Tankless Water Heater Marketing Is Everywhere

If you have had a reason to think about water heaters lately you have probably heard a lot about tankless water heaters.  In the last couple of years the tankless water heater manufacturers have really stepped up their promotional campaigns and spent some big bucks building consumer awareness and interest in their products.   Tankless water heaters have been portrayed as providing huge energy savings and endless hot water.  How much of this is true?

Just The Facts, Ma’am

Today we are going to start cutting through some of the marketing hype and find out exactly how tankless water heaters stack up in the energy savings arena.  Just how efficient are they, anyway?

Electric Tankless Water Heaters

Now is a good time to go ahead and say that we will be discussing gas (or LP) tankless heaters.  Electric tankless heaters really make no sense  99% of the time.  They just don’t do the job in most cases and usually require extensive (and expensive) rewiring.  Electricity is really a fairly expensive way to heat water in most cases.

While we are on the subject of electric water heaters lets clear up one more thing.  Even though most electric tank type water heaters are rated with a higher efficiency factor than similar gas models, they are not usually an good choice to lower your energy bills.

It’s true that electric water heaters are technically more efficient, because there is no energy wasted up the flue pipe common to all gas water heaters.  The problem is that, at least for now in most places, the electricity required to operate a water heater costs more than either natural gas or LP.  So even though it takes less energy, the energy used costs more.  Enough said.

Gas Tankless Water Heaters

Let’s talk about the possible energy savings with gas tankless water heaters.  It is a fact that gas tankless water heaters are rated almost 50% more efficient than a standard tank type gas water heater.  The reason tankless heaters beat tank type in efficiency is because there is no energy used keeping a tank full of water hot.   The only time any energy is used is when hot water is actually being used.  As soon as the faucet is turned off the heater stops heating the water.   This is one of the big selling points for tankless water heaters.

It’s Not All Hype

So it’s not all hype, tankless water heaters really are more efficient, but the payback period that is often stated in marketing literature is probably based on the best case scenario.  In the real world, the more frequently hot water is used the smaller the gap in efficiency between tankless and tank type water heaters.  This is because there is less “non use” or “temperature maintenance” energy use as the frequency increases of “hot water usage events” (a fancy way of saying a hot water faucet was turned on).  

Basically, if you are using hot water any type of heater will be using energy. It takes the same amount of BTUs to heat a gallon of water in either type of water heater.  In plain English,  the more often you use hot water the less a tankless will save you on your gas bill.

Check Your Usage Pattern

So, are tankless water heaters really more more efficient?  The answer, strictly speaking is, yes.  How much more efficient depends on your individual usage pattern.  If you are thinking about going tankless you should probably pay attention to when and how often you use hot water now.  The more frequently you use hot water, the less money you will save in energy costs.

Copyright 2008 Bryan Stevens

It’s not easy to choose a new tankless water heater.  I’m going to compare four different brands of gas units, examining the specs and features of Bosch, Reehm, Rinnai, and Takagi units.

I’ve made the table below to show what I think are the most important specifications for the tankless water heaters.  These include the maximum Input BTUs, Minimum Flow Rate to turn on the heater, the energy factor, and the temperature rise at a given flow rate.

Maximum Input BTUs

The maximum input BTUs basically tells you the size of the heater, giving you a way to quickly compare different models.  Most all tankless heaters are around 80% efficient, so any two heaters that have similar input BTU’s will have similar output capabilities.  A water heater with a higher efficiency will deliver a slightly higher flow for a given temperature rise and flow rate.

Calculating BTUs

A BTU (British thermal unit) is the amount of heat required to heat 1 pound of water 1 degree.  Water weighs about 8.3 pounds per gallon.  Heating 1 gallon of water 100 degrees would require ((8.3 pounds x 1 gallon x 100 degrees) / 80% efficiency) = 1,037 BTUs.

Heating the water takes time of course, so if we wanted to heat that gallon in one minute the Btu consumption would be (1037 BTU x 60 minutes/hour)= 62,220 BTUs per hour.

To heat 2 gallons per minute 100 degrees will require roughly 124,000 BTUs per hour.  3 gallons per minute would be 189,000 BTUs.

Minimum Flow Rate

The minimum flow rate is important because that is how much flow you need through the heater to turn it on. The problems appear when you want a low-flow of hot water.  If it takes 3/4 gallons per minute of flow to turn on the heater, and the output temperature of the heater is set for 120°, you have to mix colder water with the hot water to get the temperature you want. 

Since you have to keep the flow rate high enough to keep the heater turned on, and you have to mix it with cold, you simply cannot get a small flow of hot water. 

To make matters worse, if you are trying to take a shower, and you turn down the temperature enough for the heater to kick off, you won’t know it until that slug of cold water hits you a minute or so after you’ve adjusted the valve, depending on how long it takes the water to travel through the pipes to the fixture.

Energy Factor

The energy factor (EF) tells what a water heater’s overall energy efficiency is. It’s based on the amount of hot water produced per unit of fuel consumed over a typical day.

Factors included in the energy factor are:

Recovery efficiency – how efficiently the heat from the energy source is transferred to the water

Standby losses – the percentage of heat loss per hour from the stored water compared to the heat content of the water (water heaters with storage tanks)

Cycling losses – the loss of heat as the water circulates through a water heater tank, and/or inlet and outlet pipes.

The higher the energy factor, the more efficient the water heater is. However, higher energy factor values don’t always mean lower annual operating costs, especially when you compare fuel sources.

Temperature Rise & Flow Rate

The output flow rate is one of the most important factors since it tells you exactly how much hot water the unit can produce. Bosch and Noritz give you the temperature rise at 77° and 75° which are close enough for our comparison on their websites.  Takagi provides graphs which are nice, so I used 75° for Takagi. Rinnai only provides the flow rate at 35°, so to compare Rinnai flow rates you can figure a little less than half of the given values.

Tankless Water Heater Costs

Where I could find them I listed the lowest prices I could find online.  They may therefore be incorrect.  Keep in mind that installing the heater can be very expensive.  You may need special larger stainless steel venting, and you may have to run a larger gas line as well.  If the location does not have a power outlet you might need to run an electrical circuit since most gas tankless water heaters need power for their controls.

Gas Tankless Water Heater Comparison Table

Brand/Model          Max             Min.        EF            GPM@             Price
                              BTU             GPM                       Temp Rise

Bosch                           
Bosch 2700ES      199,000       0.65       0.82         4.2   77° F      ,100
Bosch 2400ES      175,000       0.65       0.82         3.8   77° F      ,050
Bosch 1600H        117,000        0.6        0.69         2.4   77° F      9
Noritz
Noritz N-0931M     250,000       0.5       N/A          5.6   75° F        ,399
Noritz 841             199,900        0.5       0.91         5.0   75° F       ,399
Noritz N-0751M     199,900       0.5       0.83         4.5   75° F        ,299
Noritz N-0631S     185,000        0.5       0.83         3.8   75° F       ,029
Noritz 531             140,000       0.5       0.82         3.1   75° F        9
Rinnai
Rinnai  R98LSi       237,000       0.9        N/A*         9.8  35° F
Rinnai  R50LSi       150,000       0.6        0.82          5.0  35° F       9
Rinnai  R75LSi       180,000       0.6        0.82          7.5  35° F       5
Rinnai  R94LSi       190,000       0.6        0.82          9.4  35° F 
Takagi
Takagi T-K Jr.        140,000       0.5        0.81          2.5  75° F       9
Takagi T-K3           199,000      0.5        0.84          3.6  75° F        0
Takagi T-H1           199,000      0.5        0.92          4.8  75° F