When upgrading or replacing your old water heater, it is important that you do a bit of calculating to determine how large of a unit you need. Modern water heaters are quite a bit more efficient than units produced even 10 years ago. As such, you may not actually need as large of a unit as you currently have. Choosing too small of a unit is obviously an issue since you may not have enough hot water to meet your needs. However, opting for a unit that is bigger than you need is also not ideal since it will cost you more and use more energy than necessary. With that in mind, here is a handy guide on how to calculate what size of water heater you need.

Tank vs. Tankless Water Heaters

If you’re in the market for a new water heater, the first thing you’ll need to decide is whether you want a traditional storage-tank water heater or a tankless unit. This is important as the process for determining what size of water heater you need varies quite a bit between the two types.

Tank water heaters are generally less expensive to purchase and install compared to tankless units. On the other hand, tankless units will almost always use less energy and be cheaper to operate in the long term. Another thing to consider is the fact that the average lifespan for tankless water heaters is typically around double that of tank-type units.

How to Size a Tank Water Heater

In the past, the easiest way to calculate how large of a storage-tank water heater you’d need was to base it on the number of people in the home. While this can still give you a rough estimate, it isn’t all that accurate as it really depends on how much hot water you may use in an hour or so. For this reason, we always recommend sizing a water heater based on your peak daily demand.

Peak daily demand refers to the hour of the day when your hot water usage is highest. For instance, if there are three people in the home and everyone showers between 6 and 7 a.m., this would typically be your peak daily demand time.

Once you know when during the day your hot water usage is highest, you will then need to estimate how many total gallons of hot water you use during this time. One shower uses around 20 gallons of hot water on average. If all three people in the home shower within an hour, you would use at least 60 gallons during your peak daily demand time. Additionally, if you also run your washing machine around the same time, you would need to add an extra 15 to 25 gallons per load.

After estimating your peak daily hot water usage, you can then start determining how large of a storage tank you need. However, just because you may use 60+ gallons during your peak demand time doesn’t necessarily mean that you need a 60-gallon unit. This is because modern water heaters are far more efficient and heat much more quickly than older units, which means that a 60-gallon unit will often be capable of supplying 80 or more gallons of hot water in an hour.

This is why we always recommend focusing on a metric known as First Hour Rating (FHR). FHR is a measurement required by the US Department of Energy that shows how many gallons of hot water the unit can provide in one hour. The measurement is based on the unit starting with a full tank and then continually reheating throughout the hour as more hot water is used.

FHR mainly depends on the energy source the unit uses and how efficient it is. This means that one 60-gallon unit may have an FHR of 100 gallons while another unit’s FHR may only be 70 gallons. Some highly efficient units have an FHR that is more than double their storage capacity. For instance, you can easily find 40-gallon units with an FHR of 85 gallons. This makes FHR a much more useful metric than simply guessing how many gallons you need the tank to store. As such, we would always recommend estimating what size of unit you need by calculating your peak daily demand and then searching for a unit with an FHR that is around 20 gallons higher than your estimated peak demand.

Sizing a Tankless Water Heater

A tankless water heater can basically provide an endless supply of hot water. However, these units can only supply so much hot water at a time. This means that you don’t need to be concerned about how many gallons you may need in an hour. Instead, focus on the total number of gallons you may need at one time.

Sizing a tankless water heater is done by looking at the unit’s flow rate, which is the number of gallons of hot water it can provide per minute. To determine what flow rate you need, you will still need to estimate your peak hot water demand or how much hot water you could use at once. For instance, if you regularly shower while also running the dishwasher and washing machine, this would likely be your peak demand time.

Once you’ve estimated how many hot water fixtures you may need to use at one time, you can then start to calculate how many gallons per minute you would need to supply all these fixtures at once. To simplify things, here is a quick overview of the average flow rate in gallons per minute that different household fixtures use.

  • Bathroom or kitchen faucet: 1 to 2 gallons per minute
  • Shower: 1.5 to 2.5 gallons per minute
  • Dishwasher: 3 to 4 gallons per minute
  • Washing machine: 3 to 4 gallons per minute

The flow rate for showers and sinks depends mainly on the type of faucet or showerhead. However, the flow rate for these fixtures doesn’t vary all that much as 2.5 gallons per minute is the maximum flow rate for sinks and showers allowed under U.S. law.

Moreover, the flow rate for washing machines and dishwashers can vary quite a bit depending on the type and age of the unit. If your unit was manufactured before 2010, it likely has a much higher flow rate than a newer unit would. If you’re unsure of what the flow rates for your dishwasher and washing machine are, you should be able to find this information in your owner’s manual.

After estimating how many hot water fixtures you may need at one time and how many gallons per minute each one uses, you can then calculate what flow rate you would need to meet this demand. For instance, you would typically need a tankless water heater with a flow rate of at least 7.5 gallons per minute in order to shower at the same time as both your dishwasher and washing machine are running. If you were also to use a sink at the same time, you may need a unit that has a flow rate closer to 10 gallons per minute.

If you’re still unsure of what size of water heater you need, the team at Bel-Aire Heating & Cooling is here to help. We install a wide range of water heaters, and we also specialize in water heater maintenance and repairs as well as a full range of residential and commercial HVAC services. For more information or to schedule any water heater or HVAC service in the Portage, Grand Rapids, and Three Rivers areas, contact Bel-Aire Heating & Cooling today.

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