Wrong-sized tankless is the number one reason homeowners regret going tankless. The good news: sizing isn't hard once you understand the math. This guide walks you through the same three-step process a contractor uses, applies it to your climate zone and household, then matches you to the right Rheem model — gas, electric, condensing, or combi.
Skip the guesswork. The unit on the box says 9.5 GPM; what you'll actually get depends on where you live.
Every contractor uses the same three inputs: how cold your incoming water gets, how much water you use at peak demand, and what your unit can actually deliver under those conditions. Here's how each step works.
Find the coldest your incoming water gets in winter (groundwater temperature). Subtract that from your desired tap output (typically 120°F). The difference is the rise your tankless has to deliver.
Not "how many bathrooms" — count the fixtures that might run at the exact same moment during your worst-case morning. Two showers plus a kitchen sink is about 5.5 GPM peak.
Every Rheem unit publishes flow ratings at multiple rise points. Find the model whose flow rating at your required rise meets or exceeds your peak demand. Don't compare units at 35°F if you live in Wisconsin.
Groundwater temperature swings by 30+ degrees across the country, and that swing is the single biggest factor in sizing. A unit that's perfect in Tampa is undersized in Minneapolis. Find your region below for the inlet temperature you should size around.
Note: groundwater temperatures vary by depth and source — homes on shallow wells in cold regions may see colder inlet than the table suggests. When in doubt, size up one tier.
| Fixture | Typical GPM | Low-Flow GPM |
|---|---|---|
| Standard shower | 2.0 – 2.5 | 1.5 – 1.8 |
| Rain / luxury shower | 2.5 – 4.0 | — |
| Bathroom sink (faucet) | 1.0 – 1.5 | 0.5 – 1.0 |
| Kitchen sink (faucet) | 1.5 – 2.2 | 1.0 – 1.5 |
| Dishwasher (fill cycle) | 1.0 – 1.5 | — |
| Clothes washer (fill cycle) | 1.5 – 2.5 | — |
| Bathtub fill | 3.5 – 5.0 | — |
| Soaking tub fill | 5.0 – 8.0 | — |
Add up the fixtures that realistically run at the same time. A typical 3-bedroom morning is one shower (2.0) + bathroom sink (1.0) + kitchen sink (1.5) = 4.5 GPM. A house with two simultaneous showers + a dishwasher needs 5.5 GPM minimum.
This is the chart that matters. Find your climate zone's required rise across the top, then look down the column to see what each model actually delivers — not the marketing number on the box.
| Model | 35°F Rise | 45°F Rise | 70°F Rise | 80°F Rise | Best For |
|---|---|---|---|---|---|
| Indoor Non-Condensing — RTG Series | |||||
| RTG-70 | 7.0 | 5.9 | 3.7 | 3.0 | 2 bath, warm climate |
| RTG-84 | 8.4 | 6.6 | 4.4 | 3.6 | 2–3 bath standard install |
| RTG-95 | 9.5 | 7.6 | 5.0 | 4.1 | 3+ bath, moderate climate |
| Indoor Condensing — RTGH Series | |||||
| RTGH-68 | 6.8 | 5.7 | 3.6 | 3.0 | 1–2 bath, condensing efficiency |
| RTGH-84 | 8.4 | 7.4 | 4.7 | 3.9 | 2–3 bath, long vent runs |
| RTGH-90 | 9.0 | 7.9 | 5.0 | 4.2 | 3 bath sweet spot |
| RTGH-95 | 9.5 | 8.4 | 5.5 | 4.5 | 3+ bath, cold climate |
| RTGH-95 EcoNet | 9.5 | 8.4 | 5.5 | 4.5 | Same as RTGH-95, plus WiFi |
| RTGH-SR10i IKONIC | 10.0 | 8.8 | 5.8 | 4.7 | 3+ bath, recirc pump built in |
| RTGH-SR11i IKONIC | 11.0 | 9.5 | 6.2 | 5.0 | 4+ bath, premium spec |
| Electric — RTEX Series | |||||
| RTEX-18 | 4.4 | 3.4 | 2.2 | 1.9 | Point-of-use, ADU, 1 bath warm |
| RTEX-24 | 5.9 | 4.6 | 2.9 | 2.5 | 1–2 bath warm climate |
| RTEX-27 | 6.6 | 5.2 | 3.3 | 2.8 | 2 bath warm climate |
| RTEX-36 | 8.8 | 6.9 | 4.4 | 3.7 | 2–3 bath, electric whole home |
| Combi Boilers — ThermaForce Series | |||||
| RCBH-L125i | 5.4 | 4.4 | 2.8 | 2.4 | Smaller home + space heating |
| RCBH-L160i | 6.9 | 5.7 | 3.6 | 3.0 | Mid-size home + space heating |
| RCBH-L199i | 8.4 | 7.0 | 4.4 | 3.7 | Large home + space heating |
All flow rates in GPM. Values at 70°F and 80°F rise are calculated from BTU input — confirm against current Rheem spec sheets for design-of-record sizing.
If you don't want to do the math, find the situation that sounds like yours.
Inlet ~70°F, required rise ~50°F. Two showers plus a kitchen sink at peak = 5.5 GPM. The RTG-84 covers it with headroom and is the most cost-effective indoor gas tankless in the lineup.
Inlet ~40°F, required rise ~80°F. Family of five with morning shower stack = 5.0+ GPM at high rise. Only the RTGH-95 and IKONIC series deliver this. Condensing pays for itself in cold-climate gas savings.
Inlet ~75°F, required rise ~45°F. Going electric avoids running a gas line. The RTEX-27 handles two bathrooms in this climate; step up to the RTEX-36 if a soaking tub is in play.
Low simultaneous demand, easier to run electric than gas. The RTEX-18 mounts on a closet wall, runs on a single 240V circuit, no venting required.
Combi boiler is the move. One wall-mounted unit handles space heating and domestic hot water, freeing up a closet's worth of mechanical space and reducing maintenance to a single piece of equipment.
Spec the IKONIC SR10i if you want instant hot water at distant fixtures without the complexity of a separate recirc pump. Built-in pump, EcoNet WiFi, premium 15-year warranty.
The "9.5 GPM" headline is at 35°F rise — that's a Florida number. If you live in Ohio, you need to look at the 70°F rise column, which might be closer to 5.5 GPM. Always size at your rise, not the marketing rise.
"3 bathrooms" doesn't tell you how much hot water you need — three people taking showers at 7 AM does. Some 4-bath homes with adult kids out of the house have lower peak demand than a 2-bath home with three teenagers.
Non-condensing units need stainless steel Category 3 vent. If your install requires more than 15 feet of vent, the cost difference of metal vs. PVC (which condensing uses) often exceeds the price difference between the unit tiers. Run the numbers before defaulting to non-condensing.
A 199K BTU tankless needs a properly sized gas line — typically 3/4" or 1" depending on run length. Many existing 1/2" lines from old 40-gallon tanks can't deliver enough fuel under load. Get this checked before ordering.
Tankless takes a few seconds to fire up and another 10–30 seconds for hot water to reach a distant tap. If your master bath is far from the unit, factor in either built-in recirculation (the IKONIC series) or an external recirc pump from day one.
Sizing math gets you 90% of the way. The last 10% is local context — your specific groundwater, gas pressure, vent run, and household habits. Call us at 877-881-2742 and we'll talk through it. RheemTanklessOnline.com is operated by LCP Supply alongside our flagship plumbing wholesaler PlumbersCrib.com — every order is fulfilled by an authorized Rheem dealer with full manufacturer warranty support.