A water heater swap looks like a simple in-and-out until the inspector flags the missing expansion tank, the T&P discharge piped wrong, or the vent pitched the wrong way. This guide covers the full replacement the way it passes inspection: sizing, shutoffs, connections, and the safety details that actually matter.
Most water heater replacements are straightforward: same size, same location, same fuel. The work that fails inspections is never the tank — it is the five small code items around it. Expansion tank, T&P relief discharge, dielectric unions, vent pitch, and proper shutoffs. Nail those and the swap is routine.
This guide covers tank-style replacements. Tankless units are a different job with different venting, gas, and condensate requirements — do not treat them as interchangeable.
Gas-fired water heaters connect to a gas line, and gas piping is licensed-pro work in most jurisdictions — this guide does not cover gas line installation or modification. If the unit is gas-fired, have a licensed pro make the gas connection, and pull a permit where your jurisdiction requires one.
Water heater installs sit at the intersection of plumbing and fuel codes: expansion tanks (IPC 607.3), T&P discharge (IPC 504.6), and venting and combustion air per the fuel-gas code as adopted locally. Electrical connections on electric units follow NEC 422. References are to current model codes — check your local amendments, since enforcement details vary by jurisdiction.
Match capacity to the household: a 40-gallon tank suits most 2–3 person homes, 50 gallons for larger families, and check the first-hour rating rather than tank size alone for real performance. Match the fuel type and the venting style (atmospheric, power vent, or direct vent) to what is already there — changing venting type turns a swap into a project. Measure height, diameter, and clearances before the new tank leaves the store.
Turn off the fuel — gas valve to off, or the breaker locked off for electric — and close the cold-water inlet valve. Open a hot faucet somewhere in the house to break vacuum, attach a hose to the drain valve, and drain the tank fully. A 50-gallon tank holds over 400 pounds of water; trying to move it wet is how backs and floors get destroyed.
Disconnect the water lines, the vent connector, and — on gas units — have the licensed pro disconnect the gas line and remove the old drip leg assembly. On electric units, verify zero voltage with a meter before touching conductors, and note the wire and breaker size for the new unit. Cap the water lines temporarily so the open pipes do not drain the house lines onto the floor.
Set the new tank on a solid, level surface with a drain pan underneath where required (required under tanks in or above living spaces). Connect the cold inlet through a full-port ball valve and the hot outlet, using dielectric unions or brass fittings at the steel tank nipples to stop galvanic corrosion. Keep the first few feet of pipe serviceable — the next replacement happens in 8 to 12 years.
On any closed plumbing system — which is most of them, thanks to check valves and pressure-reducing valves — thermal expansion has nowhere to go without an expansion tank (IPC 607.3). Mount it on the cold side, supported properly (a full 2-gallon tank is heavy), and pre-charge the air side to match the house water pressure. An expansion tank charged to the wrong pressure is decoration, not protection.
The temperature-and-pressure relief valve is the last defense against a tank turning into a rocket. Pipe its discharge in an approved material, full size, pitched downward, terminating 6 inches above the floor or a drain — never capped, never threaded at the end, never reduced (IPC 504.6). Inspectors check this on every single install because getting it wrong has killed people.
On atmospheric gas units, the vent connector must pitch upward at least 1/4 inch per foot toward the chimney or vent — no sags, no dips, no long horizontal runs. A vent that sags collects condensate and spills flue gas into the room. Power-vent and direct-vent units follow the manufacturer's exact venting tables; do not improvise vent lengths or terminations.
Fill the tank completely with the hot faucet open until water runs steady — firing a gas burner or energizing elements on a dry tank destroys it in minutes. Purge air from the lines, check every connection for leaks, then restore fuel: relight per the manufacturer's instructions on gas, or close the breaker on electric. Verify hot water at the tap, confirm the burner or elements cycle off at setpoint, and check the T&P discharge for weeping.
Every one of these has cost someone a callback. Learn them here instead of on the job.
The most common inspection failure. If there is a check valve or PRV anywhere upstream, thermal expansion needs somewhere to go — that is the expansion tank's whole job.
Capped, threaded, reduced, or run uphill. Any of these defeats the relief valve. Pipe it full-size, downward, ending 6 inches off the floor.
Without dielectric separation, galvanic corrosion eats the joint in a few years. Brass fittings or dielectric unions cost a few dollars and add a decade.
Energizing electric elements or firing the burner on an empty tank — dry-firing — ruins the unit instantly. Fill and purge first, every time.
A sagging or level vent connector spills combustion gas. A quarter inch per foot upward, minimum, with no dips.
The water heater you haul to the curb is not just a tank. It carries brass drain valves, brass or copper flex connectors, sometimes a full copper stub-out, and on electric units, copper wiring and elements. Most plumbers strip the brass and copper off before the tank leaves the job — it takes five minutes with a wrench.
Keep a bucket on the truck for water-heater brass: valves, fittings, and flex lines. When it fills up, call Sonny at 312-880-8174. The tank goes to the curb; the brass goes in your pocket.
No minimums — every bucket counts. On bigger lots, we handle the freight so you don't lift a finger.
Work to code. Hire a licensed pro where required. This guide explains how the work is done — it does not replace training, apprenticeship, or a license.