Takagi AT-D3U-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:

S
Samantha GardnerAug 13, 2025
Why does my Takagi AT-D3U-OS not ignite when water runs through it?
- A
  Alexander AriasAug 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.
F
Francisco HawkinsAug 17, 2025
Why is my Takagi AT-D3U-OS Water Heater water not hot enough?
- R
  Richard CooperAug 17, 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
Sheila ClarkAug 20, 2025
What to do if hot water is not available from my Takagi Water Heater?
- T
  Tiffany HallAug 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.
K
Kerri AvilaAug 25, 2025
How to get hot water faster from my Takagi AT-D3U-OS?
- M
  margaret82Aug 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.
T
Teresa AtkinsonAug 28, 2025
Why is the hot water temperature fluctuating with my Takagi AT-D3U-OS Water Heater?
- A
  Alexis Estrada DVMAug 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.
C
Chelsea SmithSep 1, 2025
What to do if my Takagi AT-D3U-OS's hot water turns cold and stays cold?
- C
  Carl JohnsonSep 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.
K
Kimberly HardinSep 4, 2025
Why is the display not working on my Takagi AT-D3U-OS Water Heater controller?
- A
  Amy BarnesSep 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
Monica LucasSep 9, 2025
Why does the fan motor keep spinning on my Takagi AT-D3U-OS after it stops?
- R
  Robert RodriguezSep 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-D3U-OS Specifications

General

Category	Water Heater
Type	Tankless
Fuel Type	Natural Gas
Flow Rate	10 GPM
Venting	Direct Vent
Heating Capacity	199, 000 BTU
Maximum BTU Input	199, 000 BTU
Minimum BTU Input	15, 000 BTU/hr
Warranty	15 years heat exchanger, 5 years parts
Energy Efficiency	0.82 UEF

Summary

Specifications

Safety Guidelines

General Safety Precautions

General safety precautions for installation and use.

Installation

General Installation Guidelines

General guidelines for installing the water heater.

Clearances

Required distances around the water heater for safe installation.

Warning for Installations

Critical safety warnings before starting installation.

High-Altitude Installations

Venting Instructions

Detailed instructions for proper venting of the appliance.

Combustion Air Supply

Requirements for providing adequate air for combustion.

Vent Length and No. of Elbows

Guidelines for maximum vent pipe length and elbow usage.

DIP Switch Settings for Vent Length

Configuring DIP switches based on vent length.

Clearances for Sidewall Terminations

Clearances for Rooftop Terminations

Vent Termination Clearances

Gas Supply and Gas Pipe Sizing

Guidelines for gas supply, pipe sizing, and connections.

Water Connections

Instructions for connecting water lines and relief valve.

Electrical Connections

Temperature Remote Controller

Easy-Link System

Multi-Unit System

Initial Operation

Steps for first-time startup and operation.

Operating Safety

Before Operating Safety

Essential safety reading before operating the unit.

Operating Instructions

Turning Off Gas to Appliance

Normal Operation

Controller Overview

Temperature Settings

Set Temperature

Temperature Settings on the PCB

Freeze Protection System

Maintenance and Service

Unit Draining and Filter Cleaning

Troubleshooting

General Troubleshooting

Water Temperature and Flow Issues

Error Codes

Error Code Overview

Fault Analysis of Error Codes

Limited Warranty

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