A tankless water heater change-out sounds like the simplest job in plumbing: old unit off the wall, new unit on the wall, hot water by lunch. Sometimes it really is that clean. Usually, though, the swap is the easy part — it's the gas and water connections around the unit that decide how the day goes.
We filmed one of our Tampa crews doing a complete tankless change-out, start to finish, and this post walks through exactly what the job involved. If you're researching a replacement, this is the honest version.
Step One: Water Off, Then the Old Unit Comes Down
Every good plumbing job starts the same way: the water gets shut off first. On this job, the old tankless — a Noritz — was coming out, and the gas and water lines both needed reconfiguring to fit the new unit.
While one tech shut the water down, the other prepped the new unit's fittings with Teflon tape on the cold, hot, and gas inlets. Prepping the new unit while the old one comes down is how a change-out stays on schedule.
The Part Nobody Warns You About: Seized Gas Fittings
Here's the moment that separates a smooth change-out from a frustrating one. The old unit's gas connector had a compression joint on the flared end that was so tight it simply would not break loose. Not with reasonable force, not at all.
So the crew cut the gas connector on the water heater side and removed the old unit with it attached. That's not a failure — on older installs, seized fittings are a normal reality, and cutting the connector is often the cleanest way through it. Once the old unit was off the wall, the old flare connection came off the gas line and a brand new gas shut-off valve (a gas cock) went in, so the new unit would have a fresh, reliable way to shut its gas off.
The lesson for homeowners: when a quote for a "simple swap" includes time for the gas line, this is why. You can't know a fitting is seized until you put a wrench on it.
Prepping the New Unit: Isolation and Descaling Valves
Before the new heater ever went up on the wall, it got fitted with a tankless valve kit — isolation valves that double as descaling (service) valves — plus the drip tee for the gas line.
The isolation valves are the part we'd point to if you asked what a "done right" install looks like. They let the unit be shut off from the house's water on both sides, which is what makes the heater serviceable: it can be isolated and descaled in place during routine maintenance instead of being pulled apart. Building that service access in at install time is far smarter than discovering you need it years later.
Hanging the Unit and Hard-Piping the Gas
The new unit didn't go back in exactly the old one's spot — it shifted a little to the left. The reason was the gas: this install hard-pipes the gas line in rigid pipe rather than reconnecting a flexible connector, and hard-piping needs a little more room to lay out properly.
Once positioned, the unit was leveled, set on its mounting screws — three across the top, more along the bottom — and secured. Then came the piping: a union fitted between the elbow and the drip tee on the gas side, and the hot, cold, and TMP (temperature and pressure relief) lines run. With the piping done, the unit was wired up and ready to be programmed.
Hard-piping is worth a sentence of explanation. It takes more layout work than a flex connector, but it gives you a rigid, permanent gas run — and on this job it let the crew retire that seized connector for good.
Leak-Test Both Sides Before First Fire
This is the step we'd never skip, and it's the last thing that happens before the new heater is allowed to make hot water:
- Water on first. Turn the water back on, let the unit fully pressurize, and check every water connection for leaks.
- Gas on second. Open the new gas shut-off and test the gas piping for leaks.
- Only then, fire it up. Power on, program the unit, and confirm it runs.
On this install, the maiden voyage went the way you want: fully pressurized, no leaks anywhere, gas piping tested, everything in order. That's the standard a change-out should be held to — tested on both the water side and the gas side before anyone calls it done.
Thinking About Your Own Tankless Replacement?
If your tankless is aging or inconsistent, our water heater installation page covers how we handle replacements, and if your existing unit still has life in it, water heater repair may be the smarter call. We serve homeowners across the area — see our Tampa plumber page.
Frequently Asked Questions
Can I buy my own tankless water heater and have you install it? Yes. The change-out in this video was exactly that — the homeowner had already purchased the new unit, and our crew handled the removal, the gas and water reconfiguration, and the install. One thing to know going in: even with the unit supplied, the connections around it may need work, like the seized gas connector we had to cut on this job.
Why would the gas line need to be reconfigured on a replacement? Because the new unit rarely lands in exactly the same spot or orientation as the old one. On this install, the heater shifted slightly to give the hard-piped gas line room to lay out correctly, and the old unit's seized flare fitting had to come off and be replaced with a new gas shut-off valve. Gas reconfiguration is a normal part of a change-out, not a sign something went wrong.
What are isolation/descaling valves, and why install them? They're a valve kit fitted to the new unit before it goes on the wall. The isolation valves let the heater be shut off from the house's water on both sides, and the service ports are what allow the unit to be descaled in place as part of routine maintenance. Installing them during the change-out means future service is straightforward instead of a teardown.
How do you make sure the new heater is safe to run? By testing before firing. We turn the water on and check every connection under full pressure, then turn the gas on and test the gas piping for leaks. The unit only gets powered up, programmed, and fired after both sides pass. On this job, everything tested clean — no leaks on water or gas.
Ready to Talk About Your Water Heater?
Whether it's a change-out like this one or you're not sure yet whether repair or replacement makes sense, we'll give you a straight answer and a firm quote. Call Sewer Studs Plumbing & Drain at (813) 680-2284, Monday through Friday, 8 AM to 5 PM.
