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Gas tankless water heaters compared on official ENERGY STAR certified ratings. Uniform energy factor (UEF) is the official efficiency number. Maximum gallons per minute is the certified lab flow, which depends on the incoming water temperature used in the test and is not a guarantee in a cold-climate install. Minimum gallons per minute is the official activation flow; a lower figure is better for low-flow fixtures. This table is limited to ENERGY STAR certified gas tankless models. Electric tankless and gas storage water heaters are not ranked here. Electric resistance tankless units generally do not appear on the ENERGY STAR water heater list we used. ENERGY STAR certified gas storage models exist, but many listings are brand rebadges with official UEF values clustered at 0.88 to 0.90, so we did not mix them into this tankless table.
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Top pick
C
Camplux RP199iB
Score 8.7/10 · Check price
Why it wins: Official UEF 0.98, maximum 5.8 GPM, minimum activation 0.6 GPM, ENERGY STAR connected-capable. Highest official uniform energy factor in this comparison (0.98). High official maximum flow (5.8 GPM).
"Not listed" means the official ENERGY STAR, DOE or manufacturer record we used does not publish that figure. Prices show "Check price" until a retailer feed provides a current price.
Buying guide: how to choose the right gas tankless water heater
Maximum GPM is a certified lab figure
ENERGY STAR maximum gallons per minute is measured under a standard test. Incoming groundwater in a cold climate is colder than the test condition, so real-world flow at a usable temperature can be lower. We score the official maximum GPM so models are compared on the same test, and we do not estimate a climate-adjusted flow.
Minimum activation flow matters for low-flow fixtures
The official minimum gallons per minute is the flow needed to start the heater. A lower figure is better for low-flow showers and faucets. Some official records list 0.1 GPM; others list 1.0 GPM. We score this field with lower values better, and we show the official number on every card.
UEF in this ENERGY STAR set is tightly grouped
Certified gas tankless UEF values in this comparison run from 0.95 to 0.98. That is a real official difference, but it is narrower than the UEF spread on heat pump water heaters. Maximum and minimum flow therefore carry substantial weight in the score. Fuel is listed as natural gas, propane, or both when the official record says so.
Safety and installation
A gas tankless water heater needs correctly sized gas supply, combustion air, venting and a licensed installer. Incomplete venting or a undersized gas line is a carbon monoxide and fire hazard. Set the temperature to reduce scald risk. Follow the maker’s clearances, freeze-protection and condensate instructions (condensing units produce condensate that must be drained). This is not a substitute for the product manual, local code or a licensed plumber.
How this ranking is calculated
Score = weighted average of normalised specs, out of 10. Full method
Uniform energy factorOfficial ENERGY STAR uniform energy factor. Higher means more hot water delivered per unit of fuel. Missing values take the category minimum. Scale: 0.95 → 0.98.
34%
Maximum flowOfficial ENERGY STAR maximum gallons per minute. This is a lab rating, not a guarantee at your incoming water temperature. Missing values take the category minimum. Scale: 3.8 → 6 GPM.
32%
Minimum activation flowOfficial ENERGY STAR minimum gallons per minute needed to start the heater. Lower is better for low-flow fixtures. Missing values take the category minimum (the highest minimum flow in the set). Scale: 1 → 0.1 GPM.
18%
Connected capableYes when the official ENERGY STAR record lists the model as connected-capable. Unstated counts as no.
Why aren't electric tankless water heaters in this ranking?
The ENERGY STAR certified water heaters dataset we used (retrieved November 13, 2026) lists hybrid/electric heat pump, gas tankless, gas storage and solar categories. It does not list electric resistance tankless models as ENERGY STAR certified in the records we pulled. We did not invent a comparison from retailer listings.
Why aren't gas storage water heaters ranked here?
ENERGY STAR certified gas storage models exist, and official UEF, first-hour rating and storage volume are complete on that list. Many of those listings are brand rebadges of the same 40- and 50-gallon platforms, with official UEF values clustered at 0.88 to 0.90. We did not mix storage tanks into this tankless table, and we did not build a separate storage ranking in this pass.
What does the official Camplux brand name mean?
The ENERGY STAR record lists the brand as “JUNSKY, CAMPLUX, GASLAND, ThermoMate” on the RT199iB and RP199iB. We display Camplux as the retail-facing name, show that official brand string in the details, and search Amazon with the cleaned model number. The UEF and flow figures are the ones on that official record.
Did you test these water heaters or list prices?
No. HeatPick ranks official ENERGY STAR certified ratings only. We do not test units, and we do not show prices or customer ratings.
Why do recirculation, vent diameter and indoor versus outdoor installation say Not listed?
Vent type is on the ENERGY STAR record and is shown when it is present (for example Power Vent or Direct Vent). Recirculation pumps, indoor versus outdoor rating and vent diameter are not reliably present in the certified-product fields we used. We did not fetch a maker product page for these models in this pass, so we leave those extras as Not listed rather than copying them from a retailer listing.