Matching the capacity of hot-water source to the needs of homeowners is a technique referred to as sizing. The key criterion is hot water storage capacity for tank water heaters. For tankless water heaters, the criterion is hot water flow rate. A properly sized water heater will meet your needs while operating more efficiently. Therefore, it is essential before purchasing a water heater to ensure if it’s the correct size.Incoming water temperature is a critical consideration, which varies by region and season. A water heater in Edmonton, either tank or tankless, will need a higher heating temperature unit input in the winter than the summer.

Bigger is not necessarily better when it comes to choosing the size of a water heater but if you install a tank that’s too small, you’ll run out of hot water before finishing the shower. However, if you get a tank that’s too big, you’re wasting money on heating so much water which you might not even use. So what do you choose??


There are two types of water heating systems: Tankless or Demand-type Water Heaters and Water Heaters with Tank.


  • Tankless or Demand-type Water Heaters:

These water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, you’ll need to determine the flow rate and the temperature rise to size a tankless water heater for its proper application in your home.

Having a tankless water heater, you may never run out of hot water. But if not sized correctly the flow rate of water can be adversely impacted. In such a case, the temperature of the shower will remain the same, but the flow could slow down to some extent.

The first step in sizing a tankless water heater is to add up the flow rates of showerheads, faucets and appliances that are likely to be used at the same time. The second step is to consider the incoming water temperature. When temperature of the incoming water goes down, higher heating temperature unit inputs will be needed.


  • Water Heater with Tank:

First hour rating is the number of gallons of hot water the heater can supply per hour. It depends on the tank capacity, source of heat and the size of the heater (burner or element). This starts with a tank full of hot water. Therefore, to properly size a storage water tank for your home, including a heat pump water heater with a tank, ensure you use the water heater’s first hour rating.

Look for water heater models with a first hour rating that matches within 1 or 2 gallons of your peak hour demand. To estimate your peak hour demand, keep in mind the number of people living in your house and determine what time of day you use the most hot water.

For sizing combination of water and space heating systems appropriately without any flaws, consult a professional plumber, whether you have a new tank or tankless water heater.

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