Condensing Boiler Plant Piping Design & Control Part 7: Calculating Boiler Cycle Time

By Chad Edmondson

How long should a boiler run before it cycles off?  And how long should it stay off before it cycles back on?

The answers to these questions will of course vary from system to system.  As a general rule, the combination of boiler run time and off time should never be any less than 10 minutes. 

Under normal operating conditions, the run time will always be dictated by two factors:

(1) Total System Volume

(2) Boiler Supply Temperature Differential

This principal goes back to our earlier blog on Specific Heat.  Remember, for every Btu that you add to a pound of water, you will increase its temperature by 1 degree.  And since there are 8.33 pounds of water in a gallon, it takes 8.33 Btus to raise the temperature of a gallon of water by one degree. 

Given the law of specific heat, you must determine the storage capacity necessary for your system in order to make sure the boiler runs at least five minutes between the start and stop of your boilers at no load conditions-- in other words, the boiler supply differential.

To understand this a little better, let’s consider a system with the following operating parameters at NO LOAD:

System Volume = 100 Gallons

System Delta T = 12° (174° Boiler ON, 186° Boiler OFF)

Based on what we learned earlier about specific heat, we can calculate that for every 100 gallons of system volume we have, we get 10,000 Btus of storage:

cycle time calc.jpg

But what about calculating boiler cycle time?   Keep in mind we have to have at least enough btu storage capability in the system to keep the boiler operating within our 12° delta T range at a minimum system load – without short cycling or going off on high limit.    It is the relationship between System Volume, Btu Output, and the System's Delta-T that determines what our run time should be.  Here are the following equations for calculating boiler cycle time:

on time off time.jpg

The 500 is a constant that comes from multiplying the weight of one gallon of water times the number of minutes in an hour, times the specific gravity, times the specific heat of the fluid.

The resulting value is your minimum run time and off time.  Just make sure that the total of these two values is at least 10 minutes or you might have to add a buffer tank.

For more information watch this video:

Condensing Boiler Variable Primary System Design