Calibration of an Outdoor Reset in a Steel Fire Tube Boiler with a 3-way Mixing Valve

The Minneapolis Energy Office has installed outdoor resets and 3-way mixing valves in three hot water multi-zone buildings with steel fire tube boilers in order to determine the cost and savings of this retrofit. The particular reset used was a sophisticated electronic control with adjustable reset ratio, adjustable intercept of the reset curve, and night setback. In the two systems installed in 1984, problems were encountered in the reset's operation and calibration. In both buildings the reset initially failed to follow the selected reset curve at all. From our experience with these buildings, it is clear that a method of determining whether the control is actually resetting correctly is needed.

In the first building, 59, the reset was operating in a very sporadic manner. The temperature of the water after the mixing valve appeared to have no relation to the outdoor temperature. The controller also was resetting too much, so that the tenants were not getting enough heat. In the second building, 10, again the distribution water temperature was not following the reset curve. However in this building, the tenants had no complaints of insufficient heat, so the problem was not obvious. This report describes for each building the process of discovering the problem, graphically displaying the data, and having the problem fixed. A suggested procedure for determining whether a control and 3-way mixing valve are working correctly is also given.

Full Report (PDF)
Calibration of an Outdoor Reset in a Steel Fire Tube Boiler with a 3-way Mixing Valve