What is your tankless heater doing? Pick the closest match.
Tap a situation for a plain-English next step. It is a guide, not a diagnosis.
Tankless Flow and Gas-Load Estimator
Pick what runs at the same time on your busiest morning. We estimate the flow (gallons per minute), the temperature rise, and the rough gas input a tankless heater would need. This is a planning guide, not a quote. A plumber confirms the real numbers on site.
How Tankless Water Heaters Work
A tankless (on-demand) water heater has no storage tank. When you open a hot tap, a flow sensor detects water moving through the unit and signals the burner to fire. Cold water passes through a heat exchanger, picks up heat, and leaves at the set temperature. When you close the tap, the burner shuts off. Because it heats only as you use it, it does not lose heat to a standing tank, and it does not run out of hot water the way a tank can, as long as demand stays within the unit's capacity.
The parts that matter
- Flow sensor. Detects water flow and starts the heating cycle.
- Burner and igniter. Heats the heat exchanger when gas flows.
- Heat exchanger. Where hot gases transfer heat to the water. This is where hard-water scale does its damage.
- Control board and display. Manages temperature, safety checks and error codes.
- Inlet filter or screen. Catches debris before it reaches the exchanger.
- Venting. Carries exhaust outdoors. Condensing models also have a condensate drain.
- Service isolation valves. Let a plumber flush the unit without draining the house.
Capacity is about flow, not gallons
A tank is sized in gallons. A tankless unit is sized by how many gallons per minute (GPM) it can heat to your target temperature, which depends on the temperature rise. The colder the incoming water, the harder the unit works and the lower the flow it can deliver. We cover sizing below and in the estimator on this page.
If you are still comparing options, our water heater installation page explains gas, electric and heat pump tanks, and our water heater repair page covers fixing an existing tank.
Tankless Water Heater Installation in San Antonio
Tankless installation is a bigger job than swapping one tank for another. The unit usually mounts on a wall, needs its own venting, and often needs a larger gas line. When we install a tankless heater, we plan the whole system rather than just set a box on a wall.
What a complete installation includes
- Load and sizing check for your household
- Gas line capacity check and upsizing if needed
- Venting route and termination to code
- Condensate drain and neutralizer for condensing units
- Service isolation valves for flushing
- Power supply and outlet for the control board
- Pressure relief and discharge routing
- Permit, testing and a walkthrough
What to question in a quote
- No mention of the gas line size
- No service valves, which makes flushing hard
- No plan for hard water
- Reusing old tank venting
- No permit
- A model chosen without a load estimate
Replacing a tank with a tankless unit
A conversion changes the plumbing, the gas supply and often the location. The old tank's vent usually cannot be reused. The gas line to the tank was sized for a burner using a fraction of what a tankless unit needs. The unit often moves to an exterior wall or a spot with a short, clean vent run. Hot and cold lines are rerouted, and a power outlet may need to be added. A plumber plans all of that before you buy.
If the gas line has to be upsized or relocated, that work is done by a licensed plumber and pressure tested before the unit is put in service. Our gas line repair and installation page explains that process, and if an old tank is being retired, our water heater repair team can tell you whether it still has useful life.
Replacing an old tankless unit
If you already have tankless and it is failing, replacement is usually simpler, since the gas, vent and water connections exist. We still check them, because old vent pipe, corroded valves and undersized gas lines get discovered at this stage.
Gas Line, Venting, Condensate and Power Requirements
The hidden cost of tankless is usually not the heater. It is the gas line, the vent and the drain. Here is what each requires.
| Requirement | Why it matters | What we check |
|---|---|---|
| Gas supply | Many tankless units are rated around 120,000 to 199,000 BTU per hour of input. A typical tank uses about 30,000 to 50,000. | Pipe size and length, total appliance load, meter capacity |
| Venting | Exhaust must exit safely. Condensing units use plastic vent pipe; non-condensing units use stainless steel vent. | Vent material, length, number of elbows, termination clearances |
| Combustion air | Burners need fresh air. Sealed-combustion models draw it from outside. | Intake path or room volume |
| Condensate drain | Condensing units produce acidic condensate that must be drained and often neutralized. | Drain access and a slope to a proper termination |
| Electrical | Gas units need a small power supply for the control board, fan and igniter. | Outlet, GFCI where needed, surge protection |
| Water connections | Service valves allow flushing and isolation. | Isolation valves, pressure relief, pipe material |
| Pressure | Very high or very low pressure affects flow and parts. | Static pressure, regulator, expansion needs |
Electric tankless is a different story
Whole-house electric tankless units can require very high electrical capacity, often several dedicated circuits and a panel with room to spare. Many homes cannot support them without costly electrical upgrades. If your home has gas, a gas unit is usually the practical choice for whole-house use. Point-of-use electric units can suit a single sink or a remote bath.
Sizing a Tankless Water Heater for Your Household
The right tankless size depends on two numbers: how many gallons per minute you need at the same time, and how much the water must be heated, called the temperature rise. Oversizing wastes money, and undersizing leads to lukewarm showers when two things run at once.
Step 1: add up peak flow
Estimate what runs together during your busiest hour. Typical hot water flows are roughly:
| Fixture or appliance | Typical hot water flow |
|---|---|
| Shower | About 1.5 to 2.5 GPM |
| Kitchen faucet | About 1.5 GPM |
| Dishwasher | About 1.5 GPM |
| Washing machine | About 2 GPM |
| Bathroom sink | About 0.5 to 1 GPM |
Step 2: find the temperature rise
Temperature rise equals your target temperature minus the incoming water temperature. If you set the heater to 120°F and the incoming water is 60°F, the rise is 60°F. Incoming water is colder in winter, so size for a cold day rather than an average one.
Step 3: check the unit's capacity at that rise
Manufacturers publish the flow each model can deliver at different temperature rises. A unit that delivers a high flow at a 40°F rise delivers much less at a 70°F rise. Always compare at your winter rise, not the marketing number.
A worked example
A family runs two showers (2 GPM each) and a dishwasher (1.5 GPM) at once: 5.5 GPM. On a 60°F winter day the rise is 60°F. The heat delivered to the water is about 5.5 x 500 x 60 = 165,000 BTU per hour, so the unit needs somewhat more than that in gas input once efficiency is considered. That points to a high-capacity unit and a robust gas line. Use the estimator on this page to try your own numbers.
Water pressure matters too. Very low pressure limits flow to the heater, and very high pressure stresses its valves and fittings. Our water pressure repair and pressure regulator replacement pages explain how we test and correct both.
Tankless Water Heater Repairs
Tankless heaters are more complex than tanks, with sensors, a control board, a fan and a precise burner. Most problems fall into a few categories, and many start with scale or a venting issue.
| Symptom | Common causes | Usually fixed by |
|---|---|---|
| No hot water, unit does not fire | No power, gas off, failed igniter or flame sensor, blocked vent | Diagnosis and part replacement |
| Fires then shuts off | Flame sensor, low gas supply, vent restriction | Cleaning, gas supply check, vent correction |
| Lukewarm or fading hot water | Scale on heat exchanger, clogged filter, undersized gas line | Descaling, filter cleaning, gas check |
| Cold blast between uses | Normal unit behavior, low flow, check valves | Settings, recirculation, buffer tank |
| Water leak | Failed fitting, valve, pressure relief, condensate line | Repair the leaking part |
| Noisy operation | Scale, fan wear, venting | Flush, service the fan, vent check |
| Error code repeats | See the next section | Depends on the category |
If the heat exchanger itself has failed from scale or corrosion, replacement of the unit may make more sense than repair. We compare the options for you in writing. If your unit is old and you are weighing a new one, see our water heater installation options. If water is leaking from the unit or its fittings, shut off the water and power and call us. Hidden pipe leaks nearby are a separate job for our leak detection team, and a flooding emergency is covered on our emergency plumber page.
Why Is My Tankless Water Heater Showing an Error Code?
Your tankless water heater is showing an error code because its control board has detected a problem with ignition, flame, airflow, temperature, water flow or condensate. Codes differ by brand and model, so the exact meaning comes from your manual. What stays consistent is the category of problem.
| Typical category | What it usually means | Safe for you to try |
|---|---|---|
| Ignition or no flame | Unit tried to light and failed. Gas supply, igniter or flame sensor. | Confirm gas valve is open. Power-cycle once. |
| Vent or air supply | Blocked exhaust or intake, or a fan problem. | Look for a visible obstruction at the outdoor terminal from the ground. |
| Overheat or high limit | Water or exhaust too hot. Often scale or low flow. | Let the unit cool, then restart once. |
| Flow or sensor | Flow too low, or a sensor is failing. Often clogged filter or scale. | Check that fixtures flow normally. |
| Condensate | Condensate drain is blocked or full. | Do not open it. Call us. |
| Control board or communication | Electronics fault. | Power-cycle once. Then call. |
What to do when a code appears
- Write down the exact codeand the brand and model number from the label.
- Power-cycle onceas the manual allows. If the code clears and stays gone, still mention it to a plumber at the next service.
- Check the basicssuch as power, gas valve open and visible vent clearance.
- Stop if it returnsand call us. Repeated resets can mask a developing problem.
Scale Buildup, Hard Water and Descaling
Yes, hard water affects tankless systems, and more severely than it affects tanks. A tankless heat exchanger has narrow passages and gets very hot. Scale builds up in those passages, restricting flow, insulating the metal from the water and forcing the unit to work harder. It is the leading cause of premature tankless failures in hard-water areas, and San Antonio is a hard-water city.
What scale does to a tankless heater
- Reduces flow and output, so hot water fades
- Raises exchanger temperature, which triggers overheat codes
- Causes noise and uneven heating
- Shortens the life of the heat exchanger and may jeopardize the warranty if the manual's maintenance is skipped
How descaling works
Descaling means circulating a descaling solution, often a mild acid such as a manufacturer-approved cleaner or white vinegar, through the heat exchanger using the service valves and a small pump, then flushing it clean. It requires isolation valves installed on the unit. If your heater has none, adding them is the first step.
How often?
Manufacturers commonly recommend flushing at least once a year, and more often in very hard water or if you have no softener. Follow your manual, because some warranties require documented maintenance. Do not wait for the heater to complain.
Protecting the heater with softening
The best long-term protection is removing the hardness before it reaches the heater. See our water softener installation page and, for taste and chlorine, whole-house water filtration. A softener plus annual service gives a tankless heater its best chance at a long life.
Tank vs. Tankless: An Honest Comparison
Tankless is not automatically better. It depends on your household, your home and your budget. Here is a straightforward comparison.
| Factor | Tank | Gas tankless |
|---|---|---|
| Hot water supply | Limited by tank size, then recovery | Continuous within the unit's flow capacity |
| Upfront installed cost | Lower | Higher, especially for conversions |
| Gas line | Usually small | Often needs a larger line |
| Venting | Common flue | Dedicated vent, condensate for condensing units |
| Typical life | About 8 to 12 years | Often 15 to 20 years with maintenance |
| Maintenance | Flush, anode check | Annual flush and descale; filter cleaning |
| Standby heat loss | Yes | None |
| Space | Floor space and a pan | Wall-mounted, saves floor space |
| Hard-water sensitivity | Moderate | High; scale control essential |
| Power outage | Gas tanks can work without power; most electric do not heat | Needs power to run |
Efficiency and savings
The U.S. Department of Energy reports that on-demand heaters can be meaningfully more energy efficient than storage tanks, with larger savings for homes that use less hot water and smaller savings for homes that use a lot. Your actual savings depend on your usage, your gas rates and your maintenance. Operating savings alone do not always repay the higher installed cost quickly, so we look at the whole picture with you.
When tankless makes sense
- You have a large household with peaks that exhaust a tank
- You want to free up floor space
- Your gas line and vent path are easy to upgrade
- You will commit to annual descaling or add a softener
When a tank is the smarter choice
- You have a tight budget and a simple tank location
- Gas line upgrades would be costly
- You are not able to maintain a tankless unit
- Your hot water demand is modest
Common Tankless Complaints and What Fixes Them
Most unhappy tankless owners are dealing with one of three things: the cold water sandwich, delays for hot water, or lukewarm output at low flow. All three have answers.
The cold water sandwich
You shower, pause briefly, and then get a short burst of cold water before the heat returns. Hot water remaining in the pipes cools, the heater shuts off, and when you restart, cold water that was in the heat exchanger flows first. A small buffer tank, a recirculation setup or adjusting use patterns can reduce it.
Waiting for hot water
A tankless heater does not make hot water arrive faster. Water still has to travel through the pipes from the heater to your tap. A far-away bathroom still waits. A recirculation pump keeps hot water moving in the lines. See our hot water recirculation pump page.
Low-flow fixtures that will not fire the unit
Tankless heaters need a minimum flow to activate. If a faucet or a very low-flow fixture runs below that, the heater may not fire. Clogged aerators can cause this too, so a quick check is worthwhile.
Two fixtures at once cause a drop
That is a sizing limit. Either a larger unit, a second unit, or staggered use is the answer. Our estimator and sizing section help you see where your limit lies.
Tankless Water Heater Cost Factors in San Antonio
Tankless cost varies more than tank cost, because the surrounding work varies. The ranges below are general industry ballpark figures for planning only. They are not a Stark quote, and your written estimate governs.
| Job type | General industry ballpark | What moves the price |
|---|---|---|
| Gas tankless, replacing an existing tankless unit | About $2,500 to $5,500 | Model, venting condition, valves, permit |
| Gas tankless, converting from a tank | About $3,500 to $8,000 or more | Gas line upsizing, new vent, relocation, condensate |
| Descale and flush service | Varies by unit and scale level | Isolation valves present, scale level |
| Service isolation valve installation | Modest add-on | Access, pipe type |
| Tankless repair | Varies widely | Part, labor, whether the exchanger is damaged |
| Recirculation or buffer add-on | Varies | Return line, controls, compatibility |
What drives the biggest swings
- Whether the gas line has to be upsized
- Venting route and wall penetrations
- Condensate drain access
- Unit capacity and brand
- Water treatment for hard water
- Permit and inspection
San Antonio Factors: Hard Water, Freezes, Gas Capacity and HOAs
Local conditions change tankless decisions here more than most people expect.
- Hard water. The biggest local factor. Plan for service valves, annual flushing and ideally a softener.
- Occasional hard freezes. The February 2021 freeze showed what happens to exposed plumbing and equipment. An outdoor tankless unit needs freeze protection and power for its protection features, and some units fail if power is lost in extreme cold.
- Gas capacity. Many San Antonio homes were built with gas lines sized for a small tank. Upgrading for tankless is common.
- Attics and garages. Heat, tight space and limited vent paths all affect where a unit can go and how vent runs are routed.
- HOA and neighborhood rules. Some communities have rules about exterior vent terminations and equipment visible from the street. Check yours before choosing an exterior-wall location.
- Permits. The City of San Antonio typically requires a permit for water heater replacement and gas work.
Building or remodeling? It is far cheaper to plan gas, vent and water for tankless at the start. Our new construction plumbing, plumbing rough-in and bathroom remodel plumbing teams can route the lines correctly the first time. If an older home has corroded supply lines, a tankless upgrade is also a good moment to look at whole-house repiping.
Seasonal Tankless Water Heater Tips in San Antonio
Winter
Colder incoming water raises the temperature rise and lowers output, so peak demand can overwhelm a marginal unit. Protect exposed pipes and outdoor units, know where your gas and water shutoffs are, and keep the unit powered during a freeze if it relies on electric freeze protection.
Spring
A good time for the annual flush and descale after the high-use winter. Clean the inlet filter, check the vent terminal for debris and nests, and test your CO alarms.
Summer
Warmer incoming water means lower temperature rise and better flow. Heat in garages and attics stresses electronics and valves, so check clearances and ventilation.
Fall
Schedule service before winter and before the holiday rush. If you are planning an upgrade or conversion, fall is a good time to avoid peak-season delays.
Our Tankless Installation and Service Process
- Intake call or text.Tell us about your household, your current heater and what is not working. Photos of the heater, meter and gas line help.
- Load and sizing check.We calculate your peak flow, temperature rise and the gas input a unit would need.
- Site review.We inspect the gas line, meter, vent path, drain, water lines, location and electrical supply.
- Options and written estimate.You get tankless and tank options in writing, including gas, venting, condensate, valves, permit and warranty.
- Your approval.Nothing starts until you approve the written estimate.
- Installation.We install the unit, gas, vent, condensate, valves and connections to code.
- Testing.We test for leaks, gas tightness, combustion, venting, temperature and flow, and confirm the controls and error history are clear.
- Walkthrough.We show you the display, the service valves and the maintenance schedule, and leave you with the information you need.
Checklists: Is Tankless Right for You and Questions to Ask
Is tankless right for my home?
· Tip: screenshot this list.
Annual tankless maintenance checklist
· Tip: screenshot this list.
Questions to ask any plumber
· Tip: screenshot this list.
Why You Should Not Install or Repair a Tankless Heater Yourself
A tankless heater is a gas appliance with an electronic control board, a precise burner and a venting system that carries exhaust. Installation and repair demand training, tools and testing.
- Gas leaks and fire. Undersized or poorly connected gas lines can leak or starve the burner.
- Carbon monoxide. Wrong venting can push exhaust into the home.
- Scalding and water damage. Pressure and temperature errors are dangerous and costly.
- Voided warranties. Many manufacturers require licensed installation and documented maintenance.
- Failed inspections and insurance problems. Unpermitted work can complicate claims and home sales.
You can verify any plumber at tsbpe.texas.gov. The safe homeowner tasks are limited: visually check the vent terminal from the ground, keep the area clear, note error codes and schedule service. Everything else is for a licensed professional.
How Our Estimates Work: No Surprises
- Sizing before pricing. We calculate your load, then recommend equipment.
- Written estimate before work. It lists the unit, gas, vent, condensate, valves, permit and any add-ons.
- Both options shown. We show you a tank and a tankless option when both are viable.
- Your approval first. Nothing begins until you say yes.
- If scope changes, we stop and talk. No unapproved add-ons on the final bill.
- Warranty in writing. Equipment and labor terms appear on your estimate.
We are located at 1011 Ada St, San Antonio, TX 78223. Verify our license at tsbpe.texas.gov before you book.
Tankless Water Heater Myths vs. Facts
| Myth | Fact |
|---|---|
| Tankless means endless hot water. | It is continuous within the unit's flow capacity. Too many fixtures at once will exceed it. |
| Any home can swap to tankless in an hour. | Gas line, venting, drain and electrical work often make it a larger job. |
| Tankless never needs maintenance. | Hard water makes annual descaling important. |
| Tankless always saves money. | Savings depend on usage, and installed cost is usually higher. |
| Tankless works in a power outage. | Most units need power to operate. |
| Error codes mean the heater is dead. | Many are scale, flow or venting problems that service can fix. |
| A softener is optional in San Antonio. | Hard water makes scale control a core part of a tankless plan. |
Tankless Water Heater FAQs for San Antonio Homeowners
Can a tankless system replace a traditional tank heater?
Yes, in most homes a tankless system can replace a tank heater, but it is a conversion rather than a swap. It usually needs a larger gas line, a dedicated vent, a condensate drain for condensing units, and a power outlet. We check your gas capacity, vent path and load first, then give a written estimate comparing tankless and tank options.
Why is my tankless water heater showing an error code?
Your tankless heater shows an error code when its control board detects a problem such as ignition or flame failure, a blocked vent, overheating, low flow or a blocked condensate drain. Codes vary by brand, so write down the exact code and model. Power-cycle once, and if it returns, call us rather than resetting repeatedly.
Does hard water affect tankless systems?
Yes. Hard water leaves scale inside the narrow passages of a tankless heat exchanger, which reduces flow, causes overheating and shortens the unit's life. San Antonio water is hard, so plan for service valves, regular flushing and ideally a water softener. Follow your manufacturer's maintenance schedule to protect the warranty.
How much does a tankless water heater cost in San Antonio?
A gas tankless unit replacing an existing tankless heater generally runs about $2,500 to $5,500 installed, while converting from a tank often costs about $3,500 to $8,000 or more because of gas line, vent and drain work. These are general industry ballparks, and your written estimate governs.
How long do tankless water heaters last?
With regular maintenance, tankless heaters often last 15 to 20 years, which is longer than most tank heaters. Hard water and skipped descaling can shorten that. Annual flushing, a clean inlet filter and a water softener give a tankless unit its best chance at a long life.
How often should a tankless water heater be flushed?
Manufacturers commonly recommend flushing a tankless heater at least once a year, and more often in very hard water or without a softener. Follow your manual, since some warranties require documented maintenance. Flushing needs service isolation valves, which we can add if your unit does not have them.
What is the cold water sandwich?
The cold water sandwich is a short burst of cold water between two periods of hot water when you pause and restart a tankless heater. It happens because the unit shuts off and cold water in the exchanger flows first. A buffer tank, recirculation setup or adjusted use patterns can reduce it.
Do I need a bigger gas line for a tankless water heater?
Often yes. Many gas tankless units are rated around 120,000 to 199,000 BTU per hour, compared with about 30,000 to 50,000 for a typical tank. A plumber must calculate your total gas load and check the line and meter capacity. Upsizing the gas line is a common part of a conversion.
What size tankless water heater do I need?
Size is based on peak gallons per minute and the temperature rise. Add up fixtures that run together, subtract your coldest incoming water from your target temperature, and check the unit's capacity at that rise. A family running two showers and a dishwasher needs roughly 5 to 6 GPM. Use our estimator for a starting point.
Is a tankless water heater worth it?
It can be, particularly for larger households, limited floor space and homes where gas and venting are easy to upgrade. It costs more to install, needs maintenance in hard water, and savings depend on usage. We compare a tank and tankless option so you can decide on real numbers.
Will a tankless water heater work during a power outage?
Most tankless heaters need electricity to run the control board, fan and igniter, so they usually will not heat water during a power outage. Some units can run on a backup power source. If outages and freezes are a concern, ask us about options.
Can you install a tankless water heater outdoors?
Yes, many gas tankless heaters are rated for outdoor installation, which can simplify venting. In San Antonio you need freeze protection and power for the unit's protection features during cold snaps. Check HOA rules about exterior equipment, and we confirm clearances before installing.
Do you repair tankless water heaters?
Yes. We diagnose error codes, ignition and flame problems, venting issues, leaks and scale-related output loss, and we descale and flush units that have service valves. If the heat exchanger has failed, we compare repair and replacement in writing so you can choose with confidence.
About This Page and Our Sources
Sources and further reading:
- U.S. Department of Energy: Tankless or Demand-Type Water Heaters — how on-demand heaters work, efficiency and lifespan
- U.S. Department of Energy: Sizing a New Water Heater — sizing basics
- ENERGY STAR: Residential Water Heaters — efficiency labels
- Texas State Board of Plumbing Examiners — verify a plumber's license
- San Antonio Water System (SAWS) — water quality and pressure information
- CPS Energy — natural gas service and safety
- City of San Antonio Development Services — permit information
Costs, timelines and code requirements vary by home, equipment and the City of San Antonio permit rules in effect at the time of the work. Nothing on this page replaces an on-site inspection by a licensed plumber. Stark Plumbing Solutions is not affiliated with SAWS, CPS Energy or any manufacturer named on this page.
