Sizing is where a lot of hot water disappointment starts. A system that is slightly too small produces a household argument every morning, and one that is too large costs money to keep hot for water nobody uses.
The two mistakes have different causes, and we see both regularly across Sunshine Coast households. Undersizing usually comes from counting bedrooms instead of people, or from a household that has grown since the last replacement. Oversizing usually comes from adding a safety margin to a number that was already right.
How demand builds: general examples, not recommendations
How demand builds: general examples, not recommendations
Factor
What drives demand
Why it matters
One or two people, one bathroom
Showers, kitchen, laundry. Usage tends to be spread rather than simultaneous.
The lowest demand profile. Recovery rate matters less because peaks are small.
Family, two bathrooms
Overlapping morning showers, dishwasher and laundry through the day.
Simultaneous use becomes the constraint. Recovery rate starts to matter as much as capacity.
Family with a bath used daily
A bath draws a large volume in a single event, often in the evening.
Adds a sharp peak that a system sized only on occupant count will miss.
Spa bath
A very large single draw, sometimes emptying a tank on its own.
Frequently the single factor that decides the system, regardless of household size.
Long or high-flow showers
Showerhead flow rate and time both multiply straight into demand.
Two households of identical size can differ substantially on this alone.
Household about to change
Children growing, someone moving in, a bathroom being added.
A system usually outlasts one significant change, so it is worth naming before sizing.
Storage: capacity and recovery, not just capacity
For a storage system there are two numbers that matter, and most conversations only mention one. Capacity is how much hot water is ready to use. Recovery rate is how quickly the system reheats once that is drawn down.
A household with a sharp morning peak and a long quiet day may do well with moderate capacity and good recovery. A household that draws hot water steadily across the day has a different requirement. Capacity alone does not describe it.
Continuous flow: it is a flow-rate question
Continuous flow units do not run out, which removes the capacity question entirely and replaces it with a different one: how much hot water per minute the unit can deliver at the temperature you want.
The practical test is simultaneous outlets. If two showers might run at once, or a shower and the kitchen, the unit needs to carry both. Undersize it and the symptom is not running out, it is the temperature dropping when someone else turns a tap on.
Incoming water temperature matters too, since the unit is raising water from whatever temperature it arrives at.
Why the number of bedrooms is a bad way to size a system
Sizing charts often work from bedrooms, because bedrooms are the one number everyone knows about a house. It is a poor proxy. Two people in a four-bedroom house use the hot water of two people. Five people in a three-bedroom house use the hot water of five.
What actually drives demand is how many people draw hot water, how long they draw it for, and how much of that happens at the same time. A household of teenagers with long showers back to back is a different problem from the same number of adults spread across the day, even though the house is identical.
Baths are the other thing charts underweight. One bath can use as much hot water as several showers, and a spa bath more again. If baths are a regular part of the week rather than an occasional thing, that changes the size before anything else does.
So the useful questions are about the household, not the building. We would rather ask five of those than read a chart at you.
What undersizing and oversizing actually cost you
Undersizing is the failure people notice. A tank that runs out mid-shower, or a continuous flow unit that goes lukewarm when a second tap opens, is a daily irritation that no amount of explaining fixes. It is also the harder mistake to correct, because the answer is usually a new system rather than an adjustment.
Oversizing is quieter and it is not free. A larger storage tank holds more water at temperature all day whether anyone uses it or not, and standing heat loss is a real running cost on an electric or gas tank. It also takes more space, costs more to buy, and on a heat pump can mean a physically larger unit in a position that was already tight.
There is a sensible margin either side, and the honest advice is to sit slightly on the generous side of what the household needs today rather than exactly on it, because households grow more often than they shrink and a system usually outlasts one change.
What is not sensible is jumping two sizes because bigger sounds safer. That is how a tank ends up in a spot where it no longer fits, or a heat pump ends up too close to a bedroom wall.
Space, tariff and what is already there
Sizing does not happen in isolation from the rest of the decision. A larger tank needs somewhere to go, and the available space sometimes caps the option before demand does.
Tariff matters for electric storage, because a controlled-load tariff heats at particular times, which puts more weight on capacity and less on recovery.
And the existing connections matter, because a system that would suit the household perfectly is not much use if the gas line or the switchboard cannot carry it without an upgrade nobody budgeted for.
How we would size it
We ask who lives there, how many bathrooms, whether anyone baths, whether hot water gets used in two places at once, and whether anything about the household is about to change.
Then we look at the space, the connections and the tariff, because those constrain the answer. The result is usually a narrow range rather than a single option, and the choice within that range comes down to budget and running cost.
If you want that worked through for your place, call 0415 611 356 or send the details.
Frequently asked questions
How do I know my system is too small?
The clearest sign is running out during ordinary use rather than during unusual use. If the second shower of the morning is cold every day, capacity or recovery is short. If it only happens when you have visitors, the system is probably sized correctly for the household and the problem is a peak rather than a pattern.
Is a bigger tank always safer?
No. An oversized storage tank keeps a larger volume of water hot around the clock, which costs more to run for hot water you never use, and it takes up more space. Oversizing is a real cost, not just wasted headroom. Matching capacity to actual demand is better than adding a margin for its own sake.
How is continuous flow sized differently?
By flow rate rather than capacity. A continuous flow unit will not run out, but it can only heat so much water per minute. The sizing question is how many outlets might run at once: two showers, or a shower and a kitchen tap. Undersize it and the temperature drops when demand overlaps.
Do baths and spa baths change the sizing?
Considerably. A bath draws a large volume in one go, and a spa bath more again. A household that baths children every evening has a very different demand profile from one of the same size that showers, even though the occupant count is identical. It is one of the first things worth mentioning when asking for a recommendation.
Should I size for the house or the household?
The household, with a thought for the future. Bedroom count is a poor proxy: two people in a four-bedroom house use two people's worth of hot water. What matters is who actually lives there, how they use it, and whether that is about to change. A system usually lasts long enough for one significant change in the household.
RCM works across Sunshine Coast and the hinterland. See the full Sunshine Coast service area for where we work regularly and where we travel on different terms.
