Plumbing
Sizing Domestic Hot Water: Finding the Peak Hour and Serving It
Domestic hot water systems are sized by the worst sixty minutes a building sees, not by the gallons it burns over a whole day.
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The hour that decides the plant
A building's daily hot water consumption is a billing number. It tells you what the gas meter or the kWh counter will read at the end of the month, and it tells you nothing whatsoever about whether the third-floor shower runs cold at ten past seven. Storage capacity and burner output are sized against demand density — how much draw arrives in one clock hour and how fast it arrives inside that hour. Two buildings can consume identical volumes over twenty-four hours and need plants that differ by a factor of three, because one spreads its draw across the day and the other concentrates it into a single savage window.
Every occupancy has that window somewhere. In dwellings it sits in the morning before the commute and again in the evening after the last meal. In a school it lands at the end of the games period, when thirty showers open within about ninety seconds of one another. In a care home it is bath rotation, which is scheduled and therefore knowable to the minute. In a hotel it is checkout morning, and it moves with the day of the week — a conference hotel on a Friday peaks differently from the same building on a Sunday. Restaurants peak on the pot-wash after service, not during it, and that surprises people who assume the kitchen is busiest when the dining room is.
So the first field task on any DHW job is not a calculation. It is an interrogation. Ask the client who uses hot water, at what hour, in what order, and whether that order is enforced by anything — a shift change, a timetable, a room-turnover schedule. Ask what happens on the busiest day of the year, and whether that day is a genuine outlier or a monthly recurrence. Written answers beat remembered ones. A facilities manager who can produce a housekeeping rota has handed you your peak hour on paper.
Where an existing system is being replaced, the building has already told you the answer and nobody has read it. Fit a clamp meter to the immersion circuit or a data logger on the cold feed for a fortnight, and the peak hour draws itself. Complaints logs work too: the times at which people said the water went cold are the times the plant ran out. Those are more reliable than a survey, because nobody misremembers a cold shower.
How the peak hour actually arrives
Within the peak hour, demand is never flat. Understanding its internal shape matters more than getting its total exactly right, because two profiles with the same hourly volume place completely different demands on a cylinder. A gym with sixty showers spread evenly across the hour can be served by recovery alone if the heat source is generous. The same sixty showers arriving in a twelve-minute block after a class finishes must come almost entirely out of stored volume, because no realistic burner recovers fast enough to keep pace.
The trade distinction is between draw that is served by stored heat and draw that is served by input. Storage buys you time; input buys you endurance. A tank-heavy, input-light plant handles short violent spikes and struggles with sustained back-to-back demand. An input-heavy, tank-light plant does the opposite: it will run a hotel laundry all afternoon and still lose a rugby team's showers. Most real buildings need a considered blend, and the blend follows the shape of the peak hour, not its size.
Three questions settle the shape. How many fixtures can plausibly be open simultaneously — not how many exist, but how many one realistic scenario opens at once? How long does each event last, and is that duration governed by anything other than the user's habits (a timed shower valve, a wash cycle, a metered tap)? And is there any recovery gap inside the hour — a five-minute lull between two class changeovers is enough for a well-matched heat source to claw back meaningful storage, whereas a continuous queue offers nothing.
Watch for the peak that stacks. A commercial kitchen that starts its pot-wash while the last covers are still leaving overlaps two demand types, and a laundry that runs its heaviest cycle during the morning shower peak converts two comfortable loads into one that fails. Sequencing is free. Before you upsize a plant, ask whether the client can move a laundry start time by ninety minutes, because that costs nothing and often removes the problem entirely.
Turning the hour into a cylinder and a burner
Once the peak hour is described — total volume, arrival shape, and the temperature it must be delivered at — the sizing itself becomes a balance between what the vessel holds and what the heat source can replace while the vessel is emptying. Usable storage is always less than nameplate volume, because stratification breaks down as the tank draws down and outlet temperature falls before the tank is nominally empty. Working on nameplate volume is one of the most common ways a correctly calculated system underperforms in service.
Storage temperature and delivery temperature are different numbers, and the gap between them is a design lever. Storing hotter and blending down at a thermostatic mixing valve multiplies effective capacity for a given tank volume, because each stored litre serves more than one litre at the outlet. That approach carries obligations: a compliant TMV arrangement, protection at every accessible outlet, and awareness that the scald risk moves from the tank to the pipework if the mixing is done badly or bypassed later by someone chasing better flow.
Legionella control pushes in the same direction as capacity and against energy efficiency, which is a genuine conflict rather than a solvable one. Storage held above the growth range and distribution kept hot throughout are the conventional controls; the specific temperatures and the frequency of monitoring are set by national guidance and the water safety regime the building falls under, so name the governing document in your submission rather than a temperature you half-remember. In the UK, water fittings and backflow requirements sit under the Water Supply (Water Fittings) Regulations; unvented hot water storage carries its own building regulations obligations and installer competence requirements. In the US, plumbing code requirements are adopted state by state, and ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems, governs water management planning for the building types it covers. Cite the designation and title, and let the authority having jurisdiction supply the numbers.
Set the delivery temperature target before sizing, not after. A system sized to deliver a comfortable shower and then asked at commissioning to feed a sanitiser rinse at a much higher temperature has been sized for the wrong job, and the fix is rarely cheap.
This is the point where the peak-hour picture you have just assembled becomes a storage volume and an input rating, so work it here rather than after the plant room layout is fixed.
Recommended tank size
57.5 gallons (recommended)
This is a rough rule-of-thumb range. Homes with multiple bathrooms used simultaneously, soaking tubs, or high-flow fixtures should size toward the higher end or consider a tankless system sized by flow rate (GPM) instead.
- Recommended range, low
- 50 gal
- Recommended range, high
- 65 gal
With the figures above, the recommended tank size comes to 57.5 gallons (recommended). The method behind this is well established, though site conditions and material batches will move it somewhat. This is presented for United States. Building in another market? Change the selector above and the units and terminology follow.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Recovery: the second half of the answer
Storage gets you through the spike. Recovery decides whether you are ready for whatever follows it, and on most jobs the second event matters more than the first. A hotel that serves its 07:00 rush beautifully and then cannot serve the 08:15 stragglers has a recovery problem, not a storage problem, and adding another cylinder will not fix it. The diagnostic is simple: if the first draw succeeds and the second fails, you are input-limited.
Recovery rate is a function of heat input, the temperature rise required, and the specific heat of water. The rise is the part people get wrong on site, because incoming main temperature is seasonal. A system commissioned in July against a warm incoming supply meets its recovery time comfortably; the same system in February faces a rise perhaps ten to fifteen degrees greater and recovers proportionally slower. If your peak hour lands in winter — and in most buildings it does, because that is when showers get longer — size against the cold-month inlet temperature and note the assumption on the drawing.
Heat pump plant changes the arithmetic further. Output falls as ambient falls and as the required flow temperature rises, so a heat pump cylinder sized on a datasheet figure taken at mild ambient will recover slowly on precisely the days it is asked to work hardest. Heat-pump-fed systems generally want more storage and more tolerance of long recovery, which usually means charging outside the peak hour rather than during it. That is a scheduling decision as much as a hardware one.
Where the peak hour has an internal lull, recovery inside the hour genuinely counts and can be credited. Where it does not — a continuous queue with no gaps — treat recovery as irrelevant to the peak itself and let storage carry the whole load, using recovery only to set how long before the next event the plant is ready again.
Having chosen a storage volume, you need to know how long the plant takes to be ready for the second wave, and that answer changes with winter inlet temperature — so check it before the cylinder is ordered.
Recovery time
58.5 minutes
This estimates recovery from a fully-drained tank using nameplate BTU input — real-world recovery is somewhat slower due to standby losses and burner cycling.
- Recovery rate
- 0.86 gal/min
For the dimensions entered, expect a recovery time of 58.5 minutes. Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States. The market selector changes the units and the trade terminology; any standard behind the formula is cited under sources.
Commissioning against the hour you designed for
A DHW system is not proven by a warm tap. It is proven by a drawdown test that reproduces the peak hour as closely as the building allows, with the tank starting at its normal set point and thermostats set as they will run. Open the number of fixtures your design assumed, hold them open for the assumed duration, and record outlet temperature at the most disadvantaged fixture — the furthest, highest, or smallest-bore one — throughout. The moment delivery temperature drops below target is the real capacity of the system, and it should arrive later than your design event ends.
Run the test in the coldest conditions available. A summer drawdown that passes tells you little. If the programme forces summer commissioning, note the inlet temperature you tested at and state the winter margin explicitly, so that a January failure is understood as a known condition rather than a defect.
Record what you measured: incoming main temperature, storage temperature at start, time to first temperature drop, and recovery time back to set point with no draw. Those four numbers let the next engineer diagnose a complaint in twenty minutes instead of re-surveying the building. Hand them to the client with the O&M, not just to the file.
Dead legs and recirculation deserve a separate look at handover. A generously sized plant still delivers a cold minute at the tap if the run to that outlet is long and unpumped, and the occupant experiences that as a sizing failure. Where a recirculation loop exists, check that it is balanced and that return temperatures at the branch ends are within the range the water safety plan demands. Loop losses also count against the plant, quietly, all day.
What changes the answer later
Peak hours drift. A residential block that was sized for families gradually fills with shift workers, and the single sharp morning peak flattens into two smaller ones — which is a system that has become oversized and now cycles badly. The opposite happens too: an office converted to a gym, a nursing home increasing its bathing frequency, a school adding a sports block. Neither change touches the pipework, and both change what the plant must do.
Fixture changes matter more than occupancy changes, and they happen without anybody telling the plumber. A refurbishment that swaps restrictive shower heads for drench heads can double the flow at every position while the building's occupancy sits still. Conversely, retrofitting flow limiters is one of the few interventions that fixes an undersized plant without touching the plant, and it is worth pricing as a serious option before condemning a cylinder.
Leave the assumptions where someone will find them. A short schedule on the plant room wall stating the design peak hour, the simultaneous fixture count assumed, the delivery temperature, and the inlet temperature used for the recovery calculation turns a future complaint into a comparison. Without it, the next engineer starts from nothing and will most likely just fit a bigger tank.
When a complaint does arrive, resist sizing anything until you have established which hour is failing and what changed. Most DHW callouts are not capacity failures at all — they are a failed thermostat, a scaled immersion, a TMV drifting, a recirculation pump that stopped in March, or a control schedule someone edited. Confirm the plant is delivering what it was designed to deliver before you conclude the design was wrong.
Storage, instantaneous, and the honest comparison
The peak hour also decides the plant type, and it decides it more cleanly than any general argument about efficiency. Instantaneous and tankless equipment sells endurance: it will run indefinitely at its rated output, and it does not stand losing heat overnight. What it cannot do is exceed that rated output for even a moment, because there is no reservoir behind it. A peak hour that is short, violent, and heavily overlapped is exactly the shape instantaneous plant handles worst, and it is also the shape most residential and sports occupancies produce.
Storage sells the opposite: a burst capability far above the heat input, followed by a period of weakness while it refills. Where the peak hour is one concentrated spike per day with hours of quiet either side, storage is usually the cheaper and simpler answer, because the input can be modest and the vessel does the work.
Many buildings genuinely want both — a buffer vessel to absorb the spike and a modulating instantaneous source to provide endurance behind it. Sizing that hybrid still starts from the same place: describe the hour, split it into the part that must come from storage and the part that can be served by input, and let those two numbers pick the equipment.
Whichever route you take, the temperature rise assumption follows the plant into service. Instantaneous units are rated at a stated rise, and their usable flow collapses as the required rise increases — the winter figure, not the datasheet headline, is the one that has to serve your peak hour.
Before you leave site
A DHW survey that captures these six things gives you enough to size the plant without a second visit.
- Incoming main temperature, measured — Take it at the coldest tap after a good run; note the month, since winter will be lower and that governs recovery.
- Simultaneous fixture count for one realistic scenario — Not the total fixture schedule — the number that can credibly be open together during the worst hour.
- Written occupancy schedule — Rotas, timetables, checkout times, laundry starts. Overlapping schedules are what turn two manageable loads into one failure.
- Delivery temperature required at each outlet type — Showers, kitchen, sanitiser rinse and laundry may differ; the highest requirement sets storage temperature and the TMV strategy.
- Existing plant nameplate and recovery behaviour — On a replacement, log the cylinder volume, input rating and the time of any reported cold-water complaints.
- Recirculation loop condition and dead-leg lengths — Loop losses count against the plant all day, and a long unpumped run reads to the occupant as an undersized system.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.
Drawn from
- ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems
- Water Supply (Water Fittings) Regulations
- ASHRAE Handbook — HVAC Applications, Service Water Heating
Guidance, not a specification. Local codes, the engineer of record and the product manufacturer’s instructions govern where they differ from anything written here.