Output vs. Supply Temperature
Percentage of rated output (at 82°C standard test condition) across the full temperature range.
Output at Your Selected Supply Temperature
Full Comparison Table
| Emitter Type | TCF | % of Rated | Output per Unit | Required Size | Min. Supply Temp |
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How to Use the Heat Emitter Calculator
Toggle on the heat emitter types you want to compare. All four are selected by default - fin-tube baseboard, panel radiators, fan coils, and radiant floor. Deselect any type that isn't relevant to your project to simplify the comparison chart and tables.
This is the temperature your boiler or heat pump will deliver. For condensing boilers and air-to-water heat pumps, this is often 40–55°C. For a standard non-condensing boiler, it might be 70–82°C. The output curve chart shows every emitter's performance across the full range, so you can see how sensitive each one is to this single variable.
Enter the design room air temperature (usually 21°C) and the room's design heat loss from a heat loss calculation. This lets the calculator show you the actual required size for each emitter type - baseboard length, radiator size, fan coil capacity, or radiant floor tube spacing - not just a percentage comparison.
The chart plots percentage of rated output against supply temperature for every selected emitter. Notice how steeply baseboard and panel radiator curves drop below 60°C, while fan coil drops more gently and radiant floor (already designed for low temperature) holds output well. This visual makes the trade-off between emitter types immediately clear.
The recommendation card identifies the best-performing emitter at your specified supply temperature and flags any emitter that would need significant oversizing to meet the room load. Use this to decide which emitter type fits your project, then proceed to the specific sizing calculator for that emitter - baseboard, radiator, or radiant floor.
Choosing the Right Heat Emitter for Low-Temperature Hydronic Systems
The heat emitter you choose has more influence on a hydronic system's real-world efficiency than the boiler itself. A 98% AFUE condensing boiler paired with undersized baseboard rated for 82°C will run at high supply temperatures most of the season, never achieving condensing mode. The same boiler paired with correctly sized radiant floor or panel radiators can run at 40–50°C nearly all winter, delivering its full rated efficiency.
Why Emitter Type Matters So Much
Every hydronic heat emitter transfers heat to the room through some combination of radiation and convection, and the heat transfer rate always depends on the temperature difference between the water and the room. The relationship is non-linear: smaller temperature differences produce disproportionately smaller heat transfer. Different emitter designs handle this differently. Fin-tube baseboard relies almost entirely on natural convection through closely spaced metal fins - when water temperature drops, the natural convection airflow weakens along with the heat transfer, compounding the loss. Fan coils use forced convection, so even with reduced water temperature, the fan keeps air moving briskly across the coil, partially compensating. Radiant floor and panel radiators sit in between, with substantial radiant heat transfer that's less temperature-sensitive than pure convection.
The Power-Law Correction
Industry-standard temperature correction factors (TCF) for hydronic emitters follow a power law: TCF = ((Tsupply − Troom) / (82 − 21))^n, where n is an exponent specific to the emitter type. Baseboard and panel radiators typically use n ≈ 1.3. Fan coils use a gentler exponent around n ≈ 1.1–1.2 because of the forced air assistance. This calculator applies emitter-specific exponents to give an accurate comparison rather than treating all emitters the same way.
Ranking Emitters for Low-Temperature Performance
From best to worst performance at reduced supply temperatures: radiant floor heating performs best because of its enormous surface area - the entire floor radiates heat, so even modest water-to-room temperature differences move significant total heat. Fan coils come next thanks to forced convection. Panel radiators are third, offering reasonable low-temperature performance especially when oversized 20–30% above standard sizing. Fin-tube baseboard performs worst at low temperatures and typically needs the most dramatic oversizing - often doubling the standard length - to work acceptably below 60°C.
Matching Emitters to Heat Source
For a standard non-condensing gas or oil boiler running 70–82°C, any emitter type works fine - they were all designed around that temperature range historically. For a condensing gas boiler aiming for consistent condensing operation, panel radiators or radiant floor are strongly preferred, paired with outdoor reset control to keep supply low on mild days. For an air-to-water heat pump - increasingly common in Canadian retrofits as part of electrification efforts - low-temperature emitters aren't optional. Heat pump efficiency (COP) drops sharply as required supply temperature rises, so radiant floor or generously sized panel radiators are essential to achieve the COP improvements that justify the heat pump investment.
Mixed-Emitter Systems
Many Canadian retrofit projects end up with mixed emitter systems: existing baseboard retained in some rooms, new radiant floor added in a renovated kitchen, perhaps a fan coil added for a finished basement. This is workable, but the system needs to run at the highest supply temperature required by any single emitter unless you use mixing valves or a buffer tank with multiple temperature zones. Use the primary-secondary piping calculator to design multi-temperature zone systems properly.
Practical Oversizing Strategy
If you want to run a low-temperature system but the existing baseboard or radiators can't be replaced economically, oversizing is the practical compromise. A panel radiator sized 30% above standard 82°C rating will deliver full design output at around 65°C instead of 82°C - not as low as radiant floor, but a meaningful step toward condensing operation. Use the radiator sizing calculator and baseboard heating calculator to work out exact oversizing requirements for your target supply temperature.
Frequently Asked Questions
Radiant floor heating performs best at low temperatures due to its large surface area - it can deliver full design output at 35–45°C supply. Fan coils come next, since the fan compensates for reduced water-to-air temperature difference, working well down to about 45–50°C. Panel radiators need roughly 55–65°C, though oversizing helps. Fin-tube baseboard performs worst, typically needing 70°C or more unless lengths are increased substantially. This ranking is why radiant floor and properly sized panel radiators are preferred for Canadian homes pairing condensing boilers or heat pumps. Use this calculator's chart to see the exact trade-off at your target supply temperature.
All hydronic emitters follow a power-law relationship with temperature, but the exponent differs by type. Baseboard and panel radiators use an exponent around 1.3, so output drops faster than linearly - at 60°C (down from 82°C standard) output falls to roughly 45–50% of rated. Fan coils have a gentler exponent near 1.1–1.2 due to forced convection, retaining around 55–60% at the same temperature. This calculator applies the correct exponent for each emitter type and shows the resulting percentage and actual output directly.