TK-310U-I Indoor Tankless Water Heater
by Takagi

Make sure there is plenty of hot water for everyone with these remarkable indoor tankless water heaters by Takagi. This relatively compact unit is powerful enough for up to three bathrooms, but doesn't take up a lot of space like the traditional water heaters. Available in Propane or Natural Gas, the tankless heater you want is right here at®!
We also offer many more tankless water heaters by Takagi:
TK-110U-I (indoor) - TK-110U-E (outdoor) - TK310U-E (outdoor) - TK-510U-I (indoor) - TK-510U-E (outdoor)

Takagi TK-310U-I - Tankless Gas Water Heater for Indoor Installations

(Replaces T-K4-IN)
TK-310U-I indoor tankless water heater

The Takagi TK-310U-I has the capacity to handle the needs for families of 4 or 5, with multiple showers going, the washing machine, and dishwasher, and you won't have to worry about running out of hot water. This slim, powerful, gas tankless water heater will easily take the place of your old 80 gallon water heater.

From Takagi's computer-controlled burners and electronic ignition (only using a specific amount of gas required for heating the water you demand), high flow rates (enough water for everyone to have their hot shower), direct venting (this is usually required when you install any gas heater in a space where you live and breathe -for safety reasons), to going tankless (no tank to maintain and heat), you're probably going to have more time and money for other things. Plus Takagi's tankless water heaters have been around for years and are of the highest quality as well as trouble-free.

The most important thing about tankless water heaters is in understanding the technology. When you open a faucet, or take a shower, the hot water will move like your "old" tank water heater, but instead of eventually running out of hot water, you now never will. Taking long showers is now possible with these tankless water heaters! Save time and money!

Use Natural Gas or Liquid Propane Gas - 8 gpm 190,000 BTU/hour

Ultra-Low NOx compliant with less than 14 ng/J (<20ppm)
Space saving size: 20-1/2" x 13-3/4" x 10"
Indoor models have a built-in temperature controller

View more detailed specifications
View flow rate and pressure loss information
Looking to install this tankless water heater outside?
Browse our selection of outdoor tankless water heaters.

TK-310U-I Indoor Tankless Water Heater = $1,284.59
 OPEN BOX SPECIAL TK-310U-I Indoor Tankless Water Heater = $1,075.99
Factory Equipped for Natural Gas with Included Liquid Propane Conversion Kit
Add installation accessories for IPS threaded connections
Add installation accessories for copper sweat connections

Please note: Some areas do not allow installation of a few of these water heaters. Read more

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Related Items & Accessories

Please note: For potential safety reasons, we do not recommend using PVC to vent gas tankless water heaters. For the best results, we recommend using Category 3 stainless steel gas vent pipe for all gas tankless water heaters.

Common Terms and Definitions
  • C, SWT, S or Socket: A regular hub or socket. The pipe fits (sweats/solders) inside this
  • S x S: Regular hub or socket on both ends
  • Mips: Male pipe threads on the outside of fitting
  • Fips: Female pipe threads on the inside of fitting
  • FTG: The same size as the pipe. Will fit inside a socket (hub) fitting
  • Hyset: Has a nailing flange on the front of the fitting
  • Dropear: Has a nailing flange on the back of the fitting
  • Nominal: Used to describe copper pipe and fittings in plumbing. Refers to the inside diameter of the pipe which varies by thickness of the pipe. The outside is a constant size and is always 1/8" larger than the nominal size. Example: 1/2" nominal is always 5/8" O.D.

Frequently Asked Questions

Q. "What type of vent do these tankless water heaters require?"
A. These tankless water heaters require Category III stainless steel vent only.

Q. "There are many companies selling all kinds of tankless water heaters. Why should I buy a Takagi tankless gas water heater from you rather than elsewhere?"
A. We are a large, reputable company that was here yesterday and we will be here tomorrow. We are often copied, but we don't believe that any one can match our service and certainly our base of satisfied customers is larger than any Internet plumbing supplier since 1995. We sell 1000's of other quality plumbing products as well, so you can rest assured that we know plumbing products. We are a very stable and reputable company. We love to sell what we believe in, and we definitely believe in Takagi.

Q. "If these units don't have a tank, how do they know when to turn on?"
A. These tankless water heaters have a flow switch built into them. When you turn on the hot side of a faucet the water begins moving. Once the water is traveling at minimum flow rate for the heater, the flow switch will turn on the gas.

Q. "Why can't I install the TK-310U-I myself?"
A. Improper installation of the TK-310U-I could create carbon monoxide gas and other dangerous gases. Installation of this unit by persons other than professionally trained technicians voids the warranty.

Q. "What's the difference between a conventional flue and a direct vent?"
A. A conventional flue is what most people are familiar with. It typically consists of a double-wall flue pipe going from the top of the heater through the roof, venting outside. The fixture draws its combustion air from the space around it (utility room, garage, hallway, etc.). A direct-vent unit vents and draws its combustion air through a specially designed pipe from the outside via an adjacent wall.

Q. "Can I use PVC to vent this tankless water heater?"
A. No. The temperature of the exhaust is too high to safely use PVC as venting material. We never recommend PVC as venting even where the ideal exhaust temperatures would technically allow it due to a much higher incident of dangerous issues associated with PVC venting on poorly-maintained gas tankless water heaters.

Q. "I live in an area where it freezes, what can I do to protect the heater from freezing?"
A. We recommend you use the Nova Vent 9007996005 backflow preventer kit to keep cold air from entering the vent and causing the heat exchanger to freeze.

Q. "Will an instantaneous water heater save me money on my utility bills?"
A. In many applications you can see a savings. There are some variables that could reduce the amount you save. One example is you will not run out of hot water, so longer showers could cut into your savings. Also, the cost of electricity, propane and natural gas in your area will affect the amount of money saved. The higher the cost for the energy type used to heat your water, the quicker you can recover the higher initial cost of installing a tankless versus tank style water heater.

  • You will never run out of hot water. "You want an hour long shower? No problem!"
  • Electric units can be installed at the point of use.
  • You're not paying energy costs to heat water 24 hours a day.
  • Our electric units don't require a T&P valve (no tank!) - if code allows.
  • Installing a tankless water heater can indeed be a bonus to the environment. (Why choose wastefulness when we're talking about finite natural resources?)
  • They all need a minimum flow rate & pressure to turn on.
  • Electric units need heavy gauge wire, while gas units need a much larger, special flue pipe and larger gas supply than a conventional water heater.
  • At times they can produce very hot water. It's easier to get scalded - but this is easily mitigated with anti-scald valves.
  • With some brands, parts are hard to find (not a negative if you originally purchase from us, as we like to carry parts for the units that we sell).
Tankless water heaters have their place. They're not "the answer" to every hot water need, but if they're used correctly they can be the solution to many of them.

Q. "Your products specify the 'temp. rise' based on the GPM flow; what do you mean by this?"
A. Temperature rise is the amount the heater raises the temperature of the incoming water, based on the flow rate shown of that water.

Q. "What's the first hour rating of this water heater?"
A. Traditional tank water heaters have a first hour rating which is determined by the amount of hot water the heater can produce in an hour when starting with a full tank of hot water. As tankless water heaters do not start with a tank of hot water, they don't really have a first hour rating the same way a tank water heater does. Instead, tankless water heaters have a gallons per hour rating, which is determined by how many gallons per hour the tankless water heater can produce when raising the incoming water temperature a specified amount.

Q. "What are the best reasons to install a Scale Inhibitor System?"
A. The system inhibits scale build-up and protects the piping from corrosion, which helps maintain efficiency and extends the life of the system.

Q. "How does the Water Heater Scale Inhibitor System prevent corrosion?"
A. It forms a thin protective layer on the inside of pipes and metal parts. This film or layer is microscopic in thickness and will not build up over time.

Q. "What happens if you don't change cartridges often enough on the Scale Inhibitor System?"
A. The original water condition and problem will reappear. Hardness will begin to build up again, which you want to avoid.

Q. "What is siliphos?"
A. Siliphos is a water treatment compound; a glass-like Polyphosphate containing Sodium, Calcium Magnesium and Silicate, formulated specifically for the treatment of water problems and to prevent the build-up of scale in plumbing. It is made from a blend of food grade organic phosphates, and has been approved by the USDA for drinking water and all food applications. It is safe for ingestion by humans, animals, and plants, and does not build up in waste water.

Q. "Is the Water Heater Scale Inhibitor System's active ingredient safe?"
A. Yes! All ingredients used are 100% pure food grade materials. They are approved by the USDA and meet purity requirements as a food additive by the World Health Organization.

Q. "Will the Water Heater Scale Inhibitor System change the taste or odor of water?"
A. No. The active ingredient is colorless, tasteless, and odorless when used as recommended.

All Takagi tankless water heaters must be installed by a licensed professional.

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