What Size Water Heater Do I Need?

What Size Water Heater Do I Need
Doesn't this look nice!

The People Speak: What Size Water Heater Do I Need?

The question, “ What size water heater do I need?” comes up quite a bit and even though it has quite a few levels of correct answers there are some simple concepts to get you a fairly accurate answer.

Take An Inventory Of Your Hot Water Use

The first question I ask of the homeowner is, how many people live in the house and what are their ages. If the household has only two people

What Size Water Heater Do I Need
The tried and true AO Smith 40 Gallon Tank Type

and they both take normal 5 to 10 minute showers a 40-gallon tank type water heater is all that is needed (I will address the use of a tankless water heater in a bit so keep your pants on). If you have one person taking a 10-minute shower with a 2.5GPM showerhead that would be 16 minutes of full hot water. However the water is always blended with cold water or you would burn your skin. You’ll get about 55 to 60 gallons of hot water out of a normal 40-gallon tank-type water heater.

If the answer to the above question is “we have four people living in our house and two are quickly approaching their teenage years” I have another answer for you. You can again do the math and realize that unless you’re a shower Nazi and relegate everyone to 5-minute showers a normal 40-gallon tank type hot water heater isn’t gonna cut it. I have a standard 50-gallon tank type and I’m just starting to stretch it’s capacity with 4 people in the house with 1 pre-teen.

What Size Water Heater Do I Need
Doesn’t this look nice!

TECHNICAL STUFF: Two of the things you must look at when asking yourself; what size hot water heater do I need?, is what’s the water heaters first hour rating and what is it’s recovery rate. Not all water heaters are built the same. Some are considered high recovery models meaning they make a lot of water in their first hour of use, they also recover very quickly. Having a very high recovery heater will allow you to use a smaller tank type heater because it produces so much hot water in it’s first hour of use. You can see an example of that here.

So if you’re children are quickly approaching their teen years it’s a good time to upsize the heater from a 40 to a high recovery 50, 65 or even a 75-gallon heater. If filling a fairly large whirlpool tub is something you like to do you’re stretching the capacity of a 50-gallon standard water heater. You can fill it but no one else can take a hot shower for about an hour or so, until the water heater catches up.


Before we talk about tankless hot water heaters here is a recap of the above:

  • How many people are in your house?
  • What are their hot water usage habits?
  • Do you like to take baths in a large and deep bathtub?
  • Ask about the first hour rating of the heater, i.e. the heater may be a 65-gallon heater that produces 116 gallons of water in the first hour. That’s quite a bit of water.
  • Ask about the recovery rate of the heater. Meaning after the first hour, how quickly is hot water produced, i.e. after the first hour a 65-gallon produces 67-gallons of hot water. BTW I’m describing a high recovery heater. Most do not produce this kind of recovery.


Speaking of Tankless Water Heaters, What Size Tankless Water Heater Do I Need?

Figuring out what size tankless heater you need is based on the same concepts but there are a few quirky things that must be taken into account. Tankless heaters are rated a little differently. They are sized by the gallons per minute (G.P.M.) they produce. So you literally need to go around the house and take inventory on how you use hot water. It’s not a bad exercise because you really get an idea of how much water is used and wasted. It’s like building a budget for your household expenses.

What Size Water Heater Do I Need
Noritz is the Cadillac of Tankless Water Heaters

All Tank less Hot Water Heaters are rated and advertised by the amount of hot water they produce at a 35° rise. Let me explain that in a bit more detail. A tankless hot water heater rated @8.4GPM will produce that amount of water if the heater has to raise the temperature of the water a total of 35 degrees. If the heater has to work harder to raise the temperature more than 35 degrees the total GPM goes down. For instance if the heater has to raise the temperate another 10 degrees the output goes down to 6.4GPM this is very significant in the upper half of the United States. The average incoming water temp in Chicago is around 55° to raise the water 60° to 115° that same heater is producing just under 5GPM. So if someone is taking a shower and washing clothes and running the kitchen sink you are starving the water heater. A tub filler that fills up a soaker tub can deliver water at 8GPM to 10GPM and the heater is made to cut off flow to the fixtures when this happens. I have seen customer’s water come out in a trickle when too much hot water is being used at a given time. In the Midwest it’s a good idea to buy the biggest tankless water heater available, which is rated at 11.2GPM. Many families who have been given the right information end up installing two tankless heaters to meet their needs.


Recap of How to Size a Tankless Water Heater:

  • How many people are living in your home?
  • What are their hot water usage habits?
  • Do a hot water survey as to how much hot water each fixture uses i.e. the showerhead uses 2.5GPM, the tub filler uses 8 GPM, the kitchen sink faucet uses 1.0GPM, the clothes washer uses 3GPM etc.
  • Look at the GPM the tankless water heater will produce and take into account incoming water temperature when sizing. IT IS THE MOST IMPORTANT DETERMINING FACTOR WHEN SIZING A TANKLESS HEATER. Tankless heaters can be a real premium comfort feature in your when sized properly but I find them undersized more often than not.

With another frigid winter blanketing a large part of the United States hot water delivery becomes very important. I hope this helps you through the buying process should you be faced with the difficult decision of purchasing a new water heater.


Happy Holidays

Sean Kavanaugh

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