Buffer Tank Sizing

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.

Let's look at sizing a buffer tank for an on/off boiler

V = t(Qhs - Qloadmin)/(500 * delta-T)

Where:

V =Minimum buffer tank volume (gallons)

t = Minimum heat source on time in minutes

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

Qloadmin= 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

Qhs = Boiler output 68,000

Qloadmin = Zone output 5000 Btuh/h

Delta-T = 20f

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

V = 63-gallon tank

 

Sizing a buffer tank for 80,000 Btu 10:1 high efficiency boiler

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

Boiler input/turndown = minimum input

80,000/10 = 8000 minimum input

V = t(Qhsmin - Qloadmin)/(500 * delta-T)

Where:

V = Minimum buffer tank volume (gallons)

t = Minimum heat source on time in minutes

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

Qloadmin= 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

Qhsmin = Boiler output 8,000

Qloadmin = 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 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 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.

What if our boiler of choice is an 80,000 Btu/h output 5:1 turndown boiler.

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

We'll add one more idea. Let's change the Delta-T to 30f

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.

Download Excel Buffer Tank Sizing Worksheet
After downloading open the form and enter the appropriate information in the green boxes on the worksheet.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per-job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc and its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.