If you are shopping for a 40-gallon gas water heater rated near 40,000 BTU, none of the supplied listings is a conventional 40-gallon tank model. Four are natural-gas tankless heaters with substantially different capacities, and one is a small electric under-sink heater. The Rheem and Westinghouse models are best suited to higher-demand homes; ThermoMate and APUS offer lower-flow tankless options. Compare the product type, gas supply, installation requirements, and hot-water demand before choosing.
| Product | Type and stated capacity | Best suited for |
|---|---|---|
| Rheem RTG Series | Outdoor natural-gas tankless; 8.4 GPM, 180,000 BTU | Higher-demand homes seeking outdoor installation |
| Westinghouse Tankless | Outdoor natural-gas tankless; 8.1 GPM, 180,000 BTU | Homes with several fixtures used together |
| ThermoMate CZ422NG | Indoor natural-gas tankless; up to 5 GPM, 113,000 BTU | Smaller demand and indoor installation |
| APUS Natural Gas Tankless | Indoor natural-gas tankless; 4.42 GPM, 95,000 BTU | Moderate demand and adjustable temperature control |
| 4-Gallon Electric Under-Sink Heater | Electric tank; 4 gallons, 120V, 1,500W | Point-of-use hot water, not whole-home replacement |
Rheem RTG Series Outdoor Tankless
Best for: Households with higher simultaneous hot-water demand that can use an outdoor tankless installation. This model is listed at 8.4 GPM and 180,000 BTU per hour, and the supplied features describe capacity for three to four simultaneous showers at a 35°F temperature rise.
The non-condensing design is intended for outdoor installation, which eliminates the need for indoor venting. It has a stated 0.82 Uniform Energy Factor and on-demand heating rather than stored tank capacity. It is a different type of equipment from a 40-gallon, 40,000-BTU storage heater, so compare its flow performance and installation needs rather than its tank size.
Limitation: It is designed for outdoor installation and needs an appropriate location and qualified installation. Its 180,000-BTU input is far above the 40,000-BTU figure in the search request.
Westinghouse 8.1 GPM Outdoor Tankless
Best for: Homes that need hot water at several fixtures at once and can install a unit outdoors. The listing specifies 8.1 GPM and 180,000 BTU, with support for three to five fixtures, including showers, sinks, dishwashers, and laundry appliances.
Its stated ±1°F temperature stability comes from self-modulating technology that adjusts gas and water flow. On-demand heating avoids storing a tank of hot water, while the compact wall-mounted design can save indoor space. Those features make this a higher-capacity alternative for buyers considering a tankless system instead of a conventional 40-gallon tank.
Limitation: The supplied details identify it as a non-condensing outdoor model. It does not provide the stored hot-water reserve or 40,000-BTU rating associated with the requested tank-heater specification.
ThermoMate CZ422NG Indoor Tankless
Best for: Buyers seeking an indoor natural-gas tankless unit for moderate household demand. It is rated up to 5 GPM and 113,000 BTU. The product information says it can provide hot water for two showers at the same time under stated conditions: 65°F inlet water and a 105°F output target.
Listed features include a smart flow sensor, temperature consistency of ±1°F, an oxygen-free copper heat exchanger, and multiple safety protections. The listing states a 0.81 UEF and freeze protection that activates at 41°F. It is a tankless alternative, not a direct replacement with the same tank volume and burner rating as the search phrase describes.
Limitation: The stated two-shower performance depends on inlet-water temperature and target temperature. Check the manufacturer’s installation requirements and local gas and venting conditions before purchase.
APUS 4.42 GPM Natural Gas Tankless
Best for: Homes with moderate hot-water demand that want an indoor tankless option with a stated adjustable temperature range. The listing gives a 95,000-BTU capacity and 4.42-GPM flow rate, plus temperature settings from 95°F to 140°F.
Features include a 0.66-GPM low-flow start, anti-freeze protection, an LED touchscreen, and temperature control stated within ±1°F. The product description says it can serve two showers and one sink, though actual flow available at a chosen temperature depends on inlet-water conditions and simultaneous demand. Its stated UEF is 0.81.
Limitation: Its maximum flow and heating input are lower than the Rheem and Westinghouse models listed above. The claims about multiple fixtures should be checked against the home’s incoming water temperature and fixture flow rates.
4-Gallon Electric Under-Sink Heater
Best for: Point-of-use needs at a kitchen, bathroom, or office where a small local supply of hot water is useful. The listing describes a 4-gallon electric tank with a 120V, 1,500W connection and wall- or floor-mounting options.
This is included only as a clearly different product type among the supplied listings. It may be relevant when a sink needs nearby hot water, but its small capacity and electric operation do not match a whole-house 40-gallon natural-gas water heater. It should not be treated as an alternative to any of the four gas tankless models for serving multiple household fixtures.
Limitation: The listing provides no gas burner rating and the capacity is 4 gallons, not 40 gallons. Do not select it as a replacement for a whole-home gas water heater.
Buying Guide: Choosing a 40-Gallon Gas Water Heater
Does the listing need to be a tank or tankless model?
A 40-gallon storage heater keeps a volume of water hot in a tank. A tankless model heats water as it flows through the unit and is usually compared by flow rate, temperature rise, and burner input. These are different ways to deliver hot water, so a tankless model’s GPM rating does not translate directly into a 40-gallon tank capacity.
If your existing heater is a 40-gallon tank, first decide whether you need a like-for-like replacement or are open to changing the system type. Tankless installation may involve different venting, gas-supply, mounting, and electrical requirements. Confirm what applies to the exact model and installation site.
What does 40,000 BTU tell you?
BTU per hour describes heating input, not storage volume. The products listed here range from 95,000 to 180,000 BTU per hour for gas tankless models, so they do not match a 40,000-BTU target. A larger input alone does not guarantee a better fit: the appropriate capacity depends on the home’s hot-water demand, incoming water temperature, and the appliance’s installation requirements.
When comparing conventional tank heaters, check both tank capacity and burner input on the product label or manufacturer documentation. Do not assume two heaters are interchangeable because they have similar tank volumes; recovery performance and venting requirements can differ.
How much hot water does your household use at once?
List the fixtures and appliances that may run simultaneously, such as showers, sinks, a dishwasher, or laundry equipment. For tankless models, compare the rated GPM at the temperature rise relevant to your location. A unit’s maximum GPM is not the same under every inlet-water condition.
The Rheem and Westinghouse listings have higher stated flow and BTU figures than the ThermoMate and APUS listings. That makes them candidates to investigate for greater simultaneous demand, not a guarantee of performance in every home. For a tank heater, consider the amount of stored hot water and how quickly it recovers after use.
Should the heater be installed indoors or outdoors?
Installation location can narrow the choices. The Rheem and Westinghouse products are described as outdoor models. ThermoMate and APUS are listed as indoor units. The supplied information does not provide every venting, clearance, or climate requirement, so consult the product’s installation manual and a qualified professional.
Outdoor installation can reduce the need for indoor vent routing, but the unit still needs a suitable location and protection appropriate to its instructions. Indoor units require an installation arrangement that meets the manufacturer’s venting and safety specifications.
What should you check before ordering?
- Fuel type: Confirm the appliance is designed for the home’s gas supply. These listings specify natural gas; do not assume they support another fuel.
- Capacity: Match tank gallons or tankless flow rate to the intended use. Verify the temperature-rise conditions behind any GPM claim.
- Installation: Check whether the unit is indoor or outdoor and review venting, mounting, gas-line, and electrical requirements.
- Temperature control: Compare the stated setpoint range and temperature stability if consistent shower temperature is important.
- Household demand: Account for the number of fixtures likely to run together, rather than choosing by BTU alone.
- Local requirements: Ask a qualified installer about applicable permits, codes, and site-specific requirements.
When is a small under-sink heater appropriate?
A 4-gallon electric heater is for a localized point-of-use need, not a whole-home gas-water-heater replacement. It may be worth considering only when a small nearby hot-water supply suits the application and the electrical and plumbing setup is appropriate. It is not a match for the requested 40-gallon gas specification.
Frequently Asked Questions
Is any listed product a 40-gallon, 40,000-BTU gas tank heater?
No. The supplied options include four natural-gas tankless heaters with 95,000–180,000 BTU ratings and one 4-gallon electric heater. None is identified as a 40-gallon gas storage heater rated at 40,000 BTU.
Can a tankless heater replace a 40-gallon tank?
It can be considered as a different system type, but it is not a direct gallon-for-gallon replacement. Compare demand, flow at the required temperature rise, fuel supply, installation requirements, and the household’s preference for stored versus on-demand hot water.
Which listed gas model has the highest stated flow rate?
The Rheem RTG Series is listed at up to 8.4 GPM. Westinghouse is listed at up to 8.1 GPM. Actual delivered flow depends on temperature rise and installation conditions.
Which listed model is intended for outdoor installation?
The Rheem RTG Series and Westinghouse tankless heater are described as outdoor models. Review each product’s installation instructions to confirm location and setup requirements.
Is the 4-gallon electric heater suitable for a whole house?
The supplied description identifies it as an under-sink heater for uses such as kitchens, bathrooms, and offices. Its 4-gallon capacity and electric operation do not match a whole-house 40-gallon gas heater.
Why does the incoming water temperature matter?
A tankless heater must raise the incoming water to the selected output temperature. Colder inlet water requires a larger temperature rise, which can reduce the flow available at that output temperature. Check the manufacturer’s capacity information for conditions relevant to your location.