Cold showers can get on anyone’s nerves, especially during frigid Denver winters. If your house seems to run low on hot water unusually fast, it may be time to change out your water heater. Learn what may be causing the loss of hot water, how to fix it, and how to select the correct size water heater if you need a replacement.

Why Does My Hot Water Run Out So Fast?

If you’re the last in a long line for the shower, it may not be surprising if the water cools off on you. But if you’re the first or second and still lose hot water, there could be an issue with your water heater system.

These are a few possible reasons your hot water runs out too fast:

  • Sediment buildup. Calcium and lime build up naturally inside water heaters over time, requiring regular system flushing. You may also consider installing a water softener to minimize the issue in the future.

  • Damaged heating element. A broken heating element requires professional repair or water heater replacement.

  • A leak in the tank. If the water runs out quickly because of a tank leak, shut off the cold-water inlet valve to prevent more water from flowing in, and call your trusted plumber for assistance. A leaky valve can easily be repaired or replaced, but the tank is a more expensive investment.

  • The tank is too small. If your hot water demand is too much for the size of your tank, you may need to upgrade to a larger tank or consider installing a new tankless water heater, which delivers endless hot water on demand.

Sizing a Standard Water Heater

Water heaters with tanks are simple to size. Determine what time of day you use the most hot water and calculate the highest volume you use in an hour. For example, showers use about 20 gallons, a top-loading clothes washer uses about 25 gallons (if you use hot water for washing clothes), and a dishwasher averages around seven gallons.

Add your total consumption during the highest hot water demand in your home. You’ll need a water heater whose capacity, or first-hour rating, is equal to or greater than your highest demand.

Choosing from Available Water Heaters

Standard residential water heater sizes range from 20- to 80-gallon tanks. Let’s run through an example household of five people to determine the ideal water heater size.

First, we need to determine what time of day the household uses the most water. In the morning, two people shower and one shaves. Someone runs the laundry in the middle of the day. Three people take evening showers, two shave, and the family runs the dishwasher at night. This household uses the most water in the evening, so we’ll calculate our peak usage using evening activities.


Gallons of Hot Water Used

Number of Times the Activity is Repeated

Gallons of Water Used









Running the Dishwasher






Average gallons used sourced from

This household uses 71 gallons of hot water at peak hours. An 80-gallon water heater should fulfill their needs, based on the family’s current schedule. If you want to err on the side of caution, opt for a water heater 10-20 gallons over your calculated peak use. This accounts for guests, particularly long showers, schedule changes, and back-to-back hours of high usage.

How to Size a Tankless Water Heater

Unlike traditional tanked water heaters, tankless water heaters heat water as you use it, providing you with a continuous hot water supply. However, there’s a limit to how much water it can heat at once (for example: running the dishwasher and clothes washer while taking a shower will likely result in reduced hot water for at least one of those events).

Here’s how to calculate what size tankless water heater to install in your house.

  1. Identify which hot water devices you intend to use. Do you have two showers that could run simultaneously with your dishwasher or washing machine?

  2. Find the flow rate. You can find the flow rate for each shower or faucet by placing a bucket under the tap and turning on the water. Measure the water collected after one minute for each fixture’s gallons per minute (GPM).

  3. Combine each of the flow rates. Once you have a flow rate for all the faucets and devices you could use simultaneously, add them to find the hot water flow rate your household needs.

Hot Water Heater Maintenance in Denver, CO

Regularly running out of hot water could indicate a problem with your water heater, but it’s also possible that the water tank is too small to support your Colorado home.

If you need a hand installing or maintaining your water heater, your Denver-licensed Benjamin Franklin Plumbing experts are ready to help. Call (303) 835-9352 today or request service online.