5 Gallon Per Minute Tankless Water Heater – One of the biggest mistakes when it comes to tankless water heaters is buying a unit that is too weak to handle all of your hot water needs.
You don’t want a heater that’s too small or a tankless heater that’s too big and wastes energy unnecessarily. A tankless water heater should be as close as possible to your home’s hot water needs.
5 Gallon Per Minute Tankless Water Heater
What size tankless water heater do I need? How many GPM tankless water heaters do you need for a gas unit and how many kW for an electric unit?
Best Tankless Water Heaters Of 2023
To correctly answer what size tankless water heater you need, you first need to figure out two things:
To properly size a tankless water heater, we need to roughly estimate our maximum hot water needs at any given time.
Most households have peak hot water needs between 21:00 and 23:00. When we shower, we brush our teeth with the hot tap and even run the dishwasher.
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We have to calculate all this hot water demand. Here’s a handy chart of how many GPMs are needed for different water fixtures:
Example: If you shower (100% flow and 110°F hot water) and use two faucets at once (100% flow and 110°F hot water), you need a tankless water heater of at least 5 GPM.
Tankless heaters can deliver between 2 and 12 GPM of hot water. How many GPM do you need? 5-10 GPM is best for most homes. Of course, the price of a tankless water heater increases along with the efficiency, we wrote about it here.
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For low water demand, electric tankless hot water heaters up to 8 GPM are suitable. For larger needs (8+ GPM), you should choose one of the best gas tankless hot water heaters here.
If you look at the specs of various tankless heaters, you’ll notice that they list maximum GPMs. In practice, the realistic maximum GMP achievable with a tankless heater is very low.
This is because the maximum water flow in GMP is measured by heating the water to 77˚F. Currently, the temperature of the water entering the pipes is very important.
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In South Texas, for example, the entering water temperature is 77˚F. For example, in Minnesota, the entering water temperature is below 37°F. That’s another 40˚F difference that a tankless water heater must overcome.
Let’s say we have a tankless heater with a maximum water flow of 10 GPM. In Texas, the inlet temperature is 77˚F, so we can get 10 GPM of 110˚F water. The heater should heat the water between 77°F and 110°F; That’s a difference of 33˚F.
On the other hand, the water temperature entering Minnesota is 37˚F. To heat water to 110˚F in Minnesota, a tankless heater can overcome a temperature difference of 73˚F instead of 33˚F in Texas.
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Are you from Minnesota or Texas? Here is an infographic for the Rinnai RU160iP SE+ Series 9 GPM Tankless Water Heater that shows the maximum water flow in your state (for the US).
Here’s another example based on the infographic above: If you live in Florida (77°F inlet temp), the Rinnai RU160iP SE+ Series Tankless Heater has a maximum water flow of 7.1 GPM . This is enough to take several showers at the same time.
On the other hand, if you live in New York City (52˚F inlet temperature), the same tankless water heater achieves a maximum water flow of 4.5 GMP. This is a direct consequence of the inlet temperature difference.
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In New York, the heater must exceed an additional 25˚F. You can take 2 or 3 showers at the same time with the same heater and the same energy consumption.
Operating costs are also worth considering, especially for larger units. You can check how much electricity large tankless electric water heaters use here. Another good informative article about propane units is how much propane these on-demand hot water heaters use.
One of the most common questions when sizing a tankless water heater is what size unit is needed for a multi-person household. Obviously, a family of 3 needs a smaller tankless water heater than a family of 5. But what are the exact GPM (gas power) or kW (electric) numbers?
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Given the different inlet water temperatures, it can be difficult to determine exactly what size tankless hot water heater you need. Here is a table with the estimates; Unfortunately, the intervals are too large. Homes in the northern United States require larger units than those in the southern United States, such as:
This data is for informational purposes only. In practice, there are many important factors, such as inlet water temperature and heavy hot water demand, that determine the exact size of tankless water heater required.
In the specification sheets you can see that the manufacturers give the maximum water volume in GMP or the maximum electrical power in kW. Generally, gas tankless water heaters have a GMP number, while electric tankless hot water heaters have a kW number.
Tankless Water Heaters: A Buyer’s Guide
As we have seen, maximum GMP is relative. It depends on where you live in the US (because it affects the temperature of the incoming water). However, the power (measured in kW) is absolute. We can compare the performance of different tankless heaters by comparing their maximum output (as we have done in the table of the best tankless heaters below).
However, it is also worth taking as an example some financial calculations. Here we recommend checking whether a tankless water heater is really worth it (we’ve done the math).
Let’s see two examples. The first is the size of a 50 gallon water heater you want to replace with a tankless water heater. The second depends on how many people live (and use the hot water) in your home. For example, what size tankless water heater does a family of 5 need?
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Here’s how it goes: You currently have a 30, 40, 50, or 80 gallon water heater and want to replace it with a tankless water heater.
The main difference is that with a water tank you have 50 liters of hot water, and with one without a tank you heat water on demand.
In an average 10-minute shower, for example, we use 10 liters of hot water. If 3 people take a shower, run a faucet or two, a dishwasher, etc., you can use up those 50 liters quickly.
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However, the case of a tankless water heater is different. There is no hot water in the tank; The tankless heater’s high-performance heat exchanger heats water when you need it to a specified maximum GMP limit.
That’s why Rinnai, the world’s best manufacturer of tankless gas heaters, offers a wide variety of models: from 7 GPM to 11 GPM:
Rinnai offers its tankless models in various sizes. They are considered the best brands of gas tankless water heaters as mentioned here.
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Note that this is only a rough estimate. It is a good idea to purchase a tankless heater that is slightly more powerful than your estimated needs. Better to be safe than sorry.
If 5 people live in the same house, they can use several taps or showers at the same time. When choosing the size of the tankless water heater, we need to consider this.
The shower is a consumer of domestic hot water that needs hot water quickly. 5 people at the same time several hot water taps, dishwashers, washing machines, etc.
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In summary, a 10GPM tankless gas heater or a 27kW electric tankless heater would be needed for a family of 5 if you live in the northern part of the US where the incoming water temperature is lower . A tankless heater has to work harder to raise the water temperature to 110°F or 120°F.
However, if you live in the South, a tankless water heater can reduce efficiency by up to 30%. So for a family of 5 in the southern United States, a 7 GPM gas tankless heater or an 18 kW tankless heater is more than enough for all hot water needs.
Remember that using tankless water heater circulation pumps can save you a lot on hot water, especially for larger units. These pumps are very useful if you want hot water quickly from a tap or shower. Then you don’t have to wait for the hot water to run out and waste that cold water.
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In most cases, only 1 tankless heater is needed for the whole house. However, if you have a very large home (2 or more bathrooms), even a large 11 GPM Rinnai gas tankless water heater may not be enough to meet your simultaneous hot water needs.
A large tankless heater takes care of most hot water needs. In cases where we need very hot water, we turn on a tankless electric water heater to help.
Another example is if you
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