How to Find The Right
Home Tankless Hot Water Heater
Last updated June1, 2009
Using energy in the most efficient manner is part of the green home profile. It's also a way to save money. Home tankless water heaters are one way to do this. With new home construction there is an obtainable efficiency target of 90 percent. Meaning that green homes, built to standards, use 1/10 the energy of traditional housing.
Retro fitting older homes to this level is probably not economically rewarding at this point, but there are steps you can take, as pointed out below.
If you have a tank water heater that is 10-20 years old, you are probably facing an efficiency rating of 40 to 50 percent. That means that about half your fuel dollar is wasted.
By upgrading to anew home tankless hot water heater, or an on-demand water heater, as they are sometimes called, you should be able to raise this number to 80-95 percent, depending on the steps you take.
Installing a whole house tankless water heater can be a big investment. Rather than settle for less take the time to find the right technology to fit your needs and how to plan for the future.
A quick energy efficiency comparison
If you do nothing more than a replacement this is energy efficiency you can expect, under the right conditions. This number will be reduced some by heat loss in the piping.
Gas storage .60-.65
Electric storage .90-95
Gas tankless .80-85
Electric tankless .95-.99
How to Choose the Right Size
Home Tankless Hot Water Heater
Rate of flow and climate are two factors that determine the horsepower of the heater you are looking for.
How to Choose the Right Size Tankless Hot Water Heater
The Climate Factor
For anyone considering a tankless water heater, the climate factor will determine how powerful a heater you will need. Tankless water heaters operate on the direct relationship between climate and flow rate - if one goes up the other goes down. The colder the climate, the fewer gallons per minute. The output of a tankless model doesn’t mean much without this info.
Smaller, less expensive electric models operate in the range of 2-3 gallons per minute (winter.) Gas and larger electric models are needed for colder climates and have higher flow rate (higher energy usage.) These offer the convenience of on-demand, unending supply of hot water but will probably not pay for themselves over the life of the unit because of the installed cost.
Step 1 - Temperature Rise
This is the measurement of water temperature for your climate. It is the difference between the water temperature (winter) as it enters the house and the temperature at the faucet.
As an example: From 50 F degrees, the water entering the house is heated to 110 F degrees at the showerhead, this is the rise. The rise in this case would be 60 F degrees.
Step 2 - Flow Rate
You also need to calculate Flow Rate - how many gallons of water you need during peak times. If you can get by with one shower or one other outlet at a time, you usage would be 1-½ to 2-1/2 gallons per minute (gpm). This is the efficient way to use an on-demand water heater. If you need more then you can estimate.
.
Or, simply use a bucket and a watch to take your own measurements, as shown above.
As an example: 1 shower + clothes washer = 4 to 5GPM
Step 3 - Figuring the size
Once you have numbers for temperature rise and flow rate, you can apply them to gas and electric model tankless heaters to find the right size.
Here is an example of max output with
a
60° F rise in temperature:
| Type |
Size |
Output |
| Electric |
17 kw |
2 gpm |
| Electric |
25 kw |
4 gpm |
| Gas |
1.17 therms |
3.2 gpm |
Here is an example of max output with
an 80° F temperature rise :
| Type |
Size |
Output |
| Electric |
25kw |
2.2 gpm |
| Gas |
1.17 therms |
2.4 gpm |
| Gas |
1.75 therms |
3.7 gpm |
| Gas |
2 therms |
4.1 gpm |
STEP 4 - Is it electric or natural gas?
Now you have an idea as to how climate affects flow rate, which of course determines energy use. It's energy you will be using for the next 10-20 years so it's worth comparing the two. Here is an example that compares electric and a natural gas.
A Simple Comparison of Electric and
Gas Tankless Heaters
Based on Virginia rates for 2009
| Type |
Power |
Max flow |
Rate |
@ |
Cost |
| Electric |
80 amp-17 kwh |
2.9 gpm |
$0.09/ kwh |
2gpm |
$1.53 hr |
| Electric |
120 amp-23 kwh |
4.5 gpm |
$0.09/ kwh |
2gpm |
$1.53 hr |
| Gas |
1.17 therms/ hr |
4.3 gpm |
$1.65/ therm |
2gpm |
$1.21 hr |
These are manufactures and utility numbers and do not have any adjustment for energy efficiency.
If you want to make your own calculations, these conversions will be handy:
1 therm = 100 cu ft of natural gas
1 therm = 29.3 kwh
1 kwh = .03412 therms
Whether it is gas or electric depends on the rates in your area.
Rates vary as these electrical (2006, cents/kwh) and gas (2007, $/1000cf) maps show.
The growing emphasis on electrical power and the decline in gas reserves point to a shift in energy use as these electrical and gas charts show.
For a comparison of models, features and prices visit our sister site: Best Tankless info.
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