We are showing how to size a buffer tank when you have an on/off or modulating boiler. When sizing the buffer tank, it should be sized for the smallest zone and a pre-determined run time. The runtime is your choice, usually between 10 and 20 minutes.

Buffer tanks reduce short cycling of the boiler. 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 the less chance of a need for 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 (Btuh)

Q_loadmin= Minimum concurrent heating load when heat source is in demand Btuh

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 Bth/h

Delta-T = 20f

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

V = 63 gallon tank

When calculating minimum input of a high efficiency boiler divide the input by ther turndown. Example of 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 Btuh

Q_loadmin= Minimum concurrent heating load when heat source is in demand Btuh

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 BTh/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 if all were the same except a 20-minute run time. would be as follows.

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

V = 12 gallon gallon tank

You can see to get a better run time would not be a drastic tank size change. You can see a delta-T change 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-T's will change the tank size. You can play with the numbers to see what fits your application best.