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Posted By admin on December 20, 2009

Introduction to the Tankless Water Heater

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.

But 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. I have one in my house and properly used these appliances are absolutely great. Improperly used 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 so I’ll lay out what you need to know about the tankless water heater so you can make an informed decision before you get, well, into hot water.

How the Tankless Water Heater 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 stand by 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 like the illustration above 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) which 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.)

Now here’s the rub. 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.

Those three factors, as you will see later in this tutorial, are what determine the type, size and possibly even the 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.
I have a point of use water heater under my kitchen sink. It creates 170 degree F hot water instantly and I love the fact that I can get a cup of tea anytime I want (like now as I’m writing this). The point of use tankless water heater is 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.

Point of use tankless water heaters are less expensive than whole house units and cost around a few hundred dollars for the unit (without installation).

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 at one time or a dishwasher, kitchen sink and lavatory hot water faucet at one time. Why the different number of fixtures? Because different fixtures use different amounts of water. Some shower heads can use six times more water than a bathroom lavatory faucet.

So 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. And the worst culprits are shower heads. That’s why you may need more than one whole house tankless water heater hooked up in parallel to meet your hot water demands, especially for simultaneous shower usage.

Whole house units are much more expensive than point of use units and cost from several hundred to several thousand dollars (without installation).

•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 110°F and you live in south Florida with groundwater at 72°F, then you need a 38°F temperature rise (110-72=3.

A tankless water heater is sized by rating its temperature rise at a given GPM. So a unit could be rated at a 33°F Temperature Rise at 2.0 GPM. Based on manufacturer’s data, this same unit could also provide a 65°F Temperature Rise at 1.0 GPM.

You see, the slower the flow of water through the unit, the more the water can be heated.

Calculating Flow Rate
he 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. To determine your required GPM, 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

As you can see, older pre-1992 faucets and shower heads can require very large water flow. So to accurately size your water heater, you need to measure the actual water flow from your faucets and shower heads which is outlined in this quick tip Quickly Determining GPM Flow Rate.

I really recommend you go through that simple exercise for bathroom faucet, shower head and kitchen sink faucets, even relatively new ones. 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. If that’s the case, you need to know before you size and install your tankless water heater.

Sizing the Tankless Water Heater (or Heaters)

OK, so you now 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 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. So 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.

Well 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. Now 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 which 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 heaters as the photo above shows.

Questions & Comments: Please feel free to add your comments and questions below. If you have any other tips, feel free to add those as well.

Categories: Knowledge Base
Tags: DIY, Do it yourself, heater, Information, tankless, water