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If you’re looking to replace your old water heater, one of the first things you’ll need to determine is what size unit you need. While you can always simply choose the same size unit as your old one, this isn’t always the best idea. There’s always the chance that your old unit is larger than you need, which means you may end up paying more than necessary. Your new unit will also use more energy. This is why we always recommend estimating your normal hot water demand to ensure you get the right-sized unit. Here’s a quick overview of how this is done.

Sizing a Water Heater by Number of People

If you’re looking for a traditional tank-style water heater, there are a few different factors you’ll want to consider when trying to decide what size unit you need. The first is the number of people who live in your home. A 30- or 40-gallon unit is typically sufficient for one or two people, while a three- or four-person family will typically need at least a 50- to 60-gallon unit. If there are five or more people in your household, you’ll definitely want a 75-gallon unit or larger.

Calculating Peak Daily Hot Water Demand

You also need to consider your peak daily hot water demand, which means determining when you use the most hot water during the day and approximately how much you use at this time. For instance, if you have a four-person family and everyone showers in the morning, this would likely be your peak demand time. Calculating how many gallons you use during this peak demand time is essential for ensuring that you choose a large enough unit so that you don’t run out of hot water during this time.

The average person uses approximately 20 gallons of water when they shower. This means that if all four people in your family shower within an hour or two of each other in the morning, you’d typically need at least an 80-gallon unit just to meet your daily showering needs. If you ever need to do laundry around the same time in the morning, this would also use up an additional 15 to 25 gallons depending on the type of washing machine you have and its age.

How to Size a Tankless Water Heater

If you have a large family or you frequently run out of hot water, you may want to consider opting for a tankless water heater instead. These units instantly heat on demand, which means you can use as much hot water as you need without ever needing to worry about running out. The fact that tankless water heaters work on demand also means that they typically use quite a bit less energy than tank-style units. Your savings will always be much higher if you don’t use all that much hot water in a typical day. However, a tankless unit will still typically save you at least a bit on energy costs even if you’re family regularly uses 80 or more gallons every day.

Although it still involves estimating your peak daily hot water demand, sizing a tankless water heater is a bit different. Since tankless water heaters work on demand, they can only produce a certain volume of hot water at one time. This means that you’re not looking at how many total gallons you need. Instead, you’ll want to focus on how many gallons of hot water you may need at one specific moment to ensure you choose a unit that can meet this demand.

To do this, you’ll want to make a list of all the different hot water fixtures you’d normally use at once, and then calculate the flow rate in gallons per minute for each of these fixtures. To make this easier, here’s an overview of various fixtures and approximately how many gallons per minute (GPM) each uses.

  • Shower: 1.5 to 3 GPM
  • Kitchen or bathroom sink: 1 to 2 GPM
  • Dishwasher: 2 to 4 GPM
  • Washing machine: 2 to 4 GPM

If you have the owner’s manual for your dishwasher or washing machine, it should tell you how many gallons per minute the unit uses. You can also easily measure the flow of your faucets and showers by using a bucket or pan to see how many gallons you have after one minute.

Once you know how many gallons of hot water per minute you may need during your daily peak demand time, you can then estimate the flow rate that your tankless water heater will need to produce. Let’s say you normally have two people who shower at the same time while you’re also doing laundry. In this case, you would need a tankless unit with a flow rate somewhere between six and 10 gallons per minute.

Houston’s HVAC Experts

At Texas Pride, our certified technicians are here to assist with any heating or air conditioning repair, installation, or maintenance services you need. We proudly service both commercial and residential customers throughout the Houston area. Our team also specialize in air purification and other indoor air quality services. For more information, give us a call today.

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