Fixtures, water heaters and service review

Fixtures and water heaters together are about 11 percent of the F25 outline, and the water service rules sit in the water supply chapter, about 13 percent more. These are the topics that apprentices work with every week, which makes them dangerous: familiarity makes candidates read quickly and answer from habit. Luis set Maya a rule for this review. "For every question, find the rule in the book before you look at the options. If you can't find it, you're answering from habit."

This lesson reviews fixtures, traps and their requirements; water temperature limits and mixing; water heater sizing, recovery and safety devices; water services; and water treatment. Each section ends with the kind of calculation or rule the exam asks about.

Fixtures and their requirements

The code sets general requirements for every fixture: smooth, non-absorbent surfaces, an overflow where one is required, a trap on every fixture except those with an integral trap, and accessible connections. It also sets how many fixtures a building needs, by occupancy and number of people, which is a building code question as much as a plumbing one. Exam questions on fixture counts give the occupancy table or extract and test whether the candidate reads the occupant load and the ratio correctly.

The rounding rule catches many candidates. When an occupancy table gives one water closet per 25 people and a floor has 60 people, the answer is three, not two. Fixture counts round up, because a fraction of a fixture cannot be installed and the requirement is a minimum.

Traps

Every trap must have a water seal deep enough to resist the pressure swings in a vented drainage system, but not so deep that solids collect. Codes commonly require a seal of at least 2 in and no more than 4 in. The trap must be the right size for the fixture, and it must not be smaller than the fixture outlet. Traps that depend on moving parts, interior partitions or concealed internal parts are generally prohibited, as are S-traps and crown-vented traps, because they lose their seal or clog.

Trap seal loss has a small number of causes, and a question that describes a symptom is asking for one of them: siphonage, from a fixture drain that is too long, too steep or unvented; back pressure, from a stack without enough venting; evaporation, in a fixture that is rarely used; capillary action, from a strand of hair or a rag hanging over the weir; and momentum, from a sudden large discharge, such as a bucket of water poured into a floor drain.

Temperature limits and mixing

Hot water must be stored hot enough to control the growth of Legionella bacteria, commonly about 140 °F, but delivered cool enough not to scald. The IPC requires shower and tub-shower valves to limit the water temperature to a maximum of 120 °F, using a pressure-balancing, thermostatic or combination valve, and requires a limiting device for bathtub and whirlpool outlets. A scald at 140 °F can happen in a few seconds; at 120 °F it takes several minutes.

Mixing calculations come up often. When hot and cold water mix, the mixed temperature is the weighted average of the two, weighted by flow. Rearranged, the equation gives the fraction of hot water in the mix.

Water heater sizing and recovery

A storage water heater's useful output in a busy hour is roughly the hot water stored, about 70 percent of the tank is usable before the outlet temperature drops, plus the water it can heat during the hour, its recovery. Recovery depends on the input, the efficiency and the temperature rise. The heat needed to raise water is the mass times the specific heat times the rise, so a gallon raised by one degree Fahrenheit needs about 8.33 Btu.

Electric heaters recover more slowly, because their elements are usually smaller in input, but they are nearly 100 percent efficient at the tank. Tankless heaters have no storage and are sized by the flow and the temperature rise at the coldest inlet temperature, which is why a tankless unit that works in summer can fall short in January.

Water heater safety devices

Every storage water heater needs a temperature and pressure relief valve, with the temperature sensor in the top portion of the tank where the water is hottest. The valve is sized to relieve at least the heater's input, and set at or below the tank's rated working pressure. Its discharge pipe must be the full size of the valve outlet, must not be reduced, trapped or valved, must drain by gravity, and must terminate where discharge is visible and will not scald anyone, not more than 6 in above the floor or to an approved receptor. Every one of those words has appeared in an exam question.

Thermal expansion is the second safety topic. When a heater warms water in a closed system, one with a check valve, backflow preventer or pressure-reducing valve on the service, the expanding water has nowhere to go. Pressure rises until the relief valve drips. The fix is a thermal expansion tank on the cold supply, sized for the heater volume and the system pressure, not a new relief valve. A dripping relief valve on a heater in a building with a new meter and check valve is a classic exam scenario.

• Seismic straps or other restraint where the code requires them.
• A drain pan with a drain where a leak would damage the building, such as a heater above a finished ceiling.
• Protection of ignition sources from flammable vapors, commonly by raising the burner in a garage or using a flammable-vapor-ignition-resistant heater.
• Combustion air and venting for gas heaters, as in lesson 7.

Water services

The water service runs from the main or the meter to the building. Its size comes from the demand and the pressure budget in lesson 5, with a minimum size set by the code, commonly 3/4 in. It must be buried below the depth of frost penetration, protected where it passes through foundations, separated from building sewers by the distance or arrangement the code requires, and made of materials rated for the pressure and for potable water. Where the service enters the building, a shutoff valve and, where required, a pressure-reducing valve and backflow preventer are installed.

Exam questions about services often test the separation rule. When a water service and a building sewer share a trench, the code requires either a minimum horizontal distance between them or that the water pipe sits on a shelf at a minimum height above the sewer, or that the sewer is made of pressure-rated pipe. Reading the full rule, not just the first distance, is the point.

Water treatment

Treatment equipment appears in exam questions about softeners, filters, disinfection and reverse osmosis. Hardness is measured in grains per gallon or in milligrams per liter as calcium carbonate, and one grain per gallon equals about 17.1 mg/L. A softener exchanges calcium and magnesium for sodium or potassium, and is sized by the hardness, the daily water use and the capacity of its resin between regenerations. Its regeneration discharge goes to the drain through an air gap.

Grains are a United States unit of mass, and many softeners sold in Canada are still rated in them, which is why the conversion is worth knowing.

A review calculator

# Fixtures, heaters and services: mixing, recovery, softener
t_hot, t_cold, t_mix, flow, minutes = 140, 50, 105, 2.0, 10  # F, F, F, gpm, min
f = (t_mix - t_cold) / (t_hot - t_cold)
print(f"Shower: hot fraction {f:.2f}, hot water used {f * flow * minutes:.1f} gal")

input_btu, eff, tank = 40000, 0.80, 50
recovery = eff * input_btu / (8.33 * (t_hot - t_cold))
print(f"Heater: recovery {recovery:.1f} gal/h, first hour about {0.7 * tank + recovery:.1f} gal")
showers = (0.7 * tank + recovery) / (f * flow * minutes)
print(f"        enough for about {showers:.1f} ten-minute showers in the first hour")

use_gal, hard_gpg = 180, 15
grains = hard_gpg * use_gal
print(f"Softener: {grains:.0f} grains a day; 32,000-grain bed lasts {32000 / grains:.1f} days")

Shower: hot fraction 0.61, hot water used 12.2 gal
Heater: recovery 42.7 gal/h, first hour about 77.7 gal
        enough for about 6.4 ten-minute showers in the first hour
Softener: 2700 grains a day; 32,000-grain bed lasts 11.9 days