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.