Why the old tank's size isn't the answer
When a heater fails, the lazy move is to buy the same size again. It's quick, it's easy to quote, and it's often wrong. The tank that was there was chosen by a builder working to a budget, or by whoever replaced it last time, for a household that may have looked nothing like yours now. A family of two that grew to five, or a house that gained a bathroom upstairs, is still being served by a tank sized for someone else.
So before you accept a number, work out what the heater is being asked to do. That's a short exercise, and it saves years of cold showers or years of paying to keep too much water hot.
A rough starting point for tanks
As a rule of thumb, one or two people do well with 30 to 40 gallons. Three or four people usually need 40 to 50. Five or more, or a home with a large soaking tub, often need 65 gallons or more. Most Rowlett houses, which are suburban single-family homes with two to four people, end up with a 40 or 50 gallon tank.
These are starting points, not answers. A family of four who all shower between 6:30 and 7:30 puts a heavier load on a tank than a family of four whose showers are spread across the day. Habits matter as much as head count.
The number that actually counts
The gallon figure on the label is the size of the tank. What you feel at the tap is a different number, called the first hour rating: how much hot water the heater can deliver in an hour of heavy use, starting from a full tank. It's printed on the yellow energy guide label and in the specs.
It matters because two 50 gallon tanks aren't equal. A gas tank with a strong burner recovers fast, so a 40 gallon gas tank can have a first hour rating that matches a 50 gallon electric tank. That's why electric tanks are often sized a step larger than gas tanks for the same household. If you can, compare first hour ratings and not just gallons.
Sizing a tankless unit is a different exercise
A tankless heater doesn't store water, so gallons don't apply. What counts is flow rate in gallons per minute, which depends on how many fixtures run at once. A typical shower uses about two gallons a minute, a kitchen tap around one and a half, a dishwasher or washing machine a bit more. Add up the worst case, such as two showers and the dishwasher on a busy morning, and that's the number the unit has to deliver.
Then there's the temperature rise. The unit has to heat the incoming water from the temperature it arrives at to the temperature you want. In a Rowlett January, the incoming water is a lot colder than in August, so the unit has to work harder and delivers fewer gallons a minute. Size for the cold month. A unit that's right in July and short in January will leave you short when it's coldest.
The tankless installation page explains the other pieces: the gas line, the vent and the drain.
Three households worked through
Here are three illustrations. They're not real homes, but the arithmetic is real.
A family of four in a three-bedroom house with two and a half baths. Two showers at once on school mornings, a dishwasher in the evening, laundry on weekends. A 50 gallon gas tank covers that with a margin. An electric tank would want to be 50 to 65 gallons for the same house.
A couple in a two-bedroom townhome with one bath. A 40 gallon tank is plenty, and a 30 would do, though the saving from the smaller one is small. The one thing to watch is a soaking tub, which can empty a smaller tank on its own.
A household of six with four baths, a big laundry load and teenagers. A single 50 gallon tank will leave the last shower cold. A 75 or 80 gallon tank, or a tankless unit sized for the busiest morning, is the answer, and both are worth quoting.
Too big is a real problem too
A bigger tank sounds safe. It isn't free. It costs more to buy, takes more floor space, and keeps more water hot around the clock, which is wasted energy in a household that never uses it. If the kids have moved out and the heater hasn't shrunk, you're heating 50 gallons for two people. Dropping a size on the replacement can lower the running cost and the purchase price together.
Space is the other limit. An attic closet or a utility alcove only takes so large a tank, and a bigger tank may not get through the hatch or fit under the vent. The size we quote has to fit where it goes, and that's checked on site before the unit is ordered.
Tankless sizing in plain numbers
As a guide, a shower uses around two gallons a minute, a bathroom faucet around one, a kitchen tap around one and a half, a dishwasher around one and a half, and a clothes washer a little over two. Say two showers and a dishwasher are running: that's about five and a half gallons a minute.
A tankless unit has to deliver that at the temperature you want. A model built for nine gallons a minute is a comfortable match in summer, but in January, when the incoming water is much colder, it may deliver closer to five or six at the same temperature. That's the reason to size for winter. If the house is larger or the household bigger, a second unit, or a bigger one, covers it.
The numbers are reasonable guides, not exact ones. Fixtures vary, and a plumber reads the actual fixtures and the incoming water temperature. The tankless installation page covers the rest.
Signs your current size is wrong
- Hot water runs out partway through a normal day, not just on a heavy one.
- The tank is fine but the second shower always runs cold.
- The water is much hotter than needed, and the tank rarely runs low, even when guests stay.
- The household has added a bathroom, a soaking tub, or a few people since the heater was installed.
If one or two of those sound familiar, say so when you call. It changes the quote.
Efficiency labels, in a sentence
The yellow energy guide on a new heater shows an efficiency number called the uniform energy factor. Higher is better. A gas tank is typically below 1, an electric tank around 0.9, and a heat pump model over 3, because it moves heat and doesn't make it. It's a useful tie-breaker between two heaters of the same size, and no help with sizing.
What to tell us when you call
Give us the age, size and fuel of the old heater, where it sits, and how many people and bathrooms are in the house. If you can, mention when the hot water gives out. From that we can tell you whether the same size is right, or whether a step up or down makes sense, and give you an estimated range for it. The written quote follows once the heater has been seen.
Sizing is part of every replacement job, and the post on reading your water heater's label shows where each number is hiding.
When two heaters beat one
In a large house, or one with a bathroom far from the heater, a single big tank may not be the best answer. The trouble is distance: the farther the heater from the fixture, the longer the wait for hot water, and the more gets wasted running the tap.
Two small answers exist. One is a second, small heater near the far bathroom, which is often a 20 or 30 gallon tank, or a small tankless unit. The other is a recirculating pump, which keeps hot water moving around the pipes so it's ready at every tap. Each has a cost: a second heater is another unit to maintain, and a recirculation pump uses energy to keep the loop warm.
If your complaint isn't running out, but waiting, size isn't the issue, and a bigger tank won't fix it. Tell the plumber which one it is. It changes what you're quoted.
A quick checklist before you call
Having these to hand makes the conversation short and the quote accurate.
- The size, fuel and age from the old heater's label.
- How many people live in the house, and how many bathrooms.
- When the hot water runs out, if it does: the second shower, the dishwasher, a bath.
- Where the heater sits, and whether it's in a closet, the garage or the attic.
- Any change coming, such as a new baby, a teenager moving in, or an addition.
That last one is easy to forget. A heater lasts around a decade, so size it for the household you expect to have, and not only the one you have today.