Pre-heated water temperature and tank size are contributors to just how quickly you get hot water. The number of heating elements within the heater and the heater’s power source, whether gas or electricity, are also factors. Larger tanks have larger volumes and take longer to heat up.
In short, less water heats more quickly than more water; so, the amount of water you are heating, and how hot you heat it, determines how quickly you get it.
Does Your Hot Water Flow Seem Too Slow?
One easy way to determine hot water heater flow rates is with the use of a formula that includes just two variables. One version of the formula looks like this:
Hot Water Tank Size + Heat Input Rate = Available Hot Water
Translation, the amount of water you are heating, plus the amount of heat you apply to it, determines how quickly you get hot water. Think about a small point-of-use water heater or water heater for an RV. Even though the heat applied to the tank is much lower than a whole-house water heater, the recovery rates are pretty quick due to the small holding tanks.
To find out if your water heater recovery rate is slow or not, apply some standard measurements used by many plumbers in the business. Calculate your hot water flow using peak usage hours, 7 a.m. until 9 a.m.
Peak usage represents the maximum load requirement for the water heater. This peak usage measurement represents the maximum output the heater will ever need to produce. If it is handling peak hours, it can handle the rest of your needs.
For instance, research data shows that during peak hours, the heater needs to produce 20 gallons of hot water, heated to 140 degrees, for the first two people to use it, totaling 40 gallons.
Add another 20 gallons for two more baths of a family of four. Dish washers need another ten gallons, and clothes washers need 20 additional gallons. The total is 90 gallons.
If you plan to use this much hot water every morning, the only solution is a 50-gallon or larger capacity hot water tank.
The maximum draw capacity of any unit is roughly 70 percent. Draw capacity means how much of the total available hot water you can pull out of the heater.
That means with a 50-gallon heater, 35 gallons of hot water are available at any given time. You need a combination of stored hot water (tank water), plus water flowing into the tank and getting heated before being drawn out, to meet a 90-gallon demand.
This estimate shows that bathing while washing dishes and clothes at the same time means running out of hot water fairly quickly. The Einsteinian solution to this major planet-stopping issue is either: don’t do it, buy a larger capacity hot water heater, or replace your tank model with a tankless water heater. Who’d a thunk it!?
Another sizing factor to consider is recovery time. While gas heaters heat water quicker, their recovery efficiency rate is lower.
For gas water heaters it is 75 percent, but for electric heaters it is 100 percent. However, even at lower recovery efficiency, gas hot water heaters (see also ‘Gas Hot Water Heater Installation‘) still produce more hot water more quickly than their electrical counterparts.
At 75% recovery efficiency, gas heaters produce 27.3 gallons per hour with a 30,000 BTU burner compared to 3.1 gallons per hour for electric heaters with a 750-watt heating element.
Now, in defense of electric water heaters, the big difference in heat levels makes for a somewhat distorted comparison. Electrical hot water heater output increases with additional heating elements and higher wattage.
As an example, at thehtrc.com, a 20,000 BTU gas heater produces 17.3 gallons of water with a 100% rise in temperature in one hour. The electrical heating element chart shows a 4,500 watt electrical element producing 20.5 gallons of 100% rise in one hour.
This electrical heating element is six times more powerful than the example above. The 20,000 BTU burner is 33% lower than in the 30,000 (BTU example).
What is a Good Water Heater Recovery Rate?
For a typical 40 to 50-gallon gas water heater, anything above 40 gallons/hour would be considered a good recovery rate. The higher BTU burner, the better it generally is for recovery.
For a typical 50-gallon electric water heater with dual heating elements, a 20 gallons/hour recovery rate is good. Single element water heaters will of course have a lower recovery rate.
The Major Factor
Don’t get confused. The amount of water getting heated, how it gets heated, and the amount used are the determining factors for how long it takes the water heater to heat up.
Heating rates list how much water gets heated per hour. The storage tank size tells you how much hot water is instantly available when you turn the tap.
If your family is large with multiple bathrooms, and multiple hot water consuming activities going on at the same time, on a regular basis, get a bigger tank. A family of four, in a 2-bath house, with a dishwasher and clothes washer needs 90 gallons of hot water during peak usage hours.
Larger tank sizes are for households that need a large quantity of hot water within a short period of time. If your family needs are less, consider taking advantage of the energy savings of a smaller tank. In this case, a 40 gallon tank will serve your needs, and you can save money by not buying a 50 gallon tank you do not need.
Bottom line: gas water heaters heat water at roughly two times the rate of electric water heaters when heating water that is around 60 degrees. See the water heater recovery table above.
If you have a gas water heater, plan to wait 45 minutes for the water to get hot after the heater has drained its capacity. If you have an electric water heater, double that to 1.5 hours.
Of course, with a tankless water heater, you never have to worry about the recovery rate. Just one more thing to consider in the tank vs tankless debate.