Outdoor Reset Control: Why a Boiler Should Care What the Weather Is Doing

A boiler that holds 180 degree water on a mild autumn afternoon is working harder than the building needs. The radiators overshoot, the thermostat shuts the burner off, the water cools, and the cycle repeats. Outdoor reset control fixes that by letting the weather decide how hot the boiler water should be.

What Outdoor Reset Does

An outdoor sensor measures the air temperature outside. The reset control uses that reading to set the boiler's target water temperature along a curve: cooler water when it's mild out, hotter water as it gets colder. The building loses heat in proportion to how cold it is, so the system delivers heat in proportion too, instead of running at one fixed temperature all winter.

Why It's Worth Adding

• Less cycling. Lower water temperatures on mild days mean fewer short burner runs and fewer on-off swings.
• Steadier comfort. Emitters run longer at a gentler temperature, which keeps rooms from swinging above and below the setpoint.
• Less standby loss. A cooler boiler and cooler piping lose less heat to the mechanical room between calls.
• Less thermal stress. Fewer hard heat-up cycles are easier on the boiler.

One Caution on Conventional Boilers

On a condensing boiler, lower return temperatures actually improve efficiency. A conventional boiler is different. If the water returns too cold, flue gases can condense inside the boiler and cause corrosion. A reset control on a non-condensing boiler therefore needs a minimum boiler temperature setting that it won't go below, and you should confirm what the boiler manufacturer requires before you set it.

What's in the Kit

Honeywell's Aquareset line is a good example of how these are packaged. The W8735S1000 outdoor reset kit includes the reset module and a C7089U outdoor sensor. The W8735Y1000 wireless kit comes with the W8735ER1000 outdoor reset module. There's also an alarm module, the W8735S3000, listed for use with certain Honeywell oil primary controls and aquastats. Confirm that the kit you choose is listed as compatible with the control already on your boiler, since these modules are designed to pair with specific Honeywell equipment.

Setting It Up

Most reset controls let you adjust the same few things: the boiler water temperature you want at your design outdoor temperature, the lowest boiler temperature you'll allow, and how steeply the curve climbs. Defaults are only a starting point. Plan to adjust after the first cold stretch, based on whether rooms ran cool or the burner was still cycling more than you expected.

Sensor Placement

The sensor reads the weather, so put it where the weather reaches it. A shaded wall, usually north-facing, away from sun, dryer vents, exhaust, and warm air from the building, gives the truest reading. A sensor in direct sun reads warm all afternoon and will tell the control the day is milder than it is.

Where This Fits With the Rest of Your Controls

Reset works alongside the boiler's limit controls, not in place of them. Your aquastat still provides the high limit, and if you run a motorized mixing valve for a radiant zone, the outdoor reset mixing valve approach does the same job at the zone instead of the boiler.

National Boiler Supply carries Honeywell outdoor reset kits and related controls. Send us the model of your boiler control and we'll confirm which kit matches it.