JAN 11, 2018

Whether you are upgrading a bathroom with a giant tub to fill with hot, relaxing water, or your current water heater is reaching the end of its useful life (about 10 years), many people find themselves at a loss when it comes time to buy a new one. Water heaters are not typically an impulse buy or one that you make on a regular basis. But, armed with the correct knowledge, you might be shocked at how much you can increase your home’s efficiency and maintain comfort by buying the right size water heater.


Before we jump headfirst into the specifics of calculating the size of water heater you need, it is important that you know some basic definitions and facts. Being able to decode the jargon used to rate water heaters will be invaluable as you begin shopping.


To begin answering the question of how big your water heater should be, it is important to know the meaning behind some prominent acronyms used for ratings of water heater power. The first, FHR, stands for “first hour rating.” When purchasing a water heater with a tank, FHR will always be listed on the yellow EnergyGuide label. This is a measure of how much hot water the unit should be expected to deliver during a busy hour — like during the morning when everyone is showering, or in the evening when laundry and dishes are being done.

The second, EF (short for “energy factor”) is a measure of, you guessed it, energy efficiency. The EF of a water heater typically decreases as size increases and is dependent on the type of water heater and other factors. Once you have determined how much hot water you need delivered (your desired FHR), you can use EF as a secondary measure to help you hone in on the perfect water heater for your household.

On the other hand, tankless or demand-type water heaters are rated based on their maximum temperature increase possible at a given flow rate. The total flow rate is determined by adding up the number of fixtures using hot water in the home (showers, laundry, dishwasher, etc.) and their respective flow rates. So, if your showers have flow rates of 2.5 gallons per minute and you expect to use the dishwasher at the same time with a flow rate of 2 gallons per minute, the total desired flow rate would be 7 gallons per minute.


Now that we have gone through some very brief explanations of how water heaters are measured, you may find yourself asking, “Where do I go from here?” While we touched on flow rates in the previous section, it is still unlikely that you know exactly how many gallons of hot water you will need at any given time — most people don’t have the flow rate of every faucet in their house memorized. And to make matters more complicated, traditional tank-style water heaters are not measured using a simple flow rate in/out calculation. In that case, you are more concerned with hot water storage capacity. Fear not! There are some practical rules of thumb that can be applied here too.

Remember earlier when we talked about FHR, the first hour rate? Essentially what is being measured is the peak demand of a water heater in your household. You will want the FHR to be slightly higher than that demand for water. It is also a fact that on average, each individual in your home uses about 12 gallons of water. So, let’s say there are four people living in your household. If everyone has to take a shower in the morning between 6am and 7am to get out the door on time, you know that you should plan on the hot water demand being around 48 gallons. So in this case, Assuming that your family is typical in the way you use hot water, you would want to look for a water heater that has a FHR of 55-60 gallons. But do not forget other factors, say for instance someone who takes baths rather than showers (a giant bathtub might require a lot of water) or if your routine dictates that dishes are being done at the same time everyone is showering.

Similarly, if you are installing a water heater in a home you are looking to rent or flip, the same math can be applied as a gauge. Add up the number of bedrooms in the house and account for one additional person on top of that.

So if B = the number of people in your household. An easy way to find out how big of a water heater you will need is to use this simple equation:

B x 12 12 = Size of the tank needed in gallons


If you still have questions about water heaters or how you should go about picking the right size water heater, do not hesitate to give us a call. We can help you come to an accurate calculation with a few questions over the phone, and while we are on the line, we can talk about potential tax rebates and schedule a time for your professional installation. Your safety is always our first concern and our professional plumbers are trained to ensure all building codes are followed.  Click here to call today!