Gas/Electric Rooftop Units: Selecting Heat Input, Fuel, and Configuration
Gas/electric rooftop units pair a DX cooling section with a gas-fired heating section in one packaged cabinet. They remain the workhorse of commercial roofs wherever natural gas is already piped to the unit, because gas heat delivers high heating capacity on cold mornings without the electrical service a comparable amount of electric heat would need. This category holds gas/electric packaged units from 3 to 25 tons from Daikin and Rheem, with heat inputs from 45,000 to 400,000 Btu/h across the listings. If the building has no gas service, look at Heat Pump Rooftop Units or Air Conditioner Rooftop Units instead. For curb, rigging, and changeout planning, see the parent Commercial Rooftop Units page.
What Is in This Category
| Brand and series | Tonnage range listed | Refrigerant | Notes from the listings |
|---|
| Daikin DSG | 3 to 25 tons | R-32 | Broadest size range; heat inputs from 45,000 to 400,000 Btu/h; some models listed as Ultra Low NOx |
| Daikin DHG | 3 to 12.5 tons | R-32 | Higher-efficiency line; ratings listed from 16.2 to 16.6 SEER2 on smaller sizes and 16 to 18.6 IEER on larger; some models listed as Ultra Low NOx |
| Daikin DP3 | 3 to 5 tons | R-32 | 13.4 SEER2; 80,000 and 120,000 Btu/h heat inputs listed; Ultra Low NOx |
| Daikin DFG | 3 to 25 tons | See spec sheet | Multi-position packaged gas/electric units; refrigerant not stated in the title |
| Rheem RGECYB | 3 to 6 tons | R-454B | Direct drive options; louvered coil guard and stainless steel heat exchanger variants |
| Rheem RGEDYB | 7.5 to 12.5 tons | R-454B | Ratings listed at 14 to 14.6 IEER |
| Rheem RGEGYB | 15 to 25 tons | R-454B | Belt drive VFD options; heat inputs up to 400,000 Btu/h listed |
Electrical configurations include 208/230V single-phase on some 3 to 5 ton Daikin models, 208/230V three-phase and 460V three-phase across most of the range, and 575V three-phase on Daikin models. Rheem gas/electric units here are listed in 208/230V and 460V three-phase.
When Gas/Electric Is the Right Fit
- Gas is already on the roof. Replacing gas/electric with gas/electric keeps the electrical load roughly where it was and avoids running new feeders.
- Cold climates and high ventilation loads. Gas heat output does not drop with outdoor temperature the way heat pump capacity does, which matters when a unit has to temper a lot of outside air for restaurants, assembly spaces, or gyms.
- Morning warm-up. Buildings that set back overnight need to recover quickly before opening. Adequate gas heat input handles that without the large electric heat kits a heat pump would need.
- Owners not ready to electrify. Where local rules allow, gas/electric is often the lowest first-cost replacement for an existing gas/electric unit.
Where it fits less well: buildings without gas service, jurisdictions restricting new gas equipment, and owners with electrification goals. In those cases, a heat pump rooftop unit is the usual alternative.
Choosing Heat Input
Most gas/electric units are offered in more than one heat input at the same cooling tonnage. In this category, a 3 ton Daikin unit, for example, is listed with several heat input options. Picking the right one is about the heating load, not the cooling tonnage.
- Start with the existing unit. If comfort was acceptable and the building has not changed, matching the existing heat input is reasonable.
- Look at ventilation. Heating outside air is often the largest part of the heating load on commercial buildings. A restaurant or classroom with high outside air needs more heat than an office of the same size.
- Don't oversize heat by habit. Excess heat input leads to short cycles and wide temperature swings. A higher heat option is not automatically better.
- Check the temperature rise. Every heat exchanger has an allowable temperature rise range at a given airflow. Confirm that your design airflow keeps the rise within the range on the spec sheet.
- Verify gas supply. Higher input means more gas flow. Check the pipe size and supply pressure, especially on long runs across a roof or when several units share one line.
Note that heat input and heating output are not the same thing. The output depends on the unit's heating efficiency, which is listed on the spec sheet.
Ultra Low NOx Models
Several Daikin listings in this category, in the DHG, DSG, and DP3 series, are identified as Ultra Low NOx. Some California air districts, including the South Coast and San Joaquin Valley districts, regulate NOx emissions from gas-fired heating equipment, and those limits can apply to the furnace section of packaged units in certain size ranges. If your job is in a district with NOx rules, confirm that the specific model and heat input you choose is certified for that district before you order. Outside those districts, a standard model is usually all that is required, though some owners choose Ultra Low NOx for their own reasons.
Heat Exchanger and Corrosion Considerations
Rheem lists stainless steel heat exchanger versions of several gas/electric models. Stainless heat exchangers are often specified for higher outside air applications, where cold incoming air can cause condensation in the heat exchanger, and for corrosive environments. Coastal sites face salt air corrosion on coils and cabinets, and some Rheem listings include louvered coil guards to protect the condenser coil from hail and physical damage. If the building is near the ocean or in an industrial area, look at the manufacturer's corrosion-resistant coil options and talk to us before ordering a standard unit.
Fuel Type: Natural Gas and Propane
Gas/electric packaged units are typically shipped for natural gas. Propane (LP) operation generally requires a manufacturer-approved conversion kit installed by a qualified technician, and the heat input may be derated. Check the installation manual for the specific model for conversion requirements and part numbers, and order the kit with the unit if the site runs on propane.
Efficiency and Refrigerant
Units under 65,000 Btu/h of cooling, generally the 3 to 5 ton sizes, carry SEER2 and EER2 ratings. Units at 6 tons and above carry EER and IEER ratings. In this category, listed values run from 13.4 to 16.6 SEER2 on the smaller units and from 13 to 18.6 IEER on the larger units. Gas heating efficiency is published separately on the spec sheet.
Daikin's current lines here use R-32 and Rheem's use R-454B. Both are A2L refrigerants, so the crew needs A2L-rated tools and training, and the installation must follow the manufacturer's instructions on leak detection and mitigation. Combining an A2L refrigerant with a gas burner in one cabinet is a normal, listed design, but it is one more reason to follow the installation manual exactly on clearances and venting.
Economizers, Venting, and Code
Energy codes based on ASHRAE 90.1 and the IECC require economizers on many units above a certain cooling capacity, depending on the climate zone. Model-specific economizers are listed in Commercial HVAC. Packaged gas units vent their combustion products through the cabinet; keep the required clearances from outside air intakes, operable windows, and property lines as stated in the installation manual and the local fuel gas code.
Frequently Asked Questions
How do I pick between two heat input options at the same tonnage?
Base it on the heating load, including outside air. If the existing heat input kept the space comfortable, matching it is reasonable. Bigger is not better if it causes short cycling.
Can I run these units on propane?
Typically yes, with a manufacturer-approved LP conversion kit installed per the installation manual. Confirm the kit part number for your model before ordering.
Do I need an Ultra Low NOx unit?
Only where an air district or local rule requires it for that equipment type and size. Check the rules for the project location and confirm the model's certification.
Why choose a stainless steel heat exchanger?
Stainless heat exchangers are typically specified where high outside air or low return air temperatures could cause condensation in the heat exchanger, or in corrosive environments. Check the manufacturer's application guidelines.
Are any of these units single-phase?
Yes. Some 3 to 5 ton Daikin gas/electric units are listed in 208/230V single-phase. Larger units are three-phase only.
What is the largest gas/electric unit in this category?
The listings run up to 25 tons, from both Daikin and Rheem, with heat inputs up to 400,000 Btu/h on some models.
