We are showing how to size a buffer tank when you have an on/off or modulating boiler. When sizing the buffer tank, use the smallest zone and a predetermined run time. The runtime is at your discretion, typically ranging from 10 to 20 minutes.

Buffer tanks reduce boiler short cycling. Most boilers are normally 100% or more oversized for the heating system. If the boiler is properly sized, short cycling will automatically be reduced. When a high-efficiency boiler is used, the higher the turndown ratio, the less likely it is to require a buffer tank.

V = t(Q_hs - Q_loadmin)/(500 * delta-T)

Where:

V =Minimum buffer tank volume (gallons)

t = Minimum heat source on time in minutes

Q_hs = Rated heat output of heat source (Btu/h)

Q_loadmin= Minimum concurrent heating load when heat source is in demand Btu/h

Delta-T = Change in average tank temperature during minimum heat source on time (f)

Let's assume the following:

t = 10-minute recovery time

Q_hs = Boiler output 68,000

Q_loadmin = Zone output 5000 Btuh/h

Delta-T = 20f

V = 10(68,000 - 5000) / (500 * 20f)

V = 63-gallon tank

When calculating the minimum input of a high-efficiency boiler, divide the input by the turndown. Example of the above boiler,

Boiler input/turndown = minimum input

80,000/10 = 8000 minimum input

V = t(Q_hsmin - Q_loadmin)/(500 * delta-T)

Where:

V = Minimum buffer tank volume (gallons)

t = Minimum heat source on time in minutes

Q_hsmin = Minimum stable heat output of heat source Btu/h

Q_loadmin= Minimum concurrent heating load when heat source is in demand Btu/h

Delta-T = Change in average tank temperature during minimum heat source on time (f) Note: The smallest zone must be less than the minimum output of the boiler or no buffer tank is required.

Let's assume the following:

t = 10-minute recovery time

Q_hsmin = Boiler output 8,000

Q_loadmin = Zone output 2000 Btu/h

Delta-T = 20f

V = 10(8,000 - 2000) / (500 * 20f)

V = 6-gallon tank

Although this will run longer cycles, I am not happy with a high-efficiency boiler running a minimum of 10 minutes. I would choose 15 - 20 minutes run times.

The difference would be if all were the same except for a 20-minute run time. would be as follows.

V = 20(8,000 - 2000) / (500 * 20f)

V = 12-gallon tank

You can see that getting a better run time would not be a drastic tank size change. You can see that changing the delta-T will change the tank size and run time.

Let's apply this.

V = 10(18,000 - 2000) / (500 * 20f)

V = 16-gallon tank

But, a 20-minute runtime would change the tank size to:

V = 20(18,000 - 2000) / (500 * 20f)

V = 32-gallon tank

V = 20(18,000 - 2000) / (500 * 30f)

V = 21.3-gallon tank

Changing run times or tank delta-Ts will change the tank size. You can adjust the numbers to find the best fit for your application.