Tankless water heater is leaking: causes, part cost
Prices checked 2026-08-28
A tankless water heater is leaking is most often a supply connection or hose problem - about 32% of reported jobs. Here is how to confirm it, what the part costs, and when to stop.
A tankless water heater has no tank to fail – it heats on demand, which means its faults are flow, gas or the heat exchanger. Scale is its slow killer.
Tankless water heater is leaking is one of the most common repair searches there is. A leak is the one fault that gets worse while you think about it. Water finds the lowest point, and the damage is rarely where the water lands. Working the list below in order finds most leaks in minutes and keeps the repair small.
Before you call anyone: 30-second checks
- Check supply and drain connections first – loose or cracked fittings cause more leaks than failed components.
- Check door gaskets, seals and pans – a worn gasket leaks only when the unit is running or full.
- Dry the area, then run the unit and watch where the first drop appears; that is your leak, not the puddle.
The likely causes, ranked
Supply connection or hose
About 32% of tankless water heater is leaking jobs come back to this cause. Restricted airflow or a blocked path is the most common and cheapest cause in this whole list – the unit cannot breathe, so it cannot do its job.
How to confirm it: Check the filter, vent or hose run for build-up and restriction; cleaning it is free, and it is the first thing to rule out.
Typical fix: usually free. A capable DIYer can usually handle this one.
Door gasket or seal worn
About 24% of tankless water heater is leaking jobs come back to this cause. The gas or fuel side – supply, valve, regulator, carburettor – is where fuel stops flowing, and it is a licensed-work zone when gas is involved.
How to confirm it: Check the supply is on and the line is clear; for fuel units, check the fuel age and the carburettor if it has sat unused.
Typical fix: $120. A capable DIYer can usually handle this one. You would be looking for a door gasket.
Drain or pump seal fault
About 22% of tankless water heater is leaking jobs come back to this cause. A motor, fan or pump with its drive components is a wear item – belts stretch, bearings dry out, capacitors lose their charge and relays stick.
How to confirm it: Listen for the motor: a hum with no movement points at a capacitor or belt; a grinding or squeal points at a bearing. Each has a different price tag.
Typical fix: $150. This one is best left to a professional. You would be looking for a pump seal kit.
Pan, tray or condensate line blocked
About 22% of tankless water heater is leaking jobs come back to this cause. Restricted airflow or a blocked path is the most common and cheapest cause in this whole list – the unit cannot breathe, so it cannot do its job.
How to confirm it: Check the filter, vent or hose run for build-up and restriction; cleaning it is free, and it is the first thing to rule out.
Typical fix: usually free. A capable DIYer can usually handle this one.
When to stop and call a pro
Water near electricity is the one case where you do not investigate – trip the breaker, shut off the supply, and call someone. Same for a leaking water heater: shut off the water and the unit before it empties a tank into your floor.
How to keep it from coming back
Most leaks are connection leaks – a supply line, a gasket, a drain. Replacing worn hoses and gaskets on schedule is cheaper than the floor repair you avoid.
Bottom line: Tankless water heater is leaking is worth diagnosing before you call anyone – the free checks above catch a surprising share of cases, and the ranked list tells you what the part should cost if it is not one of them. Run the figures through the repair-or-replace test on this page before you authorise any work, and never pay for a diagnosis without getting the part name and the price in writing first.
Likely causes, most common first
| Likely cause | How often | Typical fix | Who does it |
|---|---|---|---|
| Supply connection or hose | 32% | free | DIY-able |
| Door gasket or seal worndoor gasket | 24% | $120 | DIY-able |
| Drain or pump seal faultpump seal kit | 22% | $150 | Pro job |
| Pan, tray or condensate line blocked | 22% | free | DIY-able |
Frequencies are the share of reported jobs attributed to each cause across repair-platform and forum data - a starting point for diagnosis, not a certainty.
What this should cost to put right
Across the causes listed above the repair runs $0 to $150. A quote far above the top of that range is usually a diagnosis of convenience - get a second opinion before you authorise it.
Repair it, or replace the unit?
This is the number people get wrong most often, and it costs them the most. Run the quote through the test - it uses the mid-point cost of the fix above against a new unit and the years this one has left.
REPAIR IT
The repair is 8% of a new unit and the appliance has roughly 6 year(s) of expected life left. Fixing it is the economical call. A new unit would save about $45/year in running costs, so keep repairing until that pays back in under 5 years.
Decision rule: repair when the fix costs under 50% of a replacement and the unit is under 70% through its expected life.
Other tankless water heater symptoms
Common questions
Is it dangerous if my tankless water heater has a "is leaking" problem?
Usually not immediately - none of the likely causes on this page involve gas, fuel or a mains electrical fault. The two cautions that apply everywhere: keep water away from any electrical point, and if you ever smell gas or burning, stop diagnosing and call a professional.
How much does it cost to fix a tankless water heater with a "is leaking" problem?
Most fixes for this run $0 to $150. The most common cause, supply connection or hose, runs about nothing - it is a free check to put right, and the part you would be looking for is not usually sold separately. Get the diagnosis in writing before authorising anything above the top of the range.
Why does my tankless water heater have a "is leaking" problem?
In about 32% of reported cases it is a supply connection or hose. It is the first thing to check because it is the most common - and usually the cheapest. If that checks out clean, work down the ranked list on this page before paying for a diagnosis.
Should I repair or replace my tankless water heater?
Repair it when the fix costs under half of a replacement and the unit is under 35% through its roughly 15-year expected life. Past that point a repair is usually a deposit on the next failure - run the test on this page with your actual quote before you decide.
How this figure was built
Reviewed 2026-08-28 · quarterly.
- Bureau of Labor Statistics, Occupational Employment and Wage Statistics - construction and repair trades
- EPA Energy Guide appliance ratings
- North American Electrical Metrics / manufacturer parts catalogues
National averages. Local labour can run 20-40% above or below these figures; the calculators apply a metro factor on top.
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