Takagi AT-K5U-OS - 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:

D
Derek HobbsAug 12, 2025
Why does my Takagi AT-K5U-OS Water Heater not ignite when water runs through it?
- O
  ogoodmanAug 12, 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
James HolmesAug 16, 2025
Why is my Takagi AT-K5U-OS water not hot enough?
- A
  Alexander LesterAug 16, 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.
D
David MedinaAug 20, 2025
What to do if hot water is not available from my Takagi AT-K5U-OS?
- C
  Christopher CombsAug 20, 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.
M
Mrs. Carolyn MartinezAug 25, 2025
How to get hot water faster from my Takagi AT-K5U-OS Water Heater?
- A
  Ashley TateAug 25, 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.
M
Michael RothAug 27, 2025
Why is the hot water temperature fluctuating with my Takagi AT-K5U-OS?
- H
  heidi95Aug 27, 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.
R
roweryanAug 31, 2025
What to do if my Takagi Water Heater's hot water turns cold and stays cold?
- V
  Vanessa GoodmanAug 31, 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.
B
Bryan YoungSep 3, 2025
Why is the display not working on my Takagi AT-K5U-OS controller?
- M
  Morgan FloresSep 4, 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.
J
Jill GomezSep 7, 2025
Why does the fan motor keep spinning on my Takagi Water Heater after it stops?
- K
  Kimberly EllisSep 8, 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-OS Specifications

General

Model	AT-K5U-OS
Category	Water Heater
Type	Tankless
Fuel Type	Natural Gas
Flow Rate	5.0 GPM
Flow Rate at 45°F Rise	5.0 GPM
Efficiency	82%
Weight	60 lbs
Vent Type	Direct Vent
Energy Efficiency	0.82
Dimensions (H x W x D)	13.8 x 8.3 x 23.6 inches
Venting	2-inch PVC

Summary

Specifications

Safety Guidelines

Safety Definition

Defines terms like DANGER, WARNING, CAUTION, NOTICE for hazard identification.

General Safety Rules

Installation

General Installation Requirements

Covers general rules for installing the water heater according to codes and safety.

Clearances

Warning for Installations

Venting Instructions

General Indoor Venting

General rules for venting indoor models and air intake requirements.

Direct Venting Installation

Specific requirements for direct vent installations and pipe support.

Combustion Air From Room

General Rules for Venting Water Heaters

Combustion Air Supply

Vent Length and Elbows

Standard Vent Terminations

Installation with Vent Termination Kit

DIP Switch Settings for Vent Length

Two-Pipe, Direct-Vent Installation Examples

Horizontal Installation Examples

Vertical Installation Examples

Horizontal Installation with Vent Kit

Single Pipe with Room-Air Intake

Two-Pipe, Direct-Vent Sidewall Installation

Outdoor Installation DIP Switch Settings

Clearances for Sidewall Terminations

Clearances for Rooftop Terminations

Vent Termination Clearances

Gas Supply and Gas Pipe Sizing

General Gas Supply

General guidelines and warnings for gas supply and pipe sizing.

Gas Connections

Measuring Inlet Gas Pressure

Water Connections

Pressure Relief Valve

Electrical Connections

Easy-Link System

General Easy-Link System

Easy-Link Connection Procedures

Multi-Unit System

Initial Operation

Operating Safety

Safety Warnings Before Operation

Critical safety precautions to read before operating the unit.

Operating Instructions

Step-by-step guide for starting and operating the water heater.

Normal Operation

Built-in and Remote Controller Operation

General Normal Operation

Temperature Settings

Setting the Temperature

Temperature Settings on PCB (Without Controller)

Freeze Protection System

Maintenance and Service

Unit Draining and Filter Cleaning

Procedures for draining the unit and cleaning the inlet filter.

Troubleshooting

General Troubleshooting

General problems and solutions related to water temperature and flow.

Temperature and Amount of Hot Water Issues

Specific troubleshooting for issues with hot water availability and temperature.

Error Codes

General Error Code Information

Information on the unit's self-diagnostic functions and error code display.

Single Unit Installation Error Codes

Error codes and their meanings for single unit installations.

Fault Analysis of Error Codes

Output Temperature Chart

Limited Warranty

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