When several showers, faucets, and appliances may run at once, choosing a whole-house water heater is about more than picking the biggest flow-rate claim. You also need to match the heater to your fuel source, incoming water temperature, home’s peak demand, and electrical or venting setup. These five tankless models range from compact electric units to high-output gas and propane options. Use their stated capacities as a starting point, then confirm sizing and installation requirements with a qualified professional.
Table of Contents
Quick Comparison
| Water Heater | Fuel | Claimed capacity | Best suited for | Key installation note |
|---|---|---|---|---|
| Stiebel Eltron Tempra 24 Trend | Electric | 24 kW; flow depends on temperature rise | Homes that want electric, on-demand hot water | 240V; product data recommends at least 150A electric service |
| MIZUDO 199,000 BTU Indoor Tankless | Natural gas | Up to 10.3 GPM, as stated by the manufacturer | Homes with several bathrooms and natural-gas service | Indoor installation requires the specified direct-vent system |
| ThermoMate 36kW Electric | Electric | Estimated 7.03 GPM at a 35°F rise | Large homes with high electrical capacity | 150A maximum draw; a 200A main service is strongly recommended |
| APUS 190,000 BTU Outdoor Propane | Propane | 8.1 GPM, as stated by the manufacturer | Homes with propane that can install the heater outdoors | Outdoor placement; requires propane and a 120V power connection |
| ThermoMate 27kW Electric | Electric | 5.27 GPM at a 35°F rise | Homes needing a mid-capacity electric option | 114A maximum draw; a 200A household service is strongly recommended |
Our Picks: Best Whole House Water Heaters For High Demand reviews
Stiebel Eltron Tempra 24 Trend: A Compact Electric Option
The Tempra 24 Trend is a wall-mounted, 24 kW electric tankless heater designed to supply hot water on demand. Its compact case and lack of venting can make it an appealing alternative when you want to save floor space and avoid adding a gas exhaust route. The unit has a digital temperature display and a copper heating system.
Its self-modulating operation is intended to adjust energy use to match hot-water demand. The listed temperature range is 68°F to 140°F, but the product information does not give a specific flow rate. That makes it especially important to size it against your home’s incoming water temperature and the number of fixtures you expect to use at once.
The main trade-off is the electrical supply. The listing calls for 240V and recommends at least 150A of electric service. Before buying, have an electrician check your panel capacity and determine whether the required circuits and wiring can be added. This is a smaller-capacity choice than the high-output gas models in this roundup, and it may not suit a home with several simultaneous hot-water demands.
Pros:
- Compact wall-mounted design saves floor space.
- Does not require venting.
- Self-modulating operation and a digital temperature display.
- Includes listed leakage and parts warranties.
Cons:
- No flow-rate figure is supplied, so sizing needs careful confirmation.
- Electrical service and circuit requirements may limit where it can be installed.
Verdict: Choose the Tempra 24 if you want a compact electric heater and your home’s electrical service can support it; consider a higher-capacity gas or propane unit if your peak demand is especially heavy.
MIZUDO 199,000 BTU Indoor Natural Gas Heater: High Stated Flow
This MIZUDO indoor natural-gas model is rated at 199,000 BTU and advertises up to 10.3 GPM. The manufacturer describes it as suitable for several bathrooms and household appliances running at the same time. That makes it one of the higher-capacity options here on paper, though actual usable flow depends on how much the unit needs to raise the incoming water temperature.
The heater has a stated 0.53 GPM minimum flow activation, adjustable temperature up to 149°F, and app control with an LED display. Its direct-vent arrangement is designed to draw combustion air from outdoors and send exhaust outside through a dedicated concentric vent system. Because venting is part of the installation, confirm the required kit, route, and local code requirements before choosing an installation location.
The manufacturer lists indoor installation and CSA certification, along with a two-year full-unit warranty and five-year heat-exchanger coverage. It also says the unit is not recommended above 3,000 feet and advises draining residual water if it will sit unused for an extended period in cold climates. Those limits matter if your home is at elevation or the heater is installed in a space exposed to freezing conditions.
Pros:
- High stated output for homes with several hot-water demands.
- Low listed activation flow may suit some lower-flow fixtures.
- App control, temperature adjustment, and operating alerts are included.
- Direct-vent design brings in combustion air from outdoors.
Cons:
- Requires natural-gas service and a suitable vent installation.
- Not recommended for elevations above 3,000 feet.
Verdict: The MIZUDO is a strong candidate for a high-demand home with natural gas and a workable direct-vent route, provided its elevation and installation limits fit your property.
ThermoMate 36kW Electric: The Highest-Capacity Electric Pick
The ThermoMate 36 kW is a high-output electric tankless heater built for homes with substantial hot-water demand. The product information estimates 7.03 GPM at a 35°F temperature rise and also gives a maximum flow figure of 8.8 GPM. These are not interchangeable promises for every home: colder incoming water and a larger temperature rise can reduce the flow available at the desired hot-water temperature.
The unit uses self-modulating power and has a digital control for temperatures from 80°F to 140°F. Its compact, wall-mounted design does not need a storage tank. The listing also describes ETL certification and protections for overheating, dry firing, and freezing.
Electrical capacity is the key buying test. This heater can draw up to 150A, requires four dedicated double-pole 40A breakers and specified copper wiring, and strongly recommends a 200A main service. Those requirements can make installation costly or impractical in a home with a smaller panel. Have a qualified electrician assess the full setup before ordering.
Pros:
- High stated output for an electric tankless unit.
- Self-modulating operation and one-degree temperature adjustment.
- Compact wall-mounted design without a storage tank.
- Listed safety protections include dry-fire and overheating shutdown.
Cons:
- Requires substantial electrical capacity and multiple dedicated circuits.
- Flow falls as the required temperature rise increases.
Verdict: Pick the ThermoMate 36kW if you need a high-capacity electric heater and an electrician confirms your service can handle its demanding installation requirements.
APUS 190,000 BTU Outdoor Propane Heater: A Flexible Fuel Choice
The APUS is an outdoor, liquid-propane tankless heater rated at 190,000 BTU and 8.1 GPM. The manufacturer says it can serve multiple points at once and lists a minimum flow activation of 0.79 GPM. As with other tankless models, the advertised flow should be checked against your local incoming water temperature and the temperature you need at the tap.
Its outdoor format can free up indoor utility space and avoids the indoor vent routing needed by an indoor gas heater. The listing calls for a 120V power connection, propane, and standard 3/4-inch water and gas connections. It includes app and wired-controller temperature adjustment, with a stated range of 95°F to 149°F.
The manufacturer lists an efficiency rating of 0.81 and safeguards for freezing, overheating, and flame failure. Outdoor placement still requires careful attention to the installation instructions and local conditions; a professional can confirm whether the proposed location and propane supply are appropriate. The listed warranty is two years for components.
Pros:
- High stated flow for a propane model.
- Outdoor installation can preserve indoor space.
- App and wired-controller temperature adjustments are available.
- Includes listed freeze, overheat, and flame-failure protections.
Cons:
- Requires propane, outdoor placement, and a 120V connection.
- Its stated flow is not a guarantee of the same output in every climate or installation.
Verdict: Choose the APUS if your home uses propane and has a suitable outdoor installation spot; look at the MIZUDO instead if you have natural gas and need an indoor unit.
ThermoMate 27kW Electric: A Mid-Capacity Option for Busy Homes
The ThermoMate 27 kW is a 240V electric tankless heater rated at 5.27 GPM at a 35°F rise. It is a step below the 36 kW ThermoMate in stated output, so it may suit a home with moderate-to-high demand that does not need the largest electric unit in this selection. Its actual ability to serve multiple fixtures depends on their combined flow and the temperature rise required.
The unit has self-modulating power, a digital display, and temperature adjustment from 80°F to 140°F in one-degree steps. Its wall-mounted design keeps the footprint small. The product listing also describes an updated heating system and temperature and flow sensors, plus overheating and no-water protections.
Like the larger ThermoMate, this model needs a careful electrical check. It can draw up to 114A and calls for three dedicated double-pole 40A breakers and specified wiring. The manufacturer strongly recommends a 200A household service for stable operation. If your panel cannot support that load, an electric tankless model may require costly electrical work or may not be a practical fit.
Pros:
- Stated 5.27 GPM flow at a 35°F rise.
- Self-modulating operation adjusts power to demand.
- Compact wall-mounted form saves floor space.
- Digital temperature adjustment and listed safety protections.
Cons:
- Requires three dedicated circuits and significant electrical capacity.
- Lower stated flow than the 36 kW electric model.
Verdict: The ThermoMate 27kW is a reasonable middle ground if your electrical system can support it and your peak demand is below what the 36kW model is meant to handle.
How to Choose a Whole-House Water Heater for High Demand
Estimate your peak hot-water demand
Think about which fixtures and appliances could run at the same time, not just how many people live in your home. Add the flow needs for likely combinations, such as two showers plus a dishwasher or a shower and laundry. A plumber can help measure fixture flow and compare it with the heater’s output at your required temperature rise.
Account for incoming water temperature
Tankless flow depends on how much the heater must warm the water. A unit may deliver a higher flow when incoming water is already warm, but less when it needs a larger temperature increase. Treat advertised maximum GPM as a reference, not a guarantee. Look for a flow figure tied to a temperature rise, and ask a professional to size the unit for your climate and target temperature.
Match the heater to your available fuel
Electric models avoid gas combustion venting, but high-output units can demand substantial electrical service and multiple dedicated circuits. Natural-gas and propane models can provide high stated output, but require the correct fuel supply and appropriate venting or outdoor placement. Start with the utilities and installation space your home can support before comparing features.
Check electrical, gas, and venting requirements
Confirm voltage, breaker count, wire size, gas connection, vent route, and service capacity with qualified tradespeople. A product that fits your hot-water needs may still be a poor match if your panel, gas line, or installation location cannot support it. Electrical upgrades and vent work can change the overall project considerably.
Consider installation location and climate
An indoor gas heater needs a suitable vent route, while an outdoor model needs an appropriate exterior location and protection that follows its installation instructions. In cold climates, freeze protection and drainage guidance are important. Check the manufacturer’s requirements for the specific model and your local building rules before installation.
Compare controls, temperature limits, and warranty coverage
Digital displays, app access, and remote controls can make adjustment easier, but they do not replace correct sizing. Compare each unit’s temperature range and warranty terms, including whether coverage differs between the full unit and heat exchanger. Also consider whether the manufacturer’s installation and maintenance requirements suit your household.
Frequently Asked Questions
What size tankless water heater do I need for a high-demand home?
Size it for the total hot-water flow you may use at one time and the temperature rise needed in your area. A professional can calculate both from your fixture flow rates and incoming water temperature. Bigger stated GPM alone does not ensure the heater will meet your home’s needs.
Can an electric tankless heater supply multiple showers?
It can, if its output at your required temperature rise is high enough and your electrical service supports the unit. The 36 kW ThermoMate lists an estimated 7.03 GPM at a 35°F rise, while the 27 kW model lists 5.27 GPM at the same rise. Actual performance depends on installation conditions and simultaneous demand.
Is a gas tankless heater better for a large household?
Gas models in this selection have high stated output, but gas is not automatically the best choice for every home. You need the right fuel supply, a suitable vent route for an indoor model, or an appropriate outdoor location for an outdoor model. Compare the installation requirements and expected flow at your water temperature.
Will a tankless heater provide unlimited hot water?
Tankless units heat water on demand and do not rely on a stored tank that can run empty. However, they have a maximum heating capacity. If your household draws more hot water than the unit can heat at the required temperature, flow or temperature may fall.
Do these heaters need a professional installation?
These models involve significant electrical, gas, propane, water, or venting requirements. Use a qualified professional to confirm the home’s capacity, select a suitable location, and follow product instructions and local codes. This is especially important for the high-amperage electric models.
Which option is best if my home has propane?
The APUS is the propane model in this group. It is designed for outdoor installation and lists an 8.1 GPM flow rate. Confirm that its fuel, power, placement, and flow requirements fit your home before choosing it.
Final Recommendation
For the highest stated output in an indoor natural-gas model, the MIZUDO is the leading option here, provided you can meet its direct-vent and elevation requirements. The APUS is the propane alternative for a suitable outdoor location. If you need electric, the ThermoMate 36kW offers the highest stated capacity, but its electrical demands are substantial; the 27kW ThermoMate is a lower-output alternative. The Stiebel Eltron Tempra 24 is the compact choice when its capacity suits your needs and your electrical service can support it. In every case, confirm peak flow and installation requirements with a qualified professional before buying.
