Sizing is where pool heating quotes go wrong, because the cheapest unit on the quote is usually the smallest, and the smallest unit is the one that disappoints you in winter. Here is how the sizing logic actually works, so you can read a quote critically.
Indicative sizing by pool volume (Gold Coast, outdoor pool with cover)
| Pool volume | Heat pump class | Gas heater class | Notes |
|---|---|---|---|
| Up to 30,000 L | Roughly 13 to 19 kW | Roughly 200 to 250 MJ | Small plunge and compact pools |
| 30,000 to 50,000 L | Roughly 21 to 28 kW | Roughly 250 to 350 MJ | The most common Gold Coast family pool band |
| 50,000 to 70,000 L | Roughly 28 to 36 kW | Roughly 350 to 400+ MJ | Larger pools, spa combos |
| Any size, no pool cover | Step up a size class | Step up a size class | Evaporation is the biggest heat loss |
| Year-round swimming goal | Size generously, never minimum | Gas handles cold snaps easily | Final sizing confirmed at quote time |
All figures are indicative classes for comparing quotes, not a substitute for a site assessment. We confirm sizing at quote time against your actual pool, exposure and goals, and pricing is quoted on inspection.
How does pool heater sizing actually work?
Two numbers drive it: how much water you are heating and how fast you want it to change temperature, set against how fast the pool loses heat. A heat pump is a marathon runner: it moves heat from the air efficiently but slowly, so it is sized to hold a temperature across the season, running steadily through the day. A gas heater is a sprinter: it generates heat directly and can lift a pool several degrees overnight, which is why weekend-use households and spa owners like it. That difference is why the two are sized in different units, kilowatts for heat pumps and megajoules for gas, and why comparing a quote for each is really a comparison of two different swimming patterns, not two prices for the same thing.
Why is undersizing the classic mistake?
Because an undersized heater fails invisibly. It works, the pool gets warmer, and everyone is happy in October. Then the first real cold snap arrives, overnight losses outrun what the unit can put back during the day, and the pool sits stubbornly below target while the heater runs flat out and your electricity or gas usage climbs. The fix at that point is replacing or supplementing the unit, which nobody enjoys paying for twice. Our honest rule: on the Gold Coast, if the sizing calculation puts you between two unit sizes and you want winter swimming, take the bigger one. If you only want to stretch the season from September to May, the smaller unit is often genuinely fine, and we will tell you which camp your usage puts you in rather than defaulting to the dearer box.
Does a pool cover change the sizing?
More than any other single factor, yes. Most of a Gold Coast pool's heat loss is evaporation from the surface, and a decent cover cuts that dramatically, especially overnight when losses peak. A covered pool can often run one size class smaller, or the same size heater for noticeably less running cost. If your budget is tight, a cover plus a properly sized heater beats a bigger heater on an uncovered pool almost every time. This is also where we concede the obvious: on pure running cost for season extension, solar pool heating remains hard to beat in Queensland, and our heat pump vs gas vs solar comparison pages walk that trade-off honestly. Sizing logic still applies whichever technology you pick.
Who should install a pool heater in Queensland?
The plumbing connections are licensed plumbing work, and any gas pool heater must be installed by a licensed gas fitter to AS/NZS 5601 with the appropriate compliance certificate, including correct gas line sizing, because a 400 MJ heater is one of the hungriest gas appliances on a property and an undersized supply line starves it. Heat pumps also need a licensed electrician for the circuit. Installed pricing depends on unit, location, gas line and electrical runs, so it is quoted on inspection, and our pool heater cost pages set out what drives the number.
Sources: AS/NZS 5601 gas installations; Queensland gas work licensing under the Petroleum and Gas legislation; QBCC licensing requirements for plumbing work.
Want a pool heater sized for July, not just October? Call Hills Plumbing & Gas on 0472 657 042 or book through our Gas Fitting service page for a fixed quote with the sizing maths shown.