The Rheem IKONIC is available in three residential models — 8.4 GPM, 10.1 GPM, and 11.2 GPM at a 35°F temperature rise — and picking the right one requires three inputs: your ground water temperature zone, peak simultaneous flow demand at the installation, and desired outlet temperature. Get all three right and you spec the correct model the first time. Get any one wrong and you end up with an undersized unit that cannot keep up at peak demand or an oversized unit the client paid too much for. This guide covers the sizing process step by step using Rheem's official flow rate capacity table by zone.
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The three Rheem IKONIC residential models
All three IKONIC residential models share the same core platform — dual stainless steel condensing heat exchangers, 0.96 UEF, EcoNet Wi-Fi, LeakGuard, Hot-start programming, and the industry-leading 15-year heat exchanger warranty. The only difference between them is BTU/h input and resulting flow rate capacity.
| Model | BTU/h input | GPM at 35°F rise | GPM at 45°F rise | GPM at 67°F rise | Weight |
|---|---|---|---|---|---|
| RTGH-S84i | 157,000 | 8.4 GPM | 6.9 GPM | 4.6 GPM | 77.6 lbs (S) / 86 lbs (SR) |
| RTGH-S10i | 180,000 | 10.1 GPM | 7.9 GPM | 5.3 GPM | 77.6 lbs (S) / 86 lbs (SR) |
| RTGH-S11i | 199,900 | 11.2 GPM | 8.8 GPM | 5.9 GPM | 77.6 lbs (S) / 86 lbs (SR) |
Dimensions are the same across all three models: 28-9/16" H x 17-5/16" W x 13-11/16" D. Flow rates are calculated for a tankless set temperature of 120°F with a mixed shower temperature of 105°F.
Step 1 — find your ground water temperature zone
Rheem sizes the IKONIC using nine ground water temperature zones across the US. The zone determines your incoming water temperature — and therefore the temperature rise required to reach 120°F. The higher the temperature rise, the less GPM the unit can deliver. This is the most commonly misunderstood variable in tankless sizing.
| Zone | Ground water temp | RTGH-S84i GPM | RTGH-S10i GPM | RTGH-S11i GPM |
|---|---|---|---|---|
| Zone 1 (AK, CAN) | 37°F | 3.7 | 4.3 | 4.7 |
| Zone 2 | 42°F | 4.0 | 4.6 | 5.1 |
| Zone 3 | 47°F | 4.2 | 4.9 | 5.4 |
| Zone 4 | 52°F | 4.6 | 5.3 | 5.9 |
| Zone 5 | 57°F | 4.9 | 5.6 | 6.3 |
| Zone 6 | 62°F | 5.3 | 6.1 | 6.8 |
| Zone 7 | 67°F | 5.8 | 6.7 | 7.4 |
| Zone 8 | 72°F | 6.4 | 7.4 | 8.2 |
| Zone 9 (HI, PR) | 77°F | 7.2 | 8.3 | 9.2 |
These are the official Rheem flow rates from the IKONIC spec sheet (Form No. TK-RTGHS Rev. 5). Use the zone column that matches the installation location — not the maximum GPM figures from marketing materials.
Cold climate installations require more capacity than the max GPM numbers suggest. A Zone 2 installation (42°F ground water) gets only 5.1 GPM from the RTGH-S11i — not 11.2 GPM. Always size using the zone table, not the maximum flow rate figure. The maximum GPM at 35°F rise is a warm-climate number and will produce an undersized installation in cold climates.
Step 2 — calculate peak simultaneous flow demand
Peak simultaneous flow demand is the total GPM being drawn at the same time during the busiest moment in the household. Use Rheem's published average flow rates by application:
| Fixture | Average flow rate (GPM) |
|---|---|
| Low flow faucet | 0.5 |
| Bathroom faucet | 1.0 |
| Kitchen faucet | 1.5 |
| Shower | 2.0 |
| Dishwasher | 2.0 |
| Washing machine | 3.0 |
Add up only the fixtures that would realistically run simultaneously during the home's peak hot water period — not every fixture in the home. For most residential installations this means two to three showers, or one shower plus a dishwasher and a faucet. Rheem's bathroom count benchmark uses 2 GPM per simultaneous shower pre-mixed with cold water.
Step 3 — select the right model
With your zone GPM and peak demand determined, select the model whose flow rate at your zone meets or exceeds your peak demand. Practical examples:
| Scenario | Zone | Peak demand | Right model |
|---|---|---|---|
| 2 bed / 1 bath | Zone 8 (72°F) | ~4 GPM | RTGH-S84i (6.4 GPM) ✓ |
| 3 bed / 2 bath | Zone 8 (72°F) | ~6 GPM | RTGH-S10i (7.4 GPM) ✓ |
| 4 bed / 3 bath | Zone 8 (72°F) | ~8 GPM | RTGH-S11i (8.2 GPM) ✓ |
| 2 bed / 1 bath | Zone 3 (47°F) | ~4 GPM | RTGH-S10i (4.9 GPM) ✓ |
| 3 bed / 2 bath | Zone 3 (47°F) | ~5 GPM | RTGH-S11i (5.4 GPM) ✓ |
| 4+ bed / 3+ bath | Zone 3 (47°F) | 6+ GPM | Two RTGH-S11i cascaded |
When one unit is not enough. The IKONIC supports common venting for up to 12 units and manifold controls for up to 20 units without external control required. Two cascaded RTGH-S11i units provide up to 18.4 GPM in Zone 9 — enough for the largest residential applications. Contact our team for cascade sizing and configuration.
Recirculation models — RTGH-SR series
Each of the three IKONIC models is also available in an SR (recirculation) variant — RTGH-SR84i, RTGH-SR10i, and RTGH-SR11i — with a built-in recirculation pump that keeps hot water circulating to fixtures, eliminating cold water delays at the tap. The SR models carry the same GPM capacity as the standard models and weigh 86 lbs vs 77.6 lbs for the standard models.
Recirculation loop length: up to 400 ft on 3/4" pipe, up to 100 ft on 1/2" pipe. For installations where the unit is located far from the primary point of use, the SR model eliminates the wait time that is the most common client complaint on tankless installations in large homes.
Key IKONIC features — what every model includes
- 0.96 UEF — among the highest efficiency ratings available on a residential gas tankless unit
- Dual stainless steel condensing heat exchangers — primary and secondary, 15-year warranty
- Hot-start programming (Rheem exclusive) — keeps the unit in a ready state between back-to-back uses, avoiding cold water bursts
- EcoNet Wi-Fi — remote monitoring and control via the EcoNet app, Alexa compatible
- LeakGuard — built-in leak detection with automatic shutoff on Wi-Fi models
- Best-in-class turn down ratio (TDR) — tighter temperature control at lower demand, reducing short cycling
- Full text digital display — no dip switches, easy setup and diagnostics
- Ultra Low NOx — meets 14 ng/J NOx requirements, SCAQMD Rule 1146.2 compliant
- PVC/CPVC/Polypropylene venting — up to 70 ft on 2" pipe, 150 ft on 3" pipe
- Flex venting — through existing B-vent up to 50 ft on 2" pipe
- 1/2" gas line compatible — up to 24 feet, subject to local code
- Common venting — up to 12 units, manifold controls up to 20 units
- Outdoor conversion kit available — RTGH-SX sold separately
- Warranty — 15-year heat exchanger, 5-year parts, 1-year labor
Frequently asked questions
What is the difference between the RTGH-S84i, S10i, and S11i?
BTU/h input and flow rate capacity only — 157,000 BTU/h, 180,000 BTU/h, and 199,900 BTU/h respectively. All other features, dimensions, and warranty are identical across the three models.
Should I always spec the largest model?
Not necessarily — oversizing a tankless unit can cause short cycling at low demand, which reduces efficiency and increases wear. Spec the model whose capacity at your zone meets peak demand with a reasonable margin. In warm-climate Zone 8 or 9 installations with modest peak demand, the RTGH-S84i is often the right choice.
Does the IKONIC work with propane?
Yes — all IKONIC models ship configured for natural gas with an LP conversion kit included. Field convertible without additional parts.
What venting material does the IKONIC use?
Schedule 40 PVC, CPVC, or Polypropylene. Maximum 70 ft on 2" pipe, 150 ft on 3" pipe — reduced by 6 ft per 90° elbow on 2" pipe and 5 ft per 90° elbow on 3" pipe. Flex venting through existing B-vent allowed up to 50 ft on 2" pipe.
What is the IKONIC's minimum activation flow?
0.4 GPM activation flow, 0.26 GPM minimum continuous flow. The unit will not fire below 0.4 GPM — confirm all fixtures on the system meet this minimum.
Can I cascade IKONIC units?
Yes — common venting supports up to 12 units and manifold controls support up to 20 units without external control required using standard RJ25 communication cables. Contact our team for cascade sizing and configuration.
Shop Rheem IKONIC at PlumbersCrib.com
- Rheem IKONIC Series — all three models and SR recirculation variants at PlumbersCrib.com
- Full Rheem tankless lineup — IKONIC residential, IKONIC Commercial, and RTG Series non-condensing
- Rheem ProTech venting accessories — OEM PVC venting for IKONIC installations
Contact our team or call 877-881-5501 Monday through Friday for help sizing the right Rheem IKONIC model for your job.