How long does a tankless water heater last?
Most gas-fired tankless water heaters last 15 to 20 years. Electric tankless units typically last 8 to 10 years, though they're far less common in whole-house applications. Tank water heaters by comparison last 8 to 12 years on average.
Manufacturer rating is one thing. Actual lifespan depends heavily on water quality. A tankless unit in an area with soft water and proper annual maintenance can run past 20 years. The same unit installed in an unsoftened hard water area without descaling often fails between year 7 and year 10. Reports sometimes call out scale on visible fittings as a leading indicator that the internal heat exchanger is fouled.
What do inspectors actually check on a tankless unit?
A standard home inspection isn't a deep diagnostic. The inspector visually examines what's accessible and operates basic functions. On a tankless water heater that typically includes:
The exterior condition of the unit and its mounting. The venting material, slope, termination, and clearances. The gas supply line size relative to manufacturer requirements (tankless units typically need 3/4 inch supply minimum, sometimes 1 inch). The condensate drain on condensing models. The visible electrical connections. Operation by drawing hot water at a fixture and confirming the unit fires and produces hot water within a reasonable time.
Inspectors do not disassemble the unit, check internal scale buildup, perform combustion analysis, or test recirculation systems for proper function. Those are diagnostic services performed by a plumber or HVAC technician.
My report says the gas line is undersized. What does that mean?
Tankless water heaters draw significant gas volume during operation, much more than a tank unit. A typical tank water heater might use 40,000 BTU/hr. A whole-house tankless unit often uses 150,000 to 199,000 BTU/hr. If the existing gas line was sized for the old tank unit, it may not deliver enough gas to fire the tankless unit at full capacity.
The visible symptoms can include lukewarm output during high-demand periods, error codes on the unit's display, and short cycling. A long-term consequence is reduced combustion efficiency and accelerated wear.
Correcting an undersized line means running a larger pipe from the meter or a closer branch point. Costs vary widely depending on the distance, finish materials in the way, and local labor rates. A typical residential gas line upsize runs $400 to $2,500.
What is the condensate drain finding about?
High-efficiency condensing tankless units extract additional heat from the combustion exhaust by cooling it below the water vapor dewpoint. That creates liquid condensate that has to drain somewhere. Because the condensate is mildly acidic, it has to drain through a neutralizer before entering a household drain, or be routed to an approved location.
Common findings include: no neutralizer installed, condensate routed to an inappropriate location (an exterior wall, a sump pump basin without backup, or a floor where it can damage finishes), or the condensate line draining backward due to improper slope.
A neutralizer cartridge costs $30 to $80 in parts and needs replacement every one to three years. Most reports recommend correction at installation cost, which usually runs $150 to $400 by a plumber.
Should I be worried about scale buildup?
Scale is the biggest predictor of tankless lifespan. The heat exchanger inside the unit is a small-diameter coil that gets very hot. Minerals dissolved in the water (primarily calcium and magnesium) deposit on the heat exchanger surface as the water passes through. Over time the deposit insulates the metal, reduces flow, and stresses the unit's electronics.
The EPA notes that water quality and maintenance directly affect water heater efficiency and lifespan. For tankless units, the manufacturers I've worked with recommend annual descaling in areas with hardness above 11 grains per gallon, and every two to three years in moderate-hardness areas.
If your inspection report notes scale on the inlet and outlet fittings or scale-staining around the pressure relief valve, the unit has likely never been descaled. The internal condition is probably worse than the exterior. Many buyers ask the seller to schedule a professional descaling before closing, which runs $150 to $400 by a plumber.
The report says venting is non-compliant. Is that dangerous?
It can be. Tankless water heaters require specific venting materials and configurations that vary by model. A non-condensing tankless unit uses metal Category III stainless venting and produces very hot exhaust. A condensing unit can use PVC or polypropylene because its exhaust is much cooler. Mixing them or installing the wrong material is a real safety issue.
Common venting findings include: improper material for the unit type, improper slope (condensing units need slope back toward the unit, non-condensing need slope away), undersized vent diameter, inadequate clearances to combustibles, and improper termination distances from windows, doors, or air intakes.
Venting issues are not cosmetic. The risks include combustion gas spillage, carbon monoxide infiltration, and premature unit failure. If a report flags non-compliant venting, the next step is evaluation by a licensed plumber or HVAC contractor before continued use. Corrections range from $200 for a clearance fix to $1,500+ for a full vent replacement.
How much does tankless replacement cost?
Installed cost for a whole-house gas tankless unit typically runs $2,800 to $5,500. The range reflects unit capacity (140,000 to 199,000 BTU/hr is common for whole-house), brand and model tier, and complexity of installation.
Additional costs that can push the total higher include: gas line upsizing if the existing line is too small, venting replacement if the existing venting is incompatible, condensate drain installation, electrical work to supply the unit's controls, and water softening installation if the local water is hard.
Switching from a tank water heater to a tankless unit for the first time often costs $4,000 to $7,500 total when all the supporting work is included.
Should I have a plumber inspect a flagged tankless unit before closing?
Often yes. A standard home inspection identifies visible conditions but doesn't diagnose internal scale, combustion efficiency, or component wear. A licensed plumber's evaluation typically costs $150 to $300 and includes flow rate testing, combustion check on gas units, descaling assessment, and a recommendation on remaining service life.
If the inspection report flags multiple findings (venting, gas sizing, condensate, and scale together), a plumber's evaluation often saves money in negotiation by either confirming the system has years of life left or quantifying the actual repair scope. The unit is expensive enough that a second opinion before closing usually pays for itself.
Does a tankless water heater need its own breaker or special electrical?
Even gas-fired tankless units need electrical for their controls, ignition system, and circulation fan. Most require a dedicated 120-volt circuit on its own breaker. Some larger units or those with internal recirculation pumps need a 240-volt circuit.
Electric whole-house tankless units are a different story. They draw very high amperage, often 80 to 150 amps total across multiple circuits. Many older homes don't have the panel capacity for an electric whole-house tankless retrofit. If your report mentions electrical concerns on an electric tankless unit, panel capacity is usually the underlying issue and correction can be expensive.