Plumbing
Plumbing Water Heater Size Calculator
Size a storage water heater from the household's peak hour, not from the number of bedrooms.
Result
Storage heaters are sized on the busiest hour rather than daily total, and the number that matters on the label is the first hour rating rather than the tank volume. A tank only delivers about 70% of its capacity before the outlet runs cold.
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About this calculator
Why does a 50 gallon tank not give 50 gallons of hot water?
Because cold water enters as hot leaves and mixes. Usable output before the temperature drops is roughly 70% of the volume, which is what first hour rating measures.
Is a bigger tank always better?
No. A larger tank loses more heat standing idle, costs more to buy and takes more space. Size it to the peak hour rather than to the worst day imaginable.
Should I consider tankless instead?
If the limit is running out rather than flow rate, tankless solves it. It has its own constraints: a large gas supply or heavy electrical service, and a maximum flow that falls in winter when the incoming water is colder.
The formula
peak demand = showers x minutes x GPM x 0.7 + baths + appliance loads; tank = demand / 0.7
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