3 Steps to Determine Which Size Tankless Water Heater You Need

3 Steps to Determine Which Size Tankless Water Heater You Need

  • “How should I know what size water heater I need?!” Maybe you mumble this to yourself as you’re researching online. Perhaps you’re talking to a neighbor or loved one. You may be shouting it to the heavens as a plumber wastes your time and tracks dirt through your clean living room.

    The bottom line is, you want to know what size water heater to get, but how do you do that?

    You can determine what tankless water heater size you need with our three easy steps. Account for:

    • your household size

    • your changing water demands

    • how many “bathrooms” you have


    By the end of this article, you’ll know what size tankless water heater you need. This information came from our highly-trained technicians to make your next appointment quick and painless. So if you’re considering Monkey Wrench (for the Los Angeles area) or anyone else for that matter, read on!


    Step 1: Count the People in Your House


    Household Size

    Household size is the first thing we’ll take into consideration on our tankless sizing mission. Break out a piece of paper and a pencil to follow along.

    Our charts are based on a simple point system, so write down the points you have in each step, if any.

    This table is based on consistent household size.

    Figure 1.

    Household size table

    Got your point total? Great! Write it down and proceed to step 2.


    Step 2: Figure Out Your Changing Water Demands


    Changing Water Demands

    What are the changing water demands your house might have?

    • Using multiple fixtures at once


    Example: You have friends over for the holidays. You have a two-bathroom house, and everyone’s trying your tankless unit. One after another, they blast the hot water.

    At the same time, members of your house run the dishwasher, do laundry and wash produce in the kitchen. Your tankless heater is going to have a hard time keeping up.

    • Seasonal temperatures


    Example: Colder outdoor temperatures require more power and time to heat water and vice versa. If you live somewhere where your groundwater is 70F year-round, don’t take this into account.

    How can you fix these problems? Do a little thing called oversizing.


    Oversizing means buying a water heater that’s slightly bigger to make sure you avoid any problems from a temporary increase in water demand.

    Oversizing can also keep you from experiencing the following problems:

    • Low water pressure

    • Stop and go hot water

    • Early unit breakdown


    With these in mind, take a look at our table.

    Figure 2.

    Temporary water demands

    You may not have any extra points in this section, and that’s fine. Proceed to the next section.

    Remember! It’s always better to slightly oversize a water heater than undersize it. There’s no downside to oversizing besides a price bump.


    Step 3: Count Your “Bathrooms”


    Count Your Bathrooms

    Another aspect to consider when sizing your water heater is how many “bathrooms” you have.

    Bathrooms, in this case, refer to fixtures that use a lot of water.

    So, why do we call them bathrooms instead of tubs, showers, or fixtures? Saying everything is a tub, shower, or half of one can get confusing quickly. Rooms are easier to count.

    Figure 3.

    Number of Bathrooms

    Showers, tubs, and some non-standard fixtures can take a lot of water, so that’s why they count as at least 1 bathroom, whereas kitchen faucets, washers, etc., are half a bathroom because you use less water.


    Here are some examples of totals you might have:

    • Example A: If you have a two and a half bathroom house with tubs, a kitchen, a dishwasher, and laundry hookups, your bathroom number would be 4.

    • Example B: If you have a one bathroom home with a shower, kitchen, and laundry hookups, your number would be 2.

    • Example C: If you have a one and a half bathroom, kitchen, and laundry, your number would be 2 ½, but you would want to bump it up to 3 for oversizing.


    And now you have your final total! Proceed to the last section to get your estimated tankless water heater size.


    What Size Tankless Heater Do I Get?

    A tankless water heater installation

    We’ve talked about how water demand can change depending on the number of people in your house, the day, and how different fixtures need more or less water.

    Now, before we get into your final size, remember if you’re going to make a household change soon, you may have to bump your size up one space. An example of this would be a family member moving in or buying a shower sprayer.

    With that in mind. Add your points together and match it with the total below:

    Figure 4.

    Final tankless water heater size

    Here’s your final tankless water heater size! If you want to get yours today and you're in the Los Angeles Area call us at (310) 853-8690. If you’re not in our service area or you have a plumber you’re not ready to part with, no problem. Just give them the gallon size and tell them Monkey Wrench sent you.