Four people is the household size where hot water sizing starts to bite. Two people can get away with almost anything. Four people showering one after the other, with a dishwasher and a washing machine in the mix, will find out quickly if the unit is too small. Here is how we size for a family of four on the Gold Coast, and what each option costs installed.
Hot water for a family of four: installed cost by system type
| System type | What suits four people | Supply and install |
|---|---|---|
| Electric storage | 250 litre tank, especially on an off-peak tariff | $1,300 to $2,400 |
| Gas storage | 250 litre tank, or 170 litre where showers are spread out | $1,800 to $3,200 |
| Gas continuous flow | High-flow unit sized for two outlets running at once | $1,900 to $3,500 |
| Heat pump | Larger tank in the range, because recovery is slow | $2,500 to $4,500 after STC rebates |
All four figures are supply and install for a straightforward changeover. Where the price lands within each range depends on the brand, the tank size, whether the existing pipework, valves and location can be reused, and for gas, whether the flue and gas line are already right for the new unit.
What size storage tank does a family of four need?
For most four-person homes we fit a 250 litre tank. A storage system has two numbers that matter: how much hot water it holds, and how quickly it heats a fresh tankful. The tank size covers the morning rush, and the recovery rate decides how soon you can do it again. A smaller tank runs out during back-to-back showers, and the last person in the queue gets a cold one. On electric storage that is made worse by the tariff. An off-peak tank only heats during the overnight window, so whatever is in it first thing has to last the whole day, and that is the strongest reason to go to 250 litres on electric rather than trying to save money on a smaller cylinder. Gas storage recovers faster, so a 170 litre gas tank can carry a family of four if showers are spread across the morning and evening, but if everyone showers in the same hour, 250 litres is the safer choice. The price difference between tank sizes is small next to the cost of getting it wrong.
Is continuous flow better for a family?
Often, yes. A continuous flow unit heats water as it passes through, so it never runs out, and that removes the queue problem entirely. It is sized by flow rate rather than storage: the question is how many hot outlets you want running at the same time, not how many litres you use in a day. For a family of four we fit the high-flow unit in the brand's range, not the compact model built for a unit or a townhouse. The compact unit will run one shower comfortably and then struggle when a second tap opens, which is exactly the situation a busy household creates every morning. The other thing to check is the gas supply. A high-flow unit needs a gas line sized to feed it, and on some older homes the existing line was run for a cooktop and a small storage tank and needs upgrading. That work is quoted at the same time so there are no surprises. Installed cost sits at $1,900 to $3,500, and the gas side of the job is done under a Queensland gas work licence, which is separate from the plumbing licence.
Does a heat pump work for four people?
Yes, provided it is sized properly. A heat pump is a storage system that heats slowly and cheaply, pulling warmth from the air instead of burning it in an element. In the Gold Coast climate it is the cheapest system to run, and it pairs well with rooftop solar because you can time it to heat through the middle of the day. The trade-off is recovery. Once the tank is empty it takes longer to come back than a gas or full-rate electric unit, so for a family of four we go to the larger tank in the range rather than the smaller one, and we set the heating schedule to suit when the house actually uses hot water. Installed cost is $2,500 to $4,500 after STC rebates, which is the dearest of the four up front and usually the cheapest to run.
What drives the price within each range?
- Tank size and flow class. A 250 litre tank costs more than a 170, and a high-flow continuous unit costs more than the compact one.
- Like-for-like or a change of type. Swapping electric for electric in the same spot is the simplest job. Going from a storage tank to continuous flow means new pipework, a gas line check and a different mounting, and it sits higher in the range.
- Location. A unit in an open side yard is quicker to change than one in a cupboard or on a raised platform. Coastal installs get salt-rated valves and fittings.
- Valves and pipework. Old isolation valves, tempering valves and relief valves are replaced with the unit. We would rather include them than leave a new tank hanging off worn parts.
What we ask before quoting
How many people live in the house, when they shower, whether there is a bath in regular use, what tariff the current unit is on, whether gas is already connected, and where the unit sits. Those answers tell us the size before we look at brands. If the current system has been running out, that is the most useful piece of information you can give us, because it means the last person sized it too small and we will not repeat that.
Storage or continuous flow: which would we pick?
If gas is already at the house and the family showers in a rush, continuous flow, sized high-flow. If there is no gas and the roof has solar, a heat pump with the bigger tank. If the budget is tight and the existing unit is electric storage on off-peak, a 250 litre electric replacement is the cheapest to install and will do the job, with the honest warning that it is the dearest of the four to run. There is no single right answer for every household, which is why we ask the questions above before we quote rather than after.
Paperwork on a replacement
A like-for-like hot water replacement is notifiable work in Queensland, so we lodge a Form 4 with the QBCC after the installation. Gas units come with a gas compliance certificate from the licensed gas fitter. Both go in your records and both matter at sale time and for insurance. Owner operator, 13 plus years on the tools, QBCC plumbing licence 15243081.
Sick of the last shower going cold? Call Hills Plumbing & Gas on 0472 657 042 or book through our Hot Water Systems service page for a fixed price on a unit sized for the way your family actually uses it.