Tankless Water Heater Sizing
You’ve checked out the pros and cons of tankless water heaters here and you’ve decided that tankless is the right fit for you. Now, you have to figure out what is going to work best.
It’s time for some math. First, you have to figure out the gallons per minute (GPM) of hot water your fixtures and appliances need at any one time and add them up to determine the total usage for the household.
How to Determine Flow Rate
Typical GPM by appliance
- Bathroom Faucet: .5-1.5 GPM
- Dishwasher: 1-2.5 GPM
- Kitchen Faucet: 1.5-3 GPM
- Washing Machine: 1.5-3 GPM
- Shower: 1.5-3 GPM
- Tub: 4GPM
Case Study: 2 adults, 2 teenagers from 7AM-8AM
(Assume that at any given moment, every single person isn’t taking a shower at the same time)
2 Showers X 1.5-3 GPM = 3-6 GPM
Breakfast prep (hand washing/food prep) = 2 GPM
1 Men Shaving X .5-1.5 GPM = .5-1.5 GPM
Total Flow Rate: 5.5-9.5 GPM
Additionally, gas tankless water heaters are able to produce a larger temperature rise per GPM than electric models. Most tankless water heaters are rated for a variety of inlet temperatures. Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet/appliance.
How to Determine Your Home’s Required Temperature Rise
So, you thought the confusing part was over, huh? There’s one more step. Now, you have to determine your home’s required temperature rise by calculating the difference between your ground water temperature and your desired hot water temperature. For those of you who live in cold weather climates, not only do you get the double whammy or long, cold winters, but the cold groundwater temperature actually affects how much hot water your tankless water heater can pump out!
Typically, 105-120 degrees Fahrenheit is the target temperature. Once you subtract your ground water temperature from 105-120 degrees, this is your required temperature rise.
Calculate the difference between your ground water temperature and your desired hot water temperature – typically around 120 degrees Fahrenheit. This is your required temperature rise.
Use this Groundwater Map for help determining the groundwater temperature in your area.
Hold onto your needed flow rate and required temperature rise as you shop for your new tankless water heater.
Tankless Water Heater Shopping Made Easier
Say what?! Is your brain boggled? Luckily, a lot of tankless water heater manufacturers feel your pain and know how complicated this can be and they help you determine if a certain make or model is right for you, but you have to research a lot of makes and models to figure out which ones have that type of information for you. We have done some of the work for you here below.
For instance, check out the very economical EcoSmart ECO 27 Electric Tankless Water Heater. This unit is ideal for the northern United States, where incoming water temperatures can reach as low as 37 degrees. This unit is capable of heating nearly 3 gallons per minute at this cold temperature. That means it can handle up to two showers running at the same time and never run out of hot water. In the Southern United States, where the groundwater is warmer, it can handle up to 6 gallons a minute, which could equate to four showers and a sink running at the same time.
If you need a bit more hot water, the EcoSmart ECO 36 Electric Tankless Water Heater can heat a shower and two sinks simultaneously in colder climates with 37 degree groundwater and in warm climates, can heat three showers and 2 ½ sinks simultaneously.
The Rinnai RUC98iN Ultra Series Natural Gas Tankless Water Heater is a fantastic option if you are looking for a gas tankless water heater and have a bit more money to spend. This allows a flow rate of up to 9.8 GPM, but keep in mind, that will be lower in colder weather climates.
If you need to keep costs to the bare minimum, the Marey ECO150 Tankless Water Heater may be for you. At a price point of around $200, it’s the least expensive of all of the models that are reviewed on this page. This is going to work well for the coldest climates, but if you are in more moderate to warm climates, you can get multiple streams of hot water simultaneously. At this price, it might not hurt to try one and see how you like it and then buy another one, if you need more hot water at another source in your house! This could also be ideal for very small spaces or in an RV. If you have an RV, check out our best tankless water heaters for RVs.
Tank Water Heater Sizing
If you have decided that a tankless water heater isn’t a good fit for you, you need to ensure that you get the right size water heater with a tank. (If you haven’t read about the pros and cons of tankless water heaters, please check this out here.) I hope you like math, because this involves a bit, however, I’m going to keep this as far away as possible from 9th grade math class.
Finding Your Peak hour Demand
When you’re looking at water heaters, you need to find models with a first hour rating that matches within one gallon of your peak hour demand and this is where the math and a bit of memory and analysis comes in.
How to determine your Peak Hour Demand:
- Think about what time of day you and your family/housemates use the most hot water.
- Think about in which ways your family/housemates are using the hot water at the same time.
- Estimate the maximum usage of hot water during this time of day based on the following guide:
Common Hot Water Guzzlers
- Shower: 10 gallons
- Bath: 15 gallons
- Hand washing or food prep for 2 minutes: 4 gallons
- Shaving: 2 gallons
- Dishwasher: 6 gallons
- Washing Machine: 7 gallons
Case Study: 2 adults, 2 teenagers from 7AM-8AM
- 4 Showers X 10 Gallons = 40 Gallons
- Breakfast prep (hand washing/food prep) = 4 Gallons
- 2 Men Shaving X 2 gallon: s =4 Gallons
- Dishwasher right after breakfast = 6 Gallons
- Total Peak Hour Demand = 54 Gallons
Based on this calculation, this household would need a first hour rating of approximately 54 gallons.
Determining what size water heater to get without the math
If calculating Peak Hour Demand doesn’t sound fun, or your family has more erratic or flexible hot water usage, you can estimate your need by number of people in the household.
1-2 people: 23-36 gallon capacity
2-4 people: 36-46 gallon capacity
3-5 people: 46-56 gallon capacity
5+ people: 56+ gallon capacity
For more information on calculating the best water heater size for you, you can check out this article.
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