The kitchen moment that makes you want an upgrade
If you’ve ever started the dishwasher, rinsed a pan, and then watched the hot water fade to lukewarm halfway through, you’ve felt the “limited tank” problem in real time. In many San Antonio, TX homes, that frustration is paired with hard water that leaves white crust on faucets and showerheads. A tankless water heater is designed to fix the first issue by heating water only when you need it, but it also introduces a few new “gotchas” that homeowners don’t always see coming. Let’s break down how they work in plain English, and what actually matters when you live with one every day.
What a tankless water heater actually is
A tankless water heater is an on-demand appliance that heats water as it flows through the unit, instead of storing hot water in a big tank. The heart of the system is a heat exchanger (a metal coil that transfers heat to water passing through it). When you open a hot-water tap, a flow sensor (a small meter inside the heater) detects moving water and tells the unit to start heating.
In a gas model, the heater ignites a burner and adjusts the flame. In an electric model, it energizes heating elements. Most modern units are modulating, meaning they can ramp up or down to match the amount of hot water you’re using. That’s why a tankless can feel “endless” compared to a tank: it isn’t running out of stored hot water, it’s making hot water continuously while you draw it.
The tradeoff is that your hot water now depends on the heater responding quickly, having enough fuel or electrical power available, and staying free of mineral buildup.
Why people love tankless, and why some people regret it
Tankless water heaters can be fantastic, but the regrets usually come from sizing, setup, or maintenance, not from the concept itself.
One common surprise is the slight delay before hot water arrives. Because the unit only heats when water starts moving, you might still wait a bit at the faucet, especially if the heater is far from the kitchen. Another frustration is the “cold water sandwich,” where the water turns warm, then briefly cool, then warms again during back-to-back uses (often caused by quick on/off draws).
Undersizing is the biggest mistake. Tankless units are rated by flow, usually in gallons per minute (GPM). If the house tries to run a shower, a load of laundry, and a sink at the same time, the heater may not keep up with the temperature rise you want. And if the gas line, venting, or electrical supply isn’t matched to the new heater’s needs, you can get nuisance shutoffs, error codes, or weak performance that feels like a dud, even when the unit itself is fine.
Finally, hard-water scale can quietly choke off efficiency. In areas where mineral content is high, limescale can coat the heat exchanger and force the heater to work harder, which affects hot-water consistency and long-term reliability.
A quick checklist before you buy or replace
- Confirm how many hot-water fixtures you realistically run at once, and match the heater’s GPM rating to that demand.
- Check the “temperature rise” rating, which is how much the unit can heat incoming water at a given flow rate.
- For gas models, make sure your gas line size and meter capacity can support the higher demand.
- Verify venting requirements, because many tankless units need specific vent materials and routing.
- Plan for maintenance access so flushing the unit isn’t a once-every-five-years nightmare.
- If your home has hard water, decide up front how you’ll manage scale to protect the heat exchanger.
- Look at where the unit will mount and whether the wall, piping, and shutoff valves are in good shape.
- For condensing models, confirm there’s a proper place to route condensate (the water produced as exhaust cools).
Flow, pressure, and the myths that trip people up
Homeowners often describe a problem as “low pressure,” when it’s really “low flow.” Pressure is the force in the pipe, and flow is how much water comes out per minute. A tankless heater cares about flow because it needs enough moving water to trigger the unit and enough time to heat it.
Here’s where fixtures matter more than people think. If you have high-flow faucets or a showerhead that dumps a lot of water quickly, you can exceed the heater’s comfortable output and get lukewarm results during heavy use. If you have efficient fixtures, you may stay within the heater’s sweet spot longer. For example, WaterSense-labeled bathroom faucets are designed to use a maximum of 1.5 gallons per minute, which is lower than older standard flows, according to the EPA’s WaterSense bathroom faucet guidance. That kind of efficiency can reduce the total hot-water demand in the house.
But there’s a twist: extremely low flow at a single fixture can sometimes fail to activate a tankless unit consistently, especially if the heater’s minimum activation flow is higher than that tiny draw. That’s why a “barely cracked open” hot tap may pulse between warm and cool. The fix is often behavioral (open the tap a bit more), or it can be solved with better system design, like a dedicated recirculation setup for long pipe runs.
Hard water adds another layer. Mineral scale narrows passages and reduces heat transfer, which can make a tankless feel weaker over time even when nothing else changed. San Antonio Water System notes that typical hardness in its water supply ranges from 15 to 20 grains per gallon, which is firmly in “very hard” territory and can contribute to scaling in plumbing and appliances (SAWS water quality FAQ). In other words, if your tankless performance slowly declines over a couple of years, scale is a prime suspect.
When an upgrade uncovers problems you didn’t know you had
Replacing a water heater is one of those projects that can expose the weakest parts of an older plumbing system.
If shutoff valves haven’t been touched in years, they can be stuck or crumble internally when you finally try to close them. Old flexible supply lines might kink or leak after being moved. In some homes, the water piping has corrosion or reduced diameter that limits flow, and the tankless ends up blamed for a restriction that was already there.
Gas tankless units can also reveal undersized gas piping. The heater may fire up fine for one fixture, but cut out or throw error codes when it tries to ramp up for multiple demands. Venting and combustion air matter, too. A tankless must safely move exhaust out of the home and pull in enough fresh air for clean combustion.
If you hear banging after the install, that may be water hammer (a pressure shockwave caused by fast-closing valves). If you notice gurgling or sewer smells, it may be a drain venting issue that’s unrelated to the heater, but surfaced because you were paying closer attention during the project.
What not to do: don’t “wing it” with gas line changes, vent swaps, or improvised drain routing. Those are safety-critical details, and mistakes can be expensive and dangerous.
How to tell if it’s working the way it should
A properly installed tankless should deliver steady hot water, predictable temperatures, and clean operation without constant tinkering.
Start with a simple leak check. Look at every connection while the unit is running and again after it has cooled down. Even a slow drip can turn into cabinet damage or wall stains over time.
Next, do a real-world test. Run a hot faucet for several minutes and confirm the temperature stabilizes instead of wandering. Then try two hot-water fixtures at once. You’re not looking for perfection, you’re checking whether the system behaves consistently under normal household use.
If the unit is a condensing model, inspect the condensate drain. It should carry water away reliably without pooling. If you have a recirculation system, confirm it doesn’t cause unwanted temperature spikes or short-cycling (frequent on/off operation).
Over the next few days, glance at the mounting area for moisture, check the surrounding wall for warmth where it shouldn’t be, and listen for unusual sounds. Tankless units are not silent, but they also shouldn’t sound like they’re straining.
Finally, think about materials that touch your drinking water. If you’re replacing valves, connectors, or fittings, choose components intended for potable water contact. NSF explains that NSF/ANSI 61 sets minimum health-effects requirements for contaminants that can leach into drinking water from system components (NSF’s overview of Standard 61). That’s the kind of marking you want to see when you’re upgrading parts around the heater.
When calling a plumber saves money instead of spending it
A tankless install can be straightforward in a newer home with modern valves, accessible piping, and a properly sized gas or electrical supply. But in many San Antonio, TX neighborhoods, homes have a mix of older shutoffs, tight mechanical spaces, and water conditions that punish shortcuts.
If you want a tankless that performs like the brochure promises, professional sizing and setup is where it pays off. PlumbSmart can help you compare options and handle a full installation through our tankless water heater service page, including the details that make the difference between “endless hot water” and “why is it doing that?”
Maintenance matters too. A routine flush removes early mineral buildup before it turns into a stubborn scale layer, and it can keep performance steady year after year. If you’re trying to protect your investment, our water heater maintenance service is the kind of small, planned expense that helps you avoid bigger repairs later.
And if you’re fighting scale on faucets, glassware, and showerheads, you may get better tankless life with a whole-home approach. A properly set up softener can reduce the mineral load that forms scale, and PlumbSmart offers water softener installation if you’re ready to address the root cause instead of constantly chasing symptoms.
The three things to do for a tankless that stays reliable
Tankless water heaters work by heating water on demand through a heat exchanger, so they live and die by flow, fuel, and maintenance.
First, make sure the unit is sized for your real life, not just the number on the box. Second, plan for your water conditions, especially hard water, because scale is the slow problem that becomes the big problem. Third, don’t cut corners on installation details like gas supply, venting, condensate routing, and shutoffs, since those are the hidden reasons “good units” end up performing poorly.
Do those three things, and your tankless can feel like one of the most satisfying upgrades in the house.
