"How to Size an Inline Tankless Water Heater"
-Posted on Mar 10, 2009 - 07:22 PM
By: Adam Beazley
Tankless water heaters come in many different shapes and sizes and
there really is no "one size fits all" solution. Every home and every
end user is different, so the sizing of a tankless inline water heater
should be calculated to fit the needs of each home and user. While one
home fitted with low flow fixtures and individuals who are very
conscious about their water usage may only need one small tankless unit.
Another home with old water hog fixtures and a large family wanting to
use many fixtures simultaneously, will probably need multiple large
units to meet their needs. What it really comes down to is the end users
preference when it comes to performance.
Step 1: List your fixtures
This is a really straight forward step, just go around your home and
list the number of fixtures in your home. Separate them into groups of
similar fixture types, such as shower heads, rest room faucets and
kitchen/utility faucets. Dont forget to list your machinery which use
hot water as well, such as washing machines and dishwashers.
Step 2: Choose Your Simultaneous Needs
Now its time to figure out which of these fixtures/machinery will need
hot water at the same time. Keep in mind the more simultaneous fixtures
you need, the higher the end cost will be for your tankless water heater
system. Try to think about certain scenarios that you do not want to
happen. For instance, if someone is taking a shower and someone else
turns on the bathroom faucet, do you want the person showering to have
their water temperature fluctuate? Think about all of the possible
senarios and make a list of all of the fixtures and machines that will
all need "non fluctuating" hot water at the same time.
Step 3: Calculate GPM for selected fixtures
GPM or Gallons per minute is the standard flow rate measurement in the
US and most of your faucets and showerheads will have their GPM flow
rate stamped into the side. However, if the fixture does not have the
flow rate listed on it or you cannot read it, you have two options; take
an educated guess or manually calculate the flow rate.
| --> Educated Guess Method | |||
| low flow | Regular | Pre 1992 | |
| Restroom Faucet | 0.5 -1.5 | 2.2 | 3.0 - 5.0 |
| Kitchen Faucet | 1.5-2.0 | 2.2 | 3.0 - 7.0 |
| Utility Faucet | 1.5-2.0 | 2.2 | 3.0 - 7.0 |
| Bathtub Spout | NA | 5.0 | 5.0 - 14.0 |
| Shower Head | 1.0 - 2.0 | 2.2 | 4.0 - 8.0 |
| Appliances--> | |||
| Dishwasher | NA | 1.3 | NA |
| Cloths Washer | NA | 3.3 | NA |
This is a more accurate method of calculating the GPM of each fixture in your home. All you need is a 1 gallon container and a watch. Hold the container under the fixture and open the fixture all the way, then time how many seconds it takes to fill up the 1 gallon container. Now divide the number of seconds it took to fill up the container into 60 seconds to get your GPM calculation. (For instance, a bathtub spout that filled up the container in 15 seconds would have a GPM flow rate of 4GPM (60/15 = 4).)
--> Now Add up the Total:
After figuring out the flow rate in GPMs, of all of the fixtures and appliances you would like to use simultaneously, add them together for a total flow rate. If your total flow rate exceeds 10 GPM for a residential home, you can round it down to 10 GPM. Most homes have a 3/4" hot water main and these will generally not carry more than about 8 to 10 GPM total.
Step 4: Determine Incoming Water Temperature
The incoming water temperature of a home varies from region to region and generally gets colder the further North you go. Use the following map to determine your estimated incoming water temperature:
Step 5: Calculate the Rise
For most homes the desired water temperature of a hot shower is approximately 105°F. Unless you specifically need hotter water, use 105°F as the goal temperature and subtract the incoming water temperature determined in step 4 from the goal temperature. this will calculate the needed temperature rise for your particular region.
(For example: Louisiana has an incoming temperature of ~67°F. 105°F - 67°F = 38°F So, an individual in Louisiana will be looking for a rise of 38°F.)
Step 6: Determine Electrical Capacity
Most homes will have an electrical capacity ranging from 60amps to 200amps, while some very large houses may have a 300amp service. The easiest way to figure out the electrical capacity of your home is to have a look at your main electrical panel which hold all of the breakers for your home. The rating of a panel is usually indicated on the label inside of the panel door, although sometimes the label may be inside of the actual panel. If this is the case, It is highly recommended that you get a licensed electrician to tell you your service capacity.
Another way to determine the electrical capacity of your main electrical panel is to look at the main disconnect in the panel. The main disconnect is generally the largest breaker located at the top of the panel. This switch will have a number printed on it (ex: 150A) which will generally be the electrical capacity of the panel, unless the panel is a split buss panel, in which case an electrician will be needed to determine the capacity.
For more information and detailed instructions on determining the capacity of your electrical panel, have a look at the following website: http://www.warreninspect.com/Electric+Service+Capacity
Step 7: Making the Decision: Gas or Electric
When shopping for a tankless water heater you generally have two choices, gas or electric. Both tankless systems are much better than a typical tank type system, however determining with type of tankless is best for you will depend on a few different factors. The most obvious determining factor is weather or not you have gas services running to your home. If you do not, then your best bet will be an electric tankless water heater. However if you home does have gas services and the proper amount of electrical capacity then here are some other factors to take into consideration.
- Gas Vs Electric
gas = More expensive product
gas = higher installation cost
electric = higher efficiency
gas = higher maintenance cost
electric = requires no ventilation
electric = may require panel upgrade
While natural gas costs less in many places, the additional maintenance and installation costs required for gas tankless water heaters make electric tankless water heaters the more cost effective option. However, in some cases where the electrical panel capacity is not sufficient and a new panel must be installed, a gas tankless water heater may be more economical.
Step 8: Finding the right size
In this step we will use everything we have calculated in the previous steps. When looking for a tankless water heater, you will want one which can provide the temperature rise that was calculated in Step 4 at the required flow rate (GPM) calculated in Step 3. Minor temperature variations shouldn't affect the performance significantly. For instance, if you are looking for a tankless unit to provide a 38°F rise at 3.5GPM and the rise chart provided by the manufacturer shows a 33°F rise at 3.5GPM, then this solution should suffice. This solution would only cause a 5 degree variation in temperature which would go generally unnoticed, even in a shower.
Once the electrical tankless size is determined, check the electrical requirements and be sure that the draw (in amps) of the tankless unit is no more than half of the predetermined electrical capacity of your home. For instance, a 200 amp electrical service will work for electric tankless units drawing less than 100 amps. If a larger unit is connected and draws more than 50% of the panels rated capacity, you may experience dimming lights and other electrical phenomena while showering or using hot water.
Below is a rise to flow chart for our Titan electric tankless water heaters, which we will use for the next few examples:
Titan Tankless Water Heater Model Numbers | ||||||||||
| Flow Rate(GPM) | N-210 | N-180 | N-160 | N-120 | N-100 | N-85 | N-75 | N-64 | N-42 | N-10 |
1 GPM | 143° | 122° | 109° | 95° | 87° | 69° | 60° | 51° | 33° | 24° |
1.5 GPM | 95° | 81° | 73° | 64° | 58° | 46° | 40° | 35° | 22° | 16° |
2 GPM | 71° | 61° | 55° | 48° | 44° | 34° | 30° | 26° | 17° | |
2.5 GPM | 57° | 49° | 44° | 38° | 35° | 28° | 24° | 21° | ||
3 GPM | 48° | 40° | 36° | 32° | 29° | 23° | ||||
3.5 GPM | 40° | 35° | 31° | 28° | 25° | |||||
4 GPM | 38° | 30° | 27° | 24° | ||||||
4.5 GPM | 32° | 27° | ||||||||
5 GPM | 29° | 25° | ||||||||
EXAMPLE CALCULATION #1: Step 1: fixture list 1 kitchen faucet 3 restroom faucets 2 showers 1 dishwasher 1 cloths washer Step 2: simultaneous usage 1 shower 2.2 GPM 1 restroom faucet 1.0 GPM 1 dishwasher 1.3 GPM Step 3: total required flow rate Total GPMs- 4.5 GPM Step 4: incoming water temperature Louisiana = 67°F incoming temp Step 5: temperature rise goal 105 - 67 = 38°F temp rise goal Step 6: panel capacity 200A electrical Capacity Step 7: electric or gas No gas service, must use electric Step 8: Final Sizing and decision (1) Titan SCR4 N-210 = 32°F rise @ 4.5 GPM The N-210 max draw = 88amps Panel capacity = 200amps Usable amperage(50%) = 100amps 88amps < 100amps = plenty left over Breakers = two double pole 50amp breakers Wiring = 8AWG wire | EXAMPLE CALCULATION #2: Step 1: fixture list 1 kitchen faucet 2 restroom faucets 2 showers 1 dishwasher 1 cloths washer Step 2: simultaneous usage 1 shower 2.2 GPM 1 restroom faucet 1.0 GPM 1 dishwasher 1.3 GPM Step 3: total required flow rate Total GPMs- 4.5 GPM Step 4: incoming water temperature Kentucky = 57°F incoming temp Step 5: temperature rise goal 105 - 57 = 48°F temp rise goal Step 6: panel capacity 200A electrical Capacity Step 7: electric or gas no gas service, must use electric Step 8: Final Sizing and decision -----Senario 1----- (2) Titan SCR3s N-160 = ~49°F rise @ 4.5 GPM The N-160 max draw = 55amps (2) N-160s = 110amps Panel capacity = 200amps Usable amperage(50%) = ~100amps 110amps > 100amps = cutting it close, but will probably be ok Breakers = two double pole 60amp breakers Wiring = 6AWG wire -----Senario 2----- **Use low flow shower head (1.5GPM) for a total GPM of 3.8** (2) Titan SCR2s N-120 = 48°F rise @ 4.0 GPM The N-120 max draw = 54amps (2) N-120s = 108amps Panel capacity = 200amps Usable amperage(50%) = ~100amps 108amps > 100amps = cutting it close, but will probably be ok Breakers = two double pole 60amp breakers Wiring = 6AWG wire |
As you can see the configurations, prices and electrical needs and vary significantly depending on the scenario. In Example #2, you will notice that by simply changing a regular shower head to a low flow shower head (~$20) allowed us to save over $200 because it allowed us to use 2 smaller, cheaper models. In many situations a dual tankless setup will work better than a single larger unit, while providing redundancy in case one happens to fail.
Parallel Twin Tankless Setup
One small tankless water heater may be sufficient to provide enough hot water for an entire house in a hot climate. However, in certain cases where the household; (a) requires multiple simultaneous hot water applications (large family with multiple people showering, washing cloths or dishes at the same time); (b) is located in colder climates where the incoming water temperature is below 50 F, a parallel twin tankless setup as shown in the diagram below, may be the best option.
You may think that buying a larger unit would be better, however, larger units are basically one large box with smaller separate units inside, connected in series. Due to the fact that the NEC (National Electric Code) for these types of appliances allows no larger than a 60 AMP breaker, this forces manufacturers to split their "large units" into smaller units, contained within one large box, which must be wired separately anyway.
The advantage of a parallel twin tankless setup, as opposed to buying a larger unit or hooking up two units in series, is that you effectively double the heating power as well as the flow rate (water pressure) of your tankless water heating system, while adding redundancy to your system.
- Double Heating Capacity - Because the incoming water is
split into two, each of the tankless water heaters gets half of the
water, which allows the tankless water heaters to provide essentially
twice as much heat. A parallel twin tankless setup will provide just as
much heating capacity as a series setup, but with all of the added
advantages outlined below.
- Double your flow rate - Because you have two tankless water
heaters, the incoming cold water is split and goes into each tankless
water heater independently. This allows each tankless water heater to
output the water at its prescribed GPM output which doubles when the
water lines converge.
- Redundancy - Because a parallel twin tankless setup uses two
tankless water heaters independently, you essentially have a backup plan
in case one of your tankless water heaters needs service. By placing
shutoff valves before and after each unit, you can easily stop the water
flow to either one of the units in order to repair or replace it, while
still keeping one of your tankless water heaters in service. Unlike
large units, you will not be left with cold water while your entire unit
is out being repaired.
- Increased Product Life - In a parallel twin tankless setup,
both tankless water heaters receive equal amounts of cold water from the
incoming water supply. This cold water allows the unit to properly cool
itself after use. In a series connection or in larger units, the units
further down the line only receive heated water from the unit before,
which could hinder the ability for that unit to cool properly, thereby
shortening its life.
- 25% Higher Heating Capacity - A parallel twin tankless setup
will provide approximately 25% higher heating capacity than the largest
single box tankless unit available.
- Less Expensive - Two Titan SCR-2 tankless water heaters connected in parallel is 1/3 less expensive than larger units from our competition.
=== Save 10% from our Eco Store, when you buy 2 or more Titan tankless water heaters!
===Just use the following coupon code: twinsetup
As you can see, sizing a tankless water heater is not very hard, but it can be a little involved. Once sized and installed correctly, a tankless water heater will give you years of hot water at a fraction of the cost. Not only that, but the reduction in greenhouse gasses and toxic emissions fron using less energy will help to create a cleaner environment for everyone.
For even further savings, the overall efficiency of your water heating system can be increased by using low flow shower heads and faucets as well as energy star appliances. This will bring down your average GPM requirements, thereby requiring less energy to raise the temperature of less water. A combination of low flow fixtures and tankless water heating can save over 60% of your water heating cost.
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