Hot Water Morning Simulator: play your busiest two hours, minute by minute

01 /What does the Hot Water Morning Simulator show?

It shows the exact minute a household morning outruns its water heater, and why. Sizing charts work in hours: add up the litres your busiest hour uses, then compare the total with a first-hour rating. That answers whether a heater is big enough on average, but it cannot tell you that the third shower goes lukewarm at 7:06 while the first two were fine. The simulator can, because it replays the morning as a timeline.

It covers the 120 minutes from 6:00 to 8:00, the window in which most households stack their showers, and reruns every time you change an input. Move a shower ten minutes later, swap an electric tank for a gas one, or drop the incoming water from 15 °C to 4 °C, and the chart redraws straight away. For a tank it plots the litres of hot water ready to use; for a tankless unit it plots the litres per minute being asked for against what the burner can heat. The Play the morning button sweeps a cursor across the chart while a small gauge shows how full of hot water the tank is at each minute.

It sits beside the Water Heater Size Calculator, which works at the level of the whole hour. Use the calculator to shortlist a size and the simulator to test that size against the way your household actually lines up its mornings.

02 /What do you enter?

Two input habits matter. First, changing the heater type or the tank size resets the burner input to the default for that size, so type your label figure back in after any size change. Second, the incoming water temperature is the input people guess worst. The City of Calgary's published drinking water data for 2025 ranged from 1.0 to 20.0 °C across the year, so a single all-year number hides a large swing. Measure your own: run a cold tap for a few minutes in late winter and read it with a kitchen thermometer.

03 /Which hot water uses does it model?

It models six everyday uses, each with a fixed total flow, a use temperature and a duration. Showers, baths and the sink are mixed uses: the fixture blends tank water with cold to reach the use temperature, so only part of the flow comes from the heater. The dishwasher and warm laundry draw straight hot water.

For a mixed use, the hot water drawn at the set temperature is flow × (use − inlet) ÷ (set − inlet), with the fraction held between 0 and 1. A shower from a 55 °C tank on 7 °C water draws 7.6 × (40 − 7) ÷ (55 − 7) = 5.225 L/min of hot water, and eight minutes of that is 41.8 litres. The other 19 litres are cold water blended in at the valve. Because the fraction depends on the incoming water, the same shower takes more from the tank in winter; the hot-only appliances take the same 23 and 27 litres whatever the season.

04 /What are the four preset mornings?

The family total is four showers at 41.8 L, the sink at 9.0 L and the dishwasher's 23 L, which is 199 litres. The shape matters as much as the total. The family's showers start ten minutes apart and last eight, so each leaves a two-minute gap. The teens' long showers run back to back from 6:30 to 7:00 with no gap at all, which is why that preset is the hardest on a tank even before the two ordinary showers begin.

05 /How does the tank model work, minute by minute?

1. Start full

   At 6:00 the tank holds its usable hot water: 75% of its litres. A 50-gallon (189 L) tank starts with 141.75 L. The other quarter is not counted because cold water entering at the bottom mixes into it, so it never leaves the outlet at full temperature.

2. Draw first

   Each minute, the hot water asked for by every use running in that minute is taken from what is left.

3. Flag a short minute

   If more than 5% of that minute's demand cannot be supplied, the minute is marked short. A shortfall of 5% or less is treated as a shower you would not notice.

4. Recover

   The burner or elements then reheat kW × 60 ÷ (4.186 × (set − inlet)) litres, and the tank is not allowed to exceed its usable amount. Water needs 4.186 kJ to warm one litre by one degree.

5. Repeat

   The same draw-then-recover step runs 120 times, from 6:00 to 7:59.

The factor 0.000293071 converts BTU per hour to kilowatts, and 0.78, 0.80 and 0.95 are the representative shares of burner heat that end up in the water. In hybrid mode a heat pump tank switches to 4.5 kW for any minute in which its hot water has fallen below half of usable, which is about 71 litres on a 50-gallon tank, and returns to 1.5 kW once it climbs back above that line.

Two outputs come straight from these numbers. Recovery is the base rate in litres per hour at your temperature rise. First-hour capacity is usable storage plus 60 minutes of recovery at that base rate: 141.75 + 163.8 = 306 L for the default gas tank. It is the simulator's own estimate at your settings, so it will not match the first-hour rating printed on a label, which comes from a standard test with fixed water temperatures.

06 /How does the tankless model work?

A tankless unit has nothing stored to run down, so the simulator compares each minute's demand with a fixed capacity: L/min at the set temperature = kW × 60 ÷ (4.186 × (set − inlet)). The default 199,000 BTU/h burner delivers 55.4 kW to the water, which heats 16.5 L/min through a 48 °C rise. Any minute that asks for more is flagged, and the headline reads "Temperature drops at" that time. The default sits just under the 200,000 BTU/h (58.56 kW) ceiling Natural Resources Canada uses to define a household gas-fired instantaneous water heater.

One result surprises people. Changing the set temperature on a tankless unit barely changes how many showers it can carry. At 49 °C the capacity rises to 18.9 L/min, but each shower then draws 6.0 L/min of hot water instead of 5.2, so three showers fit at either setting. The heat a mixed shower needs depends on its own temperature and the incoming water, not on the heater's setting. Hot-only appliances are the exception: at a higher setting, the dishwasher and laundry do take more heat.

07 /How do you read the chart, the gauge and the results?

• Tank chart. The vertical axis runs from zero to the tank's full volume in litres. A dashed line marks usable hot water when full; the filled area drops during each use and climbs back as the heater recovers.
• Shaded bands. Each short minute is shaded across the chart, so you can see whether the trouble is one long stretch or several short ones.
• Use lanes. Under the chart, each use appears as a bar starting at its time, labelled SHOWER, LONG SHOWER, BATH, DISHWASHER or LAUNDRY. The two-minute sink bar is too short to hold a label; hovering over any bar gives its name and start time. Uses that overlap stack in separate lanes.
• Tankless chart. A stepped line shows the litres per minute being asked for, against a horizontal line at the unit's capacity for your temperature rise.
• Play the morning. The cursor sweeps from 6:00 to 8:00 in about seven seconds, with a clock under the button. If your device asks for reduced motion, it jumps straight to the end.
• The gauge. It shows hot water ready as a share of the whole tank, so a full tank reads 75%, matching the usable-water assumption.

The results panel gives the headline ("Hot water all morning", "Runs short at" a time, or "Temperature drops at" a time for tankless). Under it, an Enough badge confirms every use got hot water, or a Short badge names the uses running in the first short minute and counts the short minutes. For tanks it then lists usable litres, recovery, first-hour capacity and the hot water used; for tankless, the capacity and the busiest minute. When your incoming water is below 15 °C, a final line reruns the same heater with 15 °C water, which is the fastest way to see how much of a problem is seasonal.

08 /Worked examples, recomputed

These are the simulator's reference cases, worked by hand from the formulas above so you can check the tool against the arithmetic.

• 50-gallon gas, 40,000 BTU/h, 7 to 55 °C. Heat: 40,000 × 0.000293071 × 0.78 = 9.14 kW. Recovery: 9.14 × 60 ÷ (4.186 × 48) = 2.73 L/min, or 164 L/h. Usable: 189 × 0.75 = 142 L. First-hour capacity: 142 + 164 = 306 L.
• One 8-minute shower draws 41.8 L of 55 °C water, and the family preset draws 4 × 41.8 + 2 × 4.5 + 23 = 199 L. A 50-gallon gas tank covers it; its lowest point is about 78 L at 7:07.
• 50-gallon electric, two-teens preset. The long showers draw 157 L by 7:00 while the elements add only 1.3 L a minute, so the tank is nearly empty by 7:05. It runs short at 7:06, at the end of the 7:00 shower, and again through the whole 7:10 shower and the 7:20 sink: 12 short minutes.
• 120,000 BTU/h tankless, weekend preset. Heat: 120,000 × 0.000293071 × 0.95 = 33.4 kW, which is 10.0 L/min at a 48 °C rise. The bath asks for 15 × 33 ÷ 48 = 10.3 L/min of hot water, so the temperature drops from 7:00 for all 10 minutes of the fill. With 15 °C water the capacity is 12.0 L/min and nothing drops.

The tankless line looks easy only because the family's showers never overlap. Put two showers at 6:30 on a 120,000 BTU/h unit and the demand is 10.45 L/min against 10.0, so the temperature drops for all eight minutes. The heat pump lines show why hybrid mode exists: on the heat pump alone, a 50-gallon tank recovers barely a litre every two minutes.

09 /What changes the answer most?

Three inputs move the result more than tank size does: the incoming water temperature, the set temperature and the spacing between uses. Take the two-teens preset on a 40-gallon gas tank at its 36,000 BTU/h default.

Spacing is the free fix. On a 40-gallon electric tank the family preset runs short at 7:06. Deleting the dishwasher changes nothing, because the shortfall comes from the fourth shower. Starting the showers 15 minutes apart instead of 10 (6:30, 6:45, 7:00, 7:15, then the sink at 7:25 and the dishwasher at 7:35) gives hot water all morning. Raising the set temperature works too, but store water that hot only with a mixing valve at the tank or the fixtures; the temperature setting guide covers the safe way to do it.

10 /What are the limits of the model?

• Representative inputs. Burner inputs, element ratings and the efficiency factors are typical values, not your model. Enter the input from your rating label.
• The 75% usable share is a simplification. The real figure depends on the tank's shape, the dip tube, the draw rate and how long the tank has been idle.
• Hot or gone. The model treats usable water as fully hot until it runs out. A real tank fades over a minute or two as the boundary between hot and cold layers reaches the outlet.
• Recovery starts at once. The model reheats from the first litre drawn; a real thermostat waits for the water around its sensor to cool, and the model ignores standby loss and assumes a full tank at 6:00.
• Heat pump output is fixed. It uses 1.5 kW in any room, while a real heat pump delivers less in a cold basement. See heat pump water heaters.
• Tankless details are left out. The minimum flow a unit needs before it fires, its start-up delay and any flow limiting are not modelled; real units either cut the flow or let the temperature sag.
• Plumbing and edges. Pipe runs, recirculation and waiting for hot water are ignored, fixture flows are fixed, and uses that run past 8:00 are cut off at 8:00.

It is also not a diagnosis. If your own tank runs short much earlier than the simulator predicts for the same morning, the tank is probably not performing to its size: sediment, a broken dip tube or a failed element are the usual suspects, and the not enough hot water guide shows how to check each one.

11 /How do you act on the result?

1. Hot water all morning, with a margin

   Keep what you have, or replace like for like. Natural Resources Canada's Water Heater Guide advises against oversizing a tank for an infrequent high demand, because a larger tank costs more to buy and loses more heat on standby.

2. Short for a few minutes

   Reshape the morning before buying anything: space the showers, move the dishwasher and laundry after 8:00, or store hotter water behind a mixing valve. Rerun the simulator after each change.

3. Short only when the water is cold

   If the 15 °C line clears the problem, it is seasonal. The guide to why hot water runs out in winter explains the fixes that target winter specifically.

4. Short for a long stretch, or a tankless drop

   Size up with a clear target. The what size water heater guide covers first-hour ratings, and tankless water heater sizing covers flow and burner input for Canadian winter water.

5. Ready for quotes

   Tell installers the morning you tested and the result you need, such as "four showers between 6:30 and 7:10 with 4 °C water". Then request quotes and compare them on first-hour rating or flow at your temperature rise.

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