Klein NCVT3P tester
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How to replace a water heater
The maker’s step order, the stops that keep you safe, and a checker for the extra parts your house needs.
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Quick answer
Kill the breaker and prove it’s off, cool and drain the tank, then disconnect and swap. A.O. Smith’s big rule: fill the new tank until hot taps run full for three minutes before power goes on.
Step one: prove the power is off$24.98 at AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
Check it on AmazonThree tools the steps call for
All three were new and in stock on Amazon on Oct 24, 2026. Parts are on a separate page.
Channellock 440X pliers
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RIDGID 152 cutter
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| Tool | Key specs | Pick it if | Skip it if |
|---|---|---|---|
| Klein NCVT3P tester | Two modes: 70–1000 V AC and 12–1000 V AC; tip LED and beeper; flashlight; two AAA batteries; shuts off after 4 minutes idle | You need to check the heater’s wiring is dead before you open the junction box, as A.O. Smith says to. | You already own a working tester you trust; test it on a live outlet first. |
| Channellock 440X pliers | 12.05 in. long, 2.32 in. jaw capacity, 12 push-button settings, reaming feature, made in USA | Your lines meet the heater at union nuts or flexible connectors with hex nuts. | Every connection is sweated copper you’ll cut out anyway. |
| RIDGID 152 cutter | Cuts 1/4 to 2-5/8 in. tube; fold-away reamer; tool-free wheel change; 2 lb; RIDGID lists a full lifetime warranty | You have copper lines with no union and need to cut close to the old heater. | Your lines are PEX; use a PEX cutter instead. |
Connectors, the expansion tank, the drain pan and the relief valve pipe are parts, not tools, and our water heater installation kit guide covers which ones your swap needs. For PEX lines, see PEX cutters. A.O. Smith says to use two or more people to remove or install a water heater and to use an appliance dolly or hand truck, so read water heater dollies before moving day.
What does your swap need beyond the heater?
Answer five questions. Every line it returns comes from A.O. Smith’s manuals or Amtrol’s tank instructions.
A.O. Smith calls a new heater the right time to bring the house up to current code, since codes have likely changed since the old one went in. My call: run this checker before you buy anything, because an expansion tank or a PRV is a lot easier to fit while the old heater is out and the water is off.
Check the house before you buy the heater
From A.O. Smith’s step 1 and step 2 checklists.
| Check | What A.O. Smith says | Why it matters |
|---|---|---|
| Water pressure | Most codes allow up to 80 psi; it recommends 50–60 psi working pressure | High pressure can cause premature leaks in the heater |
| How to measure it | Gauge on an outside faucet; read the highest pressure through the day | Pressure is often highest at night |
| Thermal expansion | Codes require a sized, pressurized expansion tank in almost all homes | Backflow devices make most systems closed |
| Location | Indoors, near the center of the plumbing, where it won’t freeze, set vertically | Shorter hot runs; no freeze damage |
| Floor | Must hold the weight of a full heater | A full tank is far heavier than the empty one |
| Garage | Add a vehicle stop | Protects the tank from a car |
| Attics and upper floors | Avoid where a leak could damage the structure | A.O. Smith says the tank will eventually leak |
| Earthquake areas | Strap it as local code requires; California requires bracing | Keeps the tank from moving |
A.O. Smith also says that if you’ve had to replace toilet valves, had a water heater leak or repaired appliances on the plumbing, pay particular attention to pressure. Our water pressure gauge guide covers the gauge, and water pressure spikes at night explains why one daytime reading can fool you.
How to remove the old water heater
A.O. Smith’s step 3, for an electric heater, in its order.
- 1Decide if it’s your job
Read every step first. A.O. Smith says go ahead only if you can safely do the work.
- 2Breaker off
Turn off the heater’s breaker or pull its fuses.
- 3Prove it’s dead
Remove the junction box cover and check the wires with a non-contact tester.
- 4Disconnect the wires
Only after the tester shows no voltage.
- 5Run the hot water cool
Open a hot tap until the water runs cool; A.O. Smith says this can take 10 minutes or longer.
- 6Hook up a hose
Garden hose on the drain valve, other end to a drain, outside or a bucket.
- 7Cold supply off
Close the cold water valve to the heater.
- 8Drain it
Open the drain valve, and a hot tap to let air in so it drains faster.
- 9Off with the T&P pipe
Disconnect the relief valve discharge pipe. You may reuse the pipe, never the old valve.
- 10Disconnect the water lines
Unions come apart with wrenches. If you must cut, cut close to the heater and leave the pipes long.
- 11Move it out
Two or more people, per A.O. Smith.
A.O. Smith warns that sediment in the bottom of the tank can clog the drain valve. If the old tank won’t drain, it says to contact a qualified person.
My call: once the breaker is off, the hot water has run cool and the cold supply valve is closed, in A.O. Smith’s order (it warns that hot water can scald), open the drain valve before you disconnect the relief pipe or the water lines. You find out early whether the tank will drain or you need help.
What a non-contact tester can and can’t tell you
From Klein’s NCVT3P instructions, English section, Rev E.
Test it first
Klein says check it on a known live circuit before each use.
Use Mode 1 here
Klein suggests Mode 1 (70–1000 V AC) when you expect over 70 V, as on a 240 V heater circuit.
Silence isn’t proof
Klein says voltage could still be present if the tester shows nothing.
Things that hide voltage
Shielded wire, wire in grounded metal conduit, gloves between your hand and the tester, or holding it too far away.
Klein says detection is specified with the tip within 0.25 in. of the wire, held still in a bare hand, while you stand on or touch earth ground. The tip glows green when it’s on and nothing is detected. My call: test a live outlet, test the heater wires, then test the outlet again, so a dead battery can’t fool you halfway through.
Setting the new heater in its pan
A.O. Smith’s step 4, items 2 to 4.
| Rule | A.O. Smith’s number or check |
|---|---|
| Drain pan width | At least 2 in. wider than the heater’s diameter |
| Water depth in the pan | Limited to 1-3/4 in. at most |
| Pan drain | Piped to an adequate drain; most codes require it |
| T&P valve | Not touching any electrical parts |
| T&P discharge | Room to run the pipe to its own drain, not into the pan unless your code allows it |
| Service space | Room around the heater for future maintenance |
| Level | Heater stands vertically |
Set the heater gently so it doesn’t crack or dent the pan. Our water heater drain pan guide covers pan sizes and drain fittings. My call: turn the heater so the drain valve, the T&P valve and the junction box all face where you’ll stand to service them, before the lines go on.
The relief valve and its discharge pipe
A.O. Smith’s step 5. Most valves come installed; some ship in the carton.
| Rule | What A.O. Smith says |
|---|---|
| The valve | Use the new one that came with the heater; never reuse the old one |
| Pipe size | At least 3/4 in. inside diameter, sloped to drain fully |
| Pipe material | Copper or CPVC only, rated to 250°F; no PVC, iron, flexible plastic or hose |
| Where it ends | No more than 6 in. above a floor drain, or outside the building |
| Cold climates | End it indoors at a drain, since outside drains can freeze |
| Nothing in the way | No valve, cap, plug or reducer anywhere in the line |
A.O. Smith’s warning here is explosion, so this is the one step not to shortcut. Our T&P relief valve sizes guide covers replacement valves for later. My call: check the end of the discharge pipe last, with a tape measure; the 6 in. limit is easy to miss when you cut the pipe to fit.
Shutoff and mixing valves
A.O. Smith’s step 6.
If there’s no shutoff on the cold line to the heater, A.O. Smith says add one near the heater where you can reach it, and use only full-flow ball or gate valves made for drinking water; other valves can choke the flow.
It also says to put a thermostatic mixing valve at each point of use, since even a low tank setting can scald. If the heater is fed by solar or another preheater, it wants a mixing valve on the inlet to hold the incoming water to 120°F. Our thermostatic mixing valve guide explains how they work.
Connecting the water lines
A.O. Smith’s step 7. Both connections are 3/4 in. NPT.
| Your pipe | How A.O. Smith says to connect it |
|---|---|
| Copper | Solder a short pipe (about a foot) to a threaded adapter first, thread it onto the heater with sealant, then solder the house lines while wet rags keep the heater fittings cool. Or use compression fittings |
| PEX or CPVC | Fittings made for that pipe |
| Iron or PVC | Not suitable for potable water; don’t use them |
| Any pipe | A union makes the next swap easier; a dielectric union helps prevent corrosion |
| Thread sealant | Tape or pipe joint compound approved for potable water |
| Solder | 95/5 tin-antimony or equal; no lead-based solder |
Never solder while pipe is attached to the heater: A.O. Smith says the inlet and outlet contain non-metallic parts, and most models have heat traps in them that you shouldn’t remove. Double check that hot goes to hot and cold to cold. Insulate the lines, or add heat tape if the space can freeze.
On union nuts and connector nuts, use the Channellock 440X or an adjustable wrench, not a pipe wrench. RIDGID’s Pipe Wrench Use Guide says not to use its wrenches on hex or other non-round material. A pipe wrench is the right tool only on round threaded pipe, like a galvanized nipple.
If the house has a PRV over 80 psi or no expansion tank, this is the step where A.O. Smith says to fix both.
Fill the tank before power goes on
A.O. Smith’s step 8. Its manual calls powering a tank that isn’t full a common installation mistake.
- 1Pull an aerator
Remove the aerator on the nearest hot tap, so debris can wash out.
- 2Cold supply on
Open the cold valve to the heater all the way.
- 3Open a hot tap
Let it run until it flows in a full stream.
- 4Three full minutes
Keep it running full stream for three minutes.
- 5Close and check
Close the tap, put the aerator back, dry every joint, and fix any drip.
A.O. Smith calls power on a part-full tank “dry firing”: the upper element burns out, and that’s an installation error its warranty doesn’t cover. It also says almost all leaks are at connections, not the tank. If an element does burn out, our element replacement guide shows the fix.
Making the electrical connections
A.O. Smith’s step 9, after the tank is full.
- 1Breaker still off
Check again with the tester.
- 2Read the data plate
Your voltage, wire size and breaker rating and type must match the heater.
- 3Open the junction box
On top of the heater.
- 4Conduit and strain relief
Conduit if your code wants it; a UL listed or CSA approved strain relief either way.
- 5Ground, then power
Ground wire to the green screw; the two power wires joined with wire nuts or another approved method.
- 6Cover on
Secure the cover with its screws before power goes on.
A.O. Smith points to the NEC (NFPA 70) where there’s no local code. My call: if the old heater was a different wattage or the data plate asks for a different breaker than you have, stop and get an electrician; that’s a circuit change, not a swap. See 4500 vs 5500 watt elements for what wattage changes.
Setting the temperature
A.O. Smith’s step 10.
| Setting | What A.O. Smith says |
|---|---|
| Factory setting | About 120°F, to reduce the risk of scalds |
| Higher setting | More hot water and better for dishwashers and laundry |
| 140°F | Kills the bacteria behind “smelly water” |
| Any setting | Hot water can scald even at 120°F |
| If you go higher | Add thermostatic mixing valves at each point of use |
Amtrol’s sizing chart for its in-line tanks changes with the heater’s top setting (140°F, 160°F or 180°F), so if you raise the temperature later, recheck that your expansion tank is still big enough. My call: leave it at the factory setting for the first week, then adjust only if you run out.
What’s different with a gas heater
From A.O. Smith’s gas water heater manual 100366731.
| Step | What the gas manual says |
|---|---|
| Before work | If you use the main gas shutoff, also turn off every gas appliance until the install is done |
| Gas line | Sized per the National Fuel Gas Code (NFPA 54) and your gas supplier |
| At the heater | A manual shutoff, a sediment trap ahead of the gas valve, and a flexible connector or ground joint union |
| Joint sealant | Compound or tape marked as resistant to gas |
| Leak test | Soapy water on every joint before lighting; never a match or flame |
| Pressure tests | Disconnect or isolate the heater when the gas piping is tested |
| Filling | Water must flow from a hot tap before the gas goes on |
| Venting | Install the vent per code; it warns breathing carbon monoxide can cause brain damage or death |
The water side is the same as the electric steps above. My call: if you’ve never made up a gas joint or the new heater’s vent doesn’t line up with the old flue, hire that part out; a gas leak or a bad vent is a much bigger risk than a dripping union. Our gas shut-off valve guide covers the valve at the heater.
Mistakes A.O. Smith warns about
Each one is from A.O. Smith’s manual.
| Mistake | What happens | Avoid it by |
|---|---|---|
| Power on before the tank is full | Upper element burns out; not covered | Hot taps full stream for three minutes first |
| Reusing the old T&P valve | Against A.O. Smith’s instructions | Use the new valve that came with the heater |
| Discharge pipe into the pan or capped | Breaks A.O. Smith’s discharge rules | Its own line to a drain, nothing in it |
| Soldering at the heater | Heater’s non-metallic parts damaged | Solder adapters first, wet rags at the heater |
| No expansion tank on a closed system | Pressure spikes as water heats | Tank on the cold line, precharged to house pressure |
| Draining hot | Scald risk | Run hot water until it’s cool first |
A.O. Smith says a thermal expansion tank also protects the washer, dishwasher, ice maker and toilet valves, and that a toilet that runs briefly at night for no reason can be a sign of thermal expansion. Our expansion tank guide covers sizing and the precharge.
Questions people ask
Can I replace a water heater myself?
A.O. Smith says only if you can safely do the work; if not, have a qualified person do it.
How long does it take to drain a water heater?
A.O. Smith says letting the hot water run cool first can take 10 minutes or longer; draining time depends on sediment.
Why fill the tank before turning on power?
Power on a part-full tank burns out the upper element, which A.O. Smith calls dry firing.
How long should water run before I turn the power on?
Full stream from a hot tap for three full minutes, per A.O. Smith.
Can I reuse the old T&P valve?
No. A.O. Smith says use the new valve that comes with the heater.
Can I reuse the old discharge pipe?
A.O. Smith says you may be able to, if it meets the rules: 3/4 in., copper or CPVC, ending within 6 in. of a drain.
Do I need an expansion tank?
A.O. Smith says codes require one in almost all homes, because backflow devices make the system closed.
What size are water heater connections?
3/4 in. NPT on the A.O. Smith electric models in its guide.
Can I solder pipe to the water heater?
Not while it’s attached; solder adapters first and keep the heater fittings cool.
Do I need a drain pan?
A.O. Smith says most codes require one, at least 2 in. wider than the heater and piped to a drain.
What temperature should I set it to?
A.O. Smith ships at about 120°F to cut scald risk.
Should I use a pipe wrench on the union?
No. RIDGID says not to use its pipe wrenches on hex nuts; use pliers or an adjustable wrench.
What does a non-contact tester miss?
Klein says shielded wire, metal conduit and gloves can all hide voltage.
Is a gas water heater harder to replace?
It adds a gas connection, a leak test and venting, per A.O. Smith’s gas manual.
What water pressure is too high?
Most codes allow 80 psi; A.O. Smith recommends 50–60 psi.
How we picked these
Why trust me
I’m Isaac Daniels, a licensed journeyman plumber in Georgia (license #JP507711), and I’ve used all three tools here. The steps are A.O. Smith’s, in its order; tool limits are from Klein’s, RIDGID’s and Channellock’s own documents. My own advice is labeled as my call.
How I pick
- Only tools the maker’s steps call for: a tester, something for union nuts, something to cut copper.
- Three different kinds of tool, so nothing is scored against anything else.
- Each new and in stock on Amazon on Oct 24, 2026, checked through Amazon’s product data.
- Parts like connectors and expansion tanks are covered on our installation kit page, not here.
Sources
- A.O. Smith, Residential Electric Water Heater Installation Instructions and Use & Care Guide (March 2021, 100341465): getting started, installation steps 1–10.
- A.O. Smith, gas water heater manual 100366731 (assets.hotwater.com): gas supply, filling and venting sections.
- Klein Tools, NCVT3P instructions, drawing 1390427 Rev E, English section.
- RIDGID, quick-acting tubing cutters page (ridgid.com), read Oct 21, 2026; RIDGID Pipe Wrench Use Guide.
- Channellock, 440X SpeedGrip product page and technical details (channellock.com), read Oct 24, 2026.
- Amtrol, Therm-X-Trol thermal expansion tank instructions 9015-942 (models ST-5 through ST-210V).
- Amazon listing text and photos for B08DQMX7YF, B07R11V4X3 and B0009W2T3A, read through Amazon’s product data, Oct 24, 2026.
- Search volume for “how to replace a water heater” from Semrush, Oct 24, 2026.