Shopping for a new water heater? Picking the wrong size could lead to some major buyer’s remorse. Imagine getting a new water heater, only to feel a strong chill as you try to take a shower.
Every family has its own unique hot water needs, and there are factors involved with finding the right size tank/storage water heater and tankless water heater.
Sizing for a Traditional Tank Water Heater
To determine what size you need for a tank water heater, you first need to know your peak hour demand, which is the time of the day when you’re using the most hot water. You’ll also need to know how much hot water you use during that time.
Once you know that, you’ll need to match that water usage by looking for a water heater’s first hour rating: the number of gallons of hot water the heater can supply per hour (starting with a full tank). You’ll find the first hour rating of a water heater on the top left of its EnergyGuide label as “capacity (first hour rating).”
Basically, if the water heater can’t match how much you use at your highest water usage, you’ll be in for a chilly surprise.
To find your peak hour demand, first determine what part of the day (morning, noon, evening) you use the most water. Then, use this chart from Energy.gov to determine how much water you use during the hour with the most water use:
For example, let’s say you use the most water in the morning. You’ve determined that from 6:30 a.m. to 7:30 a.m., your family’s hot water usage is two showers (20 gallons), two shavings (four gallons) and one automatic dishwashing (six gallons). That means your peak hour demand is 30. Therefore, you want a tank water heater with a first hour rating of at least 30.
Sizing for a Tankless Water Heater
For a tankless water heater, you need to do two things:
- Find the flow rate you need: How much hot water you use at one time, measured in gallons per minute (gpm).
- Determine the temperature rise needed: How much the tankless water heater needs to heat the ground water to reach the desired temperature.
Both are important because the gallons of hot water a tankless water heater can provide per minute decreases the cold water it needs to warm.
Finding the Flow Rate
To find the flow rate, you need to add up the total flow rates of hot water-using appliances that you use at one time.
Here is a list of the flow rates of major hot water using appliances.
- Bathroom faucet: 0.5-1.5 gpm
- Low flow kitchen faucet: 3.0-7.0 gpm
- Shower 1.0-2.0 gpm
- Dishwasher: 1.0-2.5 gpm
- Clothes Washer: 1.5-3.0 gpm
So, if two people are showering at the same time that the dishwasher is running in the morning, the maximum flow rate you need is 3.0-6.5 gpm. Try to find the flow rate of your appliances to get accurate calculations.
Determining Temperature Rise
The higher the temperature rise, the fewer gallons per minute a tankless water heater can provide. Therefore, before you buy a tankless water heater, look at the gpm, given a particular temperature rise.
To determine the temperature rise, use this formula:
Desired hot water temperature – Incoming ground water temperature = Temperature rise
Most homes should have the desired temperature set at 120ºF. The groundwater temperature for Arizona is relatively high (72ºF-62ºF). Hence, the typical temperature rise needed in Arizona is 48ºF to 58ºF.
See what flow rate a tankless water heater can provide at those temperature needs to see if they match your home max needed flow rate.
Need Help? Call a Professional.
I know this is a lot of information to take in. But remember, a professional contractor can walk you through the details of what you need when replacing your water heater. This article will just help you understand the terms so you won’t feel lost. QCBN
If you want help choosing the right size water heater for your home, contact Pitzer’s One Hour today.
Leave a Reply