Nothing ruins a morning faster than a cold shower. Whether you’re waking up for a commute to Chicago or just trying to get the kids ready for school, running out of hot water is a headache nobody needs.
But here’s the thing: buying the biggest tank you can find isn’t the answer. An oversized unit is just an energy vampire that drives up your gas or electric bill. On the flip side, going too small leaves you shivering.
Getting the size right is a bit of a balancing act. At Cardinal Plumbing Co., we size systems for Illinois homes every day. We look at more than just the number of bathrooms. We look at how you actually live.
Here is how to figure out exactly what you need.
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Water Heater Sizing
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How Water Heater Sizing Actually Works
Most people think sizing is just about gallons. They see "50 gallons" and think, "That sounds like a lot." But in the water heater world, gallons don't tell the whole story.
Real sizing is about recovery. It’s about how fast your heater can replenish hot water while you are using it.
If you have a house full of teenagers taking back-to-back showers in Naperville, you need a unit with a high "First Hour Rating" (FHR). This number combines the tank's storage capacity with its burner power. Basically, it tells you how much hot water you can use in one busy hour before things go cold.
Sizing a Standard Tank (The "First Hour" Rule)
If you are sticking with a traditional tank water heater, forget the total gallon capacity for a second. Focus on your peak usage.
Think about your busiest hour of the day. Maybe it’s 7:00 AM when two showers are running, someone is shaving, and the dishwasher is finishing up.
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- Shower: ~20 gallons
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- Shaving: ~2 gallons
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- Dishwasher: ~6 gallons
If you use 48 gallons in that hour, buying a 40-gallon tank with a weak burner won’t cut it. You’d need a tank with a First Hour Rating of at least 50 gallons.
Quick Rule of Thumb:
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- 1–2 People: 30–40 gallon tank
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- 3–4 People: 50-gallon tank (Gas is usually better here)
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- 5+ People: 60–80 gallon tank or a high-recovery 50-gallon model
Instant Water Heater Sizing (Going Tankless)
Thinking about ditching the tank? Tankless water heater services are totally different. Since there is no storage, you don't count gallons—you count flow.
You need to know your Gallons Per Minute (GPM) demand.
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- Turn on every hot water tap you might use at once.
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- Add up their flow rates. (A standard shower is about 2.5 GPM; a faucet is 1.5 GPM).
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- If you hit 6.5 GPM total, you need a unit that can handle that specifically in our climate.
The "Illinois Factor"
This is where people get tripped up. Groundwater in Illinois is cold often around 50°F or lower in winter. Your heater has to work much harder to warm that water up to 120°F compared to a heater in Florida.
Always check the spec sheet for the GPM rating at a 70°F temperature rise. If you don't, you might find that your "8 GPM" heater only gives you lukewarm water in January.
Heat Pump Water Heater Sizing
Heat pump units are incredibly efficient because they pull heat from the air rather than creating it. But they are a bit slower to recover than a blasting gas burner.
For heat pump water heater sizing, we usually recommend going one size up. If the math says you need a 50-gallon electric tank, go for a 65 or 80-gallon heat pump.
The extra capacity acts as a battery. It stores enough hot water so the unit doesn't have to kick on its backup electric strips (which kills your efficiency savings) during those busy mornings.
A Note for the Pros: Water Heater Sizing Guide for Engineers
If you are managing a commercial building or a large luxury home, simple charts won't cut it. Our commercial plumbing team uses a more technical approach often referred to as a water heater sizing guide for engineers.
This involves using Hunter’s Curve or ASHRAE usage tables. We calculate "diversity factors"—basically the probability of every fixture being used at once.
For example, a gym in Aurora with 10 showers needs a different setup than an apartment complex with 10 showers. The gym showers run simultaneously (100% demand factor), while the apartments might only have 30% running at once.
If you have a project requiring this level of detail, check out our commercial water heater services and let us run the calculations for you.
Frequently Asked Questions
Q: Can I just replace my 40-gallon tank with a 50-gallon one?
Usually, yes. But we need to check if your current space fits the taller or wider tank. We also need to inspect your gas piping or electrical breaker to ensure it can handle the upgrade.
Q: Why does my hot water run out faster in winter?
It’s not just you. The water entering your home is colder, so your heater takes longer to heat it up. This effectively lowers your recovery rate during the months you need hot water the most.
Q: Is a tankless unit always better for large families?
Not always. "Instant" doesn't mean "infinite volume." If you have three showers running at once, a single tankless unit might struggle to keep up with the flow. In those cases, we might install two units in a series or a high-capacity commercial tank.
Need Help With the Math?
You don't have to struggle with calculators and flow charts alone. At Cardinal Plumbing Co., we can assess your home’s plumbing, check your gas line capacity, and give you honest advice on what size you actually need.
Whether you need a simple swap or a full engineering assessment for a commercial property, we’ve got you covered.
Contact us today and let’s keep the hot water running.
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