Rheem tankless sizing guide (GPM)
The most common tankless complaint — lukewarm water when two showers run at once — is almost never the unit’s fault. It is a sizing error made before the unit was ever purchased.
Tankless sizing rests on two numbers: how many gallons per minute (GPM) of hot water your household draws at its busiest moment, and how far the unit must raise the temperature of the incoming water to reach your setpoint. Get either wrong and no brand, Rheem included, can save the installation.
HeatLand Heating & Cooling — TSBC-certified and serving Metro Vancouver and the Fraser Valley since 1999 — sizes every tankless installation before quoting it. Here is how the method works, so you can sanity-check any proposal you receive.
Step 1: Calculate Your Peak Simultaneous Demand
Start by imagining your household’s busiest hot-water moment — typically a weekday morning. Which fixtures realistically run at the same time? Two showers? A shower plus the kitchen sink? Shower, dishwasher, and laundry?
Each fixture draws a flow rate measured in gallons per minute. Modern low-flow showerheads draw less than older ones; soaker tub fillers draw considerably more. Rather than quoting generic per-fixture numbers here — they vary by fixture age and type — the reliable approach is to identify your actual fixtures during a site assessment and sum the flows of the ones that genuinely overlap.
The sum is your peak demand. Be honest rather than optimistic: sizing to a polite fiction where nobody ever showers simultaneously produces exactly the lukewarm mornings tankless owners complain about. Equally, do not size for a scenario that never happens — every fixture in the house running at once — because you will pay for capacity you never use.
Step 2: Understand Temperature Rise — BC’s Hidden Variable
A tankless unit’s deliverable flow is not a fixed number; it depends on how far the unit must raise the incoming water temperature. The colder the water entering the house, the more work each gallon requires, and the fewer gallons per minute the unit can heat to your setpoint.
This is where Metro Vancouver homeowners get caught. Incoming water here is markedly colder in winter than in summer, so a unit that comfortably handles two showers in July can fall short in January if it was sized against summer conditions.
The rule is simple: size for your worst case — winter inlet temperature and full peak demand together. Manufacturers, Rheem included, publish flow-versus-temperature-rise performance for each model, and a competent installer reads your requirement against those curves for the specific models under consideration. This single step is the difference between a tankless system you forget about and one you resent every winter.
Step 3: Match to the Rheem Lineup — and Check the Gas Supply
With peak GPM and winter temperature rise established, the requirement is matched against Rheem’s condensing lineup — RTGH-designated models and the premium IKONIC line — to find units whose performance meets your worst-case demand with sensible headroom.
For high-demand households, options include stepping up within the range or, in larger homes, manifolding multiple units to share the load. For modest households, the honest recommendation may be a smaller unit than the one an online calculator suggested.
Sizing also has a gas-side consequence people miss: a larger burner needs more fuel, and the existing gas line’s diameter and length determine whether it can deliver. Verifying this is licensed work — in BC it belongs to TSBC-certified gas fitters, and HeatLand checks it during the same site visit. And the standing safety rule: any gas smell or CO alarm means stop, leave the home, and call a licensed technician from outside.
Why Professional Sizing Beats Online Calculators
Online sizing calculators are a reasonable starting point and a poor finishing point. They cannot see your actual fixtures, measure your winter inlet temperature, inspect your gas line, or ask the follow-up questions that change answers — is a bathroom renovation planned, do teenagers shower back-to-back or simultaneously, is there a soaker tub that gets used weekly?
A HeatLand sizing visit covers all of it in one pass: fixture audit, realistic peak-demand conversation, winter temperature-rise math, gas supply verification, and venting feasibility. The output is a written quote listing the specific Rheem models (and, where relevant, comparable Bradford White Infiniti options) that genuinely fit — not a shrug and a range.
Since 1999 we have corrected plenty of undersized installations done from a calculator screenshot. It is far cheaper to size correctly once. The assessment costs you a conversation; an undersized unit costs you every cold January morning until you replace it.
Quick Checklist
- List the fixtures that realistically run at the same time on your busiest morning
- Size against winter inlet temperature, not summer
- Ask the installer to show the flow-vs-temperature-rise reasoning for each proposed model
- Have the gas line verified for the proposed burner size
- Mention planned renovations that will add hot-water demand
- Insist on the sizing logic being written into the quote
| Sizing Input | Where It Comes From | Common Mistake |
|---|---|---|
| Peak simultaneous GPM | Fixture audit + honest usage discussion | Assuming showers never overlap |
| Winter inlet temperature | Local seasonal water data | Sizing against summer conditions |
| Temperature setpoint | Household preference + safety | Ignoring its effect on deliverable flow |
| Gas supply capacity | Licensed gas-line assessment | Discovering shortfall after purchase |
| Future demand | Renovation and family plans | Sizing only for today |
Frequently Asked Questions
What GPM Rheem tankless do I need for a 3-bathroom home?
Bathroom count alone cannot answer it — what matters is how many fixtures run simultaneously and your winter temperature rise. Two three-bathroom homes can need very different units. A HeatLand fixture audit and demand conversation produces the real number, and the written quote shows which Rheem models meet it with headroom.
Why does my tankless keep up in summer but not winter?
Because deliverable flow drops as temperature rise increases, and Metro Vancouver’s inlet water is much colder in winter. A unit sized against summer conditions loses effective capacity exactly when demand peaks. The fix is sizing for winter worst-case — or, for an existing undersized unit, discussing upgrade or add-on options with HeatLand.
Is it bad to oversize a tankless water heater?
Moderate headroom is sensible; dramatic oversizing wastes money on capacity you never use and can require gas-line upgrades you didn’t need. The goal is meeting your realistic winter peak with margin, not maxing out the catalogue. HeatLand quotes the fit, not the flagship, unless your demand genuinely calls for it.
Can two tankless units be installed together for a large home?
Yes. Larger households and homes with big soaker tubs sometimes manifold multiple tankless units to share peak load. It adds installation complexity — gas supply and venting for both units — so it is a site-assessment conversation. HeatLand designs and installs multi-unit configurations across Metro Vancouver and the Fraser Valley.
Reviewed by the HeatLand Heating & Cooling team — licensed HVAC contractor serving Metro Vancouver & the Fraser Valley since 1999. Specifications and rebate programs change; confirm current details with a written quote. If you smell gas or suspect carbon monoxide, stop troubleshooting, leave the area, and call a licensed technician or 911.