Water Heating Systems

Tankless Heaters

Electric and Gas Models

Sizing a Tankless Heater

Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, to size a demand water heater, you need to determine the flow rate and the temperature rise you'll need for its application (whole house or a remote application, such as just a bathroom) in your home.

First, list the number of hot water devices you expect to use at any one time. Then, add up their flow rates (gallons per minute). This is the desired flow rate you'll want for the demand water heater.

Example: let's say you expect to simultaneously run a hot water faucet with a flow rate of 0.75 gallons per minute and a shower head with a flow rate of 2.5 gallons per minute. The flow rate through the demand water heater would need to be at least 3.25 gallons per minute. To reduce flow rates, install low-flow water fixtures.

To determine temperature rise, subtract the incoming water temperature from the desired output temperature. Unless you know otherwise, assume that the incoming water temperature is 50ºF. For most uses, you'll want your water heated to 120ºF.

In this example, you'd need a demand water heater that produces a temperature rise of 70ºF - and you need at least 3.25 GPM - for most uses. For dishwashers without internal heaters and other such applications, you might want your water heated at 140ºF (60ºC). In that case, you'll need a temperature rise of 90ºF (32ºC).

Most demand water heaters are rated for a variety of inlet temperatures. Typically, a 70ºF (21ºC) water temperature rise is possible at a flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric ones. Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet. Some types of tankless water heaters are thermostatically controlled; they can vary their output temperature according to the water flow rate and inlet temperature.

Note: (a) water flow restricting valves and shower heads can reduce flow and water use by up to 50%. Source: USDA "Water Systems Handbook."

Selection Criteria
Natural or Propane Tankless Heaters

One Major Use

• Takagi T-KJr will give you about 4 GPM with 50-degree water input and a 55-degree temperature rise. This is enough for one standard shower (2.5 gpm) and a small sink at the same time. The Takagi T-KJr can also be used for heating applications up to 180 degrees.

• Takagi T-K3 will give you a max output of 6.9 GPM. A 50-degree water input and a 55-degree temperature rise will give you about 6 GPM. This is enough for two standard showers (2.5 gpm each). The Takagi T-K2 can also be used for heating applications up to 180 degrees.
  
  
• Takagi T-KD20 (Direct Vent) will give you a max output of 6.9 GPM. A 50-degree water input and a 55-degree temperature rise will give you about 6 GPM. This is enough for two standard showers (2.5 gpm each). The Takagi T-KD20 can also be used for heating applications up to 180 degrees.
  

• Takagi T-M1 will give you a max output of 9.6 GPM. A 50-degree water input and a 55-degree temperature rise will give you about 7 GPM. This is enough for two standard showers (2.5 gpm each) and a sink at the same time. In warmer climates with input water at about 60-degrees - you should be able to run 3 single-shower head showers at one time. The Takagi T-M1 can also be used for heating applications up to 180 degrees.
  
  
• Any of the Takagi Water heaters used in conjunction with a storage tank.

• Takagi T-H1 will give you a max output of 10.2 GPM. Rated for indoor/outdoor use to supply hot water to: domestic hot water systems (direct or indirect), hydronic heating systems, radiant floor heating, and combined domestic water and heating water systems.

Selection Criteria
Electric Tankless Heaters

Point of Use

• Seisco RA05, RA07, RA09, RA11 are ideal for point-of-use at single faucets. Perfect for RVs and small motor homes. Can supplement an existing tank-type heater.
  
  
• Seisco RA14 IS ideal for point-of-use applications at single showers and multiple sinks. Ideal for supplementing existing water heaters which are unable to satisfy user requirements in beach houses, hair salons, horse stables, hunting and fishing cabins. Great for small homes, condominiums and individual entities, where the incoming water temperature rarely falls below 75 degrees.
  

Three Major Uses or More

• Seisco RA18 is ideal for the whole house, apartment or condominium dwelling, where incoming water temperature rarely drops below 65 degrees F. Excellent for split systems serving individual entities such as lavatories, shower/bath rooms and kitchens.
  
  
• Seisco RA22 is ideal for the whole house, apartment or condominium dwelling where incoming water temperature rarely drops below 55 degrees F. Excellent for split systems serving individual entities such as lavatories, shower/bath rooms and kitchens.
  
  
• Seisco RA28 is ideal for the whole house and some commercial applications such as specialty restaurants, convenience stores, salons, and pet-grooming shops.

Commercial Uses

• Seisco CA12 is deal for supplementing existing tank-type heaters that are unable to satisfy user requirements in office buildings, beach houses, hair salons, horse stables, hunting and fishing cabins. Great for small offices and lavatories where incoming water temperature rarely drops below 75 degrees F.
  
  
• Seisco CA24 is perfect for commercial applications such as specialty restaurants, convenience stores, salons, and pet-grooming shops. Can be used for multiple lavatories, or to serve a cafeteria kitchen or shower stalls.

View Seisco's Temperature Rise Chart