How to determine the type and what size water heater you need

Before plunging in and hastily deciding on any particular tankless water heater, three variables determine the type, number and what size water heater you need:

1. The volume of water to be heated, measured as flow rate in gallons per minute (GPM).

2. The temperature of the cold water entering the unit.

3. What temperature you want the hot water to be as it exits the unit.

1. First of all decide on the type of tankless water heater you want -- points of use or whole house.

Point of use tankless water heaters:

Small, fitting inside a sink cabinet or a closet, these dedicated use heaters typically serve one sink/faucet or one shower. So, for example, if you want a cup of tea on demand, a point of use water heater under your kitchen sink will create 170 degree Fahrenheit hot water instantly.

Less expensive than whole house units, they cost a few hundred dollars per unit, excluding installation costs.

Whole house tankless water heaters:

Because they handle demand for more than one fixture at a time, they have a higher GPM flow. For example a unit may adequately serve two simultaneous showers; or a dishwasher, kitchen sink and lavatory hot water faucet being in use at the same time.

Which fixtures are likely to be running at any one time is important because they use different quantities of water. Shower heads are the biggest guzzlers with some using six times more water than a hot water lavatory faucet.

Depending on your requirements, you may need more than one whole house tankless water heater with multiple heat exchangers and boosters for long runs.
More expensive, they cost from several hundred to several thousand dollars, excluding installation costs.

2. How where you live affects what size water heater you need

The temperature of the cold water entering the unit is determined by where in the country you live. The colder your groundwater, the less hot water can be produced by a unit for a given GPM flow rate.

A tankless water heater which works well in Florida has to be 33% to 50% bigger in Michigan to serve the same number and types of fixtures because the groundwater coming into the unit in Florida is about 72 degrees Fahrenheit, while in Michigan it is more like 42 degrees.

3. What temperature you want the hot water to be as it exits the unit.

The temperature difference between the hot water exiting the heater and the cold water entering the unit is called the temperature rise. If you want a shower of 110 degrees Fahrenheit and live in south Florida with groundwater at 72 degrees Fahrenheit, you need a 38 degrees temperature rise (110-72=38).

Incidentally, the average groundwater temperature in the U.S. is 50 degrees Fahrenheit. Because most people prefer shower temperatures of between 105 and 120 degrees, your groundwater temperature would need to be raised by 60 to 75 degrees.

What size water heater is determined by rating its temperature rise at a given GPM so for example, a unit could be rated at a 33 degrees Fahrenheit temperature rise at 2.0 GPM. According to the manufacturer, this same unit could provide a 65 degrees Fahrenheit temperature rise at 1.0 GPM.

The reason for this is - the slower the water flows through the unit, the hotter it will be.

Calculating the flow rate:

You need to calculate the flow rate or how much hot water flow you will likely need for your house.

The Federal Energy Policy Act of 1992 required all faucet and shower fixtures made in the U.S. to have a flow rate of no more than 2.2 gallons per minute at a pressure of 60 PSI (pounds per square inch). While older fixtures use much more water than this, if you use low flow aerators you can achieve less than 2.2 GPM.

To determine your required GPM add up the number of fixtures of which type will be served by the tankless water heater:

Lavatory faucet:
low flow: 0.5 - 1.5 GPM
meets code: 2.2 GPM
pre 1992: 3.0 - 5.0 GPM

Kitchen faucet:
low flow: not suitable for washing dishes
meets code: 2.2 GPM
pre 1992: 3.0 - 7.0 GPM

Shower head:
low flow: 1.0 - 2.0 GPM
meets code: 2.2 GPM
pre 1992: 4.0 - 8.0 GPM

Other fittings:
tub: 4.0 GPM at 102 F
body jets/sprays each: 1.0 - 2.5 GPM at 104 F
washing machine: 2.0 GPM at 120 F
dishwasher: 1.5 GPM at 110 F

In order to accurately size the water heater you need, measure the actual water flow from your shower heads and faucets, even newer ones. Because the Federal Energy Policy Act of 1992 is not heavily enforced, some faucets still exceed their stated 2.2 GPM ratings and you need to know this before you install the wrong tankless water heater.

Totalling the flow rates for all the faucets and shower heads to be heated by the unit at the same time, you could have the following:

1 showerhead 4.0 GPM (pre 1992)
1 showerhead 2.2 GPM
1 lavatory faucet 2.2 GPM
1 kitchen faucet 2.2 GPM
giving us a total of 11.0 (4+2.2+2.2+2.2) gallons per minute.

To determine what size water heater you need:

From our earlier calculation, living in south Florida we determined our required temperature rise (desired hot water temperature less incoming ground water temperature) would be 38 degrees Fahrenheit.

In order to provide enough hot water for this imaginary home, we need a whole house unit capable of handling 11 GPM at a 38 degrees Fahrenheit rise.

If you wanted an electric model, Stiebel Eltron have a 240 volt Tempra 29 which can produce about 5.5 GPM at a 38 degree Fahrenheit rise, so two of these units would meet your 11.0 GPM needs.

If you live in Michigan, Minneapolis, Maine, Washington or anywhere in the northern half of the USA, your incoming ground water will be around 30 degrees Fahrenheit colder, meaning you need a unit which can handle a 68 degrees Fahrenheit rise.

Once you've determined what size water heater you need, for larger requirements it's clear you need a propane or natural gas fueled tankless water heater.