The Tankless Water Heater

So you're tired of running out of hot water in the shower and want to replace that old hot water heater. Or maybe you're considering tankless for a new project. Well, tankless hot water heaters can be a great solution to your residential hot water needs. They're also referred to as on demand hot water heaters because they don't usually store hot water, but rather create it on demand.
For all the hype surrounding these appliances there are some limitations and trade-offs to consider before you can really decide if the tankless hot water heater is for you. If used improperly they are disappointing and expensive. They come in point-of-use and whole-house versions and there are electric tankless water heaters and natural gas or propane models. Some tankless units are sized to heat a cup of tea, others to provide enough hot water for 2 or more bathrooms. Also, the region of the country you live in has a lot to do with how much hot water a tankless water heater can produce. There are a lot of factors to consider with these units. Below is some information about tankless water heaters so you can make an informed decision.

How it Works
The tankless water heater works by directly heating water on demand, as it is required. Unlike traditional hot water heaters using a storage tank, the tankless units have no storage tank and thereby have no standby heat loss. Standby heat loss is the heat lost and energy wasted by heating water only to store it in a tank and is characteristic of traditional hot water heaters. Avoiding standby heat loss is primarily how tankless water heaters make their claim of being energy efficient.
Whether a tankless water heater is point-of-use or a whole-house unit, they work the same basic way. Cold water enters the unit and is heated by a heating element (heat exchanger) that is turned on by a flow activated switch. The heat exchanger can be electric resistance heating coils or a gas fired burner using natural gas or propane. Gas units generally have more heating capacity and larger whole-house units are typically gas fired.

There are three variables that have to be considered in sizing the unit.
  • The volume of water the unit is required to heat, measured as flow rate in gallons per minute (GPM)
  • The temperature of the cold water entering the unit
  • The desired temperature of the hot water exiting the unit.

  • These three factors are what determine the type, size, and quantity of tankless water heaters you need.

    Which Type of Tankless Water Heater to Use

    There are two basic types of tankless water heaters, Point of Use and Whole House. The type you select is based upon your intended use; and your intended use will have even more to do with the cost of these units.
    Point of Use Tankless Water Heater
    The point of use tankless water heater is usually relatively small and will usually fit inside a sink cabinet or in a closet. They are typically dedicated use heaters meaning the unit serves one sink / faucet or one shower, etc.
    Whole House Tankless Water Heater
    Whole house units mean that they have higher GPM flow rate capacity and can handle demand for more than one fixture at a time. For example a unit may handle two shower fixtures or a dishwasher, kitchen sink, and lavatory hot water faucet at one time.
    Why the different number of fixtures?

    Different fixtures use different amounts of water. Some shower heads can use six times more water than a bathroom lavatory faucet. The size and number of whole house tankless water heaters you need will be largely driven by flow rate and that is determined by the number and types of fixtures you may have running at one time. Shower heads play a large factor in water consumption. Thus, you may need more than one whole house tankless water heater hooked up in parallel to meet all your hot water demands, especially for simultaneous shower usage.

    Groundwater Temperature Impact on Unit Size

    In addition to the number and type of fixtures you want simultaneously served by the tankless water heater, you will also have to consider the temperature of your groundwater. That is determined by where you live in the country. In the chart is generally accepted as the cool / warm dividing line of about 55F groundwater. The colder the groundwater gets the less hot water can be produced by a unit for a given GPM flow rate. This means a tankless water heater in Florida would have to be rated 33% to 50% larger in Michigan to serve the same number and types of fixtures.
    Well because the tankless unit heater has to warm the incoming cold water more in Michigan than Florida since the groundwater coming into the unit can be 30F colder in Michigan (42F) than Florida (72F).

    Calculating Temperature Rise

    The three variables that have to be considered in sizing and selecting the unit include:
  • The volume of water the unit is required to heat, measured as flow rate (GPM).
  • The temperature of the cold water entering the unit
  • The desired temperature of the hot water exiting the unit.

    Determine Desired Temperature Rise
    The difference between the the temperature of the hot water exiting the heater and the cold water entering the unit is called the temperature rise. If you want a shower up to 110F and you live in south Florida with groundwater at 72F, then you need a 38F temperature rise (110-72=38).
    A tankless water heater is sized by rating its temperature rise at a given GPM. A unit could be rated at a 33F Temperature Rise at 2.0 GPM. Based on manufacturer's data, this same unit could also provide a 65F Temperature Rise at 1.0 GPM. The slower the flow of water through the unit, the more the water can be heated.

    Calculating Flow Rate

    To determine your required GPM (flow rate), add up how many fixtures of what type you will have served by the tankless water heater:
    Lavatory Faucet
  • Low Flow: 0.5 -1.5 GPM
  • Meets Code* / 1992 Standard: 2.2 GPM
  • Pre-1992 Faucet: 3.0 - 5.0 GPM
    Kitchen Faucet
  • Low Flow: Not appropriate for dish cleaning
  • Meets Code* / 1992 Standard: 2.2 GPM
  • Pre-1992 Faucet: 3.0 - 7.0 GPM
    Shower Head
  • Low Flow: 1.0 - 2.0 GPM
  • Meets Code* / 1992 Standard: 2.2 GPM
  • Pre-1992 Faucet: 4.0 - 8.0 GPM

    *The Federal Energy Policy Act of 1992 required all faucet / shower fixtures made the USA to have a flow rate of no more than 2.2 GPM at 60 PSI. Often you can get below 2.2 GPM with low flow aerators but before 1992, older fixtures used much more water than 2.2 GPM.
    Older, pre-1992, faucets and shower heads can require very large water flow. In order to accurately size your water heater, you need to measure the actual water flow from your faucets and shower heads. Since the Federal Energy Policy Act of 1992 is not heavily monitored, there have been problems with faucets exceeding their stated 2.2 GPM ratings. You need to know before you size and install your tankless water heater.

    Sizing the Tankless Water Heater

    Now you know the required temperature rise (desired hot water temperature - incoming ground water temperature = temperature rise) and you have added up the required flow rates for all of the faucets and shower heads to be heated by the unit that may be on at one time.
    So let's say you require 11 GPM to accommodate 2 shower heads (1 pre 1992 at 4 GPM, 1 at 2.2 GPM), 1 lavatory faucet at 2.2 GPM and the kitchen faucet at 2.2 GPM. Based on our previous section "Calculating Temperature Rise" you need a 38°F temperature rise if you live in south Florida. You need a whole house unit capable of handling 11 GPM (4+2.2+2.2+2.2=11) at a 38°F rise and you'd like an electric model.

    That's pretty big for an electric tankless water heater but a 240 volt Tempra 29 model by Stiebel Eltron can produce about 5.5 GPM at a 38°F rise, so you need two units this size to meet the 11 GPM requirement. However, if you live in the northern half of the USA, say in Michigan or Minneapolis or Maine or Washington, then you're incoming ground water will be much colder than our Florida example. Probably at least 30°F colder, that means you'll be looking for a unit that can handle a 68°F temperature rise. Once you require a bigger unit like that, you'll have to move up to a gas or propane fueled tankless water heater or heater.

    If you plan to purchase an Electric Tankless Water Heater, consider the Electrical Requirements:

  • Voltage
  • Amperage
  • Circuit Breaker

    Many retailers sell units that will accommodate 110V, 120V, 208V, 220V, 240V, and 277V.

    Different Electric Tankless Water Heaters will have various requirements in amp draw. You will want to ensure that you can support the electrical demands of your Electric Tankless Water Heater.

    Circuit Breaker
    You must ensure that you have a circuit or circuits that will support your Electric Tankless Water Heater. It may be necessary to put your Electric Tankless Water Heater on its own circuit or circuits.

    You should consult with a qualified, licensed electrician for more information.

    Tankless Water Heater Manufacturers

    There are many different manufacturers to consider when you are shopping for your new tankless hot water heater. The following are some manufacturers to check out:
    Stiebel Eltron
    Or Click HERE to view all of our gas Tankless Water Heaters.