Takagi T-H3S-OS-N Condensing Outdoor Tankless Review

Takagi's listing states natural gas fuel, condensing operation and outdoor mounting directly. The listing title also states "8-Gallon Per Minute," which we treat as a stated maximum flow figure from the manufacturer's own title, not a number this review calculated. What the listing does not state, in the fields we draw from, is a BTU input, a temperature rise for that 8 GPM figure, a UEF, dimensions or weight.

This unit suits a household that specifically wants both things a typical listing forces you to choose between: no flue to route, and condensing efficiency. That combination fits a warm climate property (outdoor mounting is generally not advisable in a hard freeze climate) where the owner still wants the operating efficiency and lower gas usage of a condensing unit rather than settling for the non-condensing outdoor alternative.

It also suits a retrofit where an exterior wall has easy access to gas, water and a suitable condensate drain path, since eliminating the flue does not eliminate the need to route condensate somewhere.

Anywhere winter brings a hard, sustained freeze is a poor fit for any outdoor tankless heater, condensing or not, because freeze protection depends on continuous power and an outage during a freeze is exactly when protection is needed most. A property in that climate is better served by an indoor unit inside the building envelope; see our cold climate picks.

Anyone who wants to size a purchase against a published BTU input and rise curve before buying should also be cautious, since this listing states a flow figure but not the input or rise behind it. Confirming the actual specification sheet from Takagi before ordering, especially for a gas line sizing decision, is worth the extra step.

Mounting outdoors removes the flue run entirely, the same installation saving as any other outdoor unit. What an outdoor condensing unit does not remove is condensate: the unit still produces it, and it still needs to drain to a neutralizer rather than directly onto the ground or into a storm drain, which is easy to miss when there is no indoor flue or condensate line to plan around. Gas line sizing should be confirmed against Takagi's actual BTU input figure from the manufacturer, then checked against the gas line size calculator and NFPA 54 / IFGC capacity tables rather than assumed from the listing's flow number alone.

Gas and electrical connections belong to a licensed plumber or electrician. Installation labor cost estimating is covered at houseprojectcalc.com.

We did not install this unit outdoors or measure its condensing performance in that mounting configuration. This review is compiled from Takagi's published Amazon listing, including its title's stated flow figure, and the consensus of verified owner reviews on that listing, plus general knowledge of how condensing heat exchangers and outdoor mounting each work independently. Where the listing does not publish a figure, that is stated directly rather than estimated.

If the condensing-plus-outdoor combination is not required, the alternatives below simplify the installation in one direction or the other. Both accessories below are still needed regardless.