Outdoor reset is a boiler control feature that lowers or raises the supply water temperature based on outdoor temperature, so the boiler runs cooler on mild days and hotter during cold snaps.
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Quick answer
Outdoor reset is a control strategy used on many modern gas boilers where an outdoor temperature sensor feeds data back to the boiler's control board, which then adjusts the temperature of the water it sends out to radiators or in-floor loops. On a mild Metro Vancouver day, the boiler might supply relatively cool water, just enough to offset the home's modest heat loss. During a cold snap, it ramps supply water temperature up to match the greater heat loss. The goal is matching output to actual need rather than always running at one fixed high temperature.
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A home loses heat to the outdoors at a rate that depends heavily on the temperature difference between inside and outside. On a mild, damp Metro Vancouver day around 8 to 10 degrees, a house doesn't need nearly as much heat input to stay comfortable as it does during a rare cold snap dropping toward freezing. A boiler running at one fixed high supply temperature regardless of outdoor conditions is essentially oversupplying heat on mild days, which can lead to more on/off cycling and, on some systems, rooms that overshoot their target temperature before the thermostat catches up.
Outdoor reset addresses this by continuously recalculating an appropriate supply water temperature based on a reset curve, a relationship between outdoor temperature and desired supply temperature that's configured for the specific home during commissioning.
A reset curve isn't a universal setting, it's tuned to a home's heat loss characteristics, the type of emitters installed (radiant floor loops need lower supply temperatures than cast-iron radiators, for instance), and the desired indoor comfort target. During installation or commissioning, a technician sets reference points, telling the control board roughly what supply temperature to use at a few different outdoor temperatures, and the boiler interpolates between them as conditions change throughout the day.
Getting this curve right takes some judgment and, often, a bit of fine-tuning after observing how the home actually performs. A curve that's too aggressive can leave a home underheated on the coldest days; one that's too conservative loses some of the efficiency and comfort benefit outdoor reset is meant to provide.
Radiant in-floor heating in particular benefits from outdoor reset, since floor systems typically operate best at lower supply temperatures than radiators, and running them too hot for too long can, on some floor coverings, create an uncomfortably warm floor surface or overshoot room temperature. A properly tuned reset curve keeps floor supply temperature appropriately modest on Metro Vancouver's frequent mild, wet days while still ramping up adequately during the occasional cold, clear snap that follows an Arctic outflow.
Combined with a modulating boiler, outdoor reset is part of why newer hydronic systems can feel noticeably steadier and more comfortable than older, simpler on/off boiler setups.
Safety note
Any work on the gas piping, burner, venting, or pressure components of a boiler or gas water heater must be done by a Technical Safety BC licensed gas fitter. CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas, hear hissing near a gas line, or suspect a carbon monoxide problem, leave the home immediately and call FortisBC at 1-800-663-9911 from outside before doing anything else.
No, it's a feature found on many modern boilers with electronic controls, but not all boiler models include it, and even those that do need it properly commissioned to actually function.
It depends on whether the existing boiler's control board supports an outdoor sensor input. Some can be retrofitted, others can't, and a technician can assess your specific unit.
Typically on a north-facing wall or somewhere shaded from direct sun and away from heat sources like dryer vents, so it reads a representative outdoor temperature rather than a skewed local reading.
A properly configured reset curve should still ramp up supply temperature adequately for cold conditions. If a home underheats specifically during cold snaps, the curve likely needs adjustment by a technician, not disabling of the feature.
Outdoor reset typically governs space heating supply temperature. Domestic hot water, whether through an indirect tank or a combi boiler, is usually controlled separately to maintain consistent tank or delivery temperature regardless of outdoor conditions.
The clearest sign is consistent comfort across both mild and cold days without excessive cycling or overheating. If you notice either extreme, ask your technician to review and adjust the reset curve during a service visit.
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