Takagi AT-K5U-IN User Manual

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Gas Tankless Water Heater

TM

Suitable for combination potable water heating

and space-heating

Please refer to local codes for space-heating compliance.

FEATURING

 • ENDLESSHOTWATER

 • ON-DEMANDUSAGE

 • COMPACT,SPACESAVING

 • ENERGYCONSERVATION

 • COMPUTERIZEDSAFETY

 • NOPILOTLIGHT

 • ComplieswithSCAQMDRule

1146.2forNOxemissionsrequire-

mentof14ng/Jor20ppm

 • FIELDGASCONVERTIBLE

 • EASY-LINKSYSTEMAND

MULTI-UNITSYSTEM

(510U (AT-D3U-IN/OS) only)

Ifyouhaveanyquestions,please

callorwriteto:

500TennesseeWaltzParkway

AshlandCity,TN37015

TollFree:1-877-737-2840

On-Demand Water Heater

Installation Manual and Owner’s Guide

- Do not store or use gasoline or other

flammablevaporsandliquidsinthevicinity

ofthisoranyotherappliance.

- WHATTODOIFYOUSMELLGAS

 • Donottrytolightanyappliance.

 • Donottouchanyelectricswitch,donot

useanyphoneinyourbuilding.

 • Immediatelycallyourgassupplierfrom

aneighbor'sphone.Followthegas

supplier'sinstructions.

 • Ifyoucannotreachyourgassupplier,call

thefiredepartment.

- Installation and service must be performed

byaqualifiedinstaller,serviceagencyorthe

gassupplier.

WARNING

If the information in these

instructions is not followed

exactly,afireorexplosionmay

resultcausingpropertydamage,

personalinjuryordeath.

ANSI Z21.10.3 ・ CSA 4.3

510U (AT-D3U-IN/OS)

only

Models

• 110U Outdoor (AT-KJr3U-OS)

• 310U Outdoor (AT-K5U-OS)

• 510U Outdoor (AT-D3U-OS)

• 110U Indoor (AT-KJr3U-IN)

• 310U Indoor (AT-K5U-IN)

• 510U Indoor (AT-D3U-IN)

Series 200

Table of Contents

Questions and Answers:

J
Julie JohnsonAug 13, 2025
Why does my Takagi AT-K5U-IN Water Heater not ignite when water runs through it?
- J
  Jason EsparzaAug 14, 2025
  If your Takagi Water Heater doesn't ignite when water flows through it, ensure the flow rate is over 0.5 GPM. Check the filter on the cold water inlet for any blockages. Inspect for reverse or cross connections. If you're using a remote controller, ensure it's powered on. Verify that the inlet water temperature isn't too close to the set temperature. Confirm the gas supply is turned on and that the unit has power, with the power button on either the controller or inside the water heater activated.
J
joneswilliamAug 17, 2025
Why is my Takagi AT-K5U-IN Water Heater water not hot enough?
- B
  bsimonAug 18, 2025
  If your Takagi Water Heater isn't producing hot enough water, several factors could be the cause. Begin by comparing the flow and temperature using the charts. Check for cross plumbing between cold and hot water lines. Ensure the gas supply valve is fully open and the gas line is properly sized, with the supply pressure within specified limits. Verify that the set temperature isn't too low and that the Easy Link or Multi-Unit system is correctly configured.
S
Susan ThompsonAug 21, 2025
What to do if hot water is not available from my Takagi Water Heater?
- E
  Erika BarnesAug 21, 2025
  If you're not getting hot water from your Takagi Water Heater when you open a fixture, check the following: ensure the unit has a 120 VAC, 60 Hz power supply and that the power button on the controller or inside the water heater is turned on. Also, verify that both the gas and water supply valves are fully open, and the filter on the cold water inlet is clean. Make sure the hot water fixture is sufficiently open to draw at least 0.5 GPM through the water heater, and check that the unit is not frozen. Finally, inspect for cross plumbing between the hot and cold water lines, and ensure the Easy Link or Multi-Unit system is set up correctly.
W
wjonesAug 25, 2025
How to get hot water faster from my Takagi AT-K5U-IN Water Heater?
- C
  Christopher RodriguezAug 26, 2025
  The time it takes for hot water to reach your fixtures from your Takagi Water Heater depends on the length of piping and the pipe size between the two. The longer the distance or the bigger the pipes, the longer it will take to get hot water. If you want hot water more quickly, consider a hot water recirculation system.
L
Laura MerrittAug 28, 2025
Why is the hot water temperature fluctuating with my Takagi AT-K5U-IN?
- J
  Jordan FarmerAug 28, 2025
  If you are experiencing temperature fluctuations with your Takagi Water Heater, ensure the filter on the cold water inlet is clean. Check that the gas line is properly sized and the supply gas pressure is within specified limits. Also, check for any cross connection between the cold and hot water lines, and verify that the Easy Link or Multi-Unit system is set up correctly.
J
jonathanstuartSep 1, 2025
What to do if my Takagi AT-K5U-IN Water Heater's hot water turns cold and stays cold?
- D
  Dennis PatelSep 1, 2025
  If the hot water from your Takagi Water Heater turns cold and stays cold, first check if the flow rate is enough to keep the water heater running. If you have a recirculation system installed, ensure the recirculation line has enough check valves. Also, verify that the gas supply valve is fully open and the filter on the cold water inlet is clean. Lastly, ensure that your fixtures are clean of debris and obstructions.
S
sullivanryanSep 5, 2025
Why is the display not working on my Takagi AT-K5U-IN Water Heater controller?
- K
  kelleydavidSep 5, 2025
  If the built-in or remote controller of your Takagi Water Heater doesn't display anything when turned on, first ensure the unit is receiving power. For the remote controller, check that the connection to the unit is correct. Note that when the built-in controller is turned on, the STAND BY LED should be lit. Also, if the unit hasn't been used for five minutes or more, the display might turn off to save energy. Finally, in an Easy-Link System, only the parent unit's controller will display the set temperature.
M
Mikayla WilsonSep 9, 2025
Why does the fan motor keep spinning on my Takagi Water Heater after it stops?
- C
  Christopher GarzaSep 9, 2025
  It is normal for the fan motor of your Takagi Water Heater to continue running for 15 to 70 seconds after operation has stopped. This allows for quick re-ignition and purges exhaust gas from the flue.

Takagi AT-K5U-IN Specifications

General

Fuel Type	Natural Gas
Energy Factor	0.82
Efficiency	82%
Ignition Type	Electronic
Model	AT-K5U-IN
Type	Tankless

Summary

Safety Guidelines

Safety Definition

Defines DANGER, WARNING, CAUTION, and NOTICE indicators used in the manual.

Installation

General Installation Rules

Provides general guidelines and requirements for installing the water heater.

Clearances

Warning for Installations

Venting Instructions

For Indoor Models

Venting instructions specifically for indoor model installations.

Direct Venting Installation

Combustion Air from the Room

General Rules for Venting Water Heaters

Combustion Air Supply

Calculate the Air Volume of the Room

Determining room volume to ensure adequate combustion air supply.

Calculate Required Air Volume

Calculating the necessary air volume based on appliance BTU/h ratings.

Install with Outside Ventilation

Determine Type of Ventilation

Determine Minimum Free Area Required for Each Vent Opening

Vent Length and No. of Elbows

DIP Switch Settings for Vent Length

Single Pipe with Room-Air Intake

Two-Pipe, Direct-Vent Installation Examples

Clearances for Sidewall Terminations

Clearances for Rooftop Terminations

Vent Termination Clearances

Gas Supply and Gas Pipe Sizing

General

Gas Connections

Instructions and precautions for making gas connections to the water heater.

Measuring Inlet Gas Pressure

Water Connections

Pressure Relief Valve

Electrical Connections

Easy-Link System

General

Easy-Link Connection Procedures

Step-by-step guide for connecting multiple water heaters using the Easy-Link System.

Multi-Unit System

Initial Operation

Operating Safety

Normal Operation

Built-in Controller and Remote Controller

Temperature Settings

Temperature Settings on the PCB Without Controller

Freeze Protection System

Maintenance and Service

Unit Draining and Filter Cleaning

Procedure for draining the unit and cleaning the cold water inlet filter.

Troubleshooting

Error Codes

General

General explanation of the unit's self-diagnostic functions and error code display.

Single Unit Installations

For the 510U (AT-D3U-IN/OS) Model in an Easy-Link System

Explains error code display for 510U models within an Easy-Link System.

Fault Analysis of Error Codes

Limited Warranty

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