The Comprehensive Guide to Sizing a Tankless Water Heater

The Comprehensive Guide to Sizing a Tankless Water Heater

Selecting the right tankless water heater is crucial for homeowners seeking efficiency and convenience in their hot water supply. The goal of this comprehensive guide is to navigate you through the process, ensuring you choose a water heater that best suits your needs while saving money and optimizing performance.

Tankless water heaters are a game-changer in providing endless hot water on demand. They are a boon for reducing utility bills by over 30% through eliminating standby energy losses. Compact, about the size of a suitcase, and long-lasting, with a life expectancy of over 20 years, they outperform traditional tank-based units significantly.

However, their efficiency hinges on choosing the right size. A too-small system fails to meet your household’s hot water needs, while an oversized one leads to unnecessary expenditure. Proper sizing is crucial in tankless water heater installation, and this guide is designed to navigate you through that process.

Sizing Your Tankless Water Heater

Tankless or on-demand water heaters are rated by their maximum output water flow rate at a given temperature rise.

Therefore, sizing involves:

  1. Calculating Peak Hot Water Demand: Determine the gallons per minute (GPM) of hot water required during peak usage. Remember, tankless heaters heat water as it flows without storing it.
  2. Assessing Required Temperature Rise: This depends on the feed water temperature and desired output water temperature.

When a tankless water heater is undersized, it struggles to meet the demand, leading to a reduced flow rate. This reduction in flow results in a decrease in both the hot water temperature and pressure available to the user. Moreover, consistently operating at full capacity to meet the inadequate sizing strains of the system potentially leads to premature breakdowns and a shorter lifespan of the heater.

Oversizing a tankless water heater, on the other hand, primarily affects your finances. The initial cost of a larger-than-necessary system is higher, and since the heater is not utilized to its full capacity, this extra expenditure does not translate into proportional benefits. In essence, an oversized system leads to paying more for unused potential.

This guide aims to illuminate the critical factors in choosing a tankless water heater, ensuring you make an informed decision tailored to your specific needs.

Understanding Flow Rate and Capacity

Unlike traditional water heaters with a capacity measured in gallons, tankless water heaters operate based on flow rate, calculated in Gallons Per Minute (GPM).

This is the hot water the heater can supply at any moment. It’s essential to consider your household’s peak hot water usage to determine the necessary flow rate.

Tankless water heater gpm flow chart

For instance, a home with multiple showers in use simultaneously plus a running washing machine will have a higher GPM requirement compared to a household with less simultaneous usage​.

Calculating Your Specific Needs

To precisely determine the flow rate needed for a tankless water heater, a detailed assessment of your household’s water usage is key. Begin by cataloging all water fixtures – showers, kitchen and bathroom sinks, dishwashers, washing machines, etc. Each fixture has a specific flow rate, typically measured in gallons per minute (GPM).

For instance, a standard shower may use 2.5 GPM, while a kitchen sink might use 1.5 GPM. During peak times, like morning routines, multiple fixtures might be in use simultaneously. Add up the flow rates of these concurrently used fixtures to find your total peak GPM requirement.

This process helps in selecting a tankless water heater that can handle your household’s peak demand without faltering. If you overlook this step, you risk choosing a unit with a flow rate too low for your needs, leading to situations where your demand for hot water outpaces the heater’s capacity, resulting in an inadequate supply. Such precision in calculation ensures efficiency and consistent hot water availability, tailored to your home’s specific demands.

Temperature Rise: A Key Factor

Temperature rise is a crucial factor in sizing your tankless water heater. It refers to the difference between the temperature of the incoming cold water and the desired hot water temperature. This factor varies significantly based on geographic location, as colder regions have lower incoming water temperatures.

For example, if the incoming water temperature is 50°F and you desire a hot water temperature of 120°F, your temperature rise is 70°F. In colder regions, the incoming water temperature can be much lower, necessitating a higher temperature rise. This directly impacts the size and type of tankless water heater you need, as a higher temperature rise requires a more powerful unit to achieve the desired hot water temperature efficiently.

Sizing for Radiant Heat Systems

When sizing a tankless water heater for homes with radiant heat systems, it’s important to account for the continual demand these systems place on the water heater.

Radiant heating requires a steady and consistent supply of hot water, which can significantly increase the gallons per minute (GPM) and British Thermal Unit (BTU) capacity needed from your tankless heater. This constant operation can also impact the durability of the water heater, potentially leading to a reduced lifespan.

Homeowners may need to replace their heaters more frequently or explore alternative heating solutions that better align with the demands of radiant heat systems.

In conclusion, the process of sizing a tankless water heater is essential for optimal performance and efficiency. It involves understanding your household’s peak hot water demands, calculating the required flow rate (GPM), and considering the necessary temperature rise based on geographic location.

Special considerations must be made for homes with radiant heat systems requiring higher GPM and BTU capacities. Whether under-sizing or oversizing, each has its drawbacks, from reduced efficiency and potential system strain to unnecessary costs. This guide aims to provide a comprehensive understanding, ensuring you select a tankless water heater that perfectly suits your home’s needs.