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Short answer: Start with the simple things: is the gas on, does the unit have power, is the cold water valve open, and is there an error code on the display. Write the code down. In Collin County, a tankless unit that has slowly lost heat or flow is usually full of scale, and a descale fixes it. If you smell gas, stop reading and get everyone outside.
These take about five minutes. Even if they don't fix it, you'll have something useful to tell the plumber.
Don't touch light switches or anything electrical, and don't go looking for the leak. Get everyone out of the house and call your gas company from outside. Nobody should work on the water heater until they've made it safe.
If the water stays cold at every tap, the burner isn't lighting. Go back through power and gas first, since those are the usual causes, then look for a code. A blocked vent will also stop ignition. If the unit has power, gas and a clear vent and still won't fire, the likely parts are the igniter, the flame sensor or the gas valve, and that's a service call.
Plumbers call this the cold water sandwich. You run hot water, shut it off, and turn it back on a minute later. The water sitting in the pipe has cooled and the heater needs a moment to fire again, so you get a burst of cold between two hot draws. Short draws make it worse, like rinsing your hands and turning the tap off before the hot water ever arrives.
Some of it is normal for tankless. A recirculation setup or a small buffer tank cuts it down a lot, and both are worth asking about if it catches you every morning.
A tankless heater needs a minimum flow before it will fire, so a trickle from the tap can drop below that and the unit shuts off. That part is by design.
If it cuts out at a normal flow, something is restricting the water. Check the inlet screen, then the aerator on the faucet or the showerhead that's giving trouble. If those are clean, the restriction is probably scale inside the heat exchanger.
Here, that's nearly always scale. Collin County water is hard, and the minerals in it build up inside the heat exchanger. The passages narrow and the scale insulates the metal from the flame, so you get cooler water, weaker flow, or both. Eventually you get error codes.
Descaling once a year is the fix, and it's the maintenance that decides how long the unit lasts. A plumber closes the valves to the heater, connects a small pump and circulates a descaling solution through the heat exchanger to dissolve the buildup. If yours has never been descaled and it's acting up, try that before anyone talks about replacing it.
Water treatment slows scale down. Pairing a tankless unit with filtration, such as the HALO whole-home system, or with a softener is easier to plan at install time than three years later.
A few clicks when the unit starts up is normal. A whine, a rattle or a rumble that wasn't there before can be scale, a fan on its way out, or something vibrating loose. It isn't always serious, but it is worth having looked at.
Discolored or smelly hot water often comes from the pipes or the water supply rather than the heater itself. Notice whether it happens on the hot side only or on both. That one detail narrows it down a lot for whoever comes out.
Leaks are different. If water is dripping from a fitting, a valve or the unit itself, close the water valves to the heater and call. A small leak at a heat exchanger doesn't get smaller.
That's sizing, not a fault. Every tankless unit has a maximum flow it can heat, measured in gallons per minute, and it can heat less of it in winter when the water coming in is colder. Two showers and the dishwasher together can be more than a smaller unit was ever sized to do. You can stagger hot water use, or talk to us about a larger unit. More on sizing, and on what tankless does well, is in our guide to tankless water heater pros and cons.
Most codes point at ignition, flow, venting or scale, and most of those are repairable. Get the code read and the unit looked at before anyone quotes you for a new one. A tankless heater that gets descaled lasts longer than a tank heater, and the usual reason one fails early is that nobody descaled it.
Replacement starts to make sense when the heat exchanger itself has failed, or when an older unit needs a major part. Whether that's true of yours is not something anyone can tell from a code alone.
It depends on what's wrong, and we won't guess at a number without seeing the unit. What moves the price is the part (a flow sensor is a different job from a gas valve or a control board), whether it's scale or a failed component, how easy the unit is to reach, and whether the venting or the gas supply needs work too. A descale is routine maintenance, not a repair.
Genzel gives you the options and the price before any work starts, usually as Good, Better and Best choices, and financing is available. Guardian Plan members get one tankless or tank flush a year included, 15 percent off every service and no dispatch fees, for $27 a month. See what the Guardian Plan covers.
Longer than a tank heater, if it gets descaled. In a hard water area like Collin County, skipping the yearly descale is the most common reason a tankless unit underperforms and then fails early.
Usually not. Most codes point at ignition, flow, venting or scale, and most of those are repairable. Write the code down, look it up in your manual, and get it read before anyone quotes you for a new unit.
You can if you're handy. It takes a small pump, two hoses, a descaling solution and isolation valves with flush ports on the unit, and your owner's manual has the steps for your model. Most people would rather have it done once a year as part of a service visit.
Once a year for most homes here, because the water is hard. Homes with water treatment can often go longer, but have it checked every year anyway.
No. A gas tankless still needs electricity for its controls and ignition, so when the power goes out, so does your hot water.
That's the cold water sandwich: the unit restarting between short draws. A recirculation setup or a small buffer tank reduces it.
Call if the checks above don't bring it back, if the unit is leaking, if it keeps throwing the same code, or if it's been more than a year since anyone descaled it. Genzel Plumbing does tankless water heater repair and installation across Collin County, along with tank water heater repair. We're a family-owned, licensed plumber (Texas RMP-23766) in McKinney, and Robby and Jamie Genzel have run the company since they started it in 2001. Call (972) 238-5585 and you will know the price before we start.
Protecting Homes. Serving Families.
Genzel Plumbing Company, 6851 Virginia Parkway, Suite 201, McKinney, TX 75071. Call (972) 238-5585. Family owned since 2001. Licensed and insured, Texas license RMP-23766. Plumbing Done Right. People First.
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