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BAYHEAT
G-029 · REV 2 · CHECKED 2026-10-10 · MODEL v1.0.2 ▾

Drafted with AI assistance. Numbers computed by model v1.0.2 or sourced. Human review: full by BayHeat editorial desk. Licensed review: no page-specific approval recorded.

Technical review: no page-specific licensed approval recorded.

M measured · C computed · S spec · R reference · E estimate

Natural gas garage heaters

Size our insulated 3-car example in Chicago for about (): its design-day loss plus a margin. A natural gas unit heater at efficiency must burn about of gas to supply that. One input unit covers it. Its bottom must hang at least up, so the ceiling needs to be at least .

Model pick · spec-based

Mr. Heater Big Maxx 50,000 BTU forced-air unit heater (propane or natural gas)

Vented gas unit heater: 50,000 BTU/h in, 40,000 BTU/h out. Bottom must hang at least 8 ft up. BayHeat says hire a licensed gas fitter.

Paid links: we earn a commission if you buy through links on this page, at no cost to you. As an Amazon Associate, BayHeat earns from qualifying purchases.

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BayHeat advises a licensed gas fitter and the permit your town requires. Manual: in a residential garage the bottom of the heater must be at least 8 ft above the floor. Never where gasoline, solvents, paint thinner or dust are present. Source ↗

Three kinds of heater share the name

Search for a natural gas garage heater and you find three different kinds of heater. In an attached garage, only the vented kinds belong. The difference is where the exhaust goes.

TypeWhere the exhaust goesTool verdict or BayHeat rule
Vented unit heater (Mr. Heater Big Maxx class)Outdoors, through a flue pipeTool verdict: ONLY IF. BayHeat says hire a licensed gas fitter.
Separated-combustion unit heater (Modine Hot Dawg HDS class)Outdoors, and it draws its air from outdoors tooTool verdict: ONLY IF, plus whatever its own manual adds
Vent-free (unvented) wall heaterInto the garageBayHeat rule, not a tool verdict: don't buy one for an attached garage

General information. Your electrician, gas fitter, local code and the heater's manual govern.

Check my heater and garage →

Size it by BTU/h out, then check the ceiling

A gas heater has two ratings. Input is the gas it burns. Output is the heat that reaches the room. Output is input times efficiency.

The Big Maxx manual lists () in and () out for its smallest size, the MHU50. That is efficiency, and BayHeat's planner uses the same figure for every vented unit heater.

Start from the heat your garage loses, not the number on the box. The planner finds that loss at your climate station's winter design temperature, a cold night but not the coldest on record. It adds a margin, then divides by efficiency to get the gas input you need.

Heat is the first filter. Ceiling height is the second.

The manual puts the bottom of the heater at least above a residential garage floor. The cabinet and its hanging brackets sit above that line, so the unit's own height counts against your ceiling.

The manual gives that height. Its back view (page 3) shows dimension A as the cabinet height. That is for the MHU50, with the hanging brackets above it. So the MHU50 needs from floor to ceiling (). The MHU80's cabinet is , so it needs . The MHU125's is , so it needs .

Table 1 also asks for of clearance at the top. If your fitter counts that on top of the brackets, each figure rises by that much. No answer in the table below changes.

The 2-car preset's ceiling is , short for the MHU50. The standard 2-car example garage used across BayHeat has the same ceiling, so it can't take this unit. The 1-car preset's is far short.

GarageEnvelopeHeat to size forGas input neededDoes a Big Maxx fit?
1-car, ceilingTight ()Enough heat, but the ceiling rules it out. The MHU50 needs ; this ceiling is .
1-car, ceilingLeaky ()Enough heat, but the ceiling rules it out. The MHU50 needs ; this ceiling is .
2-car, ceilingTight ()Enough heat, but the ceiling rules it out. The MHU50 needs ; this ceiling is .
2-car, ceilingLeaky ()No single Big Maxx fits. The smallest size that covers the load, the MHU80, needs ; this ceiling is . Insulate first.
3-car, ceilingTight ()Yes: the MHU50, with . It clears the ceiling by .
3-car, ceilingLeaky ()No single Big Maxx fits. The smallest size that covers the load, the MHU125, needs ; this ceiling is . Insulate first.

Tight means R-13 walls, an R-30 ceiling, an insulated door kit and tight construction. Leaky means uninsulated walls, a bare drywall ceiling, a plain steel door and leaky construction. These garages are detached; an attached garage borrows some heat from the house and needs somewhat less. See the size chart for other climates.

Read the last column from the top. Only of the garages gets a Big Maxx that covers its load and clears its ceiling. The tight 3-car clears it by .

A tight garage gets far more heat from the smallest unit than it needs. Look at a smaller heater or an electric one. A leaky garage should be insulated first, because that cuts the load (see how to insulate a garage). The margin is what pushes the leaky 3-car garage past the MHU80. Without it the load is , under the MHU80's , and the MHU80 fits that ceiling ( under ). That is why insulating first matters here.

The manual lists two bigger sizes. The MHU80 burns () and puts out (). The MHU125 burns () and puts out (). BayHeat has no verified listing for either, so this page links only the MHU50.

Venting and clearances

A vented unit heater sends its exhaust outdoors through a pipe. The Big Maxx owner's manual (2019 edition) allows two routes, and both use a vent connector. Do not mix vent parts from different makers.

A wall vent must end at least from any door, window, gravity air inlet, gas or electric meter, or regulator. It must end at least below, or beside, any soffit or under-eave vent. Keep it at least from an inside corner of two outside walls.

It must end at least above the ground and the deepest snow. It must stay from any forced-air inlet, such as a dryer's fresh-air intake. Do not end it directly under roof eaves or over a walkway, where condensate can drip. These are the manual's U.S. figures from section C (pages 6 and 7), and the list is not complete.

The unit itself needs of clearance to combustibles at the top and sides, and at the rear. Keep free at the service panel (manual Table 1).

How high to hang it

The manual sets the height for a residential garage. The bottom of the heater goes at least above the floor. The heater must also sit where a vehicle can't hit it, or be shielded.

Code sets two lower floors, in short:

The manual's number is stricter than both, so for this unit the manual wins.

Air and fumes

This unit burns room air. That air must be free of chlorine, solvents, glues, paint remover and similar fumes. Otherwise the heat exchanger wears out early (manual page 5). For combustion-air sizing, the manual points to the National Fuel Gas Code (ANSI Z223.1 / NFPA 54). Your fitter applies the edition your area has adopted.

The manual also bans the unit anywhere gasoline, solvents, paint thinner, dust or unknown chemicals are, or may be, present (page 2). Move gasoline and solvents out of the garage, whatever heater you choose. A gas can and a mower in the corner is that place today.

The gas line, the permit and the installer

This is not a plug-in heater. It needs a gas pipe, a vent and a hardwired feed. The manual says a qualified installer, service agency or the gas supplier must install and service it (page 1). BayHeat says hire a licensed gas fitter and get the permit your town requires. Some places let owners do gas work in their own home, but the manual's wording still stands.

Here is what your gas fitter handles:

Spark ignition and the combustion blower both run on that power. When the power is out, this heater is off.

BayHeat's planning range for gas line, vent and labor is . It is an estimate, so get two local quotes. A detached garage may need a new buried gas line first. Ask the fitter to price that separately.

What the verdict tool says about a vented gas unit heater

Here is the Can I run it? answer for a vented gas unit heater in an attached garage. Every condition has to hold.

ONLY IF

General information. Your electrician, gas fitter, local code and the heater's manual govern.

Licensed install, correct clearances, and CO protection -- safe within code.

  1. 1.Installed and permitted by a licensed gas fitter -- this isn't a DIY connection. C BayHeat
  2. 2.Burner at least 18 in above the garage floor, and at least 6 ft up or guarded against vehicle impact. R IFGC 305.3 / 305.5 (IFGC 2021)
  3. 3.Put a UL 2034 CO alarm in the house outside each sleeping area (code, for new homes and permitted work). BayHeat also advises one by the garage door, and a low-level CO monitor in the garage rated for its temperature range while this heater runs. R IRC R315 (house alarm); BayHeat (garage door and monitor) (IRC 2021)

No gas line? A vented unit heater needs piped gas or a bulk propane tank. Small propane cylinders alone can't feed it. The propane page covers that route.

Garage heater with a thermostat

A gas unit heater takes a low-voltage wall thermostat. The manual lists terminals: R and W for an analog thermostat, plus C for a digital or Wi-Fi one. The manual suggests wire. Never connect line power to the thermostat terminals.

The spark makes radio noise. For a microprocessor thermostat, the manual suggests an isolation relay (page 9). Our example garage holds ; set yours to what you actually need.

Why a vent-free gas heater is a no in an attached garage

A vent-free heater releases its exhaust into the room. An attached garage shares walls, a door and sometimes duct paths with the house, so carbon monoxide can follow the air indoors. Ventilating the garage does not change that.

The verdict tool has no vent-free natural gas class. Its closest class, a portable unvented propane heater, gets this answer in an attached garage:

NO-GO

General information. Your electrician, gas fitter, local code and the heater's manual govern.

Unvented propane heaters aren't a safe recommendation for an attached garage.

  1. 1.An attached garage shares air paths with the house -- carbon monoxide from an unvented heater can migrate indoors even while you're awake. C BayHeat
  2. 2.Use this heater in a detached garage only, or switch to a vented or electric class for an attached one. C BayHeat

Safer alternatives for your situation: 240V hardwired electric, Vented gas unit heater, Mini-split heat pump.

BayHeat's rule: the same answer applies to a vent-free natural gas heater in an attached garage. That is a BayHeat rule, not a result from the tool. A CO alarm warns you. It does not make an unvented heater safe.

General information. Your electrician, gas fitter, local code and the heater's manual govern.

Cost per hour against electric, at state prices

At full fire the MHU50 puts () into the garage. It burns of gas an hour to do it. One therm is . Electric resistance heat needs the full of electricity for the same heat. Full fire is the most it can cost per hour. A thermostat cuts the burner off whenever the garage is warm enough.

StateGas priceElectric priceGas, per hourElectric, per hour
Illinois$1.09/therm19.2¢/kWh$0.54/hr$2.25/hr
Minnesota$1.06/therm16.3¢/kWh$0.53/hr$1.91/hr
Michigan$1.05/therm21.5¢/kWh$0.53/hr$2.52/hr
Wisconsin$1.04/therm19.0¢/kWh$0.52/hr$2.23/hr
Ohio$1.34/therm18.7¢/kWh$0.67/hr$2.19/hr
Pennsylvania$1.45/therm21.0¢/kWh$0.73/hr$2.47/hr
New York$1.70/therm29.4¢/kWh$0.85/hr$3.44/hr
Massachusetts$2.42/therm30.1¢/kWh$1.21/hr$3.53/hr
Colorado$1.08/therm16.7¢/kWh$0.54/hr$1.96/hr
Washington$1.70/therm14.4¢/kWh$0.85/hr$1.69/hr
Texas$1.88/therm16.1¢/kWh$0.94/hr$1.88/hr
California$2.13/therm33.3¢/kWh$1.06/hr$3.90/hr
US average$1.48/therm18.2¢/kWh$0.74/hr$2.13/hr

Per hour at full fire, for the same delivered heat. Computed live from EIA residential prices (natural gas: 2025 annual; electricity: 2026-07). The gas figure leaves out the utility's fixed monthly charge.

Gas costs less per hour than electric resistance heat in every one of the 51 rows of BayHeat's state price table. The closest call is Florida, where electric runs . The widest gap is Alaska, at . Your own rates decide it. A heat pump changes the electric side, so read the mini-split page too, and use the calculator for your state.

Buy this, not that

Buy: the vented unit heater, if your load and your ceiling both pass

Paid links: we earn a commission if you buy through links on this page, at no cost to you. As an Amazon Associate, BayHeat earns from qualifying purchases.

VENTED UNIT HEATER · NATURAL GAS

Mr. Heater Big Maxx 50,000 BTU forced-air unit heater (propane or natural gas)

The listing is the MHU50NG, the natural gas model. Converting it to propane takes the maker's kit, and the manual says a qualified service agency fits it. The unit needs a gas line, a vent and a feed, so budget for the fitter before the heater.

Gas input
()
Heat output
()
Vent connector
Bottom of unit above floor
at least

BayHeat advises a licensed gas fitter and the permit your town requires. Manual: in a residential garage the bottom of the heater must be at least 8 ft above the floor. Never where gasoline, solvents, paint thinner or dust are present.

Price class: $$$

Check price on AmazonPaid link

Dust in the shop

The Big Maxx burns room air. Its manual bans use where dust, solvents, paint thinner or gasoline vapor may be present. Move gasoline and solvents out of the garage, whatever heater you choose. For sawdust, a separated-combustion unit draws its combustion air from outdoors instead of from the shop. BayHeat's catalog lists the Modine Hot Dawg HDS in that class. We have no verified listing for it, so this page has no buy link for it. Its own manual governs the install, including where it may be used.

Don't buy

  • A vent-free gas heater for an attached garage. That is BayHeat's rule, as above. BayHeat lists none.
  • A unit chosen by its name. Use the sizing table. The MHU50 is too small for a leaky 2-car garage and well oversized for a tight one.
  • Any heater before you check the ceiling. The bottom of this unit has to sit at least above the floor. The MHU50 needs in all. The 1-car preset () and the 2-car preset () fall short.
  • The wrong fuel. Confirm the listing says natural gas. Swapping fuels takes the maker's kit and a qualified service agency (manual page 15).
  • A torpedo heater. BayHeat excludes open-flame forced-air heaters from every enclosed garage.

Safety scope

Put a UL 2034 CO alarm in the house outside each sleeping area. BayHeat also says to run a low-level CO monitor in the garage, rated for its temperature range, while this heater runs. Have the heater and its vent inspected once a year by a qualified service agency, as the manual says. General information. Your electrician, gas fitter, local code and the heater's manual govern.

Alarm rules depend on your place and on when the house was built. In short:

Your local code edition governs.

Safety add-on

Kidde C3010 10-year sealed-battery CO alarm (UL 2034)

Put a UL 2034 CO alarm in the house, by the garage door and outside each sleeping area.

Paid links: we earn a commission if you buy through links on this page, at no cost to you. As an Amazon Associate, BayHeat earns from qualifying purchases.

Check price on Amazon
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Data sheet: meets UL 2034 and is rated 40–100 °F for living areas. It goes in the house outside sleeping areas (IRC 2021 R315), not in an unheated garage. Source ↗

Next step

Check my heater and garage → for a verdict that fits your garage, or run the calculator with your own state's prices. To compare every fuel side by side, see garage heaters by fuel and electric vs propane vs gas.

Related

Sources

  1. [1] Mr. Heater MHU50 / MHU80 / MHU125 operating instructions and owner's manual, Enerco Group (Mr. Heater). Retrieved 2026-09-30.
  2. [2] International Fuel Gas Code, 2021, ICC. Retrieved 2026-09-25.
  3. [3] International Residential Code, 2021, ICC. Retrieved 2026-09-25.
  4. [4] Natural gas residential price by state, annual, US Energy Information Administration. Retrieved 2026-09-25.
  5. [5] Electric Power Monthly, Table 5.6.B, average retail price by state, US Energy Information Administration. Retrieved 2026-09-25.
  6. [6] Kidde C3010 sealed battery carbon monoxide alarm data sheet, Kidde. Retrieved 2026-09-30.
Size my garage — 60 s