How Long a Water Heater Replacement Takes — and Why “Same-Day, No Permit” Is a Pitch, Not a Schedule

That middle row is why one homeowner can say a tankless went in fast and another can say it took all day, and both are telling the truth. When a tankless is replacing a tankless, the gas, vent, and condensate path is already there — so it's mostly swapping hardware, not building infrastructure, and the job moves fast. Andrew F. left us a review after we replaced his failed unit on short notice:

A tank swap looks simple from the driveway. It is not one motion. A standard tank runs on about 40,000 BTUs through a 1/2-inch gas line, and every one of these steps has to happen before you get hot water back:

Drain and disconnect. The old tank has to cool and empty before it comes out. A full tank is heavy and the sediment in the bottom does not drain fast.

The expansion tank. If your cold supply passes through a check valve, a pressure-reducing valve, or a backflow preventer, the code requires a thermal expansion control device on the water heater's cold-water supply, downstream of all of them (IPC 607.3). Heated water expands, and on a closed system it has nowhere to go. Skip this part and the temperature-and-pressure valve weeps as it fights the rising pressure.

The T&P discharge line. The relief valve's discharge pipe is not a place to freehand. The code spells out fourteen separate conditions it has to meet — it can't tie into the drainage system, it has to end at an air gap in the same room, and it can't neck down smaller than the valve's outlet (IPC 504.6). That is exacting, hands-on pipe work every single time.

The sediment trap. On a gas unit, the code requires a sediment trap at the appliance connection unless the appliance already has one built in (IFGC 408.4). It's a small drip leg that catches debris before it reaches the gas valve. It gets threaded and dope-sealed by hand.

The pan and drain, where it applies. If the heater sits where a leak would cause damage — an attic, an upstairs closet, over a finished ceiling — the code requires a drain pan at least 1 1/2 inches deep with a 3/4-inch indirect drain line run to a safe point (IPC 504.7.1). Running that drain adds real time.

None of those are padding. They are the difference between a unit that's hung on the wall and a replacement that will pass an inspection.

Converting a tank to a tankless is not a bigger version of the same job. It's three trades stacked on top of the swap, which is why a full conversion runs a full day. Each piece lives on its own page, so we'll point you there rather than repeat it:

The gas. A tankless can pull up to 199,000 BTUs to flash-heat water on demand. That's why a tankless needs its own dedicated gas trunk line, and why we size the trunk before we quote anything.

The vent. A condensing tankless is a Category IV appliance. It has to exhaust through its own listed sealed vent — Schedule 40 PVC or CPVC where local code accepts it — and the old metal flue or masonry chimney cannot be reused. Each unit gets its own intake and exhaust; it can't be common-vented with the furnace. New holes get cut for that venting.

The condensate. Because it pulls the heat out of its own exhaust, a condensing unit makes acidic condensate the whole time it runs. Per the manufacturer, a neutralizer kit lets that condensate flow through media that raises its pH, which helps prevent corrosion of the drain and the public sewer. That's a new drain line and a serviceable cartridge that weren't there before.

Gas, venting, condensate, water-line changes. That's the day. It's covered in depth on the tankless conversion page.

Some replacements take more time for real, code-driven reasons — not upsells:

Your gas line was never sized for a tankless. If it was run for a 40,000-BTU tank, it was never meant to carry 199,000 BTUs, and it has to be upsized before the new unit will run without starving.

The heater sits where a leak causes damage. An attic or an upstairs closet triggers the pan-and-indirect-drain requirement above, and that drain has to be routed somewhere legal.

There's a check valve, PRV, or backflow preventer on the supply. That triggers the expansion tank, and on an older home it's often not there yet.

You're relocating the unit. Moving it changes the gas run, the vent path, and the water lines, which is closer to a conversion than a swap.

A plumber who walks the actual install before quoting can tell you which of these apply to your house. A crew quoting a flat time over the phone is guessing — or promising to skip whatever gets in the way of the clock.

Speed and price get sold the same way, and the numbers tell on themselves.

We don't publish a flat figure here, because your house decides the job — the gas line you have, where the unit sits, whether it's a swap or a conversion. What we will tell you is what's in the number: the unit, the permit, the parts the code requires, and the inspection. Not a clock, and not a corner cut to beat one.

If someone quoted you a flat “same-day” time without looking at your gas line, your vent path, or where the heater sits, you don't have a schedule — you have a sales line. We'll tell you whether yours is a few-hour swap or a full-day conversion, and what the number actually includes, before we start.

Call or text us — 405-519-1868.