Hot water system sizing: a demand calculation that works for any system type

Most hot water complaints do not come from a failed unit. They come from a unit that was the right product for the wrong demand. A household of five in a cold climate with a large bath and a teenager who showers at night will draw far more in a peak hour than the same number of people in a warmer climate with a shorter routine, and a selection made on household size alone will not separate the two.

The practical approach is to treat the job as a demand calculation. Work out the volume and the energy the household needs in its worst hour, then look at what each system type can deliver against that figure. The technology choice falls out of the calculation rather than driving it.

This matters commercially because the failure mode is slow. An undersized system passes its commissioning checks, passes practical completion, and then starts running out of hot water on the third or fourth shower of a winter morning. By the time the client rings, the retention has often been released and the conversation is about who pays to change a cylinder that is not defective.

Start with peak-hour draw, not with bedroom count

Bedroom count is a proxy that builders and salespeople use because it is easy, and it is wrong often enough to cause real disputes. The variable that determines whether a system keeps up is the volume drawn in the busiest hour, and that depends on how many people use hot water at the same time and how much each of them uses.

Build the peak-hour figure from the outlets. A shower draws a known flow rate for a known duration. A bath draws a fixed volume in one go. A kitchen sink, a dishwasher and a laundry tub each add their own draw. The demand calculation is a matter of adding up the outlets that can realistically overlap and converting that volume into the energy needed to raise it from the incoming cold water temperature to the delivery temperature.

The incoming cold water temperature is the part most often skipped. It varies with climate and season, and it sets how much work the system has to do for every litre. A sizing sheet that assumes a warm inlet will be wrong on the coldest mornings of the year, which is exactly when the demand peaks. If you are selecting a system in a climate where inlet temperatures swing widely, the design condition should be the cold end of that range.

Write the assumptions down. A demand calculation that lives only in the salesperson’s head cannot be checked when the complaint arrives, and the builder wears the outcome.

Storage and recovery are two different numbers

The most common sizing error is treating a cylinder as a single figure. It is two. Storage is how much hot water is sitting there at the start of the peak. Recovery is how fast the system reheats the cylinder while the peak is still running.

A large cylinder on a slow-recovery system will cover a short, intense draw and then fail on a long one. A smaller cylinder on a fast-recovery system will do the opposite. Neither is universally better, and which one suits the job depends on the shape of the household’s demand.

For a household where everyone showers in a tight window, storage does most of the work. For a household with staggered use across the morning and evening, recovery does more of it. If the peak-hour volume exceeds the usable storage, the difference has to be made up by recovery during the hour, and the recovery rate has to be checked at the design condition rather than at whatever condition the product data headline figure uses.

It is also worth distinguishing usable storage from nominal storage. The draw-off temperature falls as cold water enters the cylinder, so the last part of the stored volume is cooler than the first. Sizing that assumes the full nominal volume is available at delivery temperature will come up short.

Each system type has a different recovery profile

The demand calculation is technology-neutral, but the answer is not, because each system type recovers at a different rate and under different constraints.

• Electric resistance storage has a predictable recovery rate tied to the element size, and it draws heavily while recovering. Its constraint is the electrical capacity available at the site as much as the cylinder.
• Gas storage and continuous flow recover quickly, and continuous flow has effectively no storage limit, but its performance depends on the incoming water temperature and the unit’s capacity to raise it at the required flow rate. A continuous flow unit sized on a warm inlet will not hold its delivery temperature on a cold morning at the same flow.
• Heat pumps recover slowly compared with combustion or resistance, because they move heat rather than generate it directly. That makes storage volume and correct sizing more important, not less. Their output also varies with ambient conditions, so the rated performance and the delivered performance on a cold morning are not the same thing.
• Solar with electric or gas boost behaves like storage with a variable input. On a good day the boost does little; across a run of overcast days the boost carries the whole load, so the boost element or burner has to be sized for the full demand, not the average.

The Australian Government’s household guidance on heat pump water heaters is a useful reference for how these systems are specified for households, and it is worth reading before you commit a client to a particular type. The point for a builder is that the same peak-hour figure produces different answers depending on the recovery profile you are buying.

Climate and the design condition change the answer

Two identical houses with identical households will need different systems if one is on the coast and the other is on the tablelands. The difference is not the volume of water. It is the temperature the water arrives at and the ambient temperature the system operates in.

For heat pumps this is the dominant variable. A unit’s capacity at mild ambient conditions can be substantially higher than what it delivers when the air is cold, and the coefficient of performance drops with it. A selection made on the mild-condition figure will look generous on paper and come up short in the morning. The fix is straightforward: state the ambient design condition on the selection sheet and check the product data at that condition.

For continuous flow gas the equivalent variable is the incoming water temperature. The unit has to raise the water through a larger temperature difference on a cold morning to hold the same delivery temperature at the same flow. If the calculation assumed a smaller difference, the outlet will run cooler or the flow will drop when demand is highest.

For solar the variable is the solar resource across the year, and the boost has to be sized for the worst run of days rather than the average. A system that performs beautifully for most of the year and relies on a boost that is too small in a wet week will generate exactly the complaint that sizing is supposed to prevent.

Match the calculation to the energy available at the site

A demand calculation produces a requirement. The site produces constraints. The two have to be reconciled before anything is ordered.

Electrical capacity is the first constraint for heat pump and electric resistance systems. A system that recovers quickly draws a large load while it recovers, and if the switchboard and supply cannot carry that load, the recovery rate in the calculation is not achievable in practice. Check the supply capacity and the circuit arrangement before you commit to a system whose recovery depends on a heavy draw.

For heat pumps, the placement matters as much as the number. A unit that is specified for its performance at a given ambient condition will not deliver that performance if it is installed in a position where it is starved of air or exposed to conditions the data does not cover. Refrigerant work also has licensing and documentation requirements that vary by jurisdiction, and the installation has to be done by someone appropriately licensed. If you are working through refrigerant and electrical interfaces on residential systems, the material on split system air conditioner installation in NSW covers the same interface issues on a comparable installation.

There is also a scheme dimension. Small-scale technology certificates are a federal mechanism that affects the effective cost of eligible systems, and eligibility depends on the system meeting the scheme’s requirements. The Clean Energy Regulator’s information on small-scale technology certificates is the source to check before you quote a client on a system whose economics depend on the certificates. Do not quote a certificate value you have not verified for the specific model and installation.

Worked example: a four-person household in western Sydney

A builder in western Sydney is quoting a knock-down rebuild for a family of four. The client has asked for a heat pump because of the running cost, and has sent through a model they found online with a storage volume that looks generous. The builder does the demand calculation before pricing it.

The household runs three showers and a kitchen draw in a tight morning window, with a bath two or three nights a week. The builder lists the outlets, notes the flow rates, and works out the volume drawn in the busiest hour. Converting that to an energy load requires the incoming cold water temperature, so the builder uses the cold end of the range for the site rather than a mild average.

The figure that comes out is larger than the model the client selected can deliver in that hour at the design condition, because the model’s recovery is slow and the peak is short and intense. The builder puts two options to the client: a larger storage volume on the same technology, or a smaller storage volume on a system with faster recovery. Both meet the peak-hour requirement; they differ in how much of the load is carried by storage and how much by recovery during the hour.

The client chooses the larger storage option. The builder records the demand assumptions, the design condition and the product data used, and keeps them with the quote. Eighteen months later the client asks about adding a fifth occupant. Because the assumptions are on file, the conversation starts from the actual peak-hour figure rather than from an argument about whether the original system was undersized.

That is the whole commercial value of doing the calculation properly. It is not that the first selection is always right. It is that when the household changes, you can show what the system was sized for and what would need to change.

Commissioning and handover are part of the sizing

A correctly sized system can still fail its first winter if it is not commissioned to the design conditions. Delivery temperature, flow rate and control settings all have to be checked against what the sizing assumed.

At handover, walk the client through what the system was sized for. If the sizing assumed three simultaneous showers in the morning, say so. If it assumed the boost would carry the load across a run of overcast days, say so. Clients who understand the basis of the selection raise a question when their usage changes, rather than waiting for a failure.

Record the commissioning figures. Delivery temperature at the outlet, flow rates at the fixtures, and the control settings as left. If a complaint arrives later, those figures are the baseline for the conversation.

Where sizing errors surface, and who pays

Sizing errors rarely show up at inspection. They show up in use, in the coldest part of the year, when the household is doing exactly what it always does. That timing is what makes them expensive.

If the complaint arrives during the defects liability period, the builder is generally the one resolving it, even where the installed product is not defective and was installed correctly. Replacing a cylinder means a plumber, a crane or a manual handling exercise depending on the unit, an electrical or gas connection, making good, and a new compliance certificate. On a heat pump it may also mean refrigerant recovery and recharge by a licensed technician. None of that is recoverable from the manufacturer when the product worked as designed.

The defence is documentation. A demand calculation with stated assumptions, a design condition and the product data used at that condition turns a dispute about competence into a conversation about changed circumstances. Without it, the builder is arguing from memory about a decision made at quoting stage.

Safety is part of this too. Working in roof spaces and plant areas in summer to adjust or replace a system carries real heat stress risk, and Safe Work Australia’s guidance on working in heat sets out how to manage it. A sizing error that sends a crew back into a roof space in January is a safety cost as well as a financial one.

Keep the calculation current as requirements change

Requirements around hot water in residential work do move. Energy performance provisions, scheme eligibility and product performance data are all revised from time to time, and a sizing method that was correct a few years ago may not reflect the current requirement.

That is a reason to keep the calculation as a live document rather than a one-off exercise. When you are checking a requirement you are not certain about, go to the current source: the product manufacturer’s data for the specific model at the design condition, the relevant Australian Standard for the installation requirements, and the scheme administrator for certificate eligibility. Do not rely on a figure from an older job file.

For builders in NSW, the continuing professional development system is where this kind of change gets picked up systematically. The guide to how NSW builder CPD works explains the structure, and the course catalogue lists what is available across the sustainability and compliance categories. If you are weighing up what to do next, the PointCPD home page for NSW builder CPD courses is the starting point.

The sizing method itself does not change much, because it is just arithmetic applied to a household’s real demand. What changes is the data you put into it and the requirements the installation has to meet. Keep the method, update the inputs, and keep the calculation on file.

What to put on the selection sheet

A selection sheet that a plumber, an electrician and a certifier can all read is short. It states the peak-hour volume, the incoming cold water temperature used, the delivery temperature required, the usable storage and the recovery rate at the design condition, and the product data those figures came from.

It also states the assumptions about simultaneous use. That single line is what converts a sizing decision from a judgement call into a documented basis, and it is the line that ends most arguments about whether a system was undersized.

Do the calculation before the product is chosen. Check the recovery at the condition the site will actually see. Record what you assumed. Everything else in hot water sizing follows from those three.

Questions builders ask

• Heat pumps
• Hot Water
• Plumbing

Published 28 July 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.