Water Heater

What it's for

Sizes a water heater from the size of the household. For a tank you get a gallon size for gas or electric, and for an electric tank the circuit it needs: amps, breaker and wire. For tankless you get the flow in GPM, the temperature rise and the output the unit has to deliver, in Btu/h and kW. It is a sizing estimate to take to the supplier, with two installation reminders from the code in the basis row. It does not check an installation.

Step by step

The example is the screen as it opens: a gas tank for 3 bedrooms and 2.5 bathrooms.

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1. WATER HEATER TYPE — Tank or Tankless. The rows below change with it.
2. FUEL — Gas or Electric. Tank mode only. The same demand lands on a different gallon size for each fuel. With Electric two more rows appear: ELEMENT WATTS, the wattage on the nameplate (one element's rating; a standard tank heats with one at a time), and CIRCUIT VOLTAGE, 240, 208 or 120 V. They open at 4500 W and 240 V.
3. BEDROOMS — the number of bedrooms in the house. The example is 3.
4. BATHROOMS — in steps of a half, so a powder room counts as 0.5. The example is 2.5.
5. Read RESULTS: 40 gal gas, and under it Min. FHR: 62 gal (UPC Table 501.1(2)).
6. Tap BASIS & WHAT WAS ASSUMED for the installation reminders and the code edition.

Tankless mode

The same house, switched to Tankless and left on the rows as they open.

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1. Switch WATER HEATER TYPE to Tankless. FUEL goes away and two rows appear.
2. SIMULTANEOUS SHOWERS — how many showers you expect to run at the same time. It opens at 2 and cannot go below 1.
3. GROUNDWATER TEMP — the temperature of the water coming into the house, in steps of 5°F. It opens at 50°F. Colder water in means more rise for the unit to make.
4. Read RESULTS: 5.5 GPM @ 70°F rise. Under it, Min. FHR: 62 gal (UPC Table 501.1(2)) is unchanged, the next line repeats the flow and the rise, and the last gives the output the unit has to deliver: Output: 192,500 Btu/h · ≈ 56.4 kW · ≈ 235 A at 240 V if electric.

BEDROOMS and BATHROOMS stay on screen. They still feed the first-hour rating line, which a tankless unit has to meet too, but the tankless headline comes from the two rows above.

Reading the results

The headline in Tank mode is a tank size and fuel, such as 40 gal gas: the smallest tank on the screen's list whose typical first-hour rating covers the line under it. The list is 30, 40, 50 and 75 gal for gas and 40, 50, 66 and 80 gal for electric, each at the low end of what makers rate that size (a 75 gal gas tank at 113 gal).

Min. FHR is the smallest first-hour rating the house needs, in gallons, from BEDROOMS and BATHROOMS. It is read from the one published code table, UPC Table 501.1(2) as Washington adopts it (WAC 51-56-0500): 38 gal for 1 bedroom and 1 to 1.5 baths, up to 74 gal for 5 bedrooms and 2 to 2.5 baths or 4 to 6 bedrooms and 3 to 3.5 baths. The IRC sets no water-heater size. The table covers 1 to 6 bedrooms and 1 to 3.5 baths. For a house it has no cell for, such as a studio, 4 or more baths or 7 or more bedrooms, the line reads Est. FHR: … gal — rule of thumb, outside UPC Table 501.1(2), worked as 20 gal plus 12 a bedroom plus 10 a bathroom. It shows in both modes. When you shop, compare it with the first-hour rating on the heater's Energy Guide label, not with the tank's gallons.

Electric tanks over 55 gal. When the pick is a 66 or 80 gal electric tank, a line reads Over 55 gal electric: heat pump only (10 CFR 430.32(d)(1)). The federal efficiency minimum for those sizes can only be met by a heat-pump water heater. The one exception is a grid-enabled tank over 75 gal, sold for utility programs. From May 6, 2029 the same rule reaches 20 to 55 gal electric tanks, and BASIS says so.

The headline in Tankless mode is a flow and a rise, written as GPM @ °F rise, such as 5.5 GPM @ 70°F rise, and a second line repeats the GPM demand and the rise. The Output line is that flow and rise as heat: 500 × GPM × rise in Btu/h, and the same in kW. A gas unit burns more than it delivers, so compare its input rating with care. An electric unit draws the kW as current: the line gives the amps at 240 V, which can be more than the service has to spare. Ask the supplier what the unit delivers at that rise.

The circuit, for an electric tank. One more line gives the branch circuit, such as Circuit: 18.75 A · 125% = ≈ 23.44 A · 25 A breaker (30 A max) · 10 AWG Cu for 4500 W at 240 V. The amps are the watts over the volts. The NEC sizes the wire and breaker for a storage water heater at 125% of that (422.13). The breaker is the smallest standard size at or above 125% (240.6(A)). The most allowed when the nameplate marks no breaker size is 150% of the amps, next size up (422.11(E)); a size marked on the nameplate governs. The wire is the smallest copper size whose 60 °C rating covers the breaker (310.16, 240.4(D)), and the line names a larger wire when the largest breaker needs one. SIZE THE RUN IN VOLTAGE DROP, under the results card, opens Voltage Drop on this circuit: the real amps, the voltage and the wire, with CONTINUOUS LOAD (×1.25) already on, and your run length kept. Back returns here.

Red lines. In Tank mode, DEMAND EXCEEDS COMMON TANK FHR — CONSIDER TWO UNITS OR TANKLESS. means the household needs more than the largest tank on the list delivers. The headline still shows that largest tank, so do not read it as enough. Fewer BEDROOMS or BATHROOMS would clear it, but only enter what the house has. The real fix is the one the line gives. In Tankless mode, COLD GROUNDWATER — VERIFY GPM AT THIS RISE, NOT THE HEADLINE RATING. appears when GROUNDWATER TEMP goes below the 50°F it opens on — at 50°F the rise is 70°F and the line stays off. It is a warning about the unit's brochure figure, not about your inputs. MORE THAN ONE RESIDENTIAL GAS TANKLESS — 10 CFR 430.2 CAPS ONE UNDER 200,000 BTU/H INPUT appears when the output reaches 200,000 Btu/h: no single residential gas unit can deliver it.

BASIS & WHAT WAS ASSUMED opens with where the first-hour rating comes from, then, for an electric tank, the heat-pump rule and its 2029 date, then the rule behind the circuit or the output line when there is one, then the installation reminders, each with its IRC section: one on the temperature-and-pressure relief valve and its drain, with the pan a storage tank sits in where a leak would cause damage (Tank only: a tankless unit has no tank for the pan); one on appliances in a garage, raised 18" and, where a vehicle could hit one, behind a barrier; and, for a tank, one on strapping it against earthquakes in the seismic categories that ask for it (P2801.8). They show for every result. They are not checks on your job. Under them is the line IRC 2021 — verify local code and a sentence saying that states and municipalities amend the code, and to verify local amendments and the adopted edition with your building department. Your local code and your inspector govern.

This screen has no drawing and no MATERIALS list. SHARE PDF stays dimmed until you change an input, then shares the size, the circuit or output, the reminders and your inputs. SAVE puts the result in History.

Common mistakes

• Shopping by gallons alone. The minimum is a first-hour rating. Two tanks of the same gallons can have different first-hour ratings, which is why FUEL changes the answer.
• Counting a half bath as a whole one. BATHROOMS steps by 0.5 for that reason.
• Leaving GROUNDWATER TEMP at 50°F in Tankless mode. Use your own incoming water temperature. It sets the rise, and the rise sets what a tankless unit can deliver.
• Sizing an electric heater's wire on its amps alone. A storage heater is sized at 125%: 4500 W at 240 V is 18.75 A, but it needs 10 AWG, not 12.
• Expecting SIMULTANEOUS SHOWERS to change a tank size. It shows only in Tankless mode.

Related

• Plumbing — fixture units, drain size and pipe for the same house
• Electrical — has a switch for an electric water heater circuit
• Voltage Drop — the wire for an electric heater: turn on CONTINUOUS LOAD (×1.25) (NEC 422.13)
• Cut sheets and templates
• Saving and History