Capacity is flow at a rise, not a maximum
A tankless unit's capacity is a flow rate at a stated temperature rise. Navien publishes the NPE-240A2 at 11.2 GPM (42.4 L/min) at a 35 °F rise, 8.7 GPM (32.9 L/min) at 45 °F and 5.6 GPM (21.2 L/min) at 67 °F. Rinnai publishes the RU199iN as a rated flow range of 0.26 to 9.8 GPM with an 11 GPM maximum after a parameter change, and its flow curves apply only to inlet water at 21 °C or below.
The headline maximum is reached at a small rise that a Metro Vancouver winter does not provide. The number a sizing decision uses is the flow the unit holds at the rise the home actually sees, read from the manufacturer's flow-at-rise table.
Winter is the sizing condition
Metro Vancouver's water comes from mountain reservoirs, and in winter the incoming supply can be below 8 °C. Raising that to 49 °C at a shower is about a 40 °C (72 °F) rise, which puts the NPE-240A2 near its 5.6 GPM figure rather than its 11.2 GPM headline. Planning against a summer rise is how households end up short of hot water in January.
Natural Resources Canada explains why capacity classes matter as well: under the Uniform Energy Factor test, water heaters fall into very-small, low, medium and high capacity classes, and only units of the same class can be compared directly. Sizing starts from the class that matches the home, not from a single efficiency number.
Add up simultaneous demand
The sizing question is how many fixtures run at once. A shower-class fixture draws about 7.5 to 9.5 L/min (2 to 2.5 GPM), so two showers at once need roughly 17 L/min (4.5 GPM) at a 40 °C rise, close to the winter output of a single 199,000 BTU/h unit. Three simultaneous fixtures in January can exceed one unit.
FortisBC notes that some tankless units may support only one demand for hot water at a time and that a buffer tank can be added to reduce delays. Tankless units also need a minimum flow to fire, 0.4 GPM (1.5 L/min) on the RU199iN, so a trickle from a low-flow tap may not start the burner.
Check the gas supply and vent route
A 199,000 BTU/h condensing unit draws about five times a 40,000 BTU/h storage tank and connects to 3/4 in gas rather than 1/2 in. Both the RU199iN and the NPE-240A2 specify a natural gas supply pressure of 3.5 to 10.5 in. w.c. at the unit, and the supply has to hold that range under load.
Where the meter or regulator must change, the gas fitter requests the exchange from FortisBC with the delivery pressure required and the total load in BTU, and FortisBC determines the process and timeline. Venting is the other half: a condensing unit needs 2 in or 3 in PVC, CPVC, polypropylene or approved stainless to an exterior termination, plus a condensate drain and a 120 V outlet.
What the assessment measures
CANRO Heating & Cooling assesses hot-water demand and sizes the unit as separate services, and provides free written estimates for installations. The assessment counts the fixtures that run together, measures the winter inlet condition, checks the gas line, meter and supply pressure, and confirms the vent and condensate path before a model is proposed.
Sizing from measured demand rather than bedroom count or floor area is what keeps the recommendation honest. Where one unit cannot cover the peak, the options are a buffer tank, a second unit, or accepting that the third fixture waits, and the choice is made from the home's numbers.
Common sizing mistakes
The recurring mistakes are using the brochure maximum, sizing from house size, ignoring the winter inlet temperature and ignoring how many fixtures run at once. Each one points the same way: a unit that looks large on paper but falls short at the rise and simultaneous demand the home actually has.
The estimator on this site shows conversion allowances such as venting, gas supply and condensate separately, so the like-for-like and conversion costs are both visible before a written quote follows the visit.