Click a preset to auto-fill the design supply temperature above — or enter your own value directly if you've already sized emitters.
Reset Curve: Supply Temperature vs. Outdoor Temperature
Shows the calculated linear reset curve, with condensing threshold and control floors marked.
Reset Curve Reference Points
| Outdoor Temp | Supply Temp | Est. Return Temp | Condensing? |
|---|
Full Calculation Breakdown
| Parameter | Value | Notes |
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How to Use the Outdoor Reset Calculator
Enter your local design outdoor temperature — the coldest condition your heat loss calculation targets — and design indoor temperature, typically 21°C. These two anchor one end of the reset curve. Use the heat load calculator if you haven't confirmed your design heat loss and matching design temperature yet.
This is the supply water temperature your heat emitters need at design (coldest) conditions. Click an emitter type preset for a quick estimate, or enter your own value if you've already sized emitters using the heat emitter calculator or a specific baseboard/radiator/radiant floor calculator.
The minimum supply temperature floor prevents the control from calling for unrealistically low temperatures on mild days, which can cause sluggish response or short-cycling at very low output. The warm-weather shutoff temperature is the outdoor condition above which the boiler turns off entirely, since no heating is needed.
The chart plots supply temperature against outdoor temperature across the full range from design cold to warm-weather shutoff, with the ~55°C condensing threshold marked. This shows visually how much of the heating season the boiler can spend in condensing mode under this reset curve.
Most boiler outdoor reset controls accept two or more temperature pairs to define the curve — typically a "design point" (coldest outdoor temp / highest supply temp) and a "warm weather point" (mild outdoor temp / lowest supply temp). Use the reference points table to program these values into your specific boiler's control interface.
Outdoor Reset Control for Canadian Hydronic Systems
Outdoor reset control is one of the simplest and most impactful efficiency measures available for a hydronic heating system, yet it's frequently left unconfigured or poorly tuned even on boilers that support it. Properly designing and programming the reset curve is the key to unlocking a condensing boiler's rated efficiency across the full range of Canadian winter conditions, not just at one fixed operating point.
Why a Fixed Supply Temperature Wastes Energy
Without outdoor reset, a boiler runs at one fixed supply temperature — often set conservatively high to guarantee comfort on the coldest day — regardless of whether it's -25°C or +5°C outside. On a mild day, the building needs only a fraction of its design heat loss, but a fixed-temperature boiler still delivers full-temperature water, causing the heat emitters to overshoot room temperature until the thermostat cuts the call short. This produces frequent short cycling and, critically for a condensing boiler, keeps return water above the roughly 55°C condensing threshold on the very days — mild ones — that make up the majority of a typical Canadian heating season. See the condensing boiler calculator for the relationship between return temperature and boiler efficiency that outdoor reset directly influences.
How the Reset Curve Is Derived
The reset curve is a straight-line relationship (in most residential and light commercial controls) between outdoor temperature and required supply temperature, anchored by two points: the design point (design outdoor temperature paired with design supply temperature) and the warm-weather point (typically the outdoor temperature above which heating stops entirely, paired with the minimum supply temperature the control will allow). Everything between these two points follows a straight line, meaning the reset ratio — degrees of supply temperature change per degree of outdoor temperature change — is constant across the curve. This calculator computes that ratio and the resulting curve directly from your entered design parameters, rather than requiring you to look up or guess a generic value.
The Physics Behind the Curve's Slope
The reset curve's slope reflects a simple proportionality: at any outdoor temperature, the building's heat loss is proportional to the difference between indoor and outdoor temperature, and the heat emitters' output is (for baseboard and radiators) roughly proportional to a power of the difference between water temperature and room temperature. Balancing these two relationships across the full range from design temperature to warm-weather shutoff produces the linear (or near-linear) reset curve that outdoor reset controls implement. Locations with a large design temperature swing relative to their design supply temperature rise — for instance Winnipeg's steep -33°C design condition — produce a shallower reset ratio than a milder climate needing the same supply temperature rise across a smaller outdoor temperature range.
Minimum Supply Floor and Warm-Weather Shutoff
Extending the reset curve down to a theoretically calculated very low supply temperature on mild days isn't always practical. Most systems benefit from a minimum supply temperature floor — commonly 25-35°C — below which the control won't go, ensuring reasonable emitter response time and avoiding excessively low flow temperatures that can cause control instability or inadequate heat transfer in some emitter types. The warm-weather shutoff temperature defines the point above which the system turns off entirely rather than continuing to modulate down to an ever-lower supply temperature that would deliver negligible heat anyway. Both settings should be confirmed against your specific boiler manufacturer's control documentation, since exact terminology and configuration methods vary between brands.
Reset Curve and Heat Emitter Compatibility
The design supply temperature anchoring the cold end of the curve must match what your heat emitters actually need at design conditions — this is why the calculator includes quick-fill presets by emitter type. A reset curve calculated around a radiant floor's 45°C design supply keeps the boiler in the condensing zone across almost the entire heating season, since even design-day supply sits below the 55°C threshold. A curve anchored to fin-tube baseboard's 82°C design supply spends much more of the season above the condensing threshold, since even a moderately cold day still calls for a fairly high supply temperature on that steeper curve. See the heat emitter calculator to compare how different emitter choices affect the achievable reset curve and resulting condensing performance.
Outdoor Reset and Boiler Turndown Working Together
Outdoor reset and a modulating boiler's turndown ratio address complementary aspects of part-load performance. Reset control lowers the water temperature the boiler targets as outdoor conditions moderate, which helps condensing efficiency. Turndown ratio lets the boiler reduce its firing rate to match the reduced heat demand at that lower temperature without short-cycling. A boiler with excellent turndown but no outdoor reset still runs unnecessarily hot water on mild days, missing the condensing efficiency gain. A boiler with outdoor reset but poor turndown may short-cycle at the reduced heat demand even though water temperature is well controlled. Both features working together — reset curve properly configured per this calculator, paired with adequate turndown per the mod-con boiler calculator — deliver the best real-world seasonal efficiency.
Frequently Asked Questions
Outdoor reset control uses an outdoor air temperature sensor to continuously adjust the boiler's supply water setpoint according to a defined reset curve, rather than running at one fixed temperature regardless of weather. On the coldest design day, the curve calls for full design supply temperature to meet peak heat loss. On milder days, it calls for progressively lower supply temperature, since the building needs less heat and emitters can deliver it at lower water temperature. This avoids overheating, short-cycling, and reduced condensing performance that occurs when a boiler runs at one fixed high temperature year-round. Use this calculator to derive the correct curve for your specific building and emitters.
Reset ratio — how many degrees supply temperature changes per degree of outdoor temperature change — is determined by your specific heat loss and design temperature relationship, not picked arbitrarily. It equals the design supply temperature rise above room temperature divided by the design temperature difference between indoor and outdoor design conditions. A large design temperature swing (like Winnipeg's -33°C) relative to a modest supply temperature rise produces a shallower ratio than a milder climate needing the same rise across a smaller range. This calculator derives the correct ratio automatically from your entered design heat loss parameters — see the heat emitter calculator to confirm your design supply temperature requirement first.