Right-Sizing Your Water Heater: The Key to Long, Hot Showers and Small Monthly Bills

Posted: August 18, 2014

Posted date: August 18, 2014

Post by: David O’Connell President, Wilson Oil & Propane


Most of us have had the experience: we’re in the middle of a wonderful hot morning shower, when suddenly we’re shocked awake by a blast of cold water. Has our water heater failed us?

Probably not.

More likely, your water heating system has reached its limit because it isn’t properly matched to the needs of your home and family. This matching, or sizing, is the key to keeping hot water flowing for as long as you need it – and for keeping your energy bills low, too (which is important, since water heating can account for 15 percent or more of your home’s energy use).

A properly sized water heater will deliver hot water whenever and wherever you need it. To size a water heater, you need to know three things: the peak demand of your system, the capacity (for tank systems) or hot water flow rate (for tankless systems) of your water heater, and the approximate inlet temperature of water coming into your house.

Peak Demand

To find the peak demand for your system – the time of the day and time of the year when you’re using the most hot water at one time – you’ll want to conduct a simple water use audit of your home. A water use audit answers questions such as:

You can use the chart below to help estimate peak demand in your home:


Capacity and hot water flow rate

How you measure that the limits of your water heater depends on the kind of water heater you have: for water heaters with a tank, the most important sizing factor is storage capacity –how much hot water your tank can hold at one time.

An important measure related to capacity in a water tank system is recovery rate – how long it takes to reheat water once your tank is empty. A small unit with a high recovery rate could out-perform a large unit with a slow recovery rate. In terms of recovery rate, electric water heaters are the most inefficient way to heat your hot water. An oil fired water heater, indirect water heater on your boiler, propane or gas water heater all take less time to recover (i.e. reheat the tank) and cost less to operate than an electric water heater.

In tankless water heaters, the most important sizing factor is hot water flow rate –how much hot water your heater can produce and distribute, typically measured in gallons per minute. While tankless water heaters can’t really run out of hot water, the flow rate of an improperly-sized unit could slow to a trickle.

Inlet Temperature

The third factor to consider in sizing a system is the temperature of water coming into your home, also called the inlet temperature.

To heat water to acceptable temperatures for a hot shower during a Pennsylvania winter, your system may need to heat the water as much as 75 degrees – which means you’ll need more BTUs to reach that temperature than in the summer, or during winter in a warmer climate.

Putting it all Together

To find the right sized water heater for your home, find your peak demand in gallons per minute and see what tank capacity will meet that need at the coldest time of the year – that is, when the difference between the inlet and outlet water temperatures is highest.

Similarly, in a tankless system, you’ll want to make sure the hot water flow rate of the heater is fast enough to accommodate your peak demand when inlet temperatures are lowest.

Hot Tip: If you need a new water heater, Wilson has you covered!

If you need help sizing your water heater – or if you think it might be time for a new, high-efficiency unit for home –give us a call. We’ll help you choose from our line of top quality oil, gas, electric or even solar-powered water heaters, and even give you a free estimate. Contact Wilson today to learn more!