Hydronic controls · P1
Outdoor reset curve
A reset control does one thing: raise supply water temperature as it gets colder outside, along a straight line fixed by four setpoints. Enter those four and you get the ratio, the target supply at any outdoor temperature, and the whole curve.
Curve setpoints
The four values every reset control asks for, whatever it calls them.
Warmest outdoor temperature that still calls for heat.
Supply temperature at OUTDR START.
Design outdoor temperature for the location.
Supply temperature the emitters need at design.
Optional
Evaluate a specific outdoor temperature, and apply the control's clamps.
Leave blank to skip. Useful for checking a control against a thermometer.
Protects a non-condensing boiler from sustained flue-gas condensation.
Protects the emitters — radiant floors especially.
Results
Supply water rises 1.14°F for every 1°F the outdoor temperature drops.
The curve
Ten points from start to design, for a clipboard or a control screen.
| Outdoor °F | Curve °F | Commanded °F | Limit |
|---|---|---|---|
| 70.0 | 100.0 | 100.0 | |
| 62.2 | 108.9 | 108.9 | |
| 54.4 | 117.8 | 117.8 | |
| 46.7 | 126.7 | 126.7 | |
| 38.9 | 135.6 | 135.6 | |
| 31.1 | 144.4 | 144.4 | |
| 23.3 | 153.3 | 153.3 | |
| 15.6 | 162.2 | 162.2 | |
| 7.8 | 171.1 | 171.1 | |
| 0.0 | 180.0 | 180.0 |
The arithmetic
ratio = (BOIL DSGN − BOIL START)
÷ (OUTDR START − OUTDR DSGN)
supply(T) = BOIL START + (OUTDR START − T) × ratio
Clamps are applied after the curve: the maximum first, then the minimum, which is the order that keeps a boiler-protection minimum from being overridden by an emitter-protection maximum. The table shows both the raw curve value and what actually gets commanded, so you can see when a limit is doing the work.
Controls name these four setpoints differently — Weil-McLain boiler controls use OUTDR START / BOIL START / OUTDR DSGN / BOIL DSGN, Taco's iSeries-R exposes an adjustable heating curve from 0.2 to 2.2, and Comfort Calc-style controls present a ratio dial. They all describe the same line.
Notes from the field
A steeper curve is not a warmer building. If rooms are cold at design and fine in the shoulder season, the ratio is low. If they are fine at design and overheating in mild weather, BOIL START is high. The two ends adjust separately — that is the point of having four setpoints instead of one.
Mind the minimum on a cast-iron boiler. A reset curve that rides low all winter will condense flue gas unless the boiler is designed for it or protected by a bypass or a mixing device.
Give it time between changes. A high-mass system can take a day or more to settle. Change one end of the curve at a time and wait.
Sources
- Taco — OVERVIEW iSeries-R (Outdoor Reset) Mixing Valve (PA04), reset ratio definition and curve chart
- Taco — Radiant Made Easy Application Guide, iSeries-S Mixing Valve (PA06)
Setpoint naming follows the Weil-McLain boiler control manuals. Check the manual for the specific control you are commissioning: clamp ranges, warm-weather shutdown, and boiler-return protection all vary by model, and this page applies only the limits you type in.