Separated combustion (also called power-vented or direct-vent) unit heaters draw combustion air from outdoors or a dedicated intake, improving indoor air quality and safety in garages, workshops, and commercial spaces. The table below summarizes five relevant unit heaters selected for performance, venting type, and suitability for larger spaces.
| Model | Brand | BTU Rating | Coverage |
|---|---|---|---|
| Mr. Heater Big Maxx 250,000 | Mr. Heater | 250,000 BTU | Up To 8,400 Sq Ft |
| Mr. Heater Big Maxx 200,000 | Mr. Heater | 200,000 BTU | Up To 6,700 Sq Ft |
| Reznor UDXC-250 (250,000) | 1HVAC / Reznor | 250,000 BTU | Commercial/Warehouse |
| Mr. Heater MHU80NG 80,000 | Mr. Heater | 80,000 BTU | Up To 2,000 Sq Ft |
| Reznor UDXC-100 (105,000) | 1HVAC / Reznor | 105,000 BTU | Garages / Shops |
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Mr. Heater 250,000 BTU Big Maxx Unit Heater
This Mr. Heater Big Maxx model provides a 250,000 BTU heat output intended for large spaces. The product lists heating capacity up to 8,400 sq ft and an operational 82% thermal efficiency, indicating most fuel energy converts to usable heat.
Features include a heavy-duty aluminized steel heat exchanger and a standard combustion, power-vented design that routes combustion gases outside. Installation typically requires proper gas supply, venting and electrical connection for the powered vent. The unit’s heat output and efficiency make it suitable for large commercial garages, warehouses, or shops where isolated combustion improves indoor air quality.
Considerations: confirm building ventilation and clearances, ensure compatible gas type and piping, and plan for professional installation to meet local codes and to implement the power venting correctly.
Mr. Heater 200,000 BTU Big Maxx Unit Heater
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The 200,000 BTU Big Maxx model is sized for medium-to-large spaces and lists a capacity to heat up to 6,700 sq ft. It shares an 82% thermal efficiency rating and the same aluminized steel heat exchanger design as larger models in the line.
This unit is a standard combustion power-vented heater, which means combustion air and exhaust use a dedicated venting system driven by the unit’s blower or power vent. That design reduces the risk of indoor combustion byproducts entering the workspace compared with atmospheric venting.
Practical use cases include multi-bay garages, medium warehouses, and larger workshops. Confirm mounting and vent routing options for ceiling or high-wall installation and verify that the BTU rating matches the square footage and insulation of the space.
Reznor UDXC-250 Commercial Natural Gas Heater
The Reznor UDXC-250 is a commercial-grade natural gas unit rated at 250,000 BTU. The listing cites two output levels: a low fire setting for milder days and a higher output for extreme cold. The unit achieves roughly 83% thermal efficiency.
Designed for ceiling mount or suspended installations, the UDXC series includes threaded retainers at suspension points for flexible placement. The model features a low static propeller fan for quieter forced-air distribution suitable for workplaces and warehouses.
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Because this is an indoor gas unit intended for commercial spaces, installation orientation and venting must follow manufacturer and local code requirements. The dual-output capability can improve seasonal efficiency and comfort control in spaces with varying load.
Mr. Heater MHU80NG 80,000 BTU Unit Heater
The MHU80NG Big Maxx unit heater provides 80,000 BTU and lists coverage around 2,000 sq ft. This model includes two angle brackets and a propane conversion kit in its kit; it is specified for natural gas use but offers conversion provisions.
Its physical size and capacity suit single-bay garages, small workshops, or utility spaces where separated combustion or powered venting is desired. The unit supports multiple venting categories—vertical or horizontal—so verify site clearance and venting route.
Electrical requirement is minimal (listed at 120 V, 2.3 amps) and clearance recommendations (for example, minimum eight-foot clearance to floor) inform placement. For smaller commercial spaces or detached garages, this model balances capacity and footprint.
Reznor UDXC-100 105,000 BTU Unit Heater
The Reznor UDXC-100 is rated at 105,000 BTU with dual firing levels (low and high) and an 83% thermal efficiency rating. It is listed for ceilings or suspended mounting and uses a low static propeller fan for airflow.
Intended applications include garages, small warehouses, and workshop environments. The dual output settings permit lower energy use on milder days while preserving capacity for colder periods. Threaded suspension points allow flexibility in mounting orientation.
As with other commercial units, venting configuration, gas supply and clearances are core installation considerations. This model is well-suited where a mid-range BTU output is appropriate without the footprint or capacity of larger commercial heaters.
Buying Guide: Choosing A Separated Combustion Unit Heater
Selecting the right separated combustion unit heater involves matching capacity, venting, efficiency, and installation needs to your space. The following considerations will help evaluate options from small shop units to large commercial heaters.
1. Heat Output And Space Sizing
Calculate required BTU by assessing square footage, ceiling height, insulation level, and typical outdoor temperature. Higher ceilings and poor insulation raise BTU needs. Use manufacturer coverage guidance as a starting point and consult an HVAC professional for precise load calculations.
2. Efficiency Ratings
Look for thermal efficiency percentages; higher numbers mean more fuel converts to heat. Typical separated combustion units have efficiencies in the low 80s. Efficiency affects fuel use and operating cost over time, so weigh initial cost versus long-term savings.
3. Venting Type And Installation
Separated combustion units often use power venting or direct-vent systems that exhaust outside and draw combustion air from outdoors. Confirm that the unit’s venting style fits your building layout. Power-vented units require electrical supply for the vent blower.
4. Mounting Options And Air Distribution
Choose ceiling-mounted, suspended, or wall-mounted units depending on space layout. Consider fan type—propeller fans provide broad airflow for open spaces while ducted options or louvers can target specific zones. Check for multi-speed fans or dual firing settings for better control.
5. Fuel Type And Conversion
Confirm natural gas or propane compatibility. Some models include conversion kits; others are dedicated to one fuel type. Verify local gas supply capacity and line sizing to support the unit’s BTU rating.
6. Noise And Fan Type
Forced-air units vary in sound level. Low static propeller fans tend to be quieter and suitable for occupied workspaces. Evaluate noise specs and consider sound attenuation if the heater will operate near workstations.
7. Safety And Compliance
Ensure the unit meets relevant safety standards and local code requirements. Important features include proper clearances, flame safeguards, pressure switches for power vents, and adequate access panels for service. Hire licensed installers and obtain required permits.
8. Controls And Modulation
Thermostat compatibility, on/off cycling, and dual firing stages affect comfort and fuel consumption. Units with low/high fire stages can maintain efficiency across variable conditions. Consider adding a programmable thermostat or zone controls for more precise management.
9. Maintenance And Serviceability
Access to the heat exchanger, burner, and fan for cleaning and inspection reduces long-term maintenance costs. Choose models with clear service access and available replacement parts. Schedule regular inspections for vent integrity and combustion performance.
10. Budget And Total Cost Of Ownership
Compare initial equipment and installation costs against efficiency and maintenance needs. Higher efficiency and staged firing may increase upfront cost but lower operational expense. Factor in venting complexity and any structural modifications required for installation.
Comparison Perspectives
From a capacity perspective, the 250,000 BTU models suit large warehouses and multi-bay garages, while 80,000–105,000 BTU models better fit small shops and single-bay garages. Efficiency differences between 82% and 83% are modest; installation quality and proper sizing usually have a larger impact on comfort and operating cost.
From an installation perspective, ceiling-suspended units like the Reznor UDXC line provide flexible placement for commercial ceilings, while compact Big Maxx models can be mounted in tighter configurations. From a control perspective, units with multi-stage outputs offer better seasonal efficiency than single-stage, full-on/full-off units.
From a safety perspective, separated combustion with power venting improves indoor air quality by keeping combustion products out of the workspace. This is particularly important in occupied garages, automotive shops, and spaces where solvent fumes or particulate are present.
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