Hydronic radiator sizing: how to size a radiator for every room (2026)

Radiator sizing comes down to matching two numbers and getting a third one right. The first is the room's heat loss: how many watts the room loses to outside on the coldest design day, at the temperature you want to keep it. The second is the radiator's real output. Not the headline catalogue figure, but what it delivers at the water temperature your heat pump or boiler will actually run. The third is that water temperature itself, which is a design choice. Run water hotter and radiators can be smaller; run it cooler and a heat pump becomes more efficient, but the radiators must grow.

Get those right and a radiator system is quiet, fast and economical. Get them wrong and you'll have a room that never quite gets there on a July morning, or a heat pump forced to run hot and expensive to make up for undersized panels. This guide walks through each step so you can sense-check a quote, and see why we design from a calculation rather than a rule of thumb.

Every room loses heat in two ways: through its fabric (walls, ceiling, floor and windows) and through air leakage. A heat-loss calculation multiplies each surface's area by how readily it lets heat through, and by the gap between the indoor temperature you want and the outdoor design temperature, then adds an allowance for air changes. The factors that move the answer most are:

• Room volume. Floor area matters, but so does ceiling height. A 3.3 m terrace ceiling holds far more air than a 2.4 m project-home ceiling.
• Insulation. Ceiling batts make the biggest single difference, followed by wall and underfloor insulation. Many older double-brick and weatherboard homes have little or none.
• Glazing. Windows lose heat many times faster than an insulated wall. Large single-glazed sliders are often the dominant loss in a living room.
• External walls. A corner room with two external walls loses more than an internal room of the same size.
• Air leakage. Draughty floorboards, open fireplaces, old exhaust fans and gaps around doors all add load.
• Climate and setpoint. A bathroom kept warmer than a bedroom, in Canberra rather than Sydney, needs much more heat per square metre.

For early budgeting, a ballpark helps. The table gives indicative heat loads per square metre of floor area for living areas held at about 20–21 °C, with standard 2.4–2.7 m ceilings, at each city's winter design conditions. Treat it as a sense-check only. Real rooms vary widely, and the room-by-room heat-loss calculation is what we actually design from.

Move to the top of the range, or beyond it, for corner rooms, large glazed areas, high ceilings and exposed sites, and for bathrooms kept warmer than living areas. Bedrooms kept cooler sit lower. A 15 m² study in a 1990s Melbourne home, for example, lands somewhere around 1,200–1,650 W; the calculation pins down where.

The same house needs very different radiators in different cities, because heat loss scales with the gap between inside and outside on the design day. The National Construction Code divides Australia into eight climate zones. On the ABCB climate zone map, Sydney East is zone 5 (warm temperate), Sydney West and Melbourne are zone 6 (mild temperate) and Canberra is zone 7 (cool temperate).

In practice, a coastal Sydney home has a mild design day, so modest panels go a long way and the bigger risk is over-specifying. Melbourne's long, grey winters and many lightly insulated older homes push loads up and make running cost matter more. Canberra's sub-zero mornings mean radiators must be sized generously, especially if a heat pump will run them at low temperature. Western Sydney and the Blue Mountains behave more like Melbourne than the harbour. Our hydronic radiators Sydney and hydronic radiators Canberra pages cover the local detail.

European panel radiators, including the IMAS range, are tested to EN 442, and their headline output is quoted at ΔT50. ΔT (delta-T) is the difference between the radiator's mean water temperature and the room air. EN 442's standard test condition is water flowing in at 75 °C and out at 65 °C, a mean of 70 °C, in a 20 °C room: 70 minus 20 is 50 K, hence ΔT50.

That test point suits an old high-temperature boiler, but modern systems rarely run that hot. A condensing gas boiler only condenses, and reaches its rated efficiency, when return water is below about 55 °C. A heat pump is most efficient at lower flow temperatures still, and on modern heat-pump radiator systems we typically design for flow temperatures around 45–55 °C. At those temperatures the ΔT is far smaller than 50, and output falls faster than most people expect.

Radiator output doesn't fall in a straight line with temperature. EN 442 describes it with a power law: output at your actual ΔT equals the ΔT50 output multiplied by (actual ΔT ÷ 50) raised to an exponent, n, which the manufacturer measures for each model. For steel panel radiators n is typically around 1.3, and the published IMAS panel radiator data is consistent with that. Using n = 1.3 and a 20 °C room:

The manufacturer data backs this up. IMAS's vertical Type 22 panel, 600 mm wide and 1,800 mm tall, is listed at 2,376 W at 75/65 °C but 1,223 W at 55/45 °C, about 51%. A radiator picked from the catalogue at ΔT50 and run on a heat pump delivers roughly a third to a half of the number on the box. That's not a reason to avoid radiators with a heat pump; it's a reason to size them for it. Larger or deeper panels let a hydronic heat pump run cooler water, which lifts its COP and lowers running costs for the life of the system. Rooms set warmer than 20 °C shrink the ΔT further, so bathrooms need extra margin.

Steel panel radiators are described by a two-digit type. The first digit is the number of water-filled panels, the second is the number of convector fin layers welded behind them. More panels and fins mean more output from the same wall footprint, at the cost of depth and price. The IMAS steel panel range covers all four common types in heights from 300 to 900 mm and lengths to 3,000 mm, alongside the flat-fronted IM11, IM21 and IM22 panels.

*Outputs and depths from the IMAS flat-panel datasheet (IM11, IM21 and IM22, 600 mm high × 1,000 mm long). Compact steel panels of the same type sit in a similar range, but always size from the datasheet for the exact model.

The K22 is the default for good reason: it nearly doubles a K11's output for about 30 mm of extra depth. When a room needs more watts than the wall can fit at low water temperature, a K33 is the answer, or two radiators on different walls. Browse sizes and finishes on our IMAS radiators page; the range is held in Australian stock.

Take a 5 × 4 m (20 m²) living room in a 1990s Melbourne brick-veneer home with ceiling batts and a single-glazed sliding door. The table above suggests 80–110 W/m². Suppose the heat-loss calculation lands at 100 W/m², or 2,000 W at design conditions.

• Gas boiler at 70/60 °C (about 87% of rated output): you need at least 2,300 W at ΔT50. One K22 600 × 1,600 mm (2,601 W in the IMAS IM22 data) covers it with margin.
• Heat pump at 50/45 °C (about 46%): you now need roughly 4,350 W at ΔT50, for example two K22 600 × 1,400 mm panels at 2,275 W each, one under the slider and one on the other external wall.
• Heat pump at 45/40 °C (about 35%): the requirement climbs to around 5,700 W at ΔT50. That's two K22 600 × 1,800 mm panels, a K33, or an insulation and glazing upgrade that shrinks the load first.

Same room, same comfort, and up to two and a half times the rated radiator capacity depending on water temperature. That's why the heat source and radiators must be designed together, and why a quote listing radiator sizes without a design flow temperature isn't telling you enough. Our hydronic heating Melbourne team designs every job from a room-by-room calculation.

The traditional spot under the window is still the best one in most Australian homes. Glass is the coldest surface in the room. A radiator beneath it warms the cold air sliding down the pane before it pools across the floor, and offsets the chill you feel sitting near the glass. Where you can, match the radiator's length to the window's width. In well-insulated, double-glazed homes, placement is more flexible and an internal wall can work.

A few rules protect the output you've paid for:

• Leave clear space below the radiator and under any sill so air can circulate, and follow the manufacturer's bracket heights.
• Keep sofas and beds off the face of the radiator, and avoid full-length curtains that drop in front of it.
• Decorative covers and deep shelves cut output. If you want one, size up.
• Plan pipe routes early. A manifold-and-home-run layout keeps joins out of walls and makes future changes easy.

See how we install panels, columns and towel rails on our hydronic radiator heating page.

Thermostatic radiator valves (TRVs) give each room its own control. The head senses air temperature and throttles flow as the room approaches its setting. Fit one on every radiator except in the room with the main thermostat, so the two don't fight. TRV numbers are relative settings, not exact temperatures. On heat-pump systems, enough circuits (or a bypass) must stay open to keep minimum flow through the unit, and the lockshield valve at the other end is set once at commissioning to balance the system. Our hydronic valves and controls range covers TRVs, balancing valves and manifolds.

Towel rails are excellent at drying towels and modest at heating rooms. A ladder rail has far less surface than a panel of the same size, and draped towels block much of what it does emit. In a cold bathroom, especially on a low-temperature heat pump, pair a towel rail with a panel radiator or underfloor heating rather than relying on it alone. IMAS Alubain aluminium towel warmers come in heights from 745 to 1,735 mm, with an optional electric element for summer drying when the hydronic system is off.

Weighing radiators against slab heating? Our comparison of underfloor heating vs radiators sets the two side by side.

• Sizing from the ΔT50 figure on a heat-pump system. It's the most common and most expensive error.
• Using floor area alone and ignoring glazing, ceiling height and exposure.
• Copying the old radiator sizes when converting from gas to a heat pump.
• Undersizing to save on panels, which forces higher flow temperatures and higher running costs for decades.
• Oversizing without a reason, which costs money and wall space. Deliberate, calculated oversizing to lower the flow temperature is different, and often worth it.

For a room-by-room calculation and a radiator schedule for your home, browse our hydronic radiators range and send us your floor plans. We'll come back with a fixed-fee design and itemised quote.

• IMAS radiator datasheets: flat-panel (IM11, IM21, IM22) outputs at ΔT50, and vertical-panel outputs at 75/65, 70/55 and 55/45 °C.
• EN 442, the European standard for testing and rating radiators, including the ΔT50 reference condition and the exponent method for other temperatures.
• The Australian Building Codes Board climate zone map (linked above) for NCC zones by city.
• Heat pump and boiler manufacturer guidance on flow temperatures and condensing operation.
• AHHAC's own heat-loss design and commissioning experience on radiator systems across NSW, the ACT, Victoria and Queensland.

This guide was written and reviewed by the AHHAC Technical Team, licensed plumbers and hydronic engineers (NSW Lic 262528C). It will next be reviewed before winter 2027.