Calculators
Plumbing calculators: inputs you already have, one formula, one answer, with the formula printed so the number can be checked.
Each calculator takes numbers a homeowner already has, applies one formula, and returns one answer. The formula is printed on the page so the result can be checked by hand.
Water heaters 7 pages
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Water heater recovery time calculator
Recovery GPH = input x efficiency / (8.33 x temperature rise). A 4,500 W element at a 90 F rise gives 20.5 GPH, so a 40-gallon tank reheats in about 117 minutes.
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Water heater element ohms calculator and test
R = V² / P: a 4500 W, 240 V element computes to 12.8 ohms. The maker's manual passes 5 to 25 ohms; OL, under 5 or over 25 means replace. Breaker off first.
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Tankless water heater descale interval by hardness
Navien, Rinnai and Rheem all cap hardness at 200 mg/L. Navien and Rinnai band flushing by hardness; Rheem says flush annually.
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Hot water wait time calculator by pipe length
Seconds to hot water = pipe length x gallons per foot / gpm x 60. 50 ft of 3/4 in Type L copper holds 1.26 gallons: 30 seconds at a 2.5 gpm shower, 38 at 2.0 gpm.
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Water heater size calculator: first hour rating
Count each hot-water use in the busiest hour, multiply by DOE's gallons per use, add them up. DOE's own example is 66 gallons; buy a first hour rating of 66 or more.
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Tankless water heater size: GPM and temperature rise
Add the gpm of fixtures running at once, subtract incoming from outlet temperature: BTU/h = GPM x 500 x rise, kW = GPM x rise / 6.83. 4.7 gpm at 73 F is 50.2 kW.
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Expansion tank size calculator: gallons and psi
Tank gal = heater gal x expansion factor x (max psi + 14.7) / (max psi - line psi). A 50 gal heater at 140 F on 60 psi needs 1.4 gal: an Amtrol ST-5 or Watts PLT-5.
Toilets, faucets and showers 1 pages
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Shower water and energy cost calculator by gpm
Gallons = gpm x minutes; Btu = hot gallons x 8.34 x rise / UEF. An 8 minute 2.5 gpm shower is 20 gallons and 2.42 kWh electric; 1.5 gpm cuts both by 40 percent.
Dehumidifiers 1 pages
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Dehumidifier size calculator: pints/day by room
ENERGY STAR's chart: under 2,000 sq ft needs 20-30 pints/day moderately damp, 25-40 very damp, 30-50 wet; over 2,000 sq ft 30+, 40+, 50+, on the 65 F rating.
Leaks and shutoffs 1 pages
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Sump pump size calculator: GPH at head, not HP
Gallons per inch of rise is pi x radius squared / 231. An 18 in pit rising 10 in a minute needs 661 GPH at 15 ft of head; a 3/10 HP Zoeller 53 gives 1,140.
Pipes and freezing 1 pages
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Heat cable sizing calculator: length and circuit
Chemelex's formula is pipe length x spiral ratio, plus 4 ft per globe valve, 2 ft per support, 1 ft per power connection. Its own 50 ft example needs 78.0 ft.