that it can work with, thus minimizing the short cycling. Another benefit of these “new thermostats” is that they also can request a water temperature back to the boiler control. The control receives these requests from all the various zones and makes a decision based upon the zone with the highest temperature requirement. The other “lower temperature” zones then calculate their own on-time with the higher temperature water to maintain their desired setpoint. The other option would be to provide some mass to Some might say that the additional cost would be prohibitive, but the benefits certainly outweight the equipment costs. the system so that each time the boiler fired, it would have to raise the temperature of this mass X number of degrees. If the added mass is calculated correctly, the boiler would not short cycle. This added mass is known as a “Buffer Tank”. How much mass is needed? How big should this Buffer Tank be? It is actually a rather easy number to establish. It is based upon a couple of pieces of information: 1. The minimum firing on-time of the boiler (usually ten minutes), 2. BTU/H output of the boiler, 3. The minimum btu/h load of the smallest zone calling and Boiler Facts 4. The desired/acceptable temperature rise (delta T) of the tank (usually 20-40°F). When you plug in all the necessary numbers, the answer will be the suggested size of the buffer tank in gallons. For example, if we had a boiler with a BTU/H input of 150,000 and the smallest zone load was 5000 btu/h and the system could accept a 30°F rise, what size buffer tank would be needed? V= 10 (150,000-5000) = 96 gallons 30 x 500 In this particular system, with a 100 gallon buffer tank and the smallest load calling, the boiler would run for a minimum of 10 minutes and operate more efficiently. Some might say that the additional cost of the tank would be prohibitive, but when compared to the life of the system, the continuing rise in the cost of energy and the efficiency points gained by the longer “on-times”, the benefits certainly outweigh the additional equipment costs. If you have any questions or comments, e-mail me at gcarey@fiainc.com or call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM ICM/Sept/Oct 2015 13
ICM September-October 2015
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