own. A vented diesel heater adds no combustion moisture to the garage. Unvented propane and kerosene torpedo heaters do, and that water vapor condenses on cold cast iron. Cold tools can still sweat when you heat the air fast, so a steady low temperature beats quick blasts. Our diesel vs propane heater comparison goes deeper.
Can I plug a diesel heater into a regular garage outlet?
Only if it has an AC power option. The Apollo-P1 and P2 portables run on 110V AC from a lever-selected power module inside the heater. Fixed 12V heaters need a 110V-to-12V power supply that covers the 8A start-up draw. Most owners pick a 20–30A unit.
Can I leave a diesel heater on overnight to keep the garage above freezing?
Yes, and it’s one of the cheapest jobs a diesel heater does. Holding an insulated 2-car garage at 35°F (2°C) with 14°F (-10°C) outside takes about 2 kW. That’s the Low setting, around 0.2 L/hr, or about $2.40 for a 12-hour night. Have a CO alarm in place, and run it on High now and then to burn off carbon.
Do I need a permit or insurance approval to install a diesel heater in a garage?
It depends on where you live. Local codes may treat a permanently installed heater with a through-wall exhaust as a fuel-burning appliance. Some insurers also ask about fixed heaters in garages and outbuildings. Call your building department and your insurer before you drill.
Ready to Heat Your Garage or Workshop?
We have built diesel heaters in China since 2003, and our Apollo air heaters are CE, RoHS and FCC certified. Choose by how you use the garage, not by the biggest number on the box.
- Heating the same garage every winter? → Apollo-V2 5KW fixed diesel heater
- Shed or small insulated workshop? → Apollo-V1 2.5KW diesel heater
- No drilling, or heat that goes to camp and the cabin too? → Apollo-P1 / P2 portable diesel heater
Buying for a fleet or a dealership? Request a quote.
Related Guides
- How to Install a Diesel Heater: Step-by-Step Guide: tools, drilling and wiring for a fixed install.
- Diesel Heater vs Propane Heater: Complete Comparison: cost, moisture and CO side by side.
- 12V Battery for a Diesel Heater: Size, Type & Run Time: if your garage has no outlet near the heater.
