Water HeatersSolar with Gas Backup
GREENoneTEC Double GT Water Heater Manual
Setup, everyday use, maintenance and troubleshooting for the solar water heater with gas backup, with verified specifications.
- SUEF6.7
- Collector area44.6 sq. ft.
- Solar tank51 gal.
- Freeze limit19 °F
About the GREENoneTEC Double GT
GREENoneTEC model Double GT (SunPad) is a solar water heater with gas backup that meets ENERGY STAR efficiency requirements.
This direct ics system combines 44.6 sq. ft. of batch collector area, a 51-gallon solar storage tank, and a tankless backup heater (199,000 Btu/h). It is certified to SRCC OG-300 under number 30004323. The DOE classifies it in the medium usage draw pattern, so its efficiency rating assumes about 55 gallons of hot water a day.
Its Solar Uniform Energy Factor (SUEF) is 6.7: over the rating test it delivers about 6.7 units of hot-water energy for each unit of gas bought for the backup heater and controls.
Specifications
| Brand | GREENoneTEC |
|---|---|
| Model number | Double GT |
| Model name / series | SunPad |
| Water heater type | Solar with Gas Backup |
| Fuel / energy source | Natural gas |
| DOE draw pattern | Medium-Usage |
| Solar Uniform Energy Factor (SUEF) | 6.7 |
| Solar system type | Direct ICS |
| Collector type | Batch |
| Collector area | 44.6 sq. ft. |
| Solar storage tank | 51 gal. |
| Backup heater type | Tankless |
| Backup gas input | 199,000 Btu/h |
| Freeze tolerance limit | 19 °F |
| SRCC OG-300 certification no. | 30004323 |
| Connected / demand-response capable | No |
Safety information
- Set the water heater to about 120 °F (49 °C). Water at 140 °F can cause a serious burn in a few seconds; at 120 °F it takes several minutes. Homes with young children or older adults benefit from anti-scald valves at tubs and showers.
- The temperature and pressure (T&P) relief valve protects the tank from bursting. Never cap, plug or remove it, and keep its discharge pipe running full size to within about 6 in. of the floor or to an approved drain.
- Installation should be done by a licensed plumber (and electrician or gas fitter where required). Most jurisdictions require a permit.
- Install carbon monoxide alarms on every level of the home and near sleeping areas, and keep the vent and air-intake terminations clear of snow, leaves and nests.
- If you smell gas, do not operate switches or appliances; leave the building and call your gas utility or 911 from outside.
- Never store gasoline, solvents or other flammable products near the water heater.
- Where local code requires it (for example, in seismic zones), secure the tank to the wall with approved strapping.
- Solar collectors and piping can become hot enough to cause severe burns, and roof work carries a serious fall risk. Leave collector service to a qualified solar installer.
Installation and first-time setup
Use this checklist when planning the installation of the GREENoneTEC Double GT and when reviewing the finished job with your installer.
- Install the certified configuration
Solar water heaters are rated as complete systems. Install the collectors, tank, backup heater and controls that make up the SRCC OG-300 certified configuration (30004323); substitutions can change performance and incentive eligibility.
- Collector siting
In the continental U.S., collectors generally face south (within about 45°) with a tilt near the site latitude and little shading from 9 a.m. to 3 p.m. Have the roof structure checked for the added weight and flash all penetrations carefully.
- Freeze protection
This system's freeze tolerance limit is 19 °F. Keep antifreeze or drain-down protection in good order to maintain that rating.
- Mixing valve and backup
Solar-heated water can exceed safe tap temperatures on sunny days, so install a thermostatic mixing valve. Connect and set the backup heater per the manual.
- Plumbing connections
Use dielectric fittings where dissimilar metals meet, install a shut-off valve on the cold inlet, and pipe the relief valve discharge to an approved location. Add an expansion tank if the supply has a check valve or pressure-reducing valve.
- Fill and purge before start-up
Fill the tank completely and run a hot tap until water flows steadily, so no air remains, before turning on the gas and power. Then set the temperature and check every joint for leaks.
Using your water heater
Living with solar hot water
On sunny days the collectors do most of the work and the backup heater rarely runs; in cloudy weather and winter the backup makes up the difference. Run dishwashers and laundry in the afternoon or early evening, when the solar tank is hottest, and set the backup thermostat no higher than you need.
This passive design circulates water without a pump, so there is little to adjust, but the rooftop tank or collector must stay within its freeze tolerance.
Saving hot water and energy
- Fix dripping hot-water taps promptly and use WaterSense showerheads and faucet aerators.
- Insulate the first several feet of hot and cold pipes at the heater.
- Wash laundry in cold water when possible.
Care and maintenance
| When | What to do |
|---|---|
| Every year | Test the T&P relief valve by lifting its lever briefly with a bucket under the discharge pipe; if it does not reseat and stop dripping, have it replaced. |
| Every year | Drain a few gallons from the tank drain valve, or flush it fully, to remove sediment — more often in hard-water areas. |
| Every 2–3 years | Have the anode rod inspected and replaced when it is heavily corroded; this is the main defense against tank corrosion. (Some models use a powered anode — follow the manual.) |
| Every year | Inspect collectors, glazing, mounting and roof penetrations; check for leaks and confirm the controller and pump operate correctly. |
| Every month | Test carbon monoxide alarms. |
Troubleshooting
Check these common causes before calling for service.
| Problem | Possible cause | What to try |
|---|---|---|
| Backup heater runs on sunny days | Controller or sensor fault; pump not running; air or low fluid in the loop; shaded collectors. | Check the controller display and the pump; call the solar installer for loop service. |
| Water too hot at taps | Mixing valve missing, set too high or failed. | Have a thermostatic mixing valve installed or adjusted. |
| Rotten-egg smell in hot water | Bacteria reacting with the anode rod or water chemistry. | Flush the tank; a plumber can replace the anode with a different alloy or treat the water. |
| Relief valve dripping or discharging | Excess pressure from thermal expansion; failing valve; temperature set too high. | Have the pressure checked and an expansion tank added if needed; replace a faulty valve. Never plug it. |
Energy use and running costs
With an SUEF of 6.7 and the DOE medium usage load of 55 gallons a day, the backup heater would use roughly 17 therms a year — about $25.11 at $1.50 per therm.
This is an estimate based on the SUEF definition (hot-water energy delivered divided by purchased energy). Actual savings depend heavily on your climate, collector orientation and hot-water habits.
Frequently asked questions
Is the GREENoneTEC Double GT freeze tolerant?
Its freeze tolerance limit is 19 °F.
What is the SRCC certification number for the GREENoneTEC Double GT?
It is certified to SRCC OG-300 under number 30004323. Many incentive programs ask for this number.
What type of solar system is the GREENoneTEC Double GT?
It is listed as a direct ics system with batch collectors. In a direct system, household water itself flows through the collectors, so it is best suited to mild climates and good water quality. It circulates passively, without a pump.
What backup heater does the GREENoneTEC Double GT use?
It uses a tankless gas backup heater rated at 199,000 Btu/h, which heats water whenever solar energy is not enough.
Is the GREENoneTEC Double GT ENERGY STAR certified?
Yes. It appears on the ENERGY STAR certified water heater list, with a certification date of March 2022.
Sources and disclaimer
Specifications on this page come from the ENERGY STAR certified water heaters dataset published by the U.S. Environmental Protection Agency (record certified March 19, 2022), retrieved October 7, 2026.
The setup, operating, maintenance and troubleshooting guidance is written by ManualHarbor's editors for this specific product type and configuration. It is an independent guide — not a reproduction of the manufacturer's manual — and ManualHarbor is not affiliated with GREENoneTEC. Always follow the safety instructions, installation requirements and warranty terms supplied with your unit, and contact GREENoneTEC or an authorized service provider for warranty repairs and replacement parts.
Running-cost figures use assumed U.S. average utility prices and are estimates for comparison only. See our editorial methodology for details.
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