Quick Answer
The GPM (gallons per minute) rating you need for your tankless water heater depends on the number of hot water uses or “fixtures” you need to run at the same time. Most residential tankless heaters range from 2-5 GPM, with higher end models up to 10 GPM or more. As a general guide:
– For a single bathroom, look for a heater with a minimum of 2 GPM
– For a 2-3 bathroom home, look for 3-4 GPM
– For a large home with multiple bathrooms, look for 5+ GPM
You’ll also want to account for any other uses like dishwashers, washing machines, etc running at the same time. When in doubt, go with a higher GPM than you think you need to ensure adequate hot water flow.
How Do I Determine the Right GPM?
Determining the right GPM tankless water heater comes down to understanding your household’s peak hot water demand – how many fixtures you may need running hot water simultaneously. Here are some tips:
Count your hot water fixtures
Make a list of all fixtures that use hot water in your home. This includes:
– Showers
– Bathroom sinks
– Kitchen sink
– Dishwasher
– Washing machine
– Outside faucets
– Etc.
For showers and faucets, count each one individually.
Estimate flow rates
Next, figure out the flow rate of each fixture in gallons per minute (GPM):
– Showers – 2-2.5 GPM
– Bathroom sinks – 1-2 GPM
– Kitchen sinks – 1.5-2.5 GPM
– Dishwasher – 1.5-2.5 GPM
– Washing machine – 2-5 GPM
– Outside faucets – 4-8 GPM
These are general estimates – you may want to measure the actual flow rate with a “bucket test” for more accuracy. Time how long it takes to fill a 5 gallon bucket from each faucet.
Calculate the total needed GPM
Add up the flow rates for all fixtures that may run simultaneously – this is your peak demand GPM.
As an example:
– 2 showers x 2.5 GPM each = 5 GPM
– 2 bathroom sinks x 2 GPM each = 4 GPM
– Kitchen sink x 2 GPM = 2 GPM
– Dishwasher x 2 GPM = 2 GPM
– Total peak demand = 13 GPM
In this example, you would want a tankless water heater rated for at least 13 GPM to handle peak demand.
Factor in a buffer
It’s a good idea to add 20-25% more GPM capacity as a buffer, since your actual peak demand may be higher than calculated.
In the example above, adding 25% to 13 GPM means choosing a heater rated for around 16 GPM.
The buffer accounts for variations in flow rates and ensures you have adequate hot water even during periods of maximum use.
What Size Tankless Water Heater Do I Need?
Once you’ve calculated your peak GPM demand, choosing the right tankless water heater size is straightforward. Here are some general sizing guidelines:
| Peak Demand GPM | Minimum Tankless Heater Size |
|---|---|
| 1-2 GPM | 2 GPM heater |
| 2-4 GPM | 3-4 GPM heater |
| 4-8 GPM | 6-8 GPM heater |
| 8-12 GPM | 10-11 GPM heater |
| 12+ GPM | 14+ GPM heater or multiple heaters |
Choose a heater with a GPM rating at least as high as your calculated demand, or the next size up for a buffer.
In our earlier example with 13 GPM peak demand, a 16 GPM tankless heater would be appropriately sized with room to spare.
For very high demand households, multiple smaller tankless heaters can be combined. This allows flexibility to activate only the needed capacity during different demand periods.
How Many Bathrooms Can a Tankless Water Heater Support?
As a very general rule of thumb:
– 2-3 GPM tankless heater – good for 1 bathroom
– 4-5 GPM tankless heater – good for 2-3 bathrooms
– 6-8 GPM tankless heater – good for 3-4 bathrooms
– 10+ GPM tankless heater – good for 5 or more bathrooms
But the actual number of bathrooms depends a lot on your fixtures and peak flow rates. For example, a high-end master bathroom with multiple body jets may need 4 GPM just for that one shower.
It’s better to calculate your specific demand rather than going by number of bathrooms alone. Even a smaller home with only 1-2 bathrooms may need a large heater if there are high flow fixtures.
How Do I know if my Natural Gas Line is Big Enough?
Gas-fired tankless water heaters need a certain gas line capacity to function properly, measured in gas input rating (BTU/hr). Too small a gas line will starve the heater of fuel.
Here are some natural gas line sizing guidelines:
For most residential heaters:
– Up to 150,000 BTU – 1/2 inch gas line
– Up to 250,000 BTU – 3/4 inch gas line
For larger heaters:
– Up to 400,000 BTU – 1 inch gas line
– Over 400,000 BTU – 1 1/4 inch or larger gas line
This depends on the length of pipe and fittings used. Longer pipe runs may need a larger line size.
You can calculate the existing gas line size by measuring the inside diameter of the pipe. Anything 1/2″ or smaller is likely insufficient for a whole-house tankless unit.
If in doubt, consult a professional plumber to verify your gas line capacity before installing the new heater. Upgrading an undersized gas line can add cost to the installation.
Should I Get a Condensing or Non-Condensing Tankless Heater?
Condensing tankless heaters are generally more efficient (up to 98% EF) than non-condensing models (around 80% EF). But they have a few downsides:
– More expensive upfront cost
– Need PVC venting which may add installation cost
– May corrode from acidic condensate if not installed properly
Pros of condensing models:
– Higher efficiency – lower gas or electric costs
– Smaller size for equivalent GPM
– Eco-friendly – lower emissions
Pros of non-condensing models:
– Lower purchase price
– Can use existing B-venting in some retrofits
– Less complex venting requirements
For most homes, a condensing heater makes the most sense if you can fit it into your budget. But non-condensing models may be a good choice for basic needs or retrofits.
Whole House vs. Point-of-Use Tankless Heaters
Whole house tankless heaters are designed to supply hot water to an entire home. They are much higher capacity and get installed centrally to serve multiple hot water uses.
Point-of-use tankless heaters provide hot water to single remote fixtures like a bathroom or kitchen sink. They are compact and mount directly at the fixture.
Whole house tankless pros:
– One central heater for whole home
– Constant hot water at all fixtures
– May be more convenient and cost effective for larger homes
Point-of-use tankless pros:
– Small and can fit in tight spaces
– Good for remote fixtures away from central heater
– Less expensive than whole house units
– Good for homes with lower hot water needs
Many homes use a combination of both. A large whole-house unit provides hot water to most of the home, supplemented by compact point-of-use units in a few remote areas. This hybrid approach provides the best of both systems.
Electric vs. Gas Tankless Heaters
Gas tankless heaters are more common, but electric models are available if gas lines are not an option.
Pros of gas tankless:
– Often less expensive to operate than electric
– Can support higher GPM for whole house needs
– Compact – require little exterior venting
Pros of electric tankless:
– No need for gas lines – can go anywhere with sufficient electric service
– Lower flow rates suitable for point-of-use applications
– Easy to install and maintain
– Provide hot water during power outages if on a generator backed circuit
Electric tankless have limited flow capacity, usually 3-6 GPM maximum. For higher demand households, multiple electric units can be combined.
Make sure your electric panel can support the high power draw of an electric tankless. Upgrading the electric service may add cost.
Buying a Tankless Water Heater
When shopping for a tankless water heater, keep the following in mind:
– Choose an appropriate size based on your peak hot water demand
– Look for models rated for steady-state and temperature rise GPM
– Condensing models offer the highest efficiency
– Make sure your gas or electric service can support the heater capacity
– Check warranty – tankless units often have 10 years or more of coverage
– Professional installation is recommended
Leading tankless water heater brands include Rinnai, Navien, Noritz, Rheem, and Takagi. All offer a range of sizes and features across gas-fired and electric models.
Getting the right size heater with sufficient capacity for your home’s demand is key to satisfaction with tankless hot water.
Conclusion
Choosing the optimal tankless water heater for your home comes down to understanding your peak usage in gallons per minute (GPM) and selecting an appropriately sized unit with a buffer. This ensures you’ll have sufficient, consistent hot water for all your bathroom and kitchen needs. With good sizing, proper installation, and routine maintenance, a tankless water heater can provide decades of reliable service and energy savings.
