The American GB61-40T40 400 is a tall 40-gallon nominal ProLine water heater for natural gas, rated at 40,000 BTU/h input. It uses atmospheric venting and carries a six-year limited tank and parts warranty. For a replacement, our starting point is the existing gas supply and vent layout, followed by hot-water demand and physical fit.
Where this tall model fits
This 18-inch-diameter tall tank can be useful where a wider 40-gallon model would crowd the installation. Measure the door opening, pan and service access as well as the tank itself. An existing 40-gallon label does not tell us whether this particular heater will fit the same vent connection.
The listed 78-gallon first-hour rating is higher than the 62-gallon rating of the 30-gallon GB61 tall model. That makes this a useful comparison when morning demand is the concern. We still check the actual draw pattern: overlapping showers and a large tub can change the recommendation even when the existing tank is also labeled 40 gallons.
GB61-40T40 400 specifications
| Exact model | GB61-40T40 400 |
|---|---|
| Fuel | Natural Gas |
| Nominal capacity / rated storage | 40 gallons / 38 gallons |
| Input | 40,000 BTU/h |
| First-hour rating | 78 gallons, as listed by American |
| UEF | 0.64 on product page; see documentation note |
| Tank diameter | 18 inches |
| Product-page height | 58¼ inches; see drawing dimensions below |
| Drawing A: with draft hood | 61¾ inches; additional vent and service space required |
| Insulation blanket supplied | Yes |
| Limited warranty | 6-year tank and 6-year parts; conditions apply |
Fuel, venting and replacement planning
Order the factory natural-gas model GB61-40T40 400. The propane counterpart has a different series number and burner input. We verify the fuel shown on the data plate and the gas supply serving the location, rather than relying on the appearance of the old tank. Supply sizing must account for the other appliances on the system.
The atmospheric design relies on an appropriate metal vent and adequate combustion air. During a site visit, we assess the entire vent route, its condition and any appliances sharing it. The existing connector is not automatically reusable just because it reaches the new draft hood. This is also why a model chosen for a power-vent or direct-vent installation requires a separate review.
The thermopile-powered control does not need an external electrical supply. That can be useful where the present heater has no nearby outlet, but it does not settle the other installation requirements. We also check access to the shutoff, the drain arrangement, thermal-expansion provisions and the space needed to service the heater after installation.
Dimensions and manufacturer documentation notes
The exact product page lists 0.64 UEF, while the August 2025 family sheet lists 0.65 for GB61-40T40. This guide uses the product-page value and recommends checking the supplied EnergyGuide. The product page also lists a 1-inch water connection, while the linked installation manual specifies ¾-inch NPT connections. Confirm the actual fittings before ordering installation materials. The drawing shows 61¾ inches with the draft hood; 58¼ inches is the tank-top height.
The series drawing lists a 3- or 4-inch draft hood for this model family. That entry is not permission to choose a vent diameter from a shopping list: the installer must check the supplied hood, the complete vent system and the applicable installation requirements. Tank dimensions also exclude the space needed for piping, a pan where required and future service.
American lists an insulation blanket as supplied with this model. The representative manufacturer photograph does not show it. Use the instructions supplied with the actual unit when installing the blanket and preserve the required access and air openings.
Compare the other tall ProLine options
For the same fuel, compare 30-gallon GB61-30T32R 400 and 50-gallon G61-50T40R 400. Capacity, diameter and first-hour performance matter together. If the property uses propane, see the separate GB61-40T40 401 propane guide.
Browse the American Water Heaters manufacturer page and our water heater model catalog for other configurations. A replacement quote should identify the full model and fuel so you can compare like-for-like equipment.
Service and warranty considerations
We want the replacement to remain accessible after the work is complete. Leave room to reach the drain, inspect the burner area and service the anode. Discuss the maintenance schedule for your water conditions at installation; a self-cleaning dip tube does not remove the need for ongoing care.
The six-year coverage is a manufacturer’s limited tank and parts warranty. It is not a promise of six years of free installation labor. Review the certificate for eligibility, exclusions and claim requirements, and keep the purchase and installation records with the model and serial number.
Frequently asked questions
Why do the 40-gallon efficiency and connection figures differ?
American’s product page and linked family documents do not fully agree. The product page gives 0.64 UEF and a 1-inch water connection; the sheet gives 0.65 UEF and the installation manual describes ¾-inch NPT water connections. Verify the delivered unit’s label and fittings.
Is GB61-40T40 400 natural gas or propane?
GB61-40T40 400 is the factory natural gas version with 40,000 BTU/h input. Select the correct fuel model; American’s linked warranty excludes attempted conversion from one gas type to another.
How much hot water does this 40-gallon model provide?
American lists 38 gallons of rated storage and a 78-gallon first-hour rating. Actual shower and tub performance depends on incoming water temperature, fixture flow and the timing of demand. We size the replacement around those conditions.
Does it need an electrical outlet or a power-vent blower?
The listed thermopile gas control operates without external electrical power, and this is an atmospheric-vent model. Its vent system and combustion-air supply still need to be suitable for the installation.
What does the six-year warranty cover?
American lists six years of limited tank and parts coverage, subject to the written warranty. Removal and installation labor and related expenses are generally excluded. Keep the installation record and the unit’s model and serial numbers.
Official American product documents
- GB61-40T40 400 product page
- ProLine atmospheric-vent specification sheet, NRGSS00115 (August 2025)
- Residential gas FVIR installation and care manual, 100263114 (January 2015)
- American residential limited warranty, 100287889
Specifications checked October 17, 2026. Confirm the delivered unit’s data plate and supplied instructions when ordering and installing.