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Big Buddy propane heater: what the manual actually allows

By the Plain Prepper editors. Updated 2026-10-15.

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Quick answer

Mr. Heater's Big Buddy is a radiant propane heater with three settings: 4,000, 9,000 and 18,000 BTU per hour. Mr. Heater lists it for enclosed spaces up to 450 sq ft, with an oxygen depletion sensor and a tip-over shut-off. Its manual sets firm limits: LO only in a bathroom, LO or MED in a bedroom, and never run it while sleeping or unattended. It needs 18 square inches of vent at both the ceiling and the floor. A 20 lb tank must stay outdoors on a Mr. Heater hose; two 1 lb cylinders give 3 to 12 hours.

Big Buddy at a glance

Spec	Mr. Heater Big Buddy (MH18B)
Settings	LO 4,000, MED 9,000, HI 18,000 BTU/hr
Area listed	Enclosed spaces like cabins up to 450 sq ft
Fuel	One or two 1 lb cylinders, or a remote cylinder up to 20 lb with an approved hose
Safety devices	Oxygen depletion sensor, tip-over shut-off
Clearance to combustibles	Top 30 in, front 24 in, sides 6 in, rear 0 in
Ignition	Piezo, push and rotate the knob

On HI that works out to 40 BTU per hour per square foot of the listed 450 sq ft. A separate version (F274806) is sold for Massachusetts and Canada; the manual for the standard model says it is for US sale except Massachusetts.

Room limits and the vent requirement

The Big Buddy is unvented: it uses oxygen from the room. Its manual sets these limits:

• Bathroom: LO only. For emergency home use, the manual caps a bathroom at 5,000 BTU/hr.
• Bedroom: LO or MED, capped at 10,000 BTU/hr for emergency home use.
• Ventilation: 18 square inches of vent opening, for example 4.25 x 4.25 in, at the ceiling and again at floor level. In a recreational enclosure there must be a window or roof vent.
• Other appliances: do not use other fuel-burning appliances in the same space.
• Always: never leave it unattended while burning, and never operate it while sleeping.

What counts as emergency indoor use

Mr. Heater lists the Big Buddy as safe for small recreational enclosures with combustion air and ventilation, such as enclosed porches, cabins, fishing huts, trailers, tents, truck caps and vans. It also allows emergency indoor heating on 1 lb cylinders. When you run two 1 lb cylinders for emergency use, the manual defines that as:

1. The permanent heating system is temporarily out of service.
2. Heat is necessary to prevent damage to the building or contents.
3. The heater is not left unattended.
4. The 18 square inch vent area is provided.
5. The heater is removed once permanent heating is back in operation.

That is a narrow, supervised use, not a substitute for a furnace through a long outage.

Fuel: cylinders, hoses and filters

• 1 lb cylinders: one or two disposable cylinders marked for the correct fuel.
• Remote 20 lb tank: allowed only when the cylinder is located outdoors, using Mr. Heater hose F273701, F273702 or F272702 with fuel filter F273699, which must be replaced every year and must not be left out. Hose F273704, the Buddy series hose, does not need the filter.
• Accessories: Mr. Heater says unauthorized accessories are expressly prohibited, may cause serious injury and void the warranty. Some hoses sold online say only that they "fit" the Buddy, so check the part number and brand before buying.
• Altitude: above 7,000 ft the heater may burn less brightly or shut off; the manual says to provide fresh air, wait 5 minutes and relight.

Runtime by cylinder, from the manual

Supply	Runtime listed
One 1 lb cylinder	1.5 to 6 hours
Two 1 lb cylinders	3 to 12 hours
One 20 lb cylinder	24 to 108 hours
Two 20 lb cylinders	48 to 216 hours

The manual says runtime varies with the heat setting (LO, MED or HI) and the surrounding temperature, which is why each range is wide. Pair the heater with UL 2034 carbon monoxide alarms, battery-operated or with battery backup, which CPSC advises on every level and outside sleeping areas. If one sounds, CPSC says to get outside immediately and call 911.

What to look for when you buy

Questions people ask

Can you use a Big Buddy heater indoors?

Mr. Heater lists it for small, ventilated enclosures and emergency indoor heating, with 18 sq in vents at ceiling and floor, never unattended or while sleeping.

How long does a Big Buddy run on a 20 lb tank?

The manual lists 24 to 108 hours on one 20 lb cylinder, depending on the setting and temperature.

Can I use a Big Buddy in a bedroom?

Only on LO or MED, which the manual caps at 10,000 BTU/hr for emergency home use, and never while anyone is sleeping.

Do I need a fuel filter with the Big Buddy?

Yes with hoses F273701, F273702 or F272702 (filter F273699, replaced yearly); no with Mr. Heater's F273704 Buddy series hose.

Sources

• Big Buddy portable heater, Mr. Heater (Enerco)
• Portable Big Buddy radiant heater MH18B operating instructions, Mr. Heater (Enerco), hosted by Lowe's
• Carbon monoxide questions and answers, U.S. Consumer Product Safety Commission
• Keep warm and safe this winter: tips for using generators, furnaces and space heaters, U.S. Consumer Product Safety Commission

Sources checked 2026-10-15.