Point-of-Use Water Heater Buyers Guide

Hot Water Handle

The point-of-use water heaters that you see under the counter near a sink come in two different types: Tank-style and tankless. As a general rule of thumb, the more money you're willing to pay, the more hot water your water heater will be able to deliver. However, selecting the right size means you're not wasting money on an overpowered unit that will deliver more hot water than you'll ever need. 

Even though these units won't be serving your entire home, the same rules apply when selecting the right size heater. Tank-style water heaters measure capacity in terms of tank size, where tankless units uses flow rate, expressed in GPM (gallons per minute). We'll take a closer look at each later, but regardless of what type of water heater you choose, you should have a good estimate of how much hot water you'll need, especially if you plan on using it as a stand alone source of hot water.

Many point-of-use systems are designed with the option of running independently or can be installed inline with your main water heater system. Inline installation provides a boost to your main system to provide hot water fast until your larger tank can deliver.

Types of Point-of-Use Water Heaters

Tank-Style Water Heater

A tank-style water heater uses a small tank to store the water until it's needed. There are 2 factors used to determine the correct size for it to meet your needs:

  • Tank's Water Capacity: How much water the tank can hold.
  • Recovery Rate: The time needed to reheat the water within the tank.

The capacity of the tank is pretty straight forward. The larger the tank, the more hot water you'll have readily available. However, the recovery rate is determined by the efficiency and wattage of the unit's heating elements.

A higher wattage heating element should deliver a shorter recovery rate, and a shorter recovery rate means your water heater will be able to deliver hot water much faster.

Many small tank water heaters can be installed directly to the cold water line and function as a standalone unit. If this is your chosen configuration, you'll likely be fine going with a small tank for the purposes of a hand sink or other low demand outputs.

However, if you're planning on using it for a shower, dishwasher, etc. you may want to error on the side of a larger tank. A tank-style water heater can only deliver the amount of hot water within it's tank. Once the tank is depleted you'll have cold water until the unit can recover (remember the recovery rate?)

If you're looking to give your main hot water system a boost, many units are designed with the option of installing it in-line with your home's main water heating system.  When you open the hot water faucet, you'll immediately have hot water delivered by your small tank, eliminating the need to wait until the hot water runs thru the plumbing from your main unit. 

Depending on your home's hot water heating configuration, if a small tank-style heater is used as a standalone unit to service a hand sink or other small output, a smaller sized tank will most likely meet your needs. However, if it'll be used for a shower, dishwasher or other higher demand outputs, you may want to error on the side of a larger tank, or use it as a boost to your main system. 

Many top-of-the-line tank-style mini water heaters can be found for under $400, and some run far less. Although, tank systems come in many different sizes, a true "small" water heater will have a tank size of 2.7-gallons to 7-gallons. But finding the right size to meet your hot water needs is far more important than any label.

How a Point-of-Use Water Heater Tank Works
Small Water Heater

Tankless Water Heater

Tankless water heaters are sometimes called flow-thru because water enters the unit cold and leaves hot. The flow rate of a tankless water heater is the critical information needed when selecting the right size unit to meet your hot water demand.

There are 2 key pieces of information to take into account when selecting the right size unit: 

Flow Rate

The Flow Rate is measured in gallons per minute (GPM) and it's the amount of water the unit can heat at any given time. The higher the number, the more hot water your point-of-use tankless water heater can deliver. Most units fall in the range of .50 to 2.25 GPM. But getting this number right is critical, and it'll make the difference between taking a hot or cold shower. 

According to the 2010 Plumbing Standards, the average gallons per minute (GPM) is used for the following water outlets:

  • Standard Shower Head: 2.0 GPM
  • Water-Saving Shower Head: 1.5 GPM
  • Standard Hand Sink: .50 GPM
  • Bath Tub: up to 4.0 GPM
  • Kitchen Sink: 1 to 2 GPM
  • Dishwasher: 1 to 2 GPM
  • Washing Machine: 1 to 1.5 GPM

Keep in mind that these are just reference points. If you are adding a tankless water heating system to service your entire home, you'll want to be very careful to get this right. But with a Point-of-Use system there is some leeway. For instance, if you have a hand sink (.50 GPM) and a standard shower head (2.0 GPM), you might think that you should buy a unit that can deliver 2.5 GPM of hot water. However, you likely won't be servicing hot water in both outlets at the same time.

Also, if you plan on using your small tankless heater as a booster to your main water heating system (which is probably a really good idea if you are servicing a shower head) a much smaller tankless should work for you. After all, you're only trying to shorten the wait time for hot water. 

Temperature Rise

The Temperature Rise is determined by subtracting your starting temperature (the incoming water) from your end temperature (hot water). As an example, if the incoming water is 65-degrees, and your desired hot water temperature is 110, your temperature rise will be 45-degrees.

In other words, your tankless unit will need to heat the water 45-degrees in order to bring it to your desired temperature. The higher the Temperature Rise, the harder your tankless needs to work, and that means your unit will deliver a lower GPM.

If, in the Winter, your tankless needs to increase the incoming water temperature by 60-degrees (a 60-degree Temperature Rise) it may be able to deliver .45 GPM of hot water. But, in the Summer, the Temperature Rise may only be 30-degrees, in which case, the unit's GPM may increase to 1.85. That's a significant difference and something to take into account, especially in cold climates.

Tankless manufacturers generally lead with the GPM a unit can deliver, but knowing the Temperature Rise will help you accurately compare the water heater's efficiency. 

So as you can see, sizing a point-of-use heating system doesn't need to be exact. But if you have the basic information on how these units are sized, you'll be able to make an informed purchase decision and find the small water heater that best meets your needs.

How a Point-of-Use Tankless Water Heater Works

Check out our post on the best tank-style and tankless small water heaters. HERE