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What Size Tankless Water Heater To Replace a 40 Gallon?

40 & 50 Gallon Hot Water Heater Tankless Equivalent? Replied!

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    If you wanna install a new tankless water heater to replace your old 40 or 50 gallons, here’s a short direction.

    • If you’ve 1 bathroom and 2-3 family members, replace it with a 4.5 GPM tankless water heater.
    • If you have 2 bathrooms and 3-4 family members, a 6.5 GPM unit will be okay for you.
    • If you need a tankless water heater for 3 bathrooms and 4+ family members, then at least 8.5 GPM is essential.

    In case you have some additional hot water demand like bathtubs, jacuzzi, laundry, along with three large bathrooms, then 11 GPM units will be the final option to go.

    Note: Sizing a tankless water heater isn’t similar to a tank-type water heater as a tankless unit produces an unlimited hot water flowSo, both the 40 and 50 gallon tank type model is replaceable with the same tankless water heater.

    But there are some tips and tricks to calculate the perfect size of tankless water that can replace your 40 gallons unit and fulfill your hot water demand without any energy wastage.

    To learn them all, let’s dive in…

    How Many GPM is a 40 Gallon Water Heater? Let’s Find Equivalent Tankless  

    The amount of GPM is up to you. But how many minutes you’ll get the hot water flow, that depends on the tank size. Here’s an example.

    If you dispose of hot water from the 40-gallon tank to any 2.5 GPM faucet/outlet, the gradual temperature decrement will look like

    • First 4 Minutes = 160F (Max)
    • Second 4 Minutes = 135F
    • Third 4 Minutes = 116F
    • Final 4 Minutes = 91F

    If you double the flow rate, the timing will be half. (2 Mins in this case.)

    But, as you don’t need that 160F outlet temperature, you’ll mix some cold water from the tap line.

    Suppose your ambient temperature is 60F, and you need 120F hot water at any of your kitchen sink, laundry, or utility faucets.

    A typical tank-type water heater raises the temperature to 160F in MAX, and imagine 40 gallons of 160F hot water remains reserved in the tank. 

    So, you need to MIX 1 Gallon of 160F water + 0.659 Gallon of 60F water to get an output of 120F hot water. Here’s the calculation.

    40 Gallon Tank Type Water Heater Temperature Adjustment Calculation

    So, we find the mixing ratio of (160F)hot and (60F)cold water is 1: 0.659 to get the derived 120F water output.

    However, a 2.5 GPM faucet will dispose of 1.51 Gallons of 160F hot water from the tank and 0.995 Gallons of cold water from the tap line.

    But, as you know, the tank temperature will gradually decrease, and you need to mix more hot water and less cold water. 

    When you use the hot water from the tank, cold water enters inside, and the temperature gradually falls.

    Temperature Decrement of Tank Type Water Heater

    When you use 10 gallons of hot water, the tank inside temperature falls from 160F to 135F.

    Tank Type Water Heater's Temperature Loss

    After using 20 gallons of hot water, the tank inside temperature falls from 160F to 116F.

    After, after using 30 gallons of hot water, tank water temperature falls over 102F

    Finally, after using 40 gallons of tank water, the tank water temperature becomes as cold as 91.5F, which you exactly need to have a shower. So, you can’t mix any more cold water with it.

    My Preferred Tankless to Replace a 40 or 50 Gallon – Ecosmart

    Check details on Amazon

    So what size tankless water heater is equivalent to this 40 gallon unit?

    If you use 5GPM water, the 40 Gallon tank delivers hot water for 6 minutes before the temperature falls by 116F.

    But a 5 GPM tankless water heater can limitlessly deliver hot water at 120F. However, a small 3 GPM tankless water heater can do the same with a low flow rate.

    So, If your total water uses remains around 4 GPM, including bathroom, kitchen, and laundry, then a 5 GPM tankless water heater can replace a 40 Gallon unit.

    Also, you can increment or decrement the size according to your flow rate calculation.

    How many showers can you have in a 40 gallon tank? So Here’s The Equivalent Tankless  

    How many showers can 40 gallon water heater run?

    Maximum 2 Showers simultaneously. 

    The number of showers isn’t the matter here. Your concern should be how long you can take a shower with a 40 Gallon Tank water heater?

    Most dermatologists recommend that  98°F to 101°F is the best temperature for having a warm shower.

    If you use a 2.5 GPM single showerhead, then the temperature will sustain for no longer than 20 minutes.

    So, if you run 2 showers simultaneously, the maximum temperature will sustain for 10 minutes which is the average shower season.

    In case you use a kitchen faucet simultaneously, it’s hard to run more than one shower, though.

    Now, if you want a similar tankless water heater that can run 2 showers at a time, then 5 GPM is the best size. However, tankless water’s hot water flow is endless. 

    Replacing a 40 gallon and 50 gallon water heater with Gas or Electric Tankless?  

    Gas or Electric Tankless Water Heater to Replace a 40 Gallon Unit?

    50 Gallon tank-type water heater that supplies hot water to the whole home could be replaced with a 5 to 8.5GPM gas tankless water heater based on the number of your showerheads and utility faucets.

    However, you can replace a 40 gallon unit with a 3.5 GPM electric tankless water heater if you prefer a point of use installation. 

    Firstly, both the propane and natural gas tankless water heater are expensive compared to an electric tankless unit. 

    But, the monthly operational cost of a gas tankless unit is 3X cheaper if you use natural gas, or at least 2X cheaper if you prefer a propane unit.

    So, considering this, if you have a sufficient budget and want a large tankless water heater for the whole house, you should go for the gas-driven models. 

    But here’s another important fact. 

    Electric tankless water heaters are available in smaller sizes, which you won’t find between gas models. 

    The minimum gas tankless water heater for indoor installation is about 4 GPM. Though prestigious brands like RINNAI, RHEEM only manufactures larger models.

    So, when choosing a smaller unit such as 2.5~3 GPM or even 4 GPM, you should prefer the electric tankless water heater.

    Smaller tankless water heaters are mainly preferred for point of use. Or you may consider installing separately in each bathroom.

    However, various reputable brands like Stiebel Eltron and Ecosmart manufacture several smaller electric tankless units, and some come with a lifetime warranty (Such as Ecosmart). 

    Should I turn off the tankless water heater if the water is off?  

    Should I turn off tankless water heater when the water is off?

    No, you need not. 

    All the tankless water heater has TWO flow detector sensors.

    The first one detects the water flow and transmits a signal to the controller circuit whether the unit needs to be turned ON or not. Then the circuit turns ON the sparker/ignitor and opens the gas valves (in tankless gas models) or supplies power to the heating element in electric variants.

    The second sensor detects whether you increase/decrease the flow rate and transmits a signal to the controller to adjust the power to maintain your preset outlet temperature. It’s called “Auto Modulation.”

    Finally, when you turn off the showerhead/faucets, the first sensor again detects it and cuts all the powers from the heater. Also, it closes the gas valve in gas tankless heaters.

    These sensors protect the tankless water heater from any dry startup or flameout scenarios. 

    Also, there’s an additional sensor to protect the insider water being frozen, so you even don’t need to winterize those units.

    So you never need to worry about turning it off like a tank-type water heater. But yes, if you leave home for a couple of days, then you might keep the unit turned off for added safety.

    Reliable Tankless Water Heater Brands to Replace Your Old 40 Gallon Unit?  

    Reliable brands of tankless water heater to replace a 40 gallon.

    The tank-type water heaters last for decades. So when you want a replacement with a tankless one, firstly you think about reliability.

    So, I’m describing some reputable brands along with the facilities they offer.

    Reliable Brands of Gas Tankless Water Heater 


    The most reliable, most efficient, highest performer, and totally unbeatable brand of GAS tankless water heater to replace your 40-gallon old uni, that’s none other than Rinnai.

    They manufacture gas tankless water heater in 2 Series.

    Super High Efficient Models

    • RUR Model Series (96% Energy Efficient). 
    • RU Model Series(95% Energy Efficient). 
    • RUCS Model Series (93% Energy Efficient). 
    • RUS Model Series(93% Energy efficient).

    High Efficient Models

    • RL Series Models (82% Energy Efficient). 
    • RLX Series Models (82% Energy Efficient). 
    • V Series Models (82% Energy Efficient). 

    All these series come with several indoor and outdoor models.

    RUR and RU series is a bit expensive and contains a smart control feature via WIFI. They offer a 15-year warranty for heat exchangers, a 5-year warranty for parts, and a 1-year warranty for laborers.

    All the rest series models are slightly cheaper and offer a 12-years heat exchanger, 5-years labor, and 1-year parts warranty.     

    According to our analysis, users are too satisfied with Rinnai customer service.


    Rheem is another ancient American brand that’s manufacturing since 1925! 

    Rheem is cheaper than Rinnai but offers all extraordinary features like smart control, re-circulation, and condensing mechanism with almost similar energy efficiency. 

    Day by day, they’ve improved their products with advanced engineering and highly optimized mechanism. 

    Rheem also offers TWO series variants. One is Condensing Tankless, which is almost 96% efficient. Another one is the Mid Efficiency series that offers 83% efficiency.

    All these Rheem products come with a 12 year of heat exchanger, 5 years of parts, and 1 year of labor warranty.

    So, once you replace your 40-gallon unit with Rheem, you need not worry about the next 12 long years!


    Ecotemp is a Raising Superstar which was founded in 2004. 

    Since then, they have been competed with reputable brands like Rheem or Rinnai and have established their Glory.

    Ecotemp offers

    • Innovative Smart Features with Integrated WIFI
    • Extended Warranty Similar to Rheem
    • Lower Price Tag (Almost Half)

    But They don’t have any larger models, and the highest unit they offer is 6.8 GPM.

    Ecotemp offers a 12 Years heat exchanger, 5 years parts, and 1 years warranty. 

    So, if you prefer a smaller model and want to save some money, surely you can choose this brand to replace your 40-gallon unit.

    Camplux and Fogatti 

    Camplux and Fogatti are two different brands, and Camplux is a little bit more popular.

    Both of them manufacture smaller size gas tankless water heaters, and their price point is super affordable. 

    Both of the company imports their products from China. 

    But the fun fact is, they mostly copy the design and technology from Rheem and Ecotemp, so, disassembling their products, you will find almost similar accessories. 

    Accordingly, their build quality and internal components are not bad. 

    But, they offer only a 1-year Warranty, so for long-term investment, I would avoid these two brands. Though for point-of-use installation, they are fantastic.

    Reliable Brands of Electric Tankless Water Heater 

    Stiebel Eltron

    “Made in Germany,” this tag should be sufficient to describe its reliability!

    But, for their expensive labor cost, the products are slightly costly.

    Stiebel Eltron offers

    • 7 Years warranty on leakage and 3 Years on Parts.
    • An Array of models to handle “Point of use” to “Whole House” water supply.
    • 3KW to 36KW models for both 120V and 240V outlets.
    • Super responsive customer support.

    Stiebel Eltron is a long-term solution. If you don’t have any gas line and an electric tankless water heater is the only option to replace your 40 gallons, then this brand should be the first choice.


    Made in America” along with a “Lifetime Warranty,” what do you need more?

    Ecosmart is a sub-brand of Rheem. An interesting matter is, you will find precisely the same products with different branding in Rheem.

    But Rheem offers a limited-year warranty, so I would prefer Ecosmart before them.

    Ecosmart also does manufacture electric tankless water heaters from 3.5KW to 36KW, both for 120V and 240V outlets.

    According to some users’ opinions, few of them are unsatisfied with Ecosmart customer support.

    Ariston Andris

    Ariston is an Italian brand, and you will rarely find their Tankless models in the American market.

    Though, their mini-tank water heaters are popular here.

    Ariston makes some small size tankless water heater, which is around 3.5 KW, and they are only the manufacturer who offers 5 years warranty on heating elements for such smaller units.

    If you like point-of-use installation, then you should keep this brand under consideration. 


    You should only prefer this brand if you don’t want to spend much buying a tankless unit and somehow wanna fulfill your hot water demand for now.

    They import products from China; quality is Average. Also, they offer only a 1-Year warranty.

    But, again, the fun fact is, they copy the design and technology from Ecosmart. For the initial purpose, they are fantastic. So you should not underestimate them at all.

    What is the average life of a tankless water heater? (That’s Equivalent to 40 Gallon)  

    Average Lifespan of a Tankless Water Heater

    As I’ve described above, Rinnai, Rheem, and a few other gas tankless water heaters come with a 15 to 12 long years warranty period. 

    So, undoubtedly they last for a couple of decades. From the statistics we’ve found, most gas tankless water heaters from reliable brands last for longer than 20 years on average.

    Again, as you’ve seen, a few electric tankless water heater’s come with 7 years and even a lifetime warranty period.

    Frankly, electric tankless water heater lasts longer than gas, though their average lifespan is the same 20 years.

    But, here’s an essential fact to note down. 

    A tankless water heater demands routine maintenance. If you delay or neglect to do it, the gas exchanger and internal components will get damaged faster.

    As the company has claimed a lifetime warranty, of course, there’s a reason behind it. There’s almost nothing inside the unit that can be randomly defected. 

    So, if you handle the unit with care and ensure essential and routine maintenance, your tankless water heater is going to be your lifetime asset.

    How much do you save with a tankless water heater?

    How much you save with a tankless water heater?

    In a word, it’s almost 40% to 60%.

    Tank-type water heaters waste a tremendous amount of energy. Suppose you have 3 members in the family and a single bathroom with a 40 gallon hot water tank.

    When two members have completed their shower, the water temperature drops, and the unit again starts raising the temperature.

    Then the third person starts taking a shower, and simultaneously the tank-type heater keeps raising the temperature. 

    After all your showers have been completed, a whole tank of hot water remains stored inside even if you turn off the heater. So, if nobody uses that for the rest of the day, the hot water stays inside and starts losing temperature.

    Thus a vast amount of BTUs gets wasted every day beyond your consciousness.

    On the other hand, tankless water heaters only produce hot water when you need it, and it stops working as soon as you close the showerhead or any water outlets.

    Thus, users who replaced their tank-type unit with a tankless one experienced a 40% to 60% reduction to their utility bills.

    Number Of Family Members vs Size of Tankless Water Heater  

    Number of family members VS size of tankless water heater

    Sizing of a tankless water heater doesn’t depend on the number of family members. But it depends on how many bathrooms/showerheads, bidets, kitchen, or other utility sink faucets you have.

    However, you may have 6 showerheads, but only 3 of them might be used simultaneously, or three showerheads and a kitchen sink faucet might be used simultaneously, then only these outlets’ GPM need to be under calculation.

    So, no matter how many family members you have. If you simultaneously use 4 GPM of hot waters, then you should choose a 5GPM tankless unit. 

    The 30% extra power will support you on critical snowfall days, and you won’t find any extended utility bill for it.

    Cause the tankless water heater has a self-modulation mechanism. This means, if you install a 9 GPM unit but use only 3 GPM, then the water heater will utilize its 34% power only to supply that lower GPM.

    That’s an ultimate advantage of it, which you won’t get in any tank-type water heater.

    7 FAQs about Tankless Water Heater to Replace a 40 Gallon Unit