What causes Takagi Tankless Water Heater 291?

A clog in the condensate drain tube, such as scale, debris or sludge. A kinked, crushed or pinched drain tube. A drain line with a sag or a section that runs uphill, so water pools instead of flowing. Part of the drain (tube or neutralizer) sitting higher than the heater's drain port. A clogged neutralizer, if one is installed. A blockage in the condensate drain assembly inside the water heater. A drain line pushed down into the floor drain or standpipe with no air gap (Takagi calls for at least 2 inches between the line and the drain). The drain line connected straight to a condensate pump, which Takagi's manual says not to do.

How do I fix Takagi Tankless Water Heater 291?

Find the small drain tube coming from the bottom of the heater (the condensate drain port) and follow it all the way to where it empties. Look for kinks, crushed spots, sags or any section that rises. Takagi says the line has to slope downward the whole way, including when the drain isn't directly below the heater. Re-support or shorten the tube so it falls steadily. Check the end of the line: it should sit at least 2 inches above the floor drain or standpipe, not pushed down into it. If there's a neutralizer (a canister or tube filled with media, usually near the floor), check that it isn't clogged and that water can pass through it. Takagi says to replace the cartridge when the condensate pH drops below 6.0, which you can test with pH strips. Disconnect the drain tube at a convenient point and flush it with clean water to push out any clog. Wear gloves, since condensate is acidic. If the line outside the heater is clear and the code comes back, the blockage may be in the drain assembly inside the heater. That means opening the cabinet, so call a technician. Keep a working carbon monoxide alarm near any gas water heater, and turn the gas off and call for service if you notice exhaust smells or soot.

When should I call a pro for Takagi Tankless Water Heater 291?

Have a technician check and clear the condensate drain assembly inside the heater (Takagi's manual points to parts #416, 421 and 423). If the drain has to go up or a long way to reach a drain, have the installer reroute it; Takagi's manual says not to connect the drain line directly to a condensate pump. If the code persists after the drain is cleared, Takagi's manual says to consult the manufacturer.

Code: 291🚿 Takagi Tankless Water Heater

Takagi Error Code 291: Blocked Condensate Drain

Q: What does Takagi Tankless Water Heater 291 mean?

Error code 291 on a Takagi condensing tankless water heater means the condensate drain system is blocked. Condensing models make acidic water (condensate) that has to drain away by gravity, and when it can't, the heater stops with 291. Takagi's fix is to clear any blockage in the drain line, the neutralizer (if you have one) and the drain assembly inside the heater, and to make sure the drain line slopes downward with no sags. In Takagi's installation manuals for the condensing 140H, 240H, 340H and 540H models (T-H3M, T-H3J, T-H3S and T-H3) and the newer 240HX3, 340HX3 and 540HX3, code 291 is listed as "Blocked condensate drain system." The code shows on the built-in or remote controller, and the green LED on the computer board flashes five times (the same flash count as 101, 941 and 991, so read the controller if you can). These heaters pull extra heat out of the exhaust, which makes acidic condensate that has to run out of a drain port by gravity. Takagi says the drain must never be plugged, kinked or allowed to rise, and that every part of it (neutralizer and tube) has to sit lower than the heater so water doesn't back up into the heat exchanger. Unlike some codes that Takagi marks as clearing on their own when water flow stops, 291 isn't one of them, so expect it to stay until the drain is fixed. Non-condensing Takagi models (110, 310, 510) don't list a 291 code.

Reviewed 2026-10-24 · Based on 2 manufacturer & service sources

What the code means

In Takagi's installation manuals for the condensing 140H, 240H, 340H and 540H models (T-H3M, T-H3J, T-H3S and T-H3) and the newer 240HX3, 340HX3 and 540HX3, code 291 is listed as "Blocked condensate drain system." The code shows on the built-in or remote controller, and the green LED on the computer board flashes five times (the same flash count as 101, 941 and 991, so read the controller if you can). These heaters pull extra heat out of the exhaust, which makes acidic condensate that has to run out of a drain port by gravity. Takagi says the drain must never be plugged, kinked or allowed to rise, and that every part of it (neutralizer and tube) has to sit lower than the heater so water doesn't back up into the heat exchanger. Unlike some codes that Takagi marks as clearing on their own when water flow stops, 291 isn't one of them, so expect it to stay until the drain is fixed. Non-condensing Takagi models (110, 310, 510) don't list a 291 code.

Likely causes

A clog in the condensate drain tube, such as scale, debris or sludge
A kinked, crushed or pinched drain tube
A drain line with a sag or a section that runs uphill, so water pools instead of flowing
Part of the drain (tube or neutralizer) sitting higher than the heater's drain port
A clogged neutralizer, if one is installed
A blockage in the condensate drain assembly inside the water heater
A drain line pushed down into the floor drain or standpipe with no air gap (Takagi calls for at least 2 inches between the line and the drain)
The drain line connected straight to a condensate pump, which Takagi's manual says not to do

Safe DIY checks

⚠ Before you start

Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.

Find the small drain tube coming from the bottom of the heater (the condensate drain port) and follow it all the way to where it empties.
Look for kinks, crushed spots, sags or any section that rises. Takagi says the line has to slope downward the whole way, including when the drain isn't directly below the heater. Re-support or shorten the tube so it falls steadily.
Check the end of the line: it should sit at least 2 inches above the floor drain or standpipe, not pushed down into it.
If there's a neutralizer (a canister or tube filled with media, usually near the floor), check that it isn't clogged and that water can pass through it. Takagi says to replace the cartridge when the condensate pH drops below 6.0, which you can test with pH strips.
Disconnect the drain tube at a convenient point and flush it with clean water to push out any clog. Wear gloves, since condensate is acidic.
If the line outside the heater is clear and the code comes back, the blockage may be in the drain assembly inside the heater. That means opening the cabinet, so call a technician.
Keep a working carbon monoxide alarm near any gas water heater, and turn the gas off and call for service if you notice exhaust smells or soot.

🛠 When to call a pro

Have a technician check and clear the condensate drain assembly inside the heater (Takagi's manual points to parts #416, 421 and 423).
If the drain has to go up or a long way to reach a drain, have the installer reroute it; Takagi's manual says not to connect the drain line directly to a condensate pump.
If the code persists after the drain is cleared, Takagi's manual says to consult the manufacturer.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Condensate neutralizer

Takagi recommends one when local codes don't say otherwise; replace the media when condensate pH drops below 6.0.

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⚙

Condensate drain tubing

Use corrosion-resistant pipe such as PVC; Takagi says not to use metal pipe.

Search eBay →

⚙

Carbon monoxide alarm

Recommended for any home with a gas water heater.

Search eBay →

Sources

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.

Takagi Error Code 291: Blocked Condensate Drain

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Condensate neutralizer

Condensate drain tubing

Carbon monoxide alarm

Related codes

Sources