Sizing A New Water Heater

Water Heater Repair

Sizing A New Water Heater

The US Department of Energy has provided very clear guidelines on sizing a new water heater. Sizing a new water heater can be a bit tricky because you need to calculate the household’s hot water needs and find a unit that both meets those needs and cost fits within your water heater replacement budget (read: How Much Does it Cost to Replace a Water Heater in California).

Once you’ve selected a new water heater, it’s important to have it installed by a qualified professional. This will ensure that it is properly vented and that all electrical and gas connections are properly fitted.

Let’s briefly review the considerations in sizing various types of water heaters.

Sizing Tankless or Demand-Type Water Heaters

In sizing tankless water heaters otherwise called demand-type water heaters or on-demand water heaters, the key components to be sized are the water heater’s input (BTU/hr) and its capacity (gal/min).

The size of tankless water heaters is not actually determined by the size of the unit, but rather by the amount of water it can supply per minute and the temperature rise you need.

Basically, to properly size a tankless water heater, you’ll need to know: 1) the desired flow rate and 2) the temperature rise you need.

With that information, you can use one of two sizing methods to find a unit that will work for your needs

The easiest way to select the right model tankless water heater is to use an online sizing calculator. There are a number of these available from different manufacturers. Simply enter the information required and the calculator will do the rest.

If you want to size the unit yourself, here’s how:

Determine the desired flow rate

This is the number of gallons per minute (GPM) that you need the tankless water heater to supply. To find this, just look at the flow rate on the faucet that you’ll be using most often.

Most faucets have a maximum flow rate of about 2 GPM. So, if you have a single-handle faucet, you would need a tankless water heater that can provide at least 2 GPM.

But, if you have a shower head with multiple handles, each handle could have a flow rate of up to 2 GPM. So, you would need a tankless water heater that could provide at least 4 GPM.

Determine the desired temperature rise

This is the number of degrees Fahrenheit that you want the incoming water to be heated. To find this, just subtract the temperature of the coldest water supply from the temperature of the hottest water you would ever want.

For example, if your cold water supply is 60°F and you would never want water hotter than 130°F, then you would need a temperature rise of 70°F.

Now that you know the flow rate and temperature rise you need, just enter that information into our calculator and it will do the rest!

Sizing Storage and Heat Pump (with Tank) Water Heaters

On the other hand, when sizing storage hot water heaters, you need to know the number of people in your household, as well as how much water each person uses on a daily basis. From there, you can use a calculator to find out exactly what size hot water heater you need.

A good rule of thumb is each person will need an average of 13 gallons of water per day. For example, if you have a family of four, you will need at least a 50-gallon storage tank size water heater.

If you’re looking for something a little more energy efficient, you may want to consider a tankless hot water heater or a heat pump hot water heater.

These types of hot water heaters are much more efficient than traditional storage tank models, and can save you money on your energy bills in the long run.

Sizing a Solar Water Heating System

The sizing requirements for solar water heaters is a little bit different. Here we need to size both the solar collector array and the storage tank.

Sizing the solar collector array is a bit more complicated, as there are many factors that need to be considered, such as:

  • The climate you live in
  • The orientation of your roof
  • The angle of your roof
  • The amount of sunlight your home receives

Once these factors have been taken into account, you can then determine the size of the solar collector array you will need.

As a general rule of thumb, each square foot of solar collector area will provide about 1 gallon of hot water per day.

Now that you know the size of solar collector array you need, the next step is to determine the size of tank you will need to store the hot water.

What we have discussed under storage tank water heaters applies here as well. As a general rule, you will want a tank that is at least twice the size of your solar collector array.

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