Rheem RTG 8.4 GPM Outdoor Natural Gas Tankless Review

The listing publishes a 180,000 BTU input, non-condensing operation, natural gas fuel, outdoor mounting and a maximum flow figure of 8.4 GPM with no rise stated. It does not publish a UEF, dimensions, weight or minimum activation flow. Using the published BTU input and an assumed 82 percent thermal efficiency for a non-condensing unit, here is what that input produces across a range of rises.

An outdoor non-condensing unit earns its keep in a warm climate retrofit where the real cost of an indoor tankless is not the box, it is cutting a flue path through a roof or an exterior wall and running category III stainless the whole way. Mounting outdoors on an exterior wall skips that entirely: the intake and exhaust are both immediately at the unit. That is a genuine, sizeable installation saving in the right climate.

It also suits a straightforward gas line retrofit where 180,000 BTU is within what the existing service and meter can support, checked against the gas line size calculator rather than assumed.

Anywhere winter brings a hard, sustained freeze, an outdoor tankless heater is exposed in a way an indoor unit inside a heated envelope is not. Manufacturers build freeze protection into these units, but that protection depends on continuous electrical power; a winter outage during a hard freeze is exactly the scenario that damages an outdoor unit's internal plumbing. If the property is in a genuinely cold climate, an indoor unit like the RTGH-84 protected inside the building envelope is the safer long-term choice, and our cold climate tankless picks cover that case directly.

Anyone ordering by fuel type without checking twice should also pause here: this is the natural gas version, and the propane RTG-70 outdoor sibling looks nearly identical on the page. The two are not interchangeable.

The headline advantage is also the whole installation: mounting on an exterior wall with direct intake and exhaust, no flue pipe run through the house at all. What remains is sizing the gas line for the full 180,000 BTU input, which is the same NFPA 54 / IFGC capacity table exercise as any other gas appliance and does not get easier because the unit sits outside. A gas connector rated for outdoor and appliance-flex use, plus isolation valves for the annual descale, are the accessory layer this unit actually needs, since it has no vent kit to buy.

Freeze protection heater elements inside the unit need continuous power; if the property loses power during a hard freeze, that protection stops working. This is a real limitation, not a footnote, in any climate that sees sustained sub-freezing temperatures. Installation labor cost estimating belongs to houseprojectcalc.com, not this review.

We did not install this unit outdoors, run it through a freeze cycle or measure its flow. This review is compiled from Rheem's published listing specifications and the consensus of verified owner reviews on the same listing, plus the general engineering relationships common to non-condensing gas tankless heaters. Freeze exposure is a widely documented characteristic of outdoor tankless units generally, not a claim specific to hands-on testing of this model.

The propane version of this same chassis is the most common ordering mistake on this listing. The two accessories below are what an outdoor gas connection actually needs.