Code 79 with no hot water
The display shows 79 and the burner will not stay on, so fixtures go cold fast.
Start here: Start with the condensate drain line and trap, especially if the unit was working normally before this started.
Tankless water heater error help
Rinnai Tankless Water Heater Code 79
Direct answer: Rinnai code 79 usually means the heater is shutting itself down because condensate is not draining the way it should, or the unit senses water where it should not. The first things to check are the condensate drain line, trap, and any visible water collecting inside or under the heater.
Most likely: Most of the time this comes down to a blocked, kinked, frozen, or poorly sloped condensate drain path rather than a failed major component.
When a tankless throws a drainage-related shutdown, it is trying to protect itself. Reality check: this is often a wet, dirty blockage problem, not an expensive parts problem. Common wrong move: clearing the display code and walking away without fixing the drain path, because the code usually comes right back on the next call for hot water.
Don’t start with: Do not start by replacing boards or opening gas-side components. On this code, the simple drain path checks come first.
If you see water inside the cabinet or dripping below the unit,stop using it until you find whether the condensate line is blocked or leaking.
If the drain line looks clear but the code returns right away,the condensate trap or internal drain path may still be restricted and needs closer service.
Last reviewed: 2026-04-17
What code 79 usually looks like in the house
Code 79 after heavy use
The heater runs for a bit, then shuts down during showers or back-to-back hot water use.
Start here: Look for a slow drain, partial clog, or sagging hose that lets condensate back up when the unit is making more water.
Code 79 with water under the unit
You find dampness, staining, or a small puddle below the heater cabinet.
Start here: Check whether the condensate line is blocked or disconnected before assuming the heat exchanger is leaking.
Code 79 in freezing weather
The code appears after a cold snap, and the drain tubing or termination may be iced up.
Start here: Inspect the full condensate route for freezing, especially where the line runs near an exterior wall or unheated space.
Most likely causes
1. Blocked or kinked condensate drain line
This is the most common field cause. The heater makes condensate during operation, and if that water cannot leave, the unit shuts down to avoid internal overflow.
Quick check: Follow the drain tubing from the heater to its end point and look for kinks, sags, slime, debris, or a pinched section.
2. Condensate trap full of debris or not draining freely
A trap can collect sediment, scale, or gunk and slow the flow enough to trigger the code, even when the outside tubing looks fine.
Quick check: If the trap is accessible per your unit layout, look for standing water that does not move out or obvious buildup around the trap area.
3. Frozen or poorly routed condensate line
A line with bad slope, long flat runs, or exposure to cold can hold water and back up into the heater.
Quick check: Check for low spots holding water, outdoor sections exposed to freezing, or a drain line that runs uphill before dropping.
4. Internal leak or condensate sensing problem
If the drain path is open but the unit still shows 79, water may be getting where it should not, or an internal drain-related component may need service.
Quick check: Look for fresh water marks inside the cabinet edges or repeated code returns immediately after a careful drain-line check.
Step-by-step fix
Step 1: Start with the easy outside checks
Most code 79 calls are solved without taking the heater apart. You want to rule out the obvious drain path problems first.
- Turn off the heater at its power switch or unplug it if the plug is accessible.
- Let the unit sit a few minutes so you are not working around a hot cabinet.
- Look under and around the water heater for puddles, staining, or drips.
- Find the condensate drain line and trace it from the heater to where it ends or ties into a drain.
- Check for kinks, sharp bends, crushed tubing, loose connections, or a section that has sagged and is holding water.
Next move: If you find a kinked or sagging line and correct it, restore power and test hot water again. If the line looks fine from the outside, move on to checking for a clog or freeze-up.
What to conclude: Visible line damage or bad routing is often enough to cause this shutdown by itself.
Stop if:
- You smell gas.
- You see active leaking from inside the cabinet.
- The drain line is glued into a setup you are not comfortable disturbing.
- The unit is mounted where you cannot inspect it safely.
Step 2: Clear a simple blockage in the condensate path
A partial clog is more common than a failed internal part, and it can make the heater trip only during longer hot water runs.
- With power still off, disconnect only the accessible condensate tubing if it is secured with a simple clamp or slip connection.
- Check the tubing opening for slime, debris, scale, or insect blockage near the end of the line.
- Flush the removed tubing with warm water at a sink or bucket until it runs freely.
- If the line termination is dirty, clean it so water can drip or flow out without backing up.
- Reconnect the tubing with steady downward slope and no low spots that can trap water.
Next move: If the code stays gone during a full shower-length test, the blockage was likely the problem. If water still does not move freely or the code returns quickly, the trap or internal drain area may be restricted.
What to conclude: A line that was slow or blocked confirms the heater was shutting down because condensate had nowhere to go.
Step 3: Check for freezing or bad slope
Code 79 often shows up after weather changes or after someone reroutes the drain line with too much flat run.
- Inspect the full drain route for any section running through a cold garage, crawlspace, attic, or exterior wall area.
- Feel for ice-cold or visibly frozen sections if weather has been below freezing.
- Look for long horizontal runs, uphill sections, or loops that can hold water.
- Reposition accessible tubing so it drains steadily downhill from the heater to the outlet.
- If a frozen section is accessible, thaw it gently at room temperature; do not use open flame or high heat.
Next move: If correcting the slope or thawing the line restores normal operation, keep watching the drain during the next few hot water calls. If the line is open and properly routed but the code keeps returning, the problem is likely at the trap or inside the unit.
Step 4: Reset once after the drain path is corrected
After you fix the obvious cause, the heater may need a clean restart to prove the problem is actually gone.
- Restore power to the water heater.
- Clear the code using the normal user reset method for your unit, or power-cycle it if that is the only accessible reset.
- Run hot water at one fixture long enough to make the heater stay on for several minutes.
- Watch for steady operation and check the condensate line outlet for normal drainage if it is visible.
- Recheck under the unit for fresh drips after the test run.
Next move: If the heater runs through a normal hot water demand without shutting down, the drain issue was likely corrected. If code 79 comes back right away, stop resetting it and plan for service.
Step 5: Decide whether this is still a DIY cleanup or now a service call
Once the easy drain-path fixes are ruled out, the remaining causes usually involve internal access, leak tracing, or brand-specific service procedures.
- If you corrected a visible drain problem and the heater now runs normally, keep using it but monitor the drain outlet and the floor below the unit for the next day or two.
- If the code returns with a clear, properly sloped drain line, schedule service and tell them you already checked the condensate tubing, routing, and visible blockage points.
- If you found water inside the cabinet, shut the unit down and leave it off until it is inspected.
- If the unit is in a freeze-prone area, plan a permanent drain-line reroute or protection fix so the problem does not repeat.
A good result: If the heater stays on and no new water shows up, you can treat this as a corrected drainage issue.
If not: If it still shuts down, the next step is professional diagnosis of the internal condensate path or leak source.
What to conclude: At that point, buying random parts is usually wasted money. The fix depends on what is actually wet, restricted, or failing inside the heater.
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FAQ
What does code 79 mean on a Rinnai tankless water heater?
In plain terms, it usually means the heater sees a condensate drainage problem or water where it should not. The common causes are a blocked condensate line, a full trap, bad drain slope, or freezing.
Can I still use the water heater with code 79 showing?
Not a good idea. The unit is shutting down to protect itself. If you keep resetting it without fixing the drainage problem, you can end up with more water inside the cabinet or a repeat shutdown during use.
Is code 79 usually a bad control board?
No. On this code, a drain-path problem is much more likely than an electronic failure. Start with the condensate tubing, trap area, and any visible water before suspecting expensive internal parts.
Why did code 79 show up only during long showers?
That usually points to a slow drain rather than a total blockage. The heater can keep up for a short call, but during longer burner run time it makes enough condensate to back up and trip the code.
Should I replace the condensate line right away?
Only if the tubing is actually damaged, kinked beyond correction, brittle, or routed so badly that it cannot drain properly. If the line is just dirty or sagging, cleaning and rerouting it may solve the problem.
When should I call a pro for code 79?
Call for service if the drain line is clear and sloped correctly but the code comes back, if you see water inside the cabinet, or if you would need to open internal sections of the tankless water heater to keep going.