Gas Tankless Water Heater Sizing Calculator

To size a gas tankless water heater, calculate the total required flow rate (GPM) based on simultaneous hot water usage and determine the needed temperature rise. Typically, a family of four may require a unit with a minimum of 7-9 GPM capacity, while a family of five might need 9-11 GPM. Proper sizing ensures adequate hot water supply.

Gas Tankless Water Heater Sizing Calculator

Select Household Size: 1-2 People 3-4 People 5+ People Hot Water Demand (GPM): Calculate

FAQs

How do you size a gas tankless water heater? To size a gas tankless water heater, you need to consider the following factors:

• Flow Rate (GPM): Calculate the total GPM required by adding up the simultaneous hot water usage in your home. For example, if a shower uses 2 GPM, and a washing machine uses 1.5 GPM, and you want to run both simultaneously, you'd need a tankless water heater with a minimum of 3.5 GPM capacity.
• Temperature Rise: Determine the temperature rise required by subtracting the incoming water temperature from your desired hot water temperature. In the U.S., the incoming water temperature varies by region, but it's typically between 40-60°F. If you want 120°F hot water and your incoming water is 50°F, you need a 70°F temperature rise.
• Sizing Chart: Consult the manufacturer's sizing chart for their tankless water heaters. They usually provide information on flow rates and temperature rises for different models.

How big of a tankless water heater for a family of 4? For a family of four, you might need a tankless water heater with a minimum flow rate of 7-9 GPM, depending on your hot water usage patterns. This should be sufficient for showers, dishwashing, and other typical household tasks.

What size natural gas line do I need for a tankless water heater? A 3/4-inch gas line is typically adequate for most residential tankless water heaters. However, it's important to consult your local building codes and the manufacturer's recommendations to ensure proper gas line sizing.

How big of a tankless water heater do I need for a family of 5? For a family of five, you may require a tankless water heater with a minimum flow rate of 9-11 GPM to ensure an adequate supply of hot water for multiple simultaneous uses.

Should you oversize a tankless water heater? Oversizing a tankless water heater can lead to energy wastage and higher upfront costs. It's best to choose a tankless water heater that meets your specific hot water demands without significantly exceeding them.

Is a 1-inch gas line enough for a tankless water heater? In most residential applications, a 3/4-inch gas line is sufficient for a tankless water heater. A 1-inch gas line is typically not necessary, but it may be required for larger, commercial-grade tankless units.

Is a 40-gallon water heater enough for a family of 5? A 40-gallon water heater may not provide enough hot water for a family of five, especially if you have simultaneous hot water needs. It's recommended to consider a larger capacity or a tankless water heater for larger families.

Does a tankless water heater need a 3/4-inch pipe? A 3/4-inch pipe is commonly used for the gas supply line to a tankless water heater. However, you should consult the manufacturer's specifications and local building codes for precise sizing requirements.

How much hot water is used for a 5-minute shower? An estimate for a 5-minute shower at 2.5 GPM (typical flow rate for a showerhead) would be approximately 12.5 gallons of hot water.

Do you need a 220 line for a tankless water heater? Tankless water heaters typically require a 240-volt electrical connection, which is similar to a 220-volt line. Consult the manufacturer's specifications for the precise electrical requirements of your specific model.

How many BTU can a 3/4-inch gas line carry? A 3/4-inch gas line can carry approximately 200,000-250,000 BTU per hour, but the exact capacity may vary depending on factors such as gas type and local codes.

Do you need a 1/2 or 3/4 gas line for a tankless water heater? For most residential tankless water heaters, a 3/4-inch gas line is recommended. A 1/2-inch line may be sufficient for smaller units, but it's essential to check the manufacturer's specifications and local codes.

Is a tankless water heater better than a tank for a large family? Tankless water heaters are often a better choice for large families because they provide hot water on-demand, eliminating the risk of running out of hot water. However, proper sizing is crucial to ensure they meet the family's hot water needs.

How many GPM do I need for a residential tankless water heater? The required GPM for a residential tankless water heater depends on your household's hot water usage. A typical minimum would be around 5-6 GPM for a small household, but larger families may need 8-10 GPM or more.

What is the best gas tankless water heater? The best gas tankless water heater can vary depending on your specific needs and budget. Some reputable brands to consider are Rheem, Rinnai, Navien, and Noritz. Research and read reviews to find the one that suits your requirements.

What happens if a tankless water heater is undersized? If a tankless water heater is undersized, it may not be able to provide enough hot water to meet your household's demands. This can result in inadequate hot water during peak usage times.

Can you overwork a tankless water heater? Tankless water heaters are designed to handle specific flow rates and temperature rises. Exceeding these limits can overwork the unit and lead to reduced efficiency, premature wear, or insufficient hot water supply.

How many BTUs should my tankless water heater be? The required BTU rating for a tankless water heater depends on your hot water demands, temperature rise, and flow rate requirements. It's typically specified by the manufacturer and should meet your household's needs.

What size gas line do I need for 200,000 BTU? A 1-inch gas line is typically sufficient for a gas appliance with a 200,000 BTU input. However, local codes and the specific requirements of your installation should be considered.

What 2 system requirements are necessary to size a tankless water heater? The two essential system requirements for sizing a tankless water heater are flow rate (GPM) and temperature rise (°F). These factors help determine the appropriate unit for your hot water needs.

Do I need to upgrade my gas meter for a tankless water heater? In some cases, you may need to upgrade your gas meter to accommodate the increased gas demand of a tankless water heater. Consult your gas utility company and local codes for guidance.

Is there a big difference between a 40-gallon and 50-gallon water heater? The difference between a 40-gallon and 50-gallon water heater is 10 gallons, which can provide additional hot water capacity. It's a relatively modest increase and may or may not be sufficient for your needs.

How many people can shower with a 40-gallon water heater? A 40-gallon water heater can typically provide enough hot water for 2-3 consecutive showers, depending on the shower duration and flow rate.

Is a 40-gallon water heater enough for a 3-bedroom house? A 40-gallon water heater may be sufficient for a 3-bedroom house if the hot water usage is relatively low. However, it's essential to consider the number of occupants and their hot water habits.

Do I need a drain pan under a tankless water heater? A drain pan is not typically required for tankless water heaters because they don't store water. However, local building codes may have specific requirements, so it's essential to check with local authorities.

Can you use PVC pipe on a tankless water heater? Yes, PVC pipe can be used for venting on some tankless water heater models, but it must be rated for high-temperature applications and comply with local building codes.

Does a tankless water heater need a separate gas line? Tankless water heaters require a dedicated gas line to ensure a consistent and sufficient gas supply. Sharing a gas line with other appliances may lead to performance issues.

How many gallons is a 20-minute shower? A 20-minute shower at a typical flow rate of 2.5 GPM would use approximately 50 gallons of water.

How many showers before hot water runs out? The number of showers before hot water runs out depends on the capacity of your water heater and the hot water usage patterns in your household. Larger heaters can provide more consecutive showers.

What is a reasonable shower time? A reasonable shower time depends on your household's hot water capacity and your desire for water conservation. On average, a 10-15 minute shower is considered reasonable.

Can you run PEX directly to a tankless water heater? PEX can be used for the water supply lines to a tankless water heater, provided it is rated for hot water use and complies with local plumbing codes.

Why do tankless water heaters need 3 breakers? Tankless water heaters may require multiple breakers because they have electric components for controls, heating elements, and sometimes fans or pumps. Each component may have its own dedicated circuit.

Do I need to upgrade my electrical service for a tankless water heater? Depending on the electrical requirements of the tankless water heater and your existing electrical service, you may need to upgrade your electrical panel to accommodate the additional load. Consult with an electrician for a proper assessment.

What happens if a gas line is too big? If a gas line is too large for the appliance it serves, it can lead to poor gas flow, inefficient combustion, and may not allow the appliance to operate correctly. Gas line sizing should match the appliance's requirements.

What is the most common gas line size? The most common gas line size for residential applications is 3/4-inch. However, the size may vary depending on the specific gas load and local codes.

How many appliances can you have on a 3/4-inch gas line? The number of appliances that can be connected to a 3/4-inch gas line depends on their individual gas requirements and the total gas load the line can handle. It's important to ensure proper sizing for safe operation.

Can you run 2 showers with a tankless water heater? Whether you can run two showers with a tankless water heater depends on the heater's flow rate, temperature rise, and the flow rate of the showerheads. Some tankless units can handle multiple showers simultaneously, while others may require staggered use.

What is the minimum gas pressure for a tankless water heater? The minimum gas pressure requirement for a tankless water heater is typically specified by the manufacturer and can vary depending on the model. It's essential to meet or exceed this requirement for proper operation.

How much combustion air is needed for a tankless water heater? The amount of combustion air needed for a tankless water heater varies by model and can range from 1 cubic foot per 1,000 BTU to 2 cubic feet per 1,000 BTU. Proper ventilation is crucial to ensure safe operation.

How big of a tankless water heater do I need for a 4-bedroom house? The size of the tankless water heater needed for a 4-bedroom house depends on the number of occupants and their hot water usage patterns. A unit with a flow rate of 7-9 GPM may be suitable for many households.

What size tankless water heater do I need for a 4-person household? For a 4-person household, a tankless water heater with a flow rate of 7-9 GPM should be sufficient for typical hot water demands.

Can you buy too big of a tankless water heater? While it's essential to size a tankless water heater correctly, buying one that is too big may lead to energy wastage and higher upfront costs. It's best to choose a unit that meets your hot water needs without significantly exceeding them.

What is the easiest tankless water heater to install? The ease of installation depends on various factors, including your existing plumbing and gas lines. Some tankless water heaters are designed with simplified installation in mind, but the ease of installation can also vary based on your specific setup.

What size tankless water heater for a 3-bedroom house? For a 3-bedroom house, a tankless water heater with a flow rate of 6-8 GPM should be adequate for most hot water needs.

What size tankless water heater is equivalent to a 40-gallon water heater? A tankless water heater with a flow rate of around 5-6 GPM is roughly equivalent in capacity to a 40-gallon tank water heater.

What is the most reliable brand of tankless water heater? Reliability can vary, but some reputable tankless water heater brands known for quality and durability include Rheem, Rinnai, Navien, and Noritz. Reading customer reviews and consulting with professionals can help you choose a reliable unit.

Do gas tankless water heaters use a lot of gas? Gas tankless water heaters can have high gas consumption during periods of high demand, such as when multiple hot water fixtures are in use simultaneously. However, they are generally more efficient than tank water heaters because they only heat water as needed.

Is Rinnai better than Rheem? The choice between Rinnai and Rheem depends on your specific needs, budget, and preferences. Both brands offer a range of tankless water heaters, and the performance and reliability of individual models can vary. Research and compare specific models to determine the best fit for your situation.

What is the most common issue with tankless water heaters? Common issues with tankless water heaters can include mineral buildup in the heat exchanger, inadequate gas supply, and improper installation. Regular maintenance and professional installation can help mitigate these issues.

Why not install a tankless water heater? While tankless water heaters have many benefits, they may not be suitable for every situation. Reasons not to install a tankless water heater can include higher upfront costs, the need for adequate gas and electrical supply, and potential challenges with retrofitting.

What happens if a gas line is too small for a tankless water heater? If the gas line is too small for a tankless water heater, it may not provide sufficient gas flow to meet the heater's demand, resulting in reduced performance and potentially unsafe operation. Proper gas line sizing is crucial.

Why does it take so long for hot water with a tankless water heater? Tankless water heaters provide hot water on-demand, but there can still be a delay as the heater activates and heats the water. This delay may be longer if the heater is located far from the point of use.

Does water pressure decrease with a tankless water heater? Water pressure should not decrease with a tankless water heater, as long as the unit is properly sized and installed. However, flow rate may vary depending on the number of fixtures in use simultaneously.

Is higher or lower BTU better for a tankless water heater? The BTU rating of a tankless water heater should match your hot water demands. Higher BTU ratings are suitable for homes with high demand, while lower BTU units are more appropriate for lower-demand situations.

How many BTUs per hour for a tankless water heater? The number of BTUs per hour required for a tankless water heater depends on the unit's size and capacity. It can range from around 120,000 BTU/hr for smaller units to 200,000 BTU/hr or more for larger models.

What size water line do you need for a tankless water heater? The size of the water line needed for a tankless water heater depends on the unit's flow rate and local plumbing codes. Typically, 3/4-inch or 1-inch water lines are used.

Do you need a 220 line for a tankless water heater? Tankless water heaters typically require a 240-volt electrical connection, which is similar to a 220-volt line. Consult the manufacturer's specifications for the precise electrical requirements of your specific model.

What size gas line do I need for 200,000 BTU? A 1-inch gas line is typically sufficient for a gas appliance with a 200,000 BTU input. However, local codes and the specific requirements of your installation should be considered.

How many BTUs can a residential gas meter handle? The capacity of a residential gas meter can vary, but it's typically rated to handle the gas demand of typical household appliances, including a tankless water heater. However, it's essential to confirm with your gas utility company and adhere to local codes.

How many people can shower with a 50-gallon water heater? A 50-gallon water heater can typically provide enough hot water for 2-4 consecutive showers, depending on the shower duration and flow rate.

Is a 40-gallon water heater enough for a 3-bedroom house? A 40-gallon water heater may be sufficient for a 3-bedroom house if the hot water usage is relatively low and the household does not have simultaneous hot water demands.

How much hot water does a 10-minute shower use? A 10-minute shower at a typical flow rate of 2.5 GPM would use approximately 25 gallons of hot water.

Is a 40-gallon gas water heater enough for a family of 4? A 40-gallon gas water heater may be adequate for a family of four if the hot water usage is moderate and not simultaneous. However, if you have simultaneous hot water demands, you may need a larger unit or a tankless water heater.

How many people can shower with a 40-gallon water heater? A 40-gallon water heater can typically provide enough hot water for 1-2 consecutive showers, depending on the shower duration and flow rate.

How many bathrooms can you fit in a 40-gallon water heater? The number of bathrooms a 40-gallon water heater can serve depends on the hot water usage patterns. It may be suitable for a single bathroom or multiple bathrooms if usage is staggered.

Can you use a plastic drain pan under a gas water heater? Plastic drain pans are not recommended for use under gas water heaters, as they may not be heat-resistant and could melt or deform. It's best to use a metal drain pan designed for this purpose.

Where should a tankless water heater not be installed? Tankless water heaters should not be installed in locations with inadequate ventilation, where they may be exposed to freezing temperatures, or where they are not easily accessible for maintenance.

Can you use PVC on a water heater pan drain? PVC can be used for the drain line from a water heater pan, provided it complies with local plumbing codes and is installed correctly.

Can you use PEX line for a tankless water heater? PEX can be used for the water supply lines to a tankless water heater, provided it is rated for hot water use and complies with local plumbing codes.

Can I connect PEX directly to a tankless water heater? Yes, PEX can be connected directly to a tankless water heater for hot water supply lines, as long as it meets local plumbing code requirements.

Do you need special pipes for a tankless water heater? Tankless water heaters typically require durable, heat-resistant pipes for both the water supply and venting. Copper, PEX, CPVC, and special stainless steel venting materials may be used, depending on local codes and the specific unit's requirements.

Can a tankless water heater share a gas line with a furnace? A tankless water heater and a furnace can share a gas line if the gas supply is adequate to meet the combined demand of both appliances. Proper gas line sizing and adherence to local codes are essential.

Can a tankless water heater replace a gas water heater? Yes, a tankless water heater can replace a gas water heater, provided it is properly sized, installed, and meets your hot water needs.

How long of a shower is 40 gallons? The length of a shower provided by a 40-gallon water heater depends on the flow rate of the showerhead. On average, a 40-gallon water heater may provide a 10-15 minute shower.

How long of a shower is 55 gallons? A 55-gallon water heater may provide a shower of approximately 15-20 minutes, depending on the flow rate of the showerhead.

How many gallons does a 40-minute shower use? A 40-minute shower at a typical flow rate of 2.5 GPM would use approximately 100 gallons of water.

How much hot water does a 30-minute shower use? A 30-minute shower at a typical flow rate of 2.5 GPM would use approximately 75 gallons of hot water.

How much hot water does a 5-minute shower use? A 5-minute shower at a typical flow rate of 2.5 GPM would use approximately 12.5 gallons of hot water.

How many gallons of hot water does a 5-minute shower use? A 5-minute shower at a typical flow rate of 2.5 GPM would use approximately 12.5 gallons of hot water.

How much water does a 20-minute shower use? A 20-minute shower at a typical flow rate of 2.5 GPM would use approximately 50 gallons of water.

Is it OK to shower for 45 minutes? Showering for 45 minutes can consume a substantial amount of water and energy. It's generally considered excessive and not environmentally friendly. Shorter showers are more sustainable.

What is the 4-minute shower rule? The 4-minute shower rule is a water conservation guideline that encourages people to limit their shower time to four minutes to reduce water and energy consumption.

What type of piping is used for a tankless water heater? Various types of piping can be used for tankless water heaters, including copper, PEX, and CPVC, depending on local plumbing codes and the specific installation requirements.

Can I use blue PEX for hot water? Yes, blue PEX is typically used for cold water lines, while red PEX is used for hot water lines. Ensure that the PEX you use is rated for hot water applications.

Can I use flexible pipe for a tankless water heater? Flexible pipe, such as corrugated stainless steel tubing (CSST), can be used for the gas supply line to a tankless water heater. For water supply lines, flexible options like PEX are commonly used.

How many amps does a gas tankless water heater use? Gas tankless water heaters do not use electricity for heating, so they do not draw any electrical amps for heating purposes. However, they may have electrical components for controls and ignition, which typically require minimal amperage.

What size breaker do I need for a Rheem tankless water heater? The size of the breaker required for a Rheem tankless water heater depends on the unit's electrical specifications, which can vary by model. Consult the manufacturer's instructions for the correct breaker size.

Does a gas tankless water heater need a dedicated circuit? Gas tankless water heaters typically do not require a dedicated electrical circuit for heating, as they rely on gas for heating. However, they may have electrical components that require their own circuit for controls and ignition.

What do I need to know before switching to a tankless water heater? Before switching to a tankless water heater, consider your hot water needs, available gas and electrical supply, installation requirements, and local codes. Proper sizing and professional installation are essential for a successful transition.

Can a 200-amp service handle a tankless water heater? A 200-amp electrical service can generally handle the electrical demands of a tankless water heater, provided that the service has sufficient capacity for other household appliances and circuits.