Hot Water for Single-Family Residences
By Timothy Allinson, P.E.
Murray Company, Long Beach, Calif.
Last January, I was chatting at the bar prior to the ASPE Orange County Chapter meeting. My friend and associate Mike Walsh said, "You should do an article on sizing residential water heaters."
"OK." I said, "Why?"
He proceeded to recount a recent design he did for a community of high-end single-family residences (SFRs). He sized the water heaters with 75-gallon storage tanks as he felt appropriate. The developer asked why he had made them so large. Mike explained that he had calculated the volume of the largest bath and so forth. The developer said, "I've never built homes with such large heaters. Make them smaller." So he did. But, Mike went on record that 75 gallons was the right size.
Sure enough, shortly after the homes were occupied some of the homeowners complained that they were running out of hot water, and the developer had to pay Mike to revise his design for larger heaters.
How Much is Enough?
Anthony Montalto, a former associate of mine, once said, "Nobody ever complained about having too much hot water, but they sure do complain the minute they run out." His point: When it comes to hot water -- more is better. But, how much is too much?
Sizing water heaters for SFRs is a delicate subject because the design is very much dependent on the habits of the users. Europeans tend to bathe more often than Americans who are more likely to use the shower. My wife, Mila, uses the bath a lot, but she is not typical in that regard.
When sizing a water heater, the first step is to determine how many hot water consuming fixtures are in the residence. But, as my former boss Richard Reilly often said, "Fixtures don't use water, people do." Meaning, you must evaluate the population. In the case of a single-family residence, consider the number of bedrooms and assume on average that there are 1.5 people per bedroom. Then, estimate how much hot water each person will use and when. Hot water consumption is highly dependent on demographics. Let's look at an example.
My Home
I live in a three-bedroom single-family residence with 2.5 baths, meaning two bath/showers, three lavatories, a kitchen sink, dishwasher and a washing machine -- fairly standard. Our home has a 40-gallon, 50,000 BTUH gas-fired water heater -- also pretty standard. We have never run out of hot water that I recall. Why not?
If you look at the numbers from a fixture volume standpoint, the fixtures total about 150 gallons - a great deal more than the volume of the water heater.
Let's look at the numbers instead from a population standpoint. Imagine that there are 1.5 people per bedroom. That would equate to 4.5 people, which I will round to 5. In fact, there are 4 of us plus one dog, and yes, the dog uses hot water every once in a while when I drag her into the bath. So, how much hot water does each of us use?
Aidan, my 5-year-old son is wrestled into the bath every other night. While it's tough getting him in, it's even tougher getting him out. His bath uses about 50 gallons - even more than the water heater itself. However, that water is tempered with cold water from the 120-degree storage temperature to a comfortable 100 degrees, so the actual hot water consumption based on 60-degree cold water is:
T1V1 = T2V2 + T3V3
and
V3 = 50 - V2
so
100 x 50 = 120V2 + 60(50-V2)
V2 = 33.3 gallons
It is within the capacity of the 40-gallon heater, but not by much. So, why don't we run out of hot water if Mila is washing dishes while I bathe Aidan? The heater has 50,000 BTUH of firepower:
BTUH = GPH x deltaT x 8.33 #/gal
gpm = 50,000 / 60 x 40 x 8.33 = 2.5
So, the 50,000 BTUH, 40-gallon water heater has enough storage and heating capacity to supply the 2.2 gpm kitchen faucet plus a full 50-gallon bath. (Note: in the above calculation I have neglected efficiency for simplicity and used a 40-degree deltaT since the cold water in Southern California is about 60 degrees and the wash temperature at the faucet is about 100 degrees.)
After I am finished bathing Aidan and the dishes are done, the hot water heater is essentially depleted. The recovery time is:
40 gallons / 2.5 gpm = 16 minutes
That is about how long it takes me to get Nolan, our 3-month-old, ready for his bath. Nolan uses very little hot water. His plastic bathtub only holds about 5 gallons of water, and by the time I fill it up the water heater is back up to temperature.
Mila might take a bath later that night, but again the heater has had time to come back up to temperature, so she will have sufficient hot water.
I use very little hot water -- about 15 gallons during a 5-minute shower, generally at 5 a.m., so I don't much enter into the water heater equation.
As my sons get older and their bathing habits change, (if we are still in the same house I might find myself running out of hot water) but I'll deal with that if and when the time comes.
Instantaneous Heaters
Gas fired instantaneous water heaters are popular these days, partly because they offer a limitless supply of hot water. However, even with an instantaneous heater, if too many fixtures are used simultaneously, such as both showers, the clothes washer, dishwasher and sink, then the delivery temperature can drop significantly. So, even with instantaneous heaters you must try to predict the behavioral habits of the occupants.
Your Project
So, what if you don't know the behavioral habits of the residents for whom you are designing? You take your best guess. In the case of a three-bedroom residence like mine with an estimated population of 5, I would take the worst-case scenario in which all five people shower within the same hour. This would require about 75 gallons of 100-degree water, or 50 gallons of 120-degree water. I would size the heater at least this large. However, just to be safe, I would bump the heater up to 60 gallons. This is 50% larger than the heater in my home, but not all people have the behavioral habits of my family. This project might house two parents and their three teenage daughters, in which case a 75-gallon heater might be required.
Project Location
You must also account for the location of the project. Areas with cold winter climates have a cold water supply temperature of 40 degrees rather than the temperate 60 degrees of Southern California. This fact will have to be included in the calculations performed above, increasing the delatT to 80 degrees rather than 60 degrees.
Final Thought
Lastly, I would suggest erring on the side of caution. Since it is unlikely that you will know the behavioral habits of the project occupants, it is impossible to size the hot water heater with precise accuracy. Therefore, you should allow for more hot water rather than less. As mentioned, nobody ever complained about having too much hot water, except perhaps the developer who has to pay for the heater.
Timothy Allinson is a Senior Professional Engineer with Murray Company, Mechanical Contractors, in Long Beach, Calif. Prior to entering the design-build industry he worked for Popov Engineers, Inc. in Irvine, Calif, and JB&B in New York City. Tim holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed Accredited Professional. Tim is a past-president of ASPE, both the New York and Orange County Chapters, and sits on the board of the Society of American Military Engineers, Orange County Post.