Installation Requirements

The following Requirements must be adhered to when planning for and installing a Takagi tankless water heater. Failure to do so could result in improper operation, shortened unit life, voiding the warranty and/ or a compromise in safety.

Information and Guidelines such as water connections, temperature settings, recirculation systems, and initial cleaning of the water filter are also included.

Requirement #1: Gas Supply

Gas Meter Size: The larger BTU input of a tankless water heater may necessitate the installation of a larger meter. Check with the local utility.
Gas Line Size: All Takagi heaters have ¾ inch NPT connections. Gas line sizing shall be a minimum of ¾ inch inside diameter pipe. Gas lines shall be sized to allow for maximum BTU/hr input of said heater at the specified inlet pressure. These specifications can be found in the installation manual or on the rating plate located on the front cover of the heater. Please use the follow the standard set forth by the “American Gas Association.”
Gas Pressure: The minimum natural gas pressure at the unit should be 5.5 inches of water column, with a maximum of 10.5 inches. For liquid propane, the minimum is 11 inches and 14 inches maximum. Refer to the installation manual for proper specification or list the specs for every heater.
Gas Connection to the Heater: This can be done with hard pipe or with flexible line that is made specifically for gas. Please check your local codes. This piping MUST be no smaller than ¾ inch INSIDE Diameter. Just because a flex line uses ¾ inch connections doesn’t mean that it actually measure ¾ inch on the inside. In fact, most are ½ inch to 5/8 inch I.D. and will not supply enough gas to operate a tankless heater properly.
Gas Valve: High quality full port gas shutoff valve shall be installed per National/Local codes. Note: Some ball valves significantly reduce the diameter of the pipe and should therefore be avoided if possible.

Requirement #2: Venting

Intake Supply (Combustion Air): On the direct vent models, which draw the combustion air they need “directly” from the outside, the pipe diameter is 3 inches. This material is usually made of PVC, although galvanized steel (B-vent) and aluminum may also be used. Do not install a non-direct vent model, such as the T-M1 where there is an inadequate supply of combustion air to the heater area. Sizing for combustion air indoors shall follow the NFPA 54 code.
Exhaust Vent: All Takagi water heaters are Category III appliances, and therefore MUST be vented with Category III stainless steel, four inch diameter, exhaust gas vent pipe made especially for Category III appliances (the exception is the T-H1, which is a Category IV unit and uses a vent pipe approved for Category III appliances). The Category III pipe, which is available from several manufacturers, is called a “BH” vent pipe in Canada. Using another material, such as common B-vent, will damage the heater, possibly resulting in carbon monoxide poisoning. The entire exhaust vent must be sealed – from the top of the heater to the terminator. This is done by the high-temperature seals and locking devices built into the Category III approved vent pipe.
General Rules for Venting:

1. For best results, try to keep the vent system as short and straight as possible.
2. The vent pipe should be removable from the top of the heater for inspection and possible servicing of the unit.
3. The heater MUST NOT be common-vented with any other gas appliance or vent stack.
4. Do not upsize (oversize) the vent pipe. It must be 4 inches in diameter for the entire run.
5. For horizontal runs, either a) slope the pipe upwards from the heater at a rate of ¼ inch per foot and install a condensation drain; or b) slope it down and away from the heater at the same rate (25), so that the condensate will drain out.
  For any runs that are at a significant slope up to vertical and over 5 feet in length, always use a condensation drain to protect the heat exchanger, especially in cold weather. Refer to the manufacturers instructions for proper installation.


Water Connections – Pipe Sizing: The connections on the heater are ¾ inch NPT. Takagi specifies minimum ¾ inch piping to and from the heater. Using pipe smaller that ¾ inch will create flow and temperature fluctuations.

Reminder: Currently the T-K3, T-H2-DV, T-H2-OS, T-K3-Pro, T-M32, and
T-M50are the only heaters that should be connected in parallel to increase flow rate. Installing any other model in parallel will result in unsatisfactory performance in most cases.

When connecting multiple T-K3, T-H2-DV, T-H2-OS, T-K3-Pro, T-M32, and
T-M50 units together, install a check valve on each hot-water-out pipe. High quality, full port ball valves should be used on the hot and cold lines for servicing purposes. You may also want to install isolation valves for future servicing purposes.

You must also determine the correct header size, based on the number of tankless units installed. A general rule of thumb to use 1¼ inch pipe for 1 to 4 heaters; 1½ inch pipe for 5 to 10 heaters; and 2 inch pipe for up to 20 heaters.

Electrical Supply: All units run on 120 VAC 60 Hz, and draw no more power than about 92 watts or 0.92 amps. A dedicated circuit or GFCI is not needed unless dictated by local code. Units can be simply plugged into an outlet with an AC power cord (not suggested) or hard-wired. We suggest using and off/on switch in the latter case. A simple light switch will work.


Mounting the Units: All of Takagi’s current water heaters shall be wall mounted. The wall mounting brackets are attached to the heaters at the factory. Takagi water heaters are typically wall-mounted to save floor space for the end-user. Wall-mounting also makes it easier for the contractor to install and service. From the back of the unit, there is a 1-inch clearance to combustible surfaces and a zero-inch clearance for non-combustible surfaces.
  • All Takagi models come with a factory-installed mounting bracket.
Clearance Requirements: Each Takagi unit has different clearance requirements. Refer to the Takagi installation manual.
Freeze Protection: Takagi has installed freeze-protection devices to help guard against permanent damage to the heat exchanger in the event of below-freezing temperatures. Although these devices will attempt to protect the unit, there is no guarantee that they always will, such as in the case of power loss to the unit. This type of damage is not covered under warranty, so great care should be taken to avoid freezing conditions.

Each unit has a freeze-protection sensor mounted to the fan. It senses the incoming or ambient temperature. When the temperature drops below 37°F, the unit will turn on ceramic heaters located around the heat exchanger and water lines. Again – any type of power failure would cause this system not to work.

In freezing climates where the unit is installed inside, Takagi recommends that the freeze protection sensor be relocated outside if possible or at least attached to a surface that more accurately reflects the temperature of the incoming air. An extension cable is provided with each unit for this purpose.

In addition to the sensor, the backflow preventer (Part No. TV-TV03) MUST be installed wherever air at below-freezing temperatures may be drawn down the exhaust pipe into the heater a backflow preventer used by the venting manufacturer is acceptable. This device shall be installed on top of the heater. Although the backflow preventer helps to limit the flow of freezing air, it does not block all of it. In very cold climates, care must be taken to insure power to the unit, and to eliminate any negative air pressure conditions in the building that would promote cold-air flow in the heater. If this cannot be avoided, the unit should be thoroughly drained to prevent damage.
Pressure Relief Valve: A “pressure only” relief valve shall be installed according to national/state/local codes. A temperature & pressure relief valve is not required unless required by applicable codes.
Temperature Settings: All heaters come preset with a default setting of 122°F (Our T-K3 has a total of 8 settings). This temperature may be changed to three other settings via dipswitches located on the computer boards. Please see service manuals or the Takagi Tankless Translator for these settings.

Please note that studies have found that 122°F is the ideal temperature for most residential and many commercial applications. Try to avoid the old “tank habit” of raising the temperature to “get more hot water.” This will, in fact, produce a lesser volume of hot water than leaving it at its default setting.

The heaters may also be set to 17 different temperatures, using the appropriate Remote Control, depending on the heater model. See the Product Descriptions or Accessories page to find out the right Controller for your specific unit.
Outdoor Switch: Make sure this switch is set correctly for your heater’s location. The default setting for every Takagi unit is “indoor”.
Recirculation Systems: Tankless water heaters may be used with hot water recirculation systems. Tankless water heaters provide a continuous flow of water at a consistent temperature. They do not provide “instant” hot water, despite “aggressive advertising” and wishful thinking. Just as a conventional water heating system must move the hot water from the tank to the fixture or appliance – which takes time – so does a tankless system. In fact, a tankless system may take a few seconds more to turn on and heat the water. This, however, is a small price to pay for its benefits.

There are wrong ways to install a “recirc”

Wrong Way No. 1: Install a pump that is either too small, such as what was used on the previous tank-type heater; or one that is too large, such as what may have been recommended to “cover all the bases.” “ Generally, if the pump is sized too small, it will not provide enough flow to activate a Takagi tankless heater. This is especially true if the return piping is small or is an extended line with several elbows.” Takagi requires a minimum high head 1/12 hp pump.
Wrong Way No. 2: “Install a high head pump that is too large.” such as a We specify a minimum 1/12 hp pump.

This will provide enough flow to turn the heater on, but may also cause unnecessary wear on all the pipes, valves and heat exchanger. It will also waste energy. This size pump would be required, however, in a large house with long runs or with a small return pipe.

The Right Way: Use engineering formulas to estimate – as closely as possible – the approximate dynamic head loss through the recirculation loop. Then choose the correct pump from manufacturer’s data, based on flow rate at a certain head.

If these formulas are not practical, try to estimate the size of your return line run. How long and how large of a pipe run is it? In general, a long run with many elbows and a small diameter (½ inch, for example) will require a larger pump to draw the water back to the unit.

You should be looking for approximately 2 to 4 GPM of flow for most domestic hot water recirculation systems. This should be enough to turn the unit on reliably and not cause premature wear or energy waste. Use a Takagi remote to determine the flow rate, which can be adjusted using a ball valve on the first pump or the three-speed switch on the second.

Comparable pumps by other manufacturers would be a Taco 003-BC-1 for the 15-42, or a Taco 009-BF5 for the 26-96, or equal.

Having “instant hot water” at the faucet is a wonderful benefit, bit it can also be a costly one and actually defeat the purpose of tankless water heating, which is to save energy and money. Make sure you install an aquastat (a thermostat for water) and a timer on the pump to limit activation frequency and duration.

For example, the timer may be set to early morning (before showers) and afternoon/evening hours. Also, if at all possible, install insulation on the supply and return lines to avoid heat loss.
One system that performs well with tankless is a Metlund D’MAND System: the S-70T-PF-R, for small homes and the S-02T-PF-R for larger residences. Neither model needs a separate return line, because they use the existing cold-water line to return the water from the farthest fixture. Consult the manufacturer for more details.

Clean Water Filter After Initial Run: There is a small screen filter on the cold-water input connector of each heater. After running the heater for five minutes during setup, turn off the water supply and remove this screen to check for debris. Some screens may be removed easily with a half-inch copper pipe wire brush. You may choose to install an extra cartridge filter on the heater’s cold supply to avoid problems with small filter clogging prematurely.