Water heater sizing

Every line shows the quantity to buy and how StudSheet got it: the spacing, the waste and the package size, so you can check it before the supply house or the quote.

In Canada (Gas water heater, 4 people)

Set to Canada, the Sheet works the job out again for Ontario: metric sizes, bag sizes, the frost depth and the code notes. It doesn't just convert feet to meters. The first lines:

• 1 heater, Gas water heater, 189.3 L, atmospheric vent: Busiest hour (DOE worksheet, hot water per use): 4 showers × 76 L + kitchen 11 L = 314 L. A 189 L gas tank rates about 318 L in its first hour, the smallest that covers it. 4 people and 2 baths: everyone showers in that hour, at most 2 per bathroom.
• 1 valve, Ball valve, ¾ in: Cold shutoff at the heater (OBC 7.6.1.3).
• 2 connectors, Water heater connectors: Hot and cold (or hard pipe).
• 1 length of 3.66 m, Copper pipe, type L, ¾ in: Down the side of the tank to a floor drain or the floor: about 1.83 m. Copper, CPVC or PEX rated for relief lines — never PVC.
• 3 fittings, Copper fittings (sweat or press): Male adapter at the valve and two elbows (sweat or press).
• 1 tank, Thermal expansion tank, 7.6 L: Sized on the makers’ charts: about 7.6 L for a 151–189 L tank, 17 L for bigger tanks or high pressure. Likely needed: check the meter and the main for a check valve or PRV.

1. Pull the permit (or file the minor-work notice before starting).
2. Confirm the size and the fit: 50 gal, first-hour rating 84 gal; doors, stairs and the vent path; the label’s clearances.
3. Shut off the gas and the cold supply, drain the old tank to a floor drain and disconnect it.
4. Haul the old unit away.
5. Set the new unit, level.
6. Water: cold shutoff, connectors, the expansion tank on the cold side; the relief line full size to the floor or a drain.
7. Gas: the licensed gas fitter connects it — shutoff, sediment trap, connector — and leak-tests every joint.
8. Vent: the connector at the draft hood’s size, rising to the chimney or Type B vent; check for spillage at the draft hood once it fires.
9. Fill before you fire: open a hot tap until water runs without air.
10. Set the temperature (120°F), check every joint and the relief valve, and show the owner the shutoffs.
11. Final inspection; leave the manual, the permit card and the warranty registration with the owner.

Must-have tools first, then the ones that help and the ones to rent for the day.

Mistakes that cost a callback or a second trip

• Firing an empty tank: Power to an electric tank before it’s full burns out the upper element in seconds; a gas tank fired dry is damaged. Fill and purge the air first.
• A reduced or capped relief line: The relief line runs full size to an open end you can see — no valve, no cap, no threads, no trap. A capped line turns a stuck thermostat into a tank that can burst.
• Sizing by the old tank: The old tank’s gallons don’t say whether it kept up. Size by the first-hour rating against the busiest hour (a tankless by flow and rise).
• No expansion tank on a closed system: With a check valve or PRV on the main, heated water has nowhere to go: the relief valve weeps and pressure spikes wear out faucets and toilet fill valves.
• An orphaned water heater: When the furnace goes high-efficiency, the old chimney is too big for the water heater alone: it won’t draft and can spill carbon monoxide into the house. Line or resize it.

What the calculator assumes

• Tank size: the DOE’s method — the busiest hour of hot water against the first-hour rating on the label (shower 20 gal, hand dishwashing or food prep 3 gal, dishwasher 7 gal, top-loading washer 25 gal per use).
• First-hour ratings typical of current models, the label of the one you buy rules: 40 gal ≈ 70 gal, 50 gal ≈ 84 gal, 75 gal ≈ 130 gal.
• Relief line: the floor, a floor drain or outside, ending no more than 6″ above the floor or drain where it can be seen (IPC 504.6).
• Expansion tanks sized on the makers’ charts (about 2 gal for a 40–50 gal tank at normal house pressure) and charged to the house pressure; required on closed systems (IPC 607.3).
• Temperature: 120°F at the tank limits scalding; storing hotter needs an ASSE 1017 mixing valve. Shower valves limit their own outlet (ASSE 1016 / 1070).
• Gas piping is sized and tested by the licensed gas fitter (NFPA 54 / the IFGC); this Sheet lists the parts only.
• Code here: the National Standard Plumbing Code (NSPC 2021), New Jersey’s plumbing subcode. Confirm with your building department.

The Sheet sets its code notes for the job's state or province. These are New Jersey's and Ontario's; the code reference has more. Always confirm with your building department.

• Permit: minor work in New Jersey: Swapping a water heater for one of the same capacity is “minor work”: the licensed master plumber notifies the town’s construction office before starting and files the plumbing permit within five business days (N.J.A.C. 5:23-2.17A); a different size or fuel is a regular permit. Gas units: the gas connection is plumbing-subcode work by the master plumber. The inspector checks the relief line, venting and connections.
• Gas line: the gas fitter’s call: The licensed gas fitter sizes the gas pipe for this unit’s input (40,000 BTU/h — the rating plate rules) plus every other gas appliance on the line (NFPA 54 / the IFGC), connects it with a shutoff valve in the same room and a sediment trap, and leak-tests it. This Sheet lists the parts; it doesn’t size gas pipe.
• Venting: natural draft: The vent connector is the draft hood’s size or larger (3″ here), rises toward the chimney or Type B vent and is checked for spillage once it fires. If the furnace that shared the chimney went high-efficiency, the water heater may now be alone in a chimney too big for it: it needs a liner or a resized vent (NFPA 54 / the IFGC). Combustion air as the label and the code ask.
• Relief line: The relief valve’s discharge pipe is full size (¾″), rated for hot water, has no valve, no trap and no thread at its end, and runs by gravity to the floor, a floor drain or outside, ending no more than 6″ above the floor or drain where it can be seen (IPC 504.6).
• Expansion tank: likely needed: Most water meters and pressure-reducing valves make the system closed: if there’s a check valve, backflow preventer or PRV on the main, heated water has nowhere to expand, so an expansion tank goes on the cold side (IPC 607.3). Set its air charge to the house pressure.
• Ontario: TSSA for gas, the municipality for plumbing: Gas water heaters are installed by a TSSA-registered contractor with certified gas technicians. Ask your municipality whether a plumbing permit is needed for the swap. Confirm with your building department.
• Rented tank?: Many Ontario homes rent their water heater (from companies such as Enercare or Reliance): the tank belongs to them. The owner calls the rental company to end the rental or buy it out, and the old tank goes back to them, not to the scrapyard. Water heaters also can’t be sold door to door in Ontario unless the customer starts the contact.

What materials do I need to replace a water heater?

For the worked example, a gas water heater, 4 people, the Sheet counts 14 lines with waste included, starting with gas water heater, 50 gal, atmospheric vent (1 heater), ball valve, ¾ in (1 valve), water heater connectors (2 connectors), copper pipe, type L, ¾ in (1 length of 10′). Each line's math is in the table above. Put in your own sizes in the water heater calculator and every line is counted again.

How long does it take to replace a water heater, and can I do it myself?

About 5 to 7 hours for the worked example, in 11 steps. StudSheet stamps it PRO JOB: hire this out (pros get the full Sheet).

What tools do I need to replace a water heater?

15 in all. The must-haves start with appliance dolly, pipe wrenches (14″ + 18″), tongue-and-groove pliers, tubing cutter, cordless drill/driver, 4-ft level. On the Sheet you mark each one as owned, to buy or to rent.

Stuck while you replace a water heater? Ask in the Water heaters group of the StudSheet community with this Sheet attached. Homeowners who've done it and verified pros answer, and anyone can read it.

• Plumbing for contractors: run this takeoff at your own prices, send the estimate, and get it signed and the deposit paid.
• Plumbing rough-in (3 fixtures): For plumbers: supply and drain pipe by size, fittings, valves and vents for the fixtures you are adding.
• Replace a toilet (Toilet, 12″ rough-in): Measure the rough-in, pick a toilet that fits, and swap it with a new seal, bolts and supply line — plus what to do about a broken flange.
• Replace a bathroom vanity (30″ vanity): A vanity that fits the space and the pipes, its top and faucet, new supply lines, trap, shims and sealant — swapped in an afternoon.
• Replace a faucet (Kitchen faucet): A kitchen or bathroom faucet that fits your sink’s holes, new supply lines, the right sealant, and a leak check — or a cartridge for a drip.
• All 45 Sheets, across 16 trades.
• Code reference: frost depths, guard heights, stairs, retaining walls.