Floating boilers provide endless hot water when needed. In addition, you can reduce your electricity bill by more than 30% by eliminating energy losses in the reservoir. They are as big as a suitcase, save space and last up to more than 20 years, which is almost twice as long as a traditional tank unit.
However, tankless boilers are limited by the amount of water they can heat at any given time. Choose a system that is too small and unable to provide you with sufficient hot water for your household needs. Getting too big and paying too much. It is therefore essential to choose the right size – before making a purchase.
This buying guide will guide you through the process of sizing a tankless boiler and will also inform you about other aspects you should take into account when making your purchases.
- How do you determine the size of a tankless boiler?
- Step one: Flow rate – how many litres per minute for a tankless boiler.
- Second step: increasing the temperature
- Step three: On second thought.
- Table of sizes
- Example of a size chart
- Size chart for tankless water heater
- Buyer’s Guide : How to choose a tankless boiler?
- Types of tankless boilers
- Energy factor (EF)
- Frequently asked questions
How to determine the size of a non-break water heater
Simply put, tankless boilers, also called demand boilers, are designed for maximum water flow at a certain temperature rise.
To determine the size of a tankless boiler, two things have to be taken into account:
- How many liters of hot water per minute (gpm) do you need during peak hours? Remember that tankless boilers do not store water, but heat it as it flows.
- Increase in the required temperature, which is determined by the settings of the feed water temperature and the water outlet temperature.
What if you weren’t so big?
Bad idea! If the required amount of hot water is exceeded, even temporarily, most boilers automatically reduce the flow rate. This means less hot water at each outlet and thus a drop in temperature and/or pressure.
In addition, the tankless boiler, which is constantly running at full capacity, is likely to fail prematurely.
What if you’re too tall?
Excess is not that important. The only problem is the unnecessary costs upfront.
Step 1: Flow velocity – how much GPM for floating boilers
Let’s start with determining your hot water demand peak . You can use the diagram below. It gives an overview of the standard flows of different types of water systems sold in the United States. Of course you can also consult your specific valves etc. on the internet or consult their user manual.
All you have to do is determine which devices and how many devices can be used at the same time. Then summarise your consumption.
For example, if you need a shower and a kitchen mixer that operate at the same time as the maximum water flow required:
2.5 gallons per minute + 2.2 gallons per minute = 4.7 gallons per minute
WaterSense certified products require the required flow rate:
2,0 gallon/minute + 1,5 gallon/minute = 3,5 gallon/minute
Very easy! But before we start, just a few more tips:
- If you expect maximum demand at peak times such as in the morning, you can be sure that you will always have hot water available, no matter what. In addition, the tankless boiler, which is not expected to run continuously at full capacity, is likely to last longer. At the same time, with a little planning, you can significantly reduce peaks in demand. This involves showering one after the other, the dishwasher doing its job while everyone is away and cooking before or after the shower.
- Of course you have to take into account not only the number of bathrooms in your house, but also the number of people living under one roof. 2 people can only use 2 showers at the same time, even in a mansion with 5 bathrooms.
- Ask yourself this question: Will the boiler supply the whole house or only part of it?
- The following flow rates indicate the total water capacity, so that hot water is PLUS cold water. In other words, a shower with a shower head of 2.5 gpm does not mean that all the water is hot. You will probably mix it with some kind of cold to reach the desired temperature. As a result, the actual demand for hot water is slightly lower.
- Low speed aerators or brackets can be installed to reduce airflow.
|Drainage||Typical flow rate|
|Washbasin||0.5 – 1.5 gpm|
|Showerhead||2.5 (2.0*) gallons per minute|
|Bathroom faucet||2.2 (1.5*) gallons per minute|
|Bath mixer||3.0 – 4.0 gpm|
|Kitchen faucet||2.2 gpm|
|washer||Load over 23 gallon, gallon/min difficult to determine|
|Dishwasher||6 gallons per charge, gpm is difficult to determine.|
* WaterSense certified products
Take a look at this: Older devices are likely to have a higher throughput.
As you can see, we didn’t include dishwashers and washing machines. It was indeed very difficult for us to obtain reliable data on this subject. Some sources speak of 2 to 3 gpm, others of 1.5 gpm.
All you can do is operate each device individually, monitor the water meter and measure the time. This gives you an overall picture of the required flow rate.
Talk about measures: For a more scientific approach to determining peak water consumption, take a 1-gallon bucket and see how long it takes to fill a shower head, a kitchen tap, a bathroom faucet and so on. Then use the following formula to calculate the flow rate for each output:
Required flow rate = 60 /sec for filling the cup
If you don’t want to spend a whole litre on a news item, just fill in the quarter and use this form:
Air quantity = 15 / s needed for filling ¼ Cup
Step 2: Temperature rise
The next step is to determine the required temperature increase. Simply subtract the temperature of the supply water from the desired temperature of the waste water.
Required temperature rise = raw water temperature – supply water temperature
How can you determine the temperature of your feed water? You have two options:
- Measuring with a thermometer
- Use our amazing map of average groundwater temperature in the United States.
Take a look at this: These are estimates of the average temperature. Actual temperatures vary depending on the season and weather.
As you can see, the place where you live in the country plays a major role in terms of average groundwater temperature. In South Florida, where the climate is warmer, the temperature will naturally rise to 77°F. While in Alaska, parts of North Dakota, Minnesota, Wisconsin and other northern states, the temperature of groundwater can rise to 37°F.
Now this has a big influence on the amount of water that the boiler without a tank has to work to bring the water to the desired temperature. Suppose you live in Michigan with an average supply water temperature of 40°F. And you’re aiming at 110 degrees out. The required temperature increase would be
110 °F – 40 °F = 70 °F
A water heater installed in a Texan house with a water supply temperature of 70°F should use as little water as possible if
110 °F – 70 °F = 40 °F
In fact, the outlet water temperature of 105-115°F is considered ideal for daily household use. 105°F is considered to be the upper limit of the most comfortable temperature range for showering. On the counter you want something with about 110°F.
Step 3: Fold everything together
You know your hot water requirements during peak periods and the required temperature increase.
The final step is to find a tankless boiler that meets these requirements. In principle, all manufacturers supply tables with the dimensions of their products, which indicate the maximum flow rate for a given temperature rise or vice versa. Some manufacturers also specify flow rates for different water inlet and outlet temperatures.
You want to choose a system size that meets or exceeds your peak hot water requirements.
However, bear in mind that manufacturers like to promote their products with the best scenarios, so consider the information carefully. Some companies tend to overestimate what their systems can do…
Are you ready for the next step? Refer to our notes on the tankless electric water heater to find the system that best suits your needs!
Do you prefer video? Take a look at this:
Take a look at some of the size charts and you’ll soon see that gas or electricity are very important. In general, gasless boilers are more powerful, i.e. they can produce more gpm than electrical appliances for the same temperature rise.
If the required temperature increase is 70°F, the large tankless gas boiler can deliver 5.0 to 5.5 gallons per minute. The largest electrical unit (36 kW) reaches a peak of just over 3.0 gallons per minute.
Example of size chart
For your information: Here’s what the size chart for the tankless boiler looks like:
This heater can deliver 6.6 gallons per minute at 50°F and 4.8 gallons per minute at 70°F.
Do you now know the required hot water consumption and the required temperature increase? Click here for our reviews of the best electric tankless boilers!
Popular Floating Water heaters without Cooling shoes
Here are some practical examples of popular non-radiant water heaters:
|Model||Type of fuel, capacity||temperature increase||Max. GPM||# Bathroom|
|Rinnai V75iN||Natural gas, 180,000 BTU||70 °F||4.3 gallons per minute||1 – 2 bathrooms|
|50 °F||6.0 gpm||2 – 3 bathrooms|
|Rinnai RU199iP||Propane, 199,000 BTU.||70 °F||5.5 gpm||2 bathrooms|
|50 °F||7.6 gpm||3 bathrooms|
|Eltron Tempra 36 Plus Peduncle||Electric, 36 kW||70 °F||3.5 gpm||1 bathroom|
|50 °F||4.75 gallons per minute||1 – 2 bathrooms|
|Rheem RTEX-18||Electric, 18 kW||65 °F||2.0 gpm||1 bathroom|
|55 °F||2.0 gpm||1 bathroom|
|EcoSmart ECO 11||Electric, 11 kW||68 °F||1.1 litres per minute||1 bathroom|
|48 °F||1.56 gallons per minute||1 bathroom|
Copper guide: How to chooseSpecific water heater?
We’ve already talked about the size. But if you want to drive without a tank, it’s not the only thing to consider…
Tankless boiler type
Tankless boilers can be divided into categories according to fuel source and application type. Let’s start with the first one.
Type of fuel: Electricity or gas?
If you are converting your home from a tank type boiler to a tankless system, it may make sense to stick to the same type of fuel, as this can save you money on installation. When building a new home, the availability and cost of different fuels is important.
There is electricity, natural gas – generally the cheapest option, although prices vary from region to region – and propane, which is more efficient than natural gas because it has a much higher Btu.
If you’re lucky enough to have a choice, don’t forget to compare the costs of each. Natural gas is supplied via pipelines and is therefore not available everywhere. If so, he’s usually the preferred type. You have to buy propane yourself.
What is the BTU?
1 BTU = The amount of energy required to increase the temperature of a pound of water by 1°F.
The higher the efficiency of the BTU heater, the more hot water it can supply per minute.
As mentioned earlier, gas water heaters generally have more power than electrical appliances, allowing them to heat water faster. In fact, electric boilers without a boiler are only partially suitable for use throughout the house. Even the largest models can barely supply water to a house with 1.5 bathrooms, unless you live in the south.
However, electric tankless boilers have two important advantages:
- They are much easier to access than gas networks.
- And they are easier to install as long as the installation does not need to be reconnected to your home (more information below). This will save you even more money and time.
Gas-fired boilers work better in terms of energy savings, reducing operating costs.
Whole house or point of use?
You can choose between a tankless boiler for the whole house and one or more systems that work independently of each other. (Of course, you can also install two complete houses to work together, which is rare if you need it).
The advantage of sampling points is that they can be installed directly in places where hot water is needed, at least in the case of an electrical unit that allows hot water to be prepared directly. Because no ventilation is required (more information here), they can even be installed under the sink or in the cabinet.
If you use a lot of hot water – think of a jacuzzi – or in a remote area of your home that may need extra power, an on-site boiler can also be an ideal addition to the central system for the whole house. Different blocks can be used to meet the needs of families with many children.
In general, once you have reached the desired flow rate and temperature rise, you will know whether the whole house or the location is best for you. Again, there are more gas-boilerless boilers that use the whole house, while electric boilers are more site-specific, except when used in warmer climates and/or smaller locations.
Energy factor (EF)
The Energy Factor System evaluates the efficiency of energy conversion. A higher factor equals higher efficiency. This becomes important when you are looking for energy savings that directly result in lower operating costs.
Formula used to calculate the CV of a boiler:
EF = energy converted into heat from water / total energy consumption
For modern electric tankless boilers, the minimum efficiency in the USA is 0.93, although most systems easily achieve 98% or more. The energy factor therefore only plays a minor role here.
For modern tankless gas heaters, the output ranges from 0.82, the current US minimum efficiency standard, to 0.98 for high-efficiency tankless condensing gas appliances. These systems use a second heat exchanger to collect excess heat from the exhaust gases and preheat the incoming water.
FOR YOUR INFORMATION: The fact that electric tankless boilers are more energy efficient than gas boilers does not mean that they are cheaper. In fact, the opposite is true, because gas is currently the cheapest fuel.
Indicator lights (gas heater only)
Some tankless gas water heaters use a permanent pilot flame that burns continuously and therefore needs fuel to maintain the flame, even when there is no hot water. It’s clearly a waste of energy.
The only advantage of stoves with fixed indicator lamps is that they are more affordable.
The best alternatives are intermittent ignition models, where the burner only ignites when water flows.
When installing a tankless boiler, factors such as the configuration of the water supply in your home, the size of the gas pipe, the wiring and the power supply play a role.
In general, switching to the zinc-free option is expensive (if you are building a new home, you absolutely want to be zinc-free from the start). And as you can see, the installation of a gas installation is in most cases more expensive than the installation of an electrical cabinet. The good news is that your tankless boiler is then much cheaper to install because there is no need to upgrade it.
It is also recommended that you use an authorized dealer to perform the installation for you – some manufacturers will even lose the product warranty if the unit is not installed professionally.
If you are an avid do-it-yourselfer and insist on doing the installation yourself, don’t forget that there are building regulations that you must follow and you will probably have to revoke the permit.
When installing an electric boiler without a tank, the biggest problem is electricity consumption: Is the power supply in your house sufficient for the system?
For example, with the Stiebel Eltron Tempra 36 Plus, which we mentioned earlier in this manual, the recommended minimum power is 300A, plus 3 separate 50A bipolar switches. In the future, the average house will have a total capacity of 200A.
And even if you choose the small Tempra 24 Plus, you still need two 50A bipolar switches and a basic 150A service.
This means that you may need to upgrade your service panel with large cables and switches (you may be able to make these changes yourself to save money). Or you may need to market a brand new switchboard, which is a job for a qualified electrician and can easily cost $1,000 or more.
* Sign up : The cost of the floating boiler + installation price 2019.
The location of an electric water heater without a tank should at least not be a problem in terms of the current until your panel is already full.
On the note: The product descriptions must state the current requirements. If you do not know any of these parts, it is best to consult an electrician before spending money.
Would you like to know more about installing an electric tankless boiler?
What should I pay attention to when installing a tankless gas boiler? Just one word: Ventilation.
Ventilation eliminates the exhaust fumes emitted by a gas water heater. And because there are many water heaters available without gas tanks, they need special and above all more ventilation. This means that tank-less electric boilers can be installed in limited space, but with gas more space is needed.
In addition, it will probably not work if you switch from the tank to the excess use of the old ventilation. This is because the exhaust gas temperature is much higher. Metal pipes are almost always needed.
It is also important to know if the device works with direct ventilation instead of electrical ventilation. And gas water heaters without capacitors require a different ventilation than capacitors. But those are subjects for another day. All you need to know at this point is that the ventilation carried out by a certified technician comes on top of the installation costs.
Tankless water heater on street
An external tankless boiler makes ventilation completely unnecessary. Frost protection measures are available, but they are not designed for cold climate areas.
The second problem is the gas supply to the house, which must be large enough to operate a tankless installation. A new (special) pipe with a larger diameter may be necessary, although this may sometimes be sufficient to increase the supply pressure. We recommend that you make an appointment with a qualified plumber in your area to discuss your options.
At least be willing to spend a few hundred or even a few thousand dollars on it.
Finally, you should pay attention to the guarantee period and the insurance coverage. If you think your tankless boiler will last more than 20 years, you should get at least a few years warranty.
Frequently asked questions
What is the size of a boiler without bowl for a family of 5 people?
It’s almost impossible to say. This depends on the number of baths and the water used by the devices and the required temperature. Follow steps 1 to 3 of this sizing guide or use our sizing calculator for tankless boilers and you will get the answer to your question – guaranteed!
What is the size of a 50 gallon water heater that needs replacing?
Even this information is simply not enough to give a reliable answer. This depends on the number of baths and the water used by the devices and the required temperature. We advise you to follow steps 1 to 3 of this size chart or use our size chart!
How many boilers do you need without a pool?
The amount of tankless boiler you need depends on the peak demand for hot water and the required temperature increase (see design manual above).
As a rule, a single gas boiler without a tank should be sufficient to provide a medium-sized single-family home. Electric tankless boilers are ideal for small families and apartments.
If demand is very high, the installation of 2 or more stoves should be considered, either at the place of use to work separately or at a central location to work together and as a whole.
In addition, although it is more expensive to start with, it can sometimes make more sense to add two smaller units than to install a larger one.
If you have any ideas or questions about the size of your tankless boiler, please leave a comment below!
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