Water heater performance is an important consideration when selecting the right model for your home. The amount of hot water a unit can deliver is a key factor. While performance and efficiency are relevant too, understanding water heater delivery capacity will help find the hot water heater you need.
What Is First Hour Delivery?
Defining a water heater’s performance capability, first hour delivery represents the amount of hot water a fully heated unit can deliver in the first hour. This time period begins when the system has recovered to its thermostat setting and someone starts to use it, whether by taking a shower, washing the dishes, or otherwise running a hot water tap.
How Is Water Heater Delivery Measured?
The first hour rating can be misleading. It does not indicate that hot water will last for one hour when you start up the water heater. To determine water heater capacity, the 70% rule is used. When the thermostat is set to 120℉, in theory, the system should deliver 70% of tank volume when the temperature is set between 100℉ and 120℉.
This means if you have a 30-gallon hot water heater tank, you should get 21 gallons of hot water in the first hour. With a 50-gallon tank, you should get 35 gallons; and an 80-gallon tank will yield 56 gallons. This is because as hot water is being used, cold water continuously enters the tank. The two mix so the full volume of water is therefore gets cooled.
How a Water Heater Delivers Water
To better understand water heater delivery capacity, it helps to know what happens when you turn on the hot water faucet. Once you turn the handle, a dip tube on the cold-water inlet side of the unit pushes cold water into the bottom of the tank. The resulting pressure forces hot water out. When enough cold water enters the tank, there’s enough mixing to turn hot water warm, then lukewarm, and eventually cold.
The 70% rule is founded on the principle that, once 70% of available hot water has been used, tank water will become cold. To counter this, the thermostat kicks in as the water heater is used. The system starts to recover, meaning it begins heating water, even as the hot water supply is in use. Recovery capability is calculated by multiplying tank capacity by .70 plus a recovery value. That’s how you arrive at the unit’s first hour delivery.
Input wattage, for an electric water heater, and British Thermal Units (BTUs), for a gas water heater, are factored in as well. Recovery rates decrease the greater the expected temperature rise. The higher the unit’s input wattage or BTUs, the more gallons of water per hour it can handle. However, the recovery rate will generally decrease with each sequential amount of temperature rise demanded, which depends on the thermostat setting and ambient water temperature.
Factor in Hot Water Usage
When sizing a water heater for your home, it also makes sense to know what you’re using it for. A clothes washer uses 25 to 40 gallons per load while a dishwasher uses 5 to 10 gallons per load. You typically use 3 or 4 gallons when washing dishes by hand. Taking a shower requires about 3 gallons per minute, hand washing requires 1 to 2 gallons, and food preparation can consume 3 to 6 gallons.
Also consider the size of your household. Water heater sizing based on the number of people goes as follows:
- 1-2 People: 30 gallons to up to 40 to 50 gallons.
- 2-3 People: 40 to 50 gallons of water heater capacity.
- 3-4 People: 40 to as much as 75 gallons.
- 5+ People: 50 to 75 gallons.
Household size directly correlates with the level of hot water recovery needed. If you have more people using more hot water, a unit with a higher water heater delivery capacity makes sense. First hour rating is therefore an important factor to consider when shopping for a hot water heater that will meet your needs.
Contact Sky Heating & Air Conditioning
We have licensed plumbers who can help size, select, and install your water heater. Installing a new hot water heater often requires installing new plumbing lines as well and making the proper connections. Our team can also help determine if your home may benefit from a tankless water heater. And if water quality is an issue, we can install UV water filtration systems to kill bacteria and microbes. For assistance or more information, call 503-673-9083 today.