4 Factors People Overestimate When Purchasing A Tankless Water Heater

11 December 2015
 Categories: , Articles


If you are considering switching to a tankless water heater, you may start by thinking that a bigger, more powerful heater will be better for your home. However, it is important to get a water heater that suits your home's specific needs. Overestimating your needs can result in higher energy costs, a higher initial cost, and a bigger installation area. 

Groundwater or Intake Temperature

The groundwater or intake temperature is the temperature at which water enters your plumbing system. Depending on where you live and the setup of your plumbing, this temperature may vary between summer and winter months. If this is true, it is best to determine your intake temperature during the cold months so your unit will be able to adequately heat your water year round. You can get a rough estimate of your intake temperature by measuring the temperature of the cold water coming out of your faucet.

It is important to get a realistic understanding of your intake water temperature because assuming that it is colder than it is can result in you purchasing a more powerful tankless heater than is actually necessary. 

Temperature Rise 

The temperature rise is the maximum amount that you would like to raise your water temperature to. For example, if your intake temperature is 40°, and you would like to have access to water at 120°, then your temperature rise will be 80°. If you have a hot water tank, you generally want the temperature of the stored water to be above 140° to prevent the growth of bacteria that can cause Legionnaires disease. However, with a tankless water heater, you can set the water to the temperature that you need to use because the water does not sit long enough for bacteria growth to be an issue. 

For most people, the hottest water that they will need to use is for washing dishes, which is recommended to be at 120°. If you have a dishwasher, it most likely has an internal heater to achieve this temperature, so your water does not have to be set to this temperature. But if you wash by hand, you will want at least a 120° output. Otherwise, you can calculate your rise slightly above the temperature that you enjoy showering at. Many people with tankless heaters set their output temperature higher than they actually need, because they are used to using hot water tanks. 

Output or Flow 

Output is the amount of water you will need at the desired temperature, generally measured in GPM (gallons per minute). Most people calculate this by adding all of the possible sources of hot water in their house, such as the shower heads, sink faucets, and dishwasher. However, it is important to be realistic about how many of these you will use at the same time. For example, if you are a two-person household, you will probably not run both showers and the kitchen sink at the same time, but you may run the dishwasher and the kitchen sink at the same time. 

Instead of simply adding together your potential output, you should assess your current behaviors and decide how you plan to use the water that you will heat to get a better idea of the heating capacity that you will need. 


Many tankless water heaters have a higher efficiency rating than traditional water heaters because they do not lose energy heating water that is not being used. You will need to know the exact efficiency rating of the models you are deciding between to determine the size that your household will need. 

Before deciding on a tankless model, talk to an HVAC professional that can help you determine your exact needs and possible options for your household. Visit websites like http://www.smedleyservice.com for more information.