Rheem RTGH 8.4 GPM Condensing Natural Gas Tankless Review

Rheem's listing for the RTGH-84 publishes a fuel type, a BTU input, a maximum flow figure, condensing operation and indoor mounting. It does not publish the temperature rise that the 8.4 GPM figure was measured at, and it does not publish a UEF, dimensions, weight or minimum activation flow on this listing. Where a number below is not in that list, we say so rather than estimate it.

What we can compute honestly is what the published 157,000 BTU input produces across a range of rises, using Q = BTU x efficiency / (rise x 8.33 x 60) and an assumed 93 percent thermal efficiency for a condensing unit. That assumption is ours, not Rheem's, and it is stated here so the arithmetic can be checked.

The RTGH-84 sits at the size most two to three bathroom houses converge on once someone actually runs the numbers instead of buying to the box's headline GPM. In a moderate climate, a 50 F rise leaves close to 5.8 GPM by the assumption above, enough for a shower and a couple of fixtures running together. Because it condenses, it vents in PVC, which is materially cheaper and easier to route than the stainless steel a non-condensing unit needs, and that difference is often where the total project cost separates from the box price.

It also suits a household replacing an existing condensing unit where the PVC venting and condensate drain routing already exist, since that is the more expensive part of a first-time tankless retrofit. Run the actual fixture count against a rise appropriate to your groundwater temperature using the tankless sizing calculator before assuming this size is right.

A house with four or more bathrooms that regularly runs simultaneous draws, or a property in a genuinely cold groundwater region where a 70 F to 90 F rise is normal in winter, is asking more of 157,000 BTU than the assumed math above supports. The 180,000 BTU RTGH-90 in the same condensing indoor line exists specifically for that case, and it is worth the step up rather than living with a unit that runs cold water at the far end of a long shower.

Anyone who cannot route a condensate drain to a neutralizer, whether because the mechanical room sits far from a suitable drain or the local code requires a specific neutralizer model, should look at a non-condensing unit instead, which produces no condensate at all. And anyone planning an outdoor install where no flue routing of any kind is wanted should look at an outdoor-rated unit; the RTGH-84 is built for indoor mounting.

A condensing unit like the RTGH-84 vents in PVC as a Category IV appliance, which is the venting material savings that partly offsets the higher purchase price against a non-condensing unit. The condensate line needs to reach a neutralizer before it joins a copper or cast iron drain, since untreated tankless condensate runs mildly acidic. The gas line needs to be sized for the full 157,000 BTU input at whatever run length and fitting count the installation requires, which is a job for the gas line size calculator and the NFPA 54 / IFGC capacity tables behind it, not a guess based on the existing line to an old tank heater.

Gas line sizing, flue routing and the gas connection itself belong to a licensed plumber or gas fitter. Backdrafting and carbon monoxide from a bad vent connection are invisible. Installation labor cost estimating for a project like this is covered at houseprojectcalc.com, not here.

We did not install this unit, run water through it or measure its output. This review is compiled from Rheem's published specifications on the Amazon listing and the consensus of verified owner reviews on the same listing, cross-checked against the general engineering relationships that govern any gas-fired tankless heater. Where the listing is silent on a figure, that is stated plainly rather than filled in with an estimate presented as fact.

Two adjustments cover most cases: more input for a colder inlet or a bigger house, and the two accessories a condensing unit specifically needs.