Ducted heating is sized by calculating your home’s heat load — not by guesswork. Get it right and the home is warm and efficient; get it wrong and you over-spend or never get comfortable. Here’s what proper sizing accounts for.
7 min read FreshDuct Melbourne Melbourne, Victoria
Ducted heating is sized by calculating your home’s heat load — not by guesswork. Get it right and the home is warm and efficient; get it wrong and you over-spend or never get comfortable. Here’s what proper sizing accounts for.
CalculatedHeat load, not a guess
Too Big = WasteShort-cycles, costs more
Too Small = ColdNever quite copes
Ducted heating is sized by calculating your home’s heat load — floor area and layout, ceiling height, insulation, windows and orientation, and climate. It’s a calculation, not a guess, because both undersizing and oversizing cause problems.
Why Sizing Matters
Correctly sizing ducted heating is one of the most important parts of getting a system that’s warm, efficient and reliable. Size it right and the home heats evenly and economically; size it wrong — too big or too small — and you either waste gas and short-cycle, or never quite get comfortable on cold days. Sizing is a heat-load calculation for your specific home, not a guess from floor area, because the home’s characteristics all affect how much heating it needs (see the factors above).
What Sizing Depends On
Proper sizing accounts for several things: the floor area and layout (open-plan spaces and larger homes need more output); ceiling height (higher ceilings mean more air volume to heat); insulation levels (a leaky, poorly insulated home needs more); window area and orientation (large or unshaded glass loses heat); and Melbourne’s climate, which sets the design heating load. A calculation weighing all of these gives the right capacity — far more reliable than a rule of thumb.
The Cost of Undersizing
An undersized heater can’t keep up on the coldest days — it runs constantly and still leaves the home cooler than you want, exactly when you need it most. Running flat out also wears the unit and costs more for the comfort delivered. Undersizing usually happens when capacity is chosen to save money upfront, but it costs in comfort and longevity. The right size avoids this.
The Cost of Oversizing
An oversized heater warms the home fast then shuts off, cycling on and off frequently because the space cools and calls for heat again soon after. This short-cycling wastes gas, increases wear, and gives less even comfort than a right-sized unit running longer, steadier cycles. ‘Bigger to be safe’ is a common but costly mistake. See our short-cycling guide.
Getting It Sized Right
We size ducted heating with a heat-load calculation for your actual home — its area, layout, ceilings, insulation, windows and orientation — rather than copying an old unit or guessing. Correct sizing is the foundation of comfortable, efficient heating that lasts. If you’re replacing a heater, it’s the ideal time to size correctly for the home as it is now. Call 0431 918 137 or request an assessment. See our choosing guide.
Frequently Asked Questions
How is ducted heating sized for a home?
By calculating the home’s heat load — how much heating output (in megajoules or kilowatts) it needs to stay warm in Melbourne’s winter. The calculation accounts for floor area and layout, ceiling height, insulation levels, window area and orientation, and the climate. It’s a proper calculation, not a guess from floor area alone, because getting the capacity right is what makes the system efficient and comfortable.
What happens if my ducted heater is too big?
An oversized heater warms the home very quickly then shuts off, and because the home cools and calls for heat again soon after, it cycles on and off frequently (short-cycling). This wastes gas, increases wear, and gives less even comfort than a right-sized system running steady cycles. Bigger is not better with heating — correctly sized is. See our short-cycling guide.
What happens if my ducted heater is too small?
An undersized heater runs constantly and still struggles to warm the home on the coldest days, leaving you uncomfortable when you most need heat, and wearing the unit by running it flat out. It also tends to cost more for the comfort it does deliver. Matching the capacity to the home’s actual heat load avoids this. Correct sizing is the foundation of comfortable, efficient heating.
Does insulation affect what size heater I need?
Yes, significantly. A poorly insulated home loses heat faster, so it needs more heating output to stay warm — meaning a larger system and higher running costs. Improving insulation can reduce the heating capacity required and the running cost. This is why sizing accounts for insulation, and why insulation and heating are worth considering together. See our insulation library.
Can I just match the size of my old ducted heater?
Not always — like-for-like sizing can carry over an old mistake, and your home may have changed (extensions, new insulation, replaced windows). A fresh heat-load calculation ensures the new system is correctly sized for the home as it is now. We size to your actual home rather than simply copying the old unit. Call 0431 918 137 or request an assessment.
Ducted Heating Problem or Service? Talk to FreshDuct
Service, repairs, carbon monoxide testing & replacement across Melbourne — 7 days a week. Call or request a quote.
