Bosch Tankless Water Heater sizing guide in Greater Vancouver

The most common tankless complaint we get in the Lower Mainland is not a breakdown. It is a homeowner in Coquitlam saying the hot water was fine all summer and went lukewarm in December when two people used it at once.

That is a sizing story, almost every time. Someone picked a unit off a chart, or matched the tank it replaced, or assumed a four-bedroom house needs whatever a four-bedroom house is supposed to need. Tankless does not work that way. It has no reservoir to lean on, so it has to produce your peak demand in real time, at your worst-case water temperature.

What follows is how the calculation is actually done — the measurements a technician takes, the constraints that override the arithmetic, and why the gas meter in your utility room sometimes matters more than the spec sheet. HeatLand has been sizing and installing hot water systems across Metro Vancouver and the Fraser Valley since 1999. (604) 330-3812 for a free written estimate.

Two numbers decide everything: flow and rise

Every tankless sizing conversation reduces to a pair of values. Flow is how much hot water moves through the unit at once, measured in gallons or litres per minute. Rise is how many degrees the unit must add to the incoming water to reach your set point. A tankless heater’s real-world output is always stated as a flow rate at a given rise, and those two move against each other. Ask for more rise and available flow drops. Ask for more flow and the achievable rise drops.

That trade-off is why a single headline flow rate on a box is close to meaningless on its own. A unit rated for a certain flow at a mild rise will deliver considerably less when it is closing a big winter gap. Homeowners who compare only the top-line number end up undersized without knowing it.

So the sizing question becomes specific. What is the largest amount of hot water your household will call for simultaneously, on the coldest morning of the year, and can the unit meet that with the rise required at that moment?

Get those two answers honestly and the model shortlist writes itself. Guess at either one and you are relying on luck. This is the load calculation, and it is not optional busywork — it is the whole job.

Measuring your real peak demand, not your imagined one

Start with an inventory. Walk the house and list every hot water fixture: showers, tubs, bathroom and kitchen faucets, dishwasher, laundry, any outdoor or secondary suite fixtures. Then answer the harder question: which of these realistically run at the same time?

Most families have a predictable crunch. Two showers on a weekday morning. Or one shower plus the dishwasher plus a kitchen tap on a Sunday. A basement suite in an East Vancouver house changes the answer completely, because the suite’s schedule is independent of yours. Same for a household with teenagers.

You can measure actual fixture flow rather than trusting a label. Run the fixture wide open into a container of known volume and time how long it takes to fill. Do it at your showerheads especially — an old showerhead and a modern low-flow head behave very differently, and replacing one is sometimes the cheapest part of a hot water fix. Check aerators for stamped flow markings while you are there.

Add the fixtures in your realistic worst-case group. That total is your design flow. Do not design for every tap in the house running at once; nobody lives that way, and sizing for it pushes you into a unit that cycles inefficiently the rest of the year. Design for the peak you actually experience.

Winter inlet temperature is why local sizing charts matter

Sizing tables printed for a national or American audience assume an inlet temperature that may have nothing to do with your street. Groundwater and surface-supplied municipal water arrive at very different temperatures in different regions and seasons, and the Lower Mainland has its own pattern.

What matters is the coldest inlet temperature you will see, because that is the condition that produces the largest required rise and therefore the smallest available flow. Size for August and you will be fine in August. Size for the coldest week of the year and you are fine always.

The practical method is to measure. Run a cold tap until the temperature stabilises, then read it with a thermometer at the tap. Doing this in mid-winter gives real data instead of an assumption. A technician sizing your system will take this reading, or use conservative local design values if the visit happens in summer.

Location within the region shifts things too. A long service line running through cold ground in an outlying Fraser Valley property behaves differently from a short line in a dense Vancouver block, and a home on a private well has a different inlet profile again. Older homes with long uninsulated interior runs also lose heat between the unit and the fixture, which is not a sizing input but does affect perceived performance.

Set point: the number that quietly sets your rise

Your target outlet temperature is the top of the rise calculation, and it is the one variable you control directly. Set it higher and every fixture demands more of the unit. Set it lower and you gain flow capacity, immediately, for free.

There is a balance to strike and it involves real safety considerations on both ends. Water hot enough to scald is a genuine hazard, particularly with young children or elderly occupants, and mixing valves at fixtures are the standard control for that. On the other end, storage and distribution temperature has sanitary implications, which is why the industry does not simply recommend turning everything down as far as it goes. A licensed technician can advise a set point appropriate to your household, your fixtures and whether the system includes any storage or a recirculation loop.

The design mistake we see is a set point cranked up to compensate for something else. Weak hot water at a distant fixture, a fouled exchanger, or an undersized gas line will all tempt someone to turn the dial up. That masks the fault and costs fuel every day afterward.

Decide the set point deliberately during sizing, document it, and size the unit to deliver design flow at that set point using worst-case winter inlet. Then leave it alone unless a technician has a reason to change it.

Gas supply: the constraint that overrides the spec sheet

This is where a lot of tankless projects get interesting. A tankless unit fires at a far higher input rate than the storage tank it replaces, because it does in minutes what a tank does over an hour. The gas system has to deliver that input at adequate pressure while everything else in the house is also running.

A technician looks at the meter’s capacity first, then at everything already connected: furnace or boiler, range, dryer, fireplace, barbecue line, any second suite. Then at the pipe itself — diameter, material, total developed length, and the number of fittings between the meter and the appliance location. A long run to a far corner of a Surrey rancher can require a larger pipe size than a short run in a townhouse mechanical room, for the exact same unit.

When the supply cannot feed the chosen model, there are options: upsize the gas piping, request a meter or regulator change through the gas utility, relocate the unit closer to the supply, or select a model whose input the existing system can actually support. Sometimes that last option means accepting a slightly smaller unit and adjusting expectations, which is an honest conversation worth having early.

All gas work is TSBC-certified territory and typically requires a municipal permit. It is not a homeowner job under any circumstances.

Venting, clearances and older housing stock

The venting route often decides where a Bosch tankless can go, and location feeds back into the gas and water piping cost. Condensing units vent through approved plastic materials with manufacturer-specified maximum lengths, elbow allowances, slope for condensate drainback, and minimum clearances from windows, doors, air intakes, gas meters and property lines.

Older Vancouver and New Westminster housing stock complicates this. Existing chimney and B-vent chases were built for different appliances and generally cannot be reused as-is. Tight side yards make termination clearances hard to satisfy. Character homes with limited exterior wall access sometimes need a longer vent run than the manufacturer permits, which forces a different unit location.

Strata properties add rules on top of code. Many buildings restrict where anything may penetrate an exterior wall, require approval before altering common property, and have their own noise or appearance requirements. Getting strata approval in writing before the install date avoids an expensive stall.

Coastal weather matters at the termination. Wind-driven rain, salt air near the water, and snow accumulation in Fraser Valley winters all influence termination height and orientation. A termination positioned where exhaust can be pulled back into the combustion air intake will cause repeated faults that look like an equipment problem.

Combustion air supply, condensate drainage with neutralization where required, and freeze protection round out the site assessment.

Point-of-use, whole-home, and multi-unit setups

Not every hot water problem is solved by one big unit. Sizing sometimes concludes that the right answer is a different configuration.

A remote fixture at the far end of a long ranch-style house, or a suite kitchenette that runs on its own schedule, can justify a small dedicated unit near the point of use. That shortens the pipe run, cuts the wait for hot water, and takes that demand off the main unit’s peak. It also means a second gas supply and a second vent, so it is a trade, not a free win.

At the other end, homes with genuinely high simultaneous demand — several full bathrooms in use at once, a large soaker tub that has to fill in reasonable time, or a legal secondary suite with its own laundry — may be better served by linking multiple units so they share the load and stage on as demand climbs. That approach spreads runtime, keeps each unit modulating in a sensible range, and gives partial hot water if one unit needs service.

A buffer tank paired with a tankless unit is another option worth discussing, particularly where short draws or a recirculation loop cause nuisance cycling.

Which configuration fits comes out of the same load calculation. The point is that sizing is a design exercise, not a shopping decision.

What a proper sizing visit looks like, and what to ask for

A real sizing visit takes measurements, not assumptions. Expect the technician to inventory fixtures, ask pointed questions about your household’s actual morning routine, read inlet water temperature, check the gas meter and trace the existing piping, walk the proposed venting route, look at electrical availability and condensate drainage options, and note anything about the existing plumbing that will affect performance.

The output should be written. A quote worth comparing states the model recommended, the design flow and rise it was sized for, the gas work required including any pipe upsizing or utility involvement, the venting plan, condensate handling, whether isolation valves for future flushing are included, permits, and the electrical work. Two quotes are only comparable when both list all of that.

Be wary of a recommendation made over the phone from a bedroom count, or one that simply matches the previous tank. Neither method touches the flow-and-rise calculation, and both routinely produce winter disappointment.

If you are replacing a failed tank in a hurry, it is still worth the hour it takes to size properly. The unit lasts many years; the decision does not deserve fifteen minutes.

HeatLand provides a free written, no-obligation estimate with the sizing work shown, and financing is available. Licensed, insured, TSBC-certified gas fitting, serving Metro Vancouver and the Fraser Valley since 1999. Call (604) 330-3812.

Quick Checklist

• Inventory every hot water fixture, including any secondary suite, laundry and outdoor taps
• Identify your realistic peak — which fixtures actually run at the same time on a busy morning
• Measure fixture flow with a container and a stopwatch instead of trusting old labels
• Read your cold water inlet temperature at a tap in mid-winter for a true worst-case rise
• Decide your set point deliberately, with mixing valves at fixtures where scald risk exists
• Have a TSBC-certified gas fitter confirm meter capacity and pipe sizing before choosing a model
• Walk the venting route for length, elbows and termination clearances; get strata approval in writing if applicable
• Ask for a written estimate that lists the design flow and rise the unit was sized for, plus gas, vent, electrical, condensate and permits

Frequently Asked Questions

Reviewed by the HeatLand Heating & Cooling team — licensed, insured, TSBC-certified HVAC contractor serving Metro Vancouver & the Fraser Valley since 1999. Error codes, specifications, pricing and rebate programs vary by model and change over time; confirm details against your unit’s own manual, a written quote, and the manufacturer or CleanBC/FortisBC. If you smell gas or your CO alarm sounds, leave the home and call the gas utility and a licensed technician from outside.