Everyone was raised in this profession knowing how to size a water heater. However, when you said “water heater,” you meant a tank-type water heater. The standard size has been a 40-gallon water heater with an input rating ranging from approximately 34,000 Btu/hr. to 42,000 Btu/hr.
When a customer called and said they were always running out of hot water, you either suggested a 40-gallon water heater with a larger burner, or a 50-gallon water heater. (For some “McMansions” you install two 50-gallon water heaters, parallel piped, even though there normally is no justification for that much hot water.)
These days, some would accuse you of being old school if you automatically thought of tank-types when someone mentions the word “water heater.” With the advent of tankless water heaters, you have to be more specific. (By the way, I still cringe when I hear someone say, “hot water heater.” The long-standing plumbing joke is, “Why would you want to heat hot water? It's already hot.” Of course, the correct name is a cold water heater, since it is the cold water that you want to heat.)
Going TanklessIf you choose to change an installation from a tank-type water heater to a tankless water heater, the sizing becomes critical. While everyone knows how to size a tank-type water heater, it is much more difficult to size a tankless water heater.
Manufacturers have made it easier in sizing tankless water heaters by having variable burners for gas-fired units. There is a greater range of hot water supplied by the variable burner models.
It doesn't help that manufacturers promote tankless water heaters as having an unlimited supply of hot water, or never running out of hot water. They go so far as to say, “Our tankless water heater provides a home with 120 gallons of hot water an hour compared to a 40-gallon water heater that only provides 75 gallons an hour.”
This all sounds great, but if a tankless water heater is providing 120 gallons of hot water per hour, that equates to 2 gallons per minute. That's not much hot water to operate multiple fixtures. When they double the amount to 240 gallons per hour, still that is only 4 gallons per minute.
Tank-type water heaters have always been sized based on initial demand, typically the hot water needed in a 10- to 20-minute period, depending on the type of building, and the average demand for a one-hour period of time. We then size based on storage capacity and input rating.
Some contractors have attempted to size tankless water heaters based on the input rating, but that will get you in trouble. There is no way to convert the input rating of a tank-type water heater to the input rating of a tankless water heater.
I'll use my house as an example. I have three showers in the house. When my two oldest were teenagers, on a typical morning there would be three showers taken at the same time. Living in the Chicago area, during the winter months, our cold water is cold, about 45 degrees F. That means the amount of hot water required for each shower would be 1.8 gpm (each shower has a flow rate of 2.5 gpm). For three showers, the flow rate of hot water is 5.4 gpm. If I had a 40-gallon water heater, it would mean that I would have about eight minutes of a hot shower. However, I have a 50-gallon water heater, hence I am guaranteed a 10-minute shower in the morning, which is all most people need.
If I want that flow for the three showers from a tankless water heater, I need a model that provides 5.4 gpm of hot water. That, by the way, is a large residential tankless water heater. It will probably have an input rating of more than 150,000 Btu/hr. To put that into perspective, I only heat my home with 100,000 Btu/hr.
Proper SizingSo, do you always size for the maximum number of showers being taken? The answer is no. What you have to do is determine the peak flow rate of the hot water under normal high use periods of time. Sometimes that would mean considering the dishwasher, kitchen sink and washing machine at the same time. Other times it may be the kitchen sink, one lavatory and one shower.
Going back to that tankless water heater that provides 120 gallons of hot water per hour, if we consider the need of 1.8 gpm for a shower, you can take only one shower at a time with this model. You are left with only 0.2 gpm to spare.
So what happens when there is a call for a greater demand of hot water? Unlike a tank-type water heater, you don't instantly run out of hot water with only cold water coming out of the faucet or shower. For a tankless water heater, the temperature of the hot water starts to decrease. Of course, that means that you turn up the faucet to get a warmer temperature. That uses more hot water, hence the temperature continues to drop. Yes, it is a vicious cycle that you can be chasing with a tankless water heater when the demand is greater than the rating for the water heater.
The manufacturers of tankless water heaters have charts that show the flow rate vs. the temperature rise. For example, if the 2-gpm tankless heater is installed, it may raise the temperature of water 100 degrees F at 2 gpm, but at 4 gpm the temperature rise may drop to only 65 degrees F. If you have 45-degree F cold water, it is still 110 degrees of hot water, hotter than anyone takes a shower.
I should mention that the 2-gpm tankless water heaters are typically intended for either an individual fixture or a few fixtures located together. They are not sold as whole-house water heaters. The smallest tankless water heater offered for a dwelling unit is a 3-gpm model.
I have run calculations to show that a 3-gpm tankless water heater will always satisfy a home with one or one-and-a-half baths. Sometimes it can satisfy a two-bath home, but that depends on the location of the home (temperature of cold water) and the use of the dwelling unit. Many two-bath homes rarely have both showers going at the same time.
Another important factor to remember is that tankless water heaters have a minimum flow rate required to activate the water heater. That typically ranges from 0.5 to 0.75 gpm. If you trickle water out of a faucet at a lower flow rate, it will always be cold.
So when you are changing from a tank to tankless water heater, keep these sizing considerations in mind. It is not difficult to size a tankless water heater, just start figuring the number of fixtures that may be used at the same time. Then determine how much of the water is hot water since we tend to use mixed water. That sizes the tankless water heater.