For instant gas hot water across a home, choose a tankless heater sized for your incoming water temperature and the number of fixtures used at once. The Rheem RTG is best suited to larger households needing higher stated capacity; the APUS offers high flow with a built-in booster pump; and the ThermoMate models provide options for condensing efficiency or midrange output. The comparison below summarizes the listed specifications. Actual flow depends on temperature rise and installation conditions.
| Product | Stated flow and output | Best suited for |
|---|---|---|
| Rheem RTG Series | 7.0 GPM; 160,000 BTU | Large households |
| QERPRE Natural Gas | 4.3 GPM; 100,000 BTU | Smaller homes and multiple modes |
| ThermoMate Condensing | Up to 5.28 GPM; 114,000 BTU | Efficiency-focused buyers |
| ThermoMate CZ422NG | Up to 5 GPM; 113,000 BTU | Two-shower use |
| APUS Natural Gas | Up to 6.8 GPM; 140,000 BTU | Higher flow and booster-pump features |
Rheem RTG Series 7.0 GPM
Best for: Larger households that may draw hot water from several fixtures at once. The listed 160,000 BTU output and 7.0 GPM rating at a 35°F temperature rise make this the highest-capacity option in this comparison.
This indoor, non-condensing model is designed for whole-house use and uses a direct vent system. It has a wall-mount design and a listed 0.82 Uniform Energy Factor (UEF). Rheem states that it can serve two to three simultaneous showers or water points, depending on conditions.
Limitation: The 7.0 GPM figure is specified at a 35°F rise, so output may be lower when incoming water is colder or the required temperature rise is larger. Check venting and installation requirements before choosing it.
QERPRE Natural Gas 4.3 GPM
Best for: Buyers seeking an indoor unit with digital controls and preset operating modes. The heater is listed at 100,000 BTU and up to 4.3 GPM, with the product description identifying bathrooms, kitchens, and wet bars as possible water points.
Its listed V3.0 self-modulating temperature system is designed to hold water temperature within ±1°F. The three smart modes include a seasonal setting, a kitchen preset at 100°F, and a bathtub mode that alerts the user when the selected amount is reached. These controls may suit households that want more than a basic temperature setting.
Limitation: Its stated flow is lower than the 5–7 GPM options here. Consider it carefully if several high-flow fixtures are likely to run together.
ThermoMate Condensing 5.28 GPM
Best for: Households comparing condensing designs and hot-water capacity for two showers. This model lists up to 5.28 GPM, 114,000 BTU, and a 0.86 UEF. Its dual heat exchanger is described as recovering heat that conventional units may lose.
The manufacturer states that it can supply two showers at 2 GPM each with 105°F water when inlet water is 65°F. A flow sensor and control chip are described as adjusting heating power to help maintain temperature within ±1°F. The product also lists multiple protections and automatic frost protection at 41°F.
Limitation: The shower example depends on a 65°F inlet temperature and specified flow. Colder incoming water can change how much hot water the unit can deliver.
ThermoMate CZ422NG Up To 5 GPM
Best for: Buyers who want a midrange unit listed for two-shower use, along with temperature controls and safety features. It is rated up to 5 GPM and 113,000 BTU. The stated 105°F, two-shower example is based on 65°F inlet water.
The product lists an oxygen-free copper heat exchanger, up to 98% thermal efficiency, and a 0.81 UEF. Its flow sensor and control chip are described as adjusting heating 20,000 times per second to maintain temperature within ±1°F. Listed protections include freeze, flame-out, overheating, and pipe-blockage protection.
Limitation: Its maximum stated flow is below the Rheem and APUS models. Confirm that its capacity matches your household’s likely simultaneous demand.
APUS Natural Gas 6.8 GPM
Best for: Buyers prioritizing higher stated flow and a built-in booster pump. The APUS lists a 140,000 BTU burner and a maximum flow of 6.8 GPM. The manufacturer says the pump can increase water flow by up to 130% and describes use with two showers and two sinks.
The product also lists an oxygen-free copper heat exchanger, a 0.81 UEF, and a 120V AC connection. Its single-circulation feature is described as reducing cold-water wait time, while an ECO smart cycle is claimed to save energy compared with conventional models.
Limitation: The product’s highest flow and simultaneous-fixture claims should not be treated as guaranteed for every installation. Confirm required electrical, gas, and plumbing connections with a qualified installer.
Buying Guide: How To Choose An Instant Gas Water Heater
How much flow does your home need?
Estimate which fixtures may run at the same time, then compare their combined flow demand with the heater’s capacity at the temperature rise your home requires. A headline GPM rating alone is not enough: the Rheem listing, for example, specifies 7.0 GPM at a 35°F rise. Colder groundwater or a higher target temperature can reduce available flow.
Should you choose a condensing model?
A condensing heater can recover heat from exhaust, and the ThermoMate condensing model lists a 0.86 UEF. Compare that figure with the listed UEF of non-condensing options, but also check installation requirements. Do not assume every home can use the same venting or drainage setup.
What should you verify before installation?
- Confirm natural-gas supply, gas-line sizing, venting, and indoor installation requirements with a qualified professional.
- Check required electrical connections. The APUS listing specifies a 120V AC connection.
- Ask how the unit’s flow rating changes at your local incoming-water temperature and desired output temperature.
- Review the manufacturer’s installation instructions and local code requirements before purchase.
Frequently Asked Questions
Does a tankless gas water heater provide hot water instantly?
It heats water on demand rather than storing a tank of preheated water. Hot water may still take time to travel through the pipes from the heater to a faucet.
Can one unit run two showers at once?
Some models are listed for two showers under stated conditions. Actual performance depends on inlet-water temperature, shower flow, and the heater’s capacity.
What does GPM mean?
GPM means gallons per minute, a measure of water flow. Compare the rating at its specified temperature rise with the hot-water demand in your home.
Is a higher UEF always the only factor to compare?
No. UEF helps compare energy performance, but capacity, venting, fuel supply, installation requirements, and household demand also matter.
Should a gas tankless heater be professionally installed?
Professional installation is advisable because gas supply, venting, water connections, and local code requirements must be addressed. Confirm the specific requirements for the selected model.