Before you buy, learn how to size a water heater and avoid frequent homeowners' mistakes. The sizing guide, tips, calculators, and a chart will help you choose the right gas- or electric-powered tank-type model, including tankless, so you can always have enough hot water for the reliable and comfortable shower.
Sizing a water heater is important, because if you buy a small unit, it will run out of hot water fast, and if you buy larger than you need, you will pay more for hot water that you won’t even use.
Either you are looking to buy a gas or electric water heater from Rheem, AO Smith, Bradford White, State, GE, Kenmore or any other manufacturer; you will have a wide range of models available; from small point-of-use to high-recovery and ultra-efficiency type.
The worst thing you can do is to end up with the undersized tank which will bring only problems, such as uncomfortable showers, not enough hot water, heavy condensation, rusting or even worse - premature failure.
A water heater is, after home space heating, the second largest energy consumer, and with the rising costs of energy, water heating could impact your budget significantly. Before you buy a new unit, you have to think of the operating costs, and not only the costs of the fuel and heater.
One of the most frequent questions I received, and also found in many reviews are "the heater is not producing enough hot water" or "there is no hot water after 15 minutes of using it".
The above concerns should be taken into account (I mean seriously) when buying a new unit. And this is the reason why we have this article and why you are here... to learn to size a water heater accurately so you won’t spend more, or deal with the cold baths and unfinished laundry.
Before you learn how to size a water heater for a family of 3, 4, 5 or more, a small house with 1 or 2 bathrooms, or large house with 3+ bathrooms, make sure to get answers on the following questions:
As seen from the above, it is important to investigate what is the hot water usage pattern and habits of the family members at home. It is vital to know what part of the day is when the high demand for hot water occurs.
For example, in many households, the highest demand for hot water occurs between 9 and 11 PM when most of the family members are taking two or more simultaneous showers, and when the washing machine and dishwasher are running at the same time.
When checking for heaters, you will notice some important data in the specifications provided by the manufacturer: Power, Efficiency, Peak Hour Demand, First Hour Delivery, and Recovery Rate. As you analyze the technical data you become more familiar with the available models and the process on how to size a water heater properly.
The power or input is the amount of electricity (in Watts) or gas (in BTU) used in one hour, for water heating. Choosing a water heater with higher input results in faster heating.
The Peak Hour Delivery shows the highest amount of hot water (in gallons per hour - GPH) delivered by a water heater, during the busiest time.
The First Hour Rate is calculated as the maximum volume of hot water delivered during the first hour of heating.
The Recovery Rate is the amount of water (in gallons per hour) one heater can raise when the temperature increases by 60 F. As it is a part of the First Hour Rating, it can also be defined as the capacity of the heater to produce hot water in one hour.
When you compare electric vs. gas units, gas models have the higher First Hour Delivery so its tank can be smaller than the electric ones while producing the same amount of hot water. Keep in mind that only 70% of hot water of the heater's tank size is actually available for use (50-gallon tank size has 35 gallons of hot water for use).
This calculation is based on the family size of 4:
|Peak time usage
(in one hour period)
|Used Hot Water
|Hot water demand in the peak time (gal.)||90|
|Family size (# of people)||Peak demand (gallons)
|7||100 or more|
Every water heater comes with the Energy Guide label that shows the Peak Hour Demand capacity, also known as the First Hour Rating (FHR). If you are in a store and didn't have time to learn how to size a water heater you can use the FHR value instead – higher is better.
Sometimes, instead of one large unit, it is worth considering getting a water heater with the smaller tanks but with, the higher recovery rates. If the large amounts of hot water are required, you can install multiple units, plumbed in parallel.
Tankless water heaters, as opposed to tank-type models, do not store hot water inside the tank. They use the heat exchanger where water while passing, is heated - on demand.
This is why we use a different approach when sizing and trying to answer the question: "how big a water heater do I need".
The following should be considered:
Tip: Use the map to get the temperature of groundwater temperature in your region.
All the tips for sizing the water heater from the above text are useful for the homeowners who are buying a new or replacing the old heater. Always consider your future family needs for hot water; so buying a larger tank size is a good idea if running out of hot water often, your family grows or planning to add some new fixtures.
The main criteria when sizing tank-type water heaters are the tank capacity and recovery rate (in gallons per hour – GPH), while for tankless is hot water flow rate (in gallons per minute – GPM).