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What Size Tankless Water Heater For 3 Bathrooms?

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    You need an 8 to 11 GPM tankless water heater for 3 bathrooms. But…

    Perfectly sizing a tankless water heater might be done in two different ways. 

    • You may install a single tankless water heater for all 3 bathrooms.
    • Or you may install 3 separate tankless water heaters for each of those bathrooms.

    Both setups have advantages and drawbacks. (Described Bellow!)

    However, before installation, you need to know a few critical facts about tankless water heaters. 

    Let’s dive in…

    1. Sizing a Single Tankless Water Heater for 3 Bathroom

    Sizing a Single Tankless Water Heater for 3 Bathroom

    Here’re the possible water outlets that your bathroom might have and their average flow rate.

    1. Showerhead – 2.5 GPM
    2. Hand washing sink – 1.5 GPM
    3. Bidet – 1.5 GPM
    4. Jacuzzi – 1.2 to 1.5 GPM
    5. Bathtub – 4 to 7 GPM

    Combining all these appliances, the total GPM is around 11 to 14 GPM.

    If you skip the TUB and the JACUZZI, then it’s only 5.5 GPM.

    Now the fact is, you never ever will use all these appliances together! But you have another two bathrooms, which might have similar water outlets or a few more/less.  

    Additionally, you have a few outlets in the kitchen and laundry, and there’s a hot water demand of an additional 2.5 GPM.

    Here’s the essential part of this calculation.

    There’s a very minimum possibility that all 3 bathrooms will be used simultaneously, only then if you have multiple family members who need to shower at the same time and go to the office/school in the morning.

    But, there’s a huge possibility that your kitchen sink and at least one bathroom will sometimes be used simultaneously.

    So, choosing a minimum of 8 GMP tankless water heater will be an intelligent choice. Cause, at a time, it will able to handle

    3 Showers of 3 Separate bathrooms (2.5 x 3 = 7.5 GPM) 

    (Third bathroom user might use the bidet instead of the shower.)


    2 showers of 2 bathrooms (2X2.5 = 5 GPM)

    And a Kitchen/Laundry (1X2.5 = 2.5 GPM)

    So, you’ll use 7.5 GPM out of 8 GPM capacity. However, you might use another hand washing sink if required!

    If you want to use the bathtub or have a few more essential hot water demands, then 11 GPM will be the last option to go.

    But here’s an important fact to keep in mind. Frankly, the outlet temperature of a tankless water heater remains around 120F to 140F, which isn’t required for having a shower. You will need to mix it with a few percentages of cold water.

    Though in the kitchen or laundry, a higher temperature is essential. 

    Overall, the 8 GPM should be okay for you if you aren’t from the coldest states of America. If you are, then 11 GPM is the perfect option for you.

    Advantage of Having a Single Tankless Water Heater

    • Installation is hassle-free and cost-efficient
    • Maintainance is easier.
    • Price is more affordable considering 3 separate items.
    • Both gas and electric tankless water heater is available of this size.
    • Larger units are more energy efficient!


    • Higher operational cost.
    • If the unit gets defective, that will ruin all 3 bathrooms.
    • Require more maintenance.

    You can’t maintain the temperature for different bathrooms/kitchens separately.

    In case you wanna replace your tank-type heater with tankless, you may read my guideline on the Size of Tankless Water Heater To Replace a 40 Gallon.

    2. Size of Separate Tankless Water Heater for 3 Bathroom

    Generally, 3~5 GPM tankless water heater is sufficient to handle a single bathroom. 

    Notably, a 3 GPM tankless water heater is 3 times cheaper than an 8~9 GPM unit, along with the installation cost. But, only the electric tankless water heater is available of this size.

    But, your location and the ambient temperature are the primary matter of consideration here. 

    The tankless water heater has a limitation in temperature raising ability. As you know, when the water passes through the heat exchanger, it consumes the temperature from the heating element.

    So, there’s a concise amount of time to raise that water temperature, and here comes the limitation of the tankless water heater.

    Explanation of the Limitation 

    Suppose you’ve installed a 3GPM tankless water heater and preset the output temperature at 120F. Unfortunately, it doesn’t guarantee you to get an outlet water flow with exactly 120F temperature. 

    Cause, when you preset an electric 3GPM tankless water heater to 120F, it’s capable of raising water temperature by 35F (mostly) with that specific water flow. 

    So, if your ambient water temperature remains at 80F, your outlet temperature will be at 115F. 

    In case your ambient temperature falls to 65F, then your outlet temperature will be around 100F.

    So, if you want to maintain the perfect 120F temperature, you would require to decrease the water flow by 1GPM or more.

    If you are from northern America or any of the coldest states, this issue will suffer you. To avoid this trouble, you might choose a 5 GPM unit for each bathroom so that you can get at least 3 GPM water flow, always with the perfect outlet temperature.

    3. Gas or Electric, which type of tankless water heater will be a good choice?

    Gas or Electric, which type of tankless water heater

    If you want to install a single unit for all 3 bathrooms, then a gas tankless water heater will be the best solution. Otherways, if you’re going to install separate heaters for each bathroom, then an electric tankless water heater is a better option to go.

    As for single installation, 8+ GPM is the minimum size tankless water heater for 3 bathrooms. 

    So, several GAS models and a few electric models are available with 8+ GPM flow rate capacity.

    On the other hand, if you want separate installation for each bathroom, you’ll require a 3 to 5 GPM tankless water heater. So, many electric models but a few gas-driven models are available in that case.

    About Electric Models 

    Only 36KW Electric tankless water heaters can deliver hot water with 8 GPM flow rate maintaining your preset temperature.

    The most prestigious brands that offer this size are

    1. Stiebel Eltron
    2. Rheem and
    3. Ecosmart

    These 3 brands also manufacture lower GPM tankless water heaters if you want to install them separately in every three bathrooms. 

    18KW to 24KW water heaters can deliver hot water with a 3 to 5 GPM flow rate. Along with those upper brands, a few different brands manufacture these low GPM tankless water heaters. Those are

    • Eemax
    • Camplux
    • Eco touch
    • Ariston Andris

    Electric tankless water heaters are cheaper than gas. 

    Depending on the brand, 18KW will cost around $350~$400

    24KW will cost about $450~$550, and 36KW will cost around $600~$750.

    Also, the installation cost of electric tankless water heaters is significantly cheaper than gas and effortless. You can do it in the DIY process.

    But, the drawback is the cost of electricity is 2X~3X higher than natural gas. 

    About Gas Driven Models

    Gas tankless water heaters are expensive, but the operation cost is 3X lower than electricity!

    Prestigious manufacturers that offer 8~11 GPM gas water heaters are

    • Rinnai
    • Rheem
    • Camplux

    Among them, I prefer Rinnai and Rheem the most. But Rinnai is energy efficient, according to my experiment.

    If you like separate installation, then a few manufactures offer 4~5 GPM models. They are

    • Fogatti
    • Camplux
    • TCMT

    Here’s my preference is Fogatti. 

    But one essential fact is, lower sized gas water heaters aren’t much energy efficient.

    Depending on brand reputation, 4.5 GPM unit costs around $500

    7.5 to 8 GPM is available by $800 to $900.

    11 GPM model is too expensive, which is around $1400 to $1700.

    Accordingly, their installation cost is enormous and requires hiring professionals.

    But, once you install the item, the rest of the life will remain in charm with the 3X less monthly utility bill and lower maintenance fee.

    13 FAQs to Know Before Choose a Tankless Water Heater for 3 Bathroom

    How many showers can a tankless water heater run?  

    Shortly, a 3GPM tankless water heater runs 1 Showerhead in winter and 2 showerheads in summer. 

    An 8 GPM showerhead runs 3 showerheads in winter and 5 showerheads in summer.

    Standard showerheads are of 1.8 to 2.5 GPM. There are a few showerheads with an extended 3.5 to 4 GPM.

    Now, a tankless water heater delivers hot water with 120F to 140F output temperature, which is too high to take a shower, so you need to mix some cold water.

    So, if you mix 1.5-gallon hot water with 1-gallon cold water, that will be a comfortable temperature for the skin with a total 2.5 GPM water flow. In this scenario, a 3 GPM tankless water heater will be able to handle 2 showerheads simultaneously.

    But, if your ambient temperature falls, then your tankless water heater will be unable to raise the temperature sufficiently. Then you might require to use that hot water without mixing any cold water, so the 3 GPM tankless water heater will be able to handle a single showerhead.

    Can a tankless water heater fill a tub?  

    Yes, it can. 

    A tub tap is generally of 4 to 7 GPM. If you install a single tankless water heater in the bathroom, that would be a minimum of 3 GPM, but you’ll need to mix some cold water.

    So, there’s no problem in filling a tub with a tankless water heater.

    Why does hot water run out after shower?  

    This scenario might happen if your tankless water heater is placed far from the bathroom or the showerhead.

    It’s not like the water heater is still producing hot water, but the unused water in the pipeline remains hot and comes out of the showerhead.

    Additionally, the heat exchanger transmits heat for a while after shutting down the water heater so that it might occur. 

    But, if hot water comes out endlessly after turning the heater off, then your tankless water heater might be defective.

    How long does shower water stay hot?  

    Tankless water heaters produce on-demand hot water, so the water will remain hot until you close the showerhead.

    You don’t need to turn off the tankless water heater; just closing the faucet or the shower head will automatically turn it off.

    Cause the water heater can sense the water flow and control the heating elements in an automatic process.

    What happens if tankless water heater is undersized?

    If your tankless water heater is undersized, two scenarios could happen.

    • You will get perfect temperature hot water with an insufficient flow rate.
    • You will get hot water at a high flow rate, but the outlet temperature will fall badly.

    How do I calculate what size Tankless water heater I need?

    You may calculate it following these simple steps.

    1. Note all your outlets and their GPM flow rate where you’re planning to use hot water. (Such as kitchen sink, shower head, bidet, etc.)
    2. Now think which outlets will be at use SIMULTANEOUSLY, and add those GPMs.
    3. Now choose 30% larger than your summation result. For example, if you require 3 GPM at a time, choose a tankless water heater with a 3.9 GPM capacity.

    This 30% extension is for those worst weather days during winter when the ambient temperature falls badly.

    Size of Tankless Water Heater for Different Numbers of Family Members.  

    You don’t need to worry about the number of family members while using a tankless water heater.

    That’s a matter of consideration while choosing a tank-type water heater. Cause if one member takes 10 Gallons of hot water for shower, then 3 members will require 30 Gallons.

    But in a tankless water heater, you only need to calculate the maximum flow rate GPM you require. 

    If you install one 3 GPM tankless water heater, that will produce an unlimited amount of hot water by maintaining the flow rate. 

    So, if you have large family members and have separate bathrooms, calculate those showerhead’s flow rates and install a unit following my upper guideline. 

    What is a good size tankless water heater? (For Efficiency)  

    For electric tankless water heaters, all are similarly efficient. But smaller one requires less expensive electrical wirings, breakers, and other equipment. 

    So, installation is easier and cheaper for smaller size units.

    But, for gas tankless water heaters like RINNAI, larger models are more efficient than smaller ones. Though they are expensive enormously.

    An 11 GPM Rinnai tankless water heater is 96%+ efficient, while an 8 GPM one has a maximum of 88% to 90% efficiency.

    So, for electric tankless water, the smaller, the better, and for gas tankless water heater, the larger, the better.

    What is the highest gpm tankless water heater?  

    Rinnai, Rheem, and a few different manufacturers offer an 11 GPM gas tankless water heater that is maximum in size.

    Stiebel Eltron, Eco Smart, and a few different electric tankless water heater manufacturers offer the highest 36KW, which can deliver 8.7 GPM water in MAX flow rate.

    Are tankless water heaters cheaper?  

    Yes and No.

    The Rinnai, 11 GPM tankless water heater’s price is around $1700, which is more expensive than most 40-gallon tank-type water heaters. 

    But there are many gas tank-type water heaters like AO Smith, which are 3X expensive than this unit.

    However, 8.5 to 9 GPM units are cheaper than 40-gallon units.

    So, the buying price depends on size and brand. But, the operational cost of a tankless water heater is at least 40% less than a tank-type water heater. 

    Cause it doesn’t reserve any unnecessary hot water and make an energy wastage.

    Do tankless water heaters need maintenance?  

    Yes, it requires cleaning and sometimes changing some heating elements or heat exchanger by times.

    Generally, every six months to 1 year, you would require to flush the unit and clean with vinegar or tank cleaner depending on your water chemistry.

    The highest it may cost you is around $20.

    If you need to replace any heat exchanger, that might be done under warranty. 

    So, maintenance doesn’t cost too much.

    How much does a plumber charge to flush a water heater?  

    If you can’t do it yourself, you might need to hire a professional and pay $100 to $145, depending on the state.

    But, it’s a straightforward procedure, and you should try at least before attempting to hire anyone.

    What is the downside of a tankless water heater?  

    1. It requires more maintenance than tank-type units.
    2.  MAX temperature isn’t too high.
    3. Sometimes defective item starts malfunctioning, and you might get a sudden cold water touch during the shower.
    4. Lifespan isn’t as long as a tank-type unit.