Tankless Water Heater Questions, Answered Briefly
Most tankless water heater questions come down to four things: sizing the unit for winter water, keeping scale out of the heat exchanger in our hard water, keeping it powered so its controls and freeze protection work, and catching small leaks at valves early. The short answers below cover each one and point to our deeper guides.
These answers are meant to be quick. If your unit is leaking or showing a code right now, jump to the leaks and errors group. For a puddle under any water heater, our leak repair team in Apache Junction is open 24/7.
Buying and Sizing
How do I read a tankless unit's GPM rating?
A flow rating in gallons per minute only means something next to the temperature rise it was measured at. One unit might list a high GPM at a small rise and a much lower GPM at a large one. Compare models at the same rise, and use a winter rise for our area, since incoming water is coolest then. Our tankless installation page shows how we size for the cold months.
What's the difference between condensing and non-condensing units?
A condensing unit has a second heat exchanger that pulls extra heat from the exhaust, so it wastes less fuel. That cooler exhaust can vent through plastic pipe, but it makes acidic water that needs a drain. A non-condensing unit is simpler, with no condensate, but needs metal venting rated for hot exhaust. Compare the efficiency figures on the EnergyGuide labels, as our tank vs tankless guide suggests.
Can two tankless units work together in a large home?
Yes. Many brands let two or more units be linked so they share the load, firing the second one only when demand calls for it. It's a common answer for big homes with several bathrooms. The catch is supply: the gas line and meter have to carry both units at full fire, or the panel has to handle the electric load. Our installation guide covers gas line sizing.
Hard Water and Descaling
What's in a tankless descaling solution?
Most products made for tankless heaters are mild acids, often citric-based, that dissolve calcium and magnesium scale. Some homeowners use plain white vinegar instead. Either way, follow your manual, since some makers name what they approve. The solution is pumped through the heat exchanger, then flushed out with clean water. The steps and timing are laid out on our tankless water heater repair page.
What is a scale-prevention cartridge on a tankless line?
It's a small filter housing on the cold line, usually filled with a slow-dissolving media that helps keep minerals from sticking as scale. It doesn't soften the water or remove hardness, and the cartridge must be replaced on the maker's schedule. It's a lighter option than a full softener. Our post on whether a softener is worth it for a water heater compares the choices.
Why does my unit show a scale or maintenance warning?
Some models watch for signs that scale is building, such as the heat exchanger running hotter than it should, and flash a reminder or code. It usually means descaling is due. If the warning comes back within months of a descale, the water is winning, and treatment or a shorter schedule makes sense. Our local hard water guide explains why buildup is so fast here.
Leaks and Errors
Is white crust on the valves under my unit a leak?
Often, yes, just a slow one. Hard water that seeps from a union, valve stem or hose-port cap dries into white or green crust before you ever see a drip. Wipe it, dry the area, and check again the next day. If it's damp, the fitting needs tightening or a new washer, or the valve needs replacing. Our guide to tankless leaks lists the other places water escapes.
Why does my tankless unit click or whir without heating?
A short whir when you open a tap is normal: the fan purges the burner before lighting. Repeated clicking with no flame means the igniter is trying and failing, often from a dirty flame sensor, low gas supply or a blocked intake. After a few tries the unit locks out with a code. Write it down; the causes behind ignition codes are on our tankless repair page.
Where should the acidic condensate drain?
Condensing units make a steady trickle of mildly acidic water. It should run through a drain line to a proper drain, often through a neutralizer that raises its pH first. Letting it drip onto concrete, metal or a roof can damage them over time. If the line clogs or comes loose, water pools under the unit. Our tankless install page covers condensate and venting needs.
Costs and Lifespan
What raises the cost of a tankless repair?
The part matters most: a flame sensor, filter or valve costs far less than a fan, control board or heat exchanger. Access adds time on roofs, in attics or tight closets, and a unit choked with scale may need descaling before it can be tested. For a local benchmark, Angi lists about $646 as the average for water heater repairs of all kinds in Apache Junction. Our repair cost guide breaks that range down.
Is paying for yearly descaling worth it?
In our water, usually. Scale is the main thing that shortens a tankless unit's life here, and a failed heat exchanger is the costliest repair it can need. A yearly descale, inlet filter cleaning and a quick look at valves and venting costs a fraction of that. Units looked after this way can reach 20 years or more, as our post on water heater lifespans in Arizona explains.
Does a tankless warranty cover labor?
Often only for a short time, if at all. Many tankless warranties cover the heat exchanger for longer than other parts, and labor for a much shorter period or not at all. Registration and install rules can change what's covered, and scale damage may be excluded. Read the terms with the model number in hand. If a repair isn't covered, our replacement cost page shows what a new unit runs here.
Snowbirds and Outages
Does a tankless water heater work during a power outage?
Most don't. Even gas tankless units need electricity for the fan, igniter and controls, so the hot water stops when the power does. Some owners run a plug-in unit from a generator, but only as the manual allows and never with the generator indoors or in a garage. Our post on monsoon power surges and water heaters covers what to check when the power comes back.
Does an outdoor tankless unit need a cover for summer storms?
Outdoor-rated units are built to handle rain, so a cover usually isn't needed, and a homemade one can block the intake or exhaust. Some makers sell pipe covers that tidy and protect the lines underneath. After a monsoon storm, check that wind-blown debris hasn't clogged the vent area. Seasonal owners can ask about this during our snowbird water heater service visit.
Is my outdoor unit safe if the power fails on a frozen night?
Not for long, since its freeze protection runs on electricity. Our freeze protection guide covers what to do when a cold night and an outage arrive together.
More Tankless Water Heater FAQs
Do tankless water heaters need any upkeep besides descaling?
Yes, a little. Clean the small inlet water filter, and on units that pull indoor air, the air intake screen too. Check the vent and intake for blockage, make sure the condensate line drains freely, and on condensing units, replace the neutralizer media when it's spent. Look over the relief valve and isolation valves for drips. Most of this fits into the yearly descaling visit.
Does a water softener mean I can skip descaling my tankless unit?
Usually not completely. Softened water slows scale a great deal, so many owners can descale less often, but the manual's schedule still applies, and some buildup and debris still reach the heat exchanger. Follow what your model's manual says for treated water, keep a log of each service, and watch for any scale warning on the display.
Is it normal for a tankless vent to puff out vapor on cold mornings?
Yes. Exhaust from a gas tankless unit carries water vapor, and on cool mornings it shows up as a white plume, much like your breath does. Condensing units make the most. It's only a concern if water drips back inside the unit, the plume smells unusual, or the vent is near a window or air intake that it shouldn't be.
Can I install a tankless water heater myself?
We don't advise it. A tankless unit usually needs gas line or electrical changes, new venting, a condensate drain and a relief valve, all set by the maker's instructions and local code. A first-time conversion normally needs a permit, and mistakes in gas or venting work can be dangerous. Many manufacturers also tie warranty coverage to a proper installation.
Does a tankless unit need its own gas shutoff valve?
Yes. Codes generally require an accessible manual shutoff valve near each gas appliance, so the unit can be turned off for service without cutting gas to the whole home. A sediment trap on the gas line is usually required too. Know where yours is, since it's the valve you'd close before any repair on the unit.