Field guide · Hydronic controls

Outdoor reset curve

A reset control raises supply water as it gets colder outside along one straight line. The line is fixed by four setpoints — two outdoor temperatures and two supply temperatures — and the ratio is just the slope between those ends. This guide walks the naming and the field questions; the numbers live in the calculator.

Estimate only

Starting point for commissioning talk-throughs, not a substitute for a licensed hydronic design or the control manufacturer's manual. Clamp ranges, warm-weather shutdown, and boiler-return protection vary by model — check the book for the control you are setting.

What the four setpoints mean

Two outdoor points, two supply points — same line every brand names differently.

OUTDR START — outdoor temperature where heat is first needed (warm end of the curve).
BOIL START — supply water temperature commanded at that outdoor temperature.
OUTDR DSGN — design (coldest) outdoor temperature for the job.
BOIL DSGN — supply temperature required at design conditions.

Weil-McLain boiler controls use those four names. Taco's iSeries-R exposes an adjustable heating curve (typically 0.2 to 2.2). Comfort Calc-style boards present a ratio dial. They all describe the same straight line between the warm end and design.

Ratio is the slope

Reset ratio is degrees of supply rise per degree of outdoor drop:

ratio = (BOIL DSGN − BOIL START)

÷ (OUTDR START − OUTDR DSGN)

supply(T) = BOIL START + (OUTDR START − T) × ratio

A steeper curve is not automatically a warmer building. If rooms are cold at design and fine in the shoulder season, the ratio is low (or BOIL DSGN is low). If they are fine at design and overheating in mild weather, BOIL START is high. The two ends adjust separately — that is why you have four setpoints instead of one knob.

Need the ratio, target supply at today's outdoor temp, and the ten-point curve table from your four setpoints? Run the outdoor reset calculator.

Min and max clamps

Real controls clamp the raw curve. A minimum supply protects a cast-iron boiler from riding low all winter and condensing flue gas (unless the boiler is designed for it or protected by mixing / bypass). A maximum supply protects emitters — radiant slabs and high-temp radiators care about different ceilings.

Order matters: apply the maximum first, then the minimum, so a boiler-protection floor is not overridden by an emitter ceiling. The calculator table shows both the raw curve value and what is actually commanded so you can see when a limit is doing the work.

Wondering what supply the curve (and your clamps) command at a given outdoor temperature? Plug the four points into the outdoor reset calculator.

Give the building time

High-mass systems can take a day or more to settle after a curve change. Change one end at a time and wait before you chase the other. Mixing on a low-temperature loop is a separate question — see the injection mixing GPM → Cv guide.

Next step

What ratio and target supply do your four setpoints actually produce — and where do the min/max clamps bite? Enter OUTDR START, BOIL START, OUTDR DSGN, and BOIL DSGN in the calculator.

Open outdoor reset calculator

Sources

Reset ratio definition and curve language follow Taco iSeries literature. Setpoint naming follows Weil-McLain boiler control manuals. This guide does not invent curve math — run the four points in the calculator.