The one “small” problem that makes your kitchen feel broken
If your hot water starts out fine but turns lukewarm halfway through doing dishes, it can make an ordinary evening feel like a chore. In San Antonio, TX, that “random” drop-off often comes down to a not-so-random culprit: mineral buildup inside the tankless unit. Tankless water heaters are great at making hot water on demand, but they rely on clean passages and efficient heat transfer. When scale starts lining the inside, the heater has to work harder to deliver the same comfort.
Cleaning (descaling) your tankless water heater is one of those homeowner tasks that pays you back fast: steadier hot water, fewer weird temperature swings, and less stress on expensive parts.
What you’re actually cleaning inside a tankless unit
A tankless water heater doesn’t store hot water like a traditional tank. Instead, it heats water as it flows through a heat exchanger (the metal “maze” that transfers heat from the burner or electric elements into your water). That heat exchanger is the main thing you’re cleaning.
When people say “flush” or “descale,” they mean circulating a mild acidic solution through the heat exchanger to dissolve mineral deposits. Scale is the hard crust left behind when water with dissolved minerals (mostly calcium and magnesium) is heated and those minerals come out of solution.
Most modern units also have a cold-water inlet screen (a small filter that catches debris) and, on many setups, service valves (also called isolation valves) that let you connect hoses for maintenance without shutting down the whole house.
Why homeowners run into trouble with DIY cleaning
Most tankless cleaning problems aren’t about effort, they’re about details. Here’s what commonly trips homeowners up.
First, not every tankless system is set up for easy flushing. If your unit doesn’t have isolation valves, trying to rig something together can turn into a leak at best, or a flooded cabinet at worst. Second, people tend to rush the shutdown. Gas units should be turned off at the control panel and the gas supply valve closed; electric units should be powered down at the breaker. Skipping that step can trigger error codes, surprise ignitions, or damaged components.
Another issue is using the wrong chemical strength. Straight industrial descalers or heavy acids can be too aggressive for seals and internal surfaces. On the other hand, a “tiny splash” of vinegar in a bucket won’t do much if scale has been building for a while. Homeowners also forget the inlet screen, so the unit looks “cleaned” but still struggles because debris is restricting flow.
Finally, overtightening hose connections is a classic. Tankless service ports and hose fittings don’t need brute force. Cranking down too hard can distort washers, crack plastic fittings, and create a slow leak that only shows up days later.
A practical cleaning checklist that works in real life
This is the basic approach plumbers use, adapted for a careful homeowner. Always check your manufacturer’s manual first for any model-specific steps.
- Turn off the unit (controller off), then shut off gas or flip the electric breaker off
- Close the hot and cold isolation valves on the tankless service valve kit
- Relieve pressure by opening a hot water faucet briefly, then close it
- Connect two washing-machine hoses to the service ports (one to the cold service port, one to the hot service port)
- Put the hose ends into a bucket with a small submersible pump, creating a loop that circulates solution through the heater
- Fill the bucket with clean water and white vinegar (or a manufacturer-approved descaling solution) and circulate it for the recommended time
- Dump the solution, refill the bucket with clean water, and circulate fresh water to rinse
- Close service ports, remove hoses, open the isolation valves, and restore power or gas
- Run a hot faucet for a few minutes to purge air and check for steady temperature
- Clean the cold-water inlet screen if your model has one, then recheck for leaks
A quick reality check: if your setup doesn’t have service valves, or the valves are seized and won’t turn, it’s smarter to pause and get help than to force it.
Pressure, flow, and the “my tankless is weak” myth
When a tankless heater feels “weak,” it’s usually not running out of hot water. It’s either struggling to sense enough flow to fire consistently, or it’s losing heat transfer efficiency because of scale.
Here’s the difference in plain terms. Pressure is the push behind the water (what you feel when a spray hits your hand). Flow is how much water is actually moving through the pipe, measured in gallons per minute. Tankless units care about flow because they need enough movement to trigger heating and stay stable.
Hard-water scale narrows internal passages, which can reduce flow and make temperatures swing. That’s why two people can take back-to-back showers and swear the heater is “inconsistent” even though the unit is technically working. In homes where hot water demand stacks up, efficient fixtures can also help the system stay comfortable. For example, EPA WaterSense labeled bathroom faucets use a maximum of 1.5 gallons per minute compared with a common standard flow of 2.2 gpm, cutting demand without making the sink feel unusable when you choose a good-quality model, as explained on the EPA WaterSense bathroom faucets page.
And if you’re in an area with very hard water, scale isn’t a “maybe.” SAWS notes typical hardness in their supply often falls in the 15 to 20 grains per gallon range, which is considered very hard and can drive faster mineral buildup in heaters and fixtures, according to the San Antonio Water System water quality FAQs. That’s why some households need cleaning more often than the generic “once a year” advice floating around online.
What cleaning can reveal, and what not to do next
A proper flush can expose issues that were being “masked” by a restricted system, and it helps to know what’s normal and what’s not.
If your shutoff valves won’t budge, don’t force them. Stuck valves can snap internally, and then you’ve got a bigger repair on your hands. If you see green crusty buildup around fittings, that’s often corrosion starting at a joint. Cleaning the heater won’t fix that, and cranking fittings tighter can make it worse.
If you notice gurgling sounds or sewer smell nearby, don’t ignore it. Some tankless units produce condensate (water created from exhaust cooling) and route it to a drain line. A dry trap or a poorly routed condensate line can let odors travel back. Pouring chemicals down drains to “solve the smell” is a bad move because it can damage pipes and still won’t fix a venting or trap issue.
Water hammer is another surprise: that loud bang when a faucet or valve shuts quickly. Once flow is restored after cleaning, hammer can become more noticeable, especially if your home has older piping or worn shutoff valves. Don’t try to “solve” it by partially closing valves everywhere. That creates new restrictions and can shorten the life of appliances.
One more important “don’t”: avoid harsh acids unless your manufacturer explicitly allows them. Tankless systems have seals, sensors, and internal surfaces that are built for potable water systems, not strong chemical cleaning. When you’re unsure, the safest path is a mild solution, proper time, and a full rinse.
How to tell you did it right without guessing for weeks
Right after cleaning, the goal is simple: normal operation with no leaks, no error codes, and stable hot water.
Start with the obvious. Look at every connection you touched: service ports, hose bib caps, nearby unions, and the cold-water inlet area. Wipe them dry and come back in ten minutes. A slow drip will show up as a fresh wet spot.
Then check performance in a way that matches real daily use. Run a hot faucet and watch for temperature hunting (hot-cold-hot cycling). A brief wobble at startup can be normal, but it shouldn’t bounce wildly. If you have a recirculation system, confirm it’s running the way it did before and not stuck in a constant loop.
Over the next couple of days, glance inside the cabinet or mechanical area with a flashlight. Leaks after tankless maintenance often show up as a tiny bead of water that only forms when the unit heats up and parts expand. Catching that early prevents warped cabinets, damaged drywall, and mold headaches.
If you replaced or cleaned any parts that contact drinking water, it’s also smart to keep an eye out for odd tastes or odors. Reputable fixtures and components are typically certified to health-effect standards intended for drinking water contact. NSF explains that NSF/ANSI 61 sets minimum health-effects requirements for chemical contaminants that can leach from drinking-water system components, which is why certifications matter when water touches internal parts, seals, or fittings, as described on NSF’s overview of NSF/ANSI 61.
When calling a plumber is the cheapest option
DIY cleaning is doable, but there are a few moments when it’s smarter to hand it off, especially in San Antonio, TX where hard water can turn small problems into repeat problems.
If your unit lacks isolation valves, the “simple flush” becomes a project that can involve shutting down water to the whole house and modifying piping. That’s a good time to bring in a pro for a clean, safe setup and a quick performance check. PlumbSmart can handle routine upkeep through tankless water heater service and preventative water heater maintenance so you’re not waiting for a cold-shower emergency.
If you’re flushing often and scale keeps coming back fast, it may be time to address the cause instead of repeating the symptom. A properly sized softener can reduce the minerals that create scale in the first place, which helps protect the heater, faucets, and appliances. If that sounds like your house, PlumbSmart can help with water softener installation and setup that matches your household’s real water use.
And if you see active leaks, corrosion, or a shutoff valve that won’t close fully, don’t wait. Those are the kinds of “small” plumbing issues that get expensive when they turn into water damage.
The three habits that keep tankless hot water reliable
If you only remember three things, make them these.
Make it easy on yourself by keeping your system serviceable. Isolation valves and clean hose connections turn maintenance into a routine task instead of a stressful repair.
Descale the right way: mild solution, enough time to work, and a full rinse so you don’t leave anything behind that can irritate seals or sensors.
Plan for your water conditions. With the hard water common in this area, a regular cleaning schedule and the right water treatment can keep your tankless heater efficient, steady, and ready for real everyday use.
