The nameplate GPM is measured at the easiest condition
Rheem and Noritz each sell on a single flow figure quoted at the smallest temperature rise they tabulate. Noritz lists the EZ111DV inside the EZTR75 bundle at 11.1 GPM, at a 30°F rise. Read down the same tables and the numbers drop quickly. The Rheem delivers 7.9 GPM at a 45°F rise and 5.3 GPM at 67°F. The Noritz delivers 7.8 GPM at 50°F, 6.5 GPM at 60°F and 5.5 GPM at 70°F. For both, the figure at a 67-70°F rise is roughly half the nameplate.
A related headline-versus-reality gap turns up in patio heater BTU ratings.
Working out the rise your house needs
Temperature rise is your output setpoint minus the temperature of the water arriving at the heater. The manufacturer tables are indexed by rise, so this is the one number you need before you can read them. A thermometer under a cold tap in the coldest month gives you the incoming figure. Subtract it from the output temperature you intend to set and find that row.
Then add up the fixtures you expect to run together. For this guide that is two showers and a dishwasher. Each fixture’s rated flow appears on its own specification, and the total is the load the table has to meet in your row, not in the 35°F row. If the total exceeds the figure for your rise, the heater is undersized for that combination at that time of year.
Only three of the five publish a figure at a realistic rise
This is where the set divides. Noritz publishes a 13-point table running from a 30°F rise to 140°F: 11.1 GPM at 30°F, 9.7 at 40°F, 8.7 at 45°F, 7.8 at 50°F, 6.5 at 60°F, 5.5 at 70°F, 4.8 at 80°F, 4.3 at 90°F, 3.9 at 100°F, and on down to 2.8 GPM at 140°F. Rheem publishes three points, 10.1 GPM at 35°F, 7.9 at 45°F and 5.3 at 67°F, which is enough to size against if your rise falls near one of them. A.O. Smith’s ATHR-199X3 has one confirmed point, 5.9 GPM at a 67°F rise, which matches the maximum flow on its ENERGY STAR High-Usage record.
Neither Rinnai unit has a manufacturer flow-by-rise table in the documents retrieved for this guide. For a buyer whose whole problem is flow at a high rise, this gap is the deciding weakness of both Rinnai picks.
At the one rise where three units can be compared, the spread is narrow. A.O. Smith’s 5.9 GPM at 67°F, Noritz’s 5.5 GPM at 70°F and Rheem’s 5.3 GPM at 67°F sit within 0.6 GPM of one another, and the Noritz figure is quoted at a slightly harder rise than the other two.
Gas input: 180,000 against 199,900 BTU/h
Four of the five publish a maximum input of 199,000 or 199,900 BTU/h. Rheem’s is 180,000 BTU/h, about 10 percent less heat to spend. At 45°F, the one rise both manufacturers tabulate, Rheem publishes 7.9 GPM against the Noritz’s 8.7 GPM. Above 67°F, Rheem publishes nothing. Noritz does, all the way down to 2.8 GPM at 140°F.
Minimum input matters at the other end of the range, because it is the smallest flame the burner can hold. A.O. Smith publishes 9,000 BTU/h and, per retailer specification data, a 22:1 turndown ratio. Rheem publishes 11,000 BTU/h, both Rinnai units 15,000 BTU/h, and Noritz 18,000 BTU/h with a 10/11:1 turndown per unit.
Recirculation ships standard on two of these
Rheem’s SR-Series includes a built-in recirculation pump as standard, plus Hot Start Programming, which the brochure describes as minimizing cold-water bursts when a faucet is reopened within five minutes. A.O. Smith’s ATHR-199X3 has an integrated pump with an onboard display that holds up to two recirculation schedules.
Noritz lists an External Recirculation Pump Kit, RPK-EXT, as an optional accessory to the EZ111DV. Rinnai’s SENSEI RX chart describes the RX199iN as Smart-Circ capable once a pump or pump accessory kit is installed. Rinnai’s page lists the RUR199iN as recirculation capable without stating whether a pump ships inside.
Fitting it where the tank was
Widths are close: 17.31 in for the Rheem, 18.5 in for the Noritz and both Rinnai units. Heights run from 27.1 in for the Noritz case, through 28.56 in for the Rheem and 30.11 in for the Rinnai RX, to 31.3 in for the Rinnai RUR. Depth spans 11.2 in on the Noritz to 13.69 in on the Rheem. A.O. Smith is the outlier. Its hotwater.com page lists 34 in high by 16.53 in wide with no depth, while its ENERGY STAR record lists 27.0 by 17.0 by 16.0 in. Both cannot be right, and neither could be confirmed. Measure the space against the larger figures or get the current spec sheet from A.O. Smith before committing to a tight closet.
Three of the five publish a weight: 55 lb for the Rinnai RX, 70 lb for the Noritz unit (81 lb for the EZTR75 bundle, which includes the kits), and 73 lb for the Rinnai RUR.
Venting is where Rheem is the most specific. Its brochure publishes maximum runs of 70 ft in 2-in PVC or 150 ft in 3-in PVC, and up to 50 ft of flex venting through an existing B-vent, the option aimed at houses that already have a B-vent chase. It can take combustion air from the room, subject to National Fuel Gas Code room-volume rules, or from outside through a direct vent. Noritz’s EZ111DV is a direct-vent unit using 2-in or 3-in Schedule 40 PVC/CPVC, flexible polypropylene or an outdoor vent cap, and the EZTR75 bundle includes a 25-ft flex vent kit, though the spec sheet states no maximum run. Rinnai lists concentric, PVC, CPVC and polypropylene for the RX199iN, and polypropylene, PVC/CPVC and common venting for the RUR199iN, without a single maximum run in the documents retrieved. A.O. Smith publishes a 2.00-in flue and ABS, PVC, CPVC and polypropylene as acceptable materials.
Indoor and outdoor are not interchangeable. The RUR199iN is indoor only. The RX199iN installs indoors or outdoors with the RXOVC vent cap accessory. The Rheem is an indoor unit with an RTGH-SX outdoor conversion kit sold separately, and the A.O. Smith needs a separate Outdoor Vent Cap Kit for outdoor placement. The Noritz spec sheet lists an outdoor vent cap among its venting options.
When the simpler unit is enough
If your incoming water keeps the rise near 45°F, the published tables show 7.9 GPM from the Rheem and 8.7 GPM from the Noritz. If the sum of your fixtures sits comfortably below those rows, the decision turns on recirculation, footprint and UEF instead. In that situation the Rinnai RX199iN’s 0.98 UEF, 55 lb weight, 0.26 GPM activation flow and indoor-or-outdoor flexibility are real advantages, and the absence of a published table costs you less because you are not operating near the edge of the curve.
If the rise is 60°F or more and the load is still two showers and a dishwasher, buy only from a published figure at that rise, which in this set means the Rheem, the Noritz or the A.O. Smith. Where one unit’s row does not cover the load, Noritz’s spec sheet describes an optional 2-unit Quick Connect that pairs two EZ111DVs for twice the capacity, with a combined modulating range of 18,000 to 399,800 BTU/h.
Figures that do not size the unit
UEF is a gas-cost variable, not a capacity one. The spread here is 0.93 for the Rinnai RUR, 0.95 for the A.O. Smith and the Noritz (per the spec sheet; Noritz’s current product page lists 0.96-0.98 for the latest revision), 0.96 for the Rheem and 0.98 for the Rinnai RX. Confirm the figure against the exact model number, because the Noritz discrepancy shows how one family can carry two values.
Minimum activation flow decides whether a trickle at one sink fires the burner, not whether two showers run warm. The Rinnai RX publishes 0.26 GPM (0.5 L/min). Rheem publishes 0.4 GPM to activate and 0.26 GPM minimum continuous flow. Noritz and A.O. Smith publish 0.4 GPM, the latter through a retailer listing. Rinnai does not publish a figure for the RUR199iN in the retrieved material.
Warranty terms are published for four of the five. Noritz covers the heat exchanger for 25 years on residential use, the longest in this set. Rheem, Rinnai RX and A.O. Smith each cover it for 15 years, with 5 years on parts; Rinnai adds 4 more years of labor to its 1 year if the RX is registered within 90 days. Rheem and Rinnai RX are both approved to 10,200 ft altitude. The RUR199iN’s product page states SCAQMD approval and an Ultra Low NOx rating alongside residential ENERGY STAR certification; the A.O. Smith carries Ultra-Low NOx in its product name, and no other unit’s retrieved documents state either.
