How to Replace an Atmospheric Gas Tank Water Heater

Why this matters

An atmospheric gas heater swap looks like the simplest job on the truck, and it is the one most likely to leave the customer with a hazard nobody can see: a vent connector that spills flue gas into the basement, a gas union that weeps, or a closed system with no expansion tank that starts the relief valve dripping a month later. Every one of those passes the "it makes hot water" test. This HowTo is the technical sequence from shutting the old heater down to proving the new one drafts. The day's logistics (permit, haul-away, customer walkthrough) are owned by Water Heater Changeout: Day of Install; the paperwork by The Water Heater Permit and Inspection SOP.

Gas work here is done only where your plumbing license covers gas appliances, which varies by state and locality. Where it does not, the gas connection and lighting go to the licensed gas fitter.

Before you start

Have on hand: the new heater with its installation instructions (they are part of the listing), dielectric unions or nipples, a ball valve if the cold inlet has a gate or globe valve, an expansion tank and a tire gauge if the system is closed, pipe and fittings for a new sediment trap and union, connector parts, sheet metal screws, leak-check solution or an electronic gas detector, a smoke pencil, a personal CO monitor, a drain hose, a hand truck, and a pressure gauge on a hose thread.

Check before touching anything:

• Gas odor. If you smell gas on arrival, everyone leaves the house, nobody operates a switch, a light or a phone inside, and the call to the gas utility is made from outside. The swap waits.
• The existing venting. Read it against Atmospheric Water Heater Vent Connector Rules: rise, clearance, screws, entry height on a shared vent. Problems found now are priced now, not discovered after the new tank is in.
• Static water pressure. Thread the gauge onto a hose bib with no water running. Over 80 psi means a PRV conversation first. Write the number down; it is also the expansion tank precharge.
• Closed or open system. A check valve or PRV at the meter makes the system closed and needs an expansion tank; see Expansion Tank Sizing for Closed-Loop Domestic Water Heaters.
• Combustion air and clearances for the room, per Combustion Air Requirements Reference.
• Old connector joints sealed with white paper tape. On older installs that tape can contain asbestos, which releases fibers when torn or sanded. Do not disturb it; leave the section intact and handle it under your shop's asbestos procedure, or by an abatement contractor where your state requires one.

The finished connections

 COLD SUPPLY   HOT TO      VENT CONNECTOR, single
    |          HOUSE       wall, rising to B-vent
 [BALL] cold      |               /
    |  shutoff    |              /
    +-[EXP TANK]  |       +-DRAFT HOOD-+
    |             |       +-----+------+
 [DIEL]        [DIEL]  <- dielectric unions
    |             |
 +--+-------------+-----------+
 |  cold in      hot out [T&P]+-> discharge pipe,
 |                            |   full size, down
 |      40 GAL ATMOSPHERIC    |   to its approved
 |      GAS TANK              |   termination
 |                            |          GAS DROP
 |                     [GAS   |              |
 |                      CTRL]-+-[UNION]--+ [SHUTOFF]
 |                            |          |   |
 |                            |          +-[TEE]
 +----------------------------+              |
   [drain valve]                         [CAP] sediment
                                          trap nipple

The procedure

1. Shut the gas off at the appliance shutoff and turn the gas control to OFF. The burner must not try to light on an empty or partly drained tank, and the line must be dead before the union is broken. Skip it and you open a live gas line. Leave the house gas on so other appliances keep running.

2. Shut the cold supply valve and open a hot faucet upstairs. The open tap breaks the vacuum so the tank drains. If the cold valve does not fully close (water keeps coming from the hot tap), go to the main; see Valves on a Residential Water System and Where Each Belongs.

3. Drain the tank to a floor drain or outside. The water is at the heater's set temperature: route the hose to the drain before opening the valve, keep hands off the hose end, and keep children and pets out of the path. A full 40 gallon tank weighs well over 300 lb; nobody moves that. If sediment plugs the drain valve, open the cold valve briefly to stir it.

4. Disconnect the gas. Break the union, cap or plug the open line, and remove the old sediment trap if it will be rebuilt. A capped line cannot leak while the tank is out.

5. Disconnect the vent connector and the water lines. Pull the screws at the draft hood and lift the connector off. Cut the water lines above the old dielectric fittings, which are usually the corroded part; reusing them puts the oldest pipe back on the newest tank.

6. Move the old tank out and set the new one. An empty tank is still heavy and awkward: two people, a hand truck and straps, lifting with the legs. Set the new heater level, in a pan where the location needs one (see Water Heater Placement: Pans, Elevation and Access), with the clearances on its label, the draft hood relief opening clear, and seismic straps where your jurisdiction requires them.

7. Make the water connections with dielectric transitions. Copper joined directly to the steel tank nipple corrodes at the joint. Solder the copper side before threading anything onto the tank so torch heat never reaches the tank's lined nipples; the soldering steps are owned by How to Sweat a Copper Water Joint That Passes First Time. Lead-free solder and flux on potable water.

8. Install the expansion tank if the system is closed. Set its precharge with the tire gauge to the static pressure you measured, before it sees water. A tank left at factory precharge on a higher-pressure house is partly full of water before it starts and cannot absorb expansion. Full steps: How to Install a Thermal Expansion Tank and Set Its Precharge.

9. Install the T&P valve if it is not factory installed, then the discharge pipe. Full size, no valve, no trap, rated for the temperature, terminating where the adopted plumbing code (IPC or UPC, as adopted) allows. Owned by The T&P Discharge Pipe and the Rules Other Trades Break.

10. Make the gas connection. From the appliance shutoff, a tee with a capped nipple below it as the sediment trap, the branch to a union, then the gas control. The fuel gas code (IFGC or NFPA 54, as adopted) requires, in substance, a sediment trap ahead of the heater's gas control; it catches scale and moisture before they reach the control; the union lets the next plumber pull the heater without unthreading the drop. Gas-rated dope or tape on male threads only, and a backup wrench on the control so its body is not twisted in the tank.

11. Install the vent connector. Rising at least 1/4 in per foot, three screws per joint, the clearance to combustibles its type requires, and the smaller connector entering higher on a shared vent, per Atmospheric Water Heater Vent Connector Rules.

12. Fill the tank before lighting. Close the drain valve, open the cold supply, and leave the upstairs hot tap open until water runs without spitting air. Lighting an empty or part-full tank overheats the bottom and can ruin it in minutes. Check every water joint at full pressure.

13. Turn the gas on and leak check every joint you touched. Open the appliance shutoff and put leak-check solution on the union, the tee, the trap cap and the gas control inlet, or sweep them with an electronic detector. Growing bubbles mean a leak: gas off, remake the joint, retest. Never use a flame. A new or extended gas line, not just a reconnection, gets a full pressure test per Gas Line Pressure Test + Leak Detection SOP.

14. Light the heater per the label on the tank and set the control to the customer's setting, commonly 120 F. Wear the CO monitor from here on; if it alarms, shut the gas control off and ventilate before going further.

15. Run the spillage test. Close exterior doors and windows, run every exhaust fan and the dryer, close the heater room door if there is one, and let the heater fire for the warm-up period in its instructions. Hold a smoke pencil at the draft hood relief opening: the smoke must be drawn in. Repeat with the furnace firing if it shares the vent.

Finish test

Done means each of these was observed, not assumed: no bubbles at any gas joint you made; no drips at any water joint at full pressure; the expansion tank precharge equals the measured static pressure; the T&P discharge runs full size to its termination; the burner lights and holds; and smoke at the draft hood is drawn in after the maker's warm-up period under worst-case depressurization. A heater that spills fails, even if it spills only with the dryer running.

Worked example: a 40 gallon swap on a closed system

A 12 year old 40 gallon atmospheric heater leaking from the tank. 3/4 in copper water, 1/2 in black iron gas drop, sharing a B-vent with an 80 percent furnace. The meter has a dual check valve, so the system is closed.

Pre-checks. Static pressure at the hose bib: 62 psi, under the 80 psi limit, so no PRV work. Precharge target: 62 psi. The existing connector entered the common vent below the furnace connector, a retrofit error from when the furnace was replaced. That was priced and approved before starting.

Swap. Gas off at the appliance shutoff, cold off, drained to the floor drain (about 45 minutes; the drain valve clogged once and cleared on a cold-water stir). The old dielectric nipple on the cold side was corroded through. New dielectric unions, a new 3/4 in full-port cold ball valve, and a 2 gallon expansion tank raised from its factory 40 psi to 62 psi with the tire gauge before it went on.

Gas and vent. New tee, 3 in capped nipple as the trap, new union. The 3 in connector was re-piped into a tee above the furnace connector, rising 9 in over 4 ft against the 1 in minimum, three screws per joint.

Start-up. Filled until the upstairs hot tap ran solid; no drips at 62 psi. Gas on, solution on four joints: the union showed slow bubbles, was remade and retested clean. Lit per the label, set at 120 F.

Spillage test. Doors closed, bath fan, kitchen hood and dryer running. After the warm-up period in the heater's instructions, smoke at the relief opening drew in with the heater alone and again with the furnace firing. The CO monitor read zero throughout.

What skipping two steps would have cost. Without the expansion tank, the first long heating cycle pushes house pressure up behind the check valve until the T&P lifts, which is the callback in The Relief Valve That Started Dripping After the Meter Change. With the connector left entering below the furnace, the heater might have passed on a mild day and spilled on the first cold morning the furnace ran long.

References

• IFGC 2021 or NFPA 54, as adopted, for appliance connection, sediment trap and venting
• IPC 2021 or UPC 2021, as adopted, for water heater installation and relief valve discharge
• Water heater manufacturer's installation instructions, part of the listing
• See related: Water Heater Changeout: Day of Install
• See related: Gas Line Pressure Test + Leak Detection SOP