Measured Energy Savings from Outdoor Resets in Modern, Hydonically Heated Apartment Buildings

Since 1982 the Minneapolis Energy Office and Minnegasco have been testing outdoor reset and cutout controls in modern, hydronically heated apartment buildings. The tests show that in most buildings the reset and cutout together reduce space heating costs by 10 to 20 percent with a payback of less than one year.

An outdoor reset varies the temperature of the water in the heating distribution system in response to outdoor temperature. An outdoor cutout shuts off the circulating pumps and prevents the boiler from firing when the outdoor temperature is warm enough that no heat is needed. Outdoor resets that control the temperature of the boiler water directly were installed in three buildings heated with gas-designed cast iron boilers, along with outdoor cutouts. The devices were evaluated by running the buildings alternately under reset/cutout control and constant temperature control at two week intervals over two heating seasons. Annual savings averaged 6700 Btu/sqft-yr, or 18% of annual space heating costs. At the present price of $0.585 per therm, the savings in dollars ranged from $159 to $1393 per year. The reset and cutout together cost $450 installed, so in all cases but one the estimated payback was less than one year. Limited tests comparing the reset and cutout together with the reset alone were inconclusive.

Temperature data from hallways, apartments and boiler rooms indicate that the outdoor reset improves the seasonal efficiency of the boiler, reduces heat loss from the distribution piping, and limits the tenants' ability to keep their apartments at excessive temperatures.

The savings estimates for cast iron boiler systems are supported by analyses of monthly gas data for five other buildings in which resets were installed. These buildings already had cutouts, and the boiler water temperatures had been reset manually to varying degrees in the heating season before the automatic reset devices were installed, so the savings would be expected to be somewhat less. Savings averaged 6200 Btu/sqft-yr, or 10% of total annual gas use (approximately 13% of annual space heating cost). All five buildings showed paybacks of less than one year for the reset alone.

A different type of outdoor reset that mixes hot boiler water with cooler return water to provide the desired supply temperature was installed in a building heated by a steel fire tube boiler. The building was run alternately in manual reset mode and automatic reset mode over one heating season. Savings were estimated at 4600 Btu/sqft-yr or $1025, for a system with an installed cost of about $3000.
An outdoor reset is probably the most cost-effective major retrofit for hydronically heated apartment buildings with cast iron boilers. Further work is needed to assess the cost-effectiveness for buildings with steel fire tube boilers.

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Measured Energy Savings from Outdoor Resets in Modern, Hydonically Heated Apartment Buildings